07ddec159 CDMA Concepts (1)

A REPORT

ON

CODE DIVISION MULTIPLE ACCESS

BY:

JAGDISH S.SHETE 07DDEC159

AT

BHARAT SANCHAR NIGAM LTD.

DEHRADUN

AN INTERNSHIP PROGRAM-2 STATION

OF

FACULTY OF SCIENCE AND TECHNOLOGY

DEHRADUN

(29TH OCT,2010)

|JAGDISH S SHETE 07DDEC159 1

A REPORT

ON

CODE DIVISION MULTIPLE ACCESS

BY:

JAGDISH S.SHETE 07DDEC159

PREPARED IN PARTIAL FULFILLMENT OF THE

IP401 INTERNSHIP PROGRAM-2 COURSE

AT

BHARAT SANCHAR NIGAM LTD.

DEHRADUN

AN INTERNSHIP PROGRAM-2 STATION

OF

FACULTY OF SCIENCE AND TECHNOLOGY

DEHRADUN

(29TH OCT,2010)


TABLE OF CONTENT

SERIAL

NO.

CONTENT PAGE

NO

I ACKOWLEDGEMENT 5

1 OVER VIEW OF ORGANISATION 6

2 INTRODUCTION OF CDMA 8

3 BIRTH OF CDMA 8

4 HISTORY OF CDMA 8

5 DIGITAL REVOLUTION AND EVOLUTION 9

6 CDMA PRINCIPLE 10

7 CDMA MODULATION 11

8 CDMA FOR CELLULAR 11

9A NETWORK STRUCTURE DESIGNED FOR

PACKETIZED COMMUNICATION12

10.1 THE MOBILE STATION (MS) 12

10.2 THE RADIO ACCESS NETWORK (RAN) 13

10.3 THE BASE STATION TRANSCEIVER SUBSYSTEM 14

10.4 THE BASE STATION CONTROLLER (BSC) 14

10.5 MOBILE SWITCHING CENTER (MSC) 14

10.6 THE PACKET CONTROL FUNCTION (PCF) 14


10.7 OPERATION AND ADMINISTRATION (OAM) 14

11 SPREAD SPECTRUM PRINCIPLE 15

12DIFFERENT CODES:WALSH CODE ,LONG CODE,SHORT CODE

16

13

ADVANTAGES OF CODES:CAPACITY,VOCODER AND VARIABLE DATA RATES ,LESS (OPTIMUM) POWER PER CELL,SEAMLESS HAND-OFF ,NO FREQUENCY PLANNING ,MULTIPLE DIVERSITY

17

14 PHYSICAL AND LOGICAL CHANNELS: 21

15 DETECTING AND SOLVING SOME COMMON PROBLEMS IN CDMA2000 1X NETWORKS

30

16 FEATURES OF CDMA 34

17 CDMA AND WILL 34

18 CDMA VS GSM (THE OLD HORSE) 35

18.1 VOICE ENCODING ,SPECTRAL EFFICIENCY,IN BUILDING COVERAGE

36

19 ADVANTAGES OF CDMA&DISADVANTAGES 38

20 CONCLUSION 40

21 BIBLIOGRAPHY 41

ACKNOWLEDGEMENTI Would Like To Express My Gratitude To MR. ARVIND KUMAR , DGM ( CMTS), Bharat Sanchar Nigam Ltd , Dehradun For Allowing Me To


Take Up This Course On IP-2(Internship Program 2) And Providing Me With All The Facilities Required For Completing This Course.

Special Thanks Are Due To MR. S.N.UNIYAL, SDE (CMTS )And MR. DINESH KUMAR (JTO) For Allowing Me To Take Up CODE DIVISION MULTIPLE ACCESS And Providing Me With The Guidance At Every Step IP2

Thanks To Prof.G.P.Shrivastva And Prof.R.C.Ramola,Dean And

Prof.Anilesh IP Faculty For Giving This Opportunity Above All, I Am Thankful To The Almighty ,My Friends Who Have Helped Me And Have Been There With Me Throught out The Preparation Of This Report

1. INTRODUCTION

1.1 BSNL


BSNL formed in October, 2000, Bharat Sanchar Nigam Limited (known as BSNL, India Communications Corporation Limited) is a state-owned telecommunication company in India. BSNL is the sixth largest cellular service provider, with over 57.22 million customers as of December 2009 and the largest land line telephone provider in India. Its headquarters are at Bharat Sanchar Bhawan, Harish Chandra Mathur Lane, Janpath , New Delhi. It has the status of Mini Ratna , a status assigned to reputed public sector companies in India.

BSNL is India's oldest and largest Communication Service Provider (CSP). Currently has a customer base of 90 million as of June 2008.It has footprints throughout India except for the metropolitan cities of Mumbai and New Delhi which are managed by MTNL. As on March 31, 2008 BSNL commanded a customer base of 31.55 million Wire line , 4.58 million CDMA-WLL and 54.21 million GSM Mobile subscribers. BSNL's earnings for the Financial Year ending March 31, 2009 stood at INR 397.15b (US$7.03 billion) with net profit of INR 78.06b (US$ 1.90 billion). BSNL has an estimated market value of $ 100 Billion. The company is planning an IPO within 6 months to offload 10% to public in the Rs 300-400 range valuing the company at over $100 billion.

BSNL has set up a world class multi-gigabit, multi-protocol convergent IP infrastructure that provides convergent services like voice, data and video through the same Backbone and Broadband Access Network. At present there are 0.6 million Data One broadband. The company has vast experience in Planning, Installation, network integration and Maintenance of Switching & Transmission Networks and also has a world class ISO9000 certifiedtelecomTrainingInstitute.

Scaling new heights of success, the present turnover of BSNL is more than Rs.351,820 million (US $ 8 billion) with net profit to the tune of Rs.99,390 million (US $ 2.26 billion) for last financial year. The infrastructure asset on telephone alone is worth about Rs.630,000 million (US $ 14.37 billion).

1.2 PRESENT AND FUTURE

BSNL (then known as Department of Telecom) had been a near monopoly during the socialist period of the Indian economy. During this period, BSNL was the only telecom service provider in the country (MTNL was present only in Mumbai and New Delhi). During this period BSNL operated as a typical state-run organization, inefficient, slow, bureaucratic, and heavily unionized. As a result subscribers had to wait for as long as five years to get a telephone connection.

The corporation tasted competition for the first time after the liberalization of Indian economy in 1991. Faced with stiff competition from the private telecom service providers, BSNL has subsequently tried to increase efficiencies itself. DoT veterans, however, put the onus for the sorry state of affairs on the Government policies, where in all state-owned service providers were required to function as mediums for achieving egalitarian growth


http://en.wikipedia.org/wiki/Mahanagar_Telephone_Nigam

http://en.wikipedia.org/wiki/New_Delhi

http://en.wikipedia.org/wiki/Mumbai

http://en.wikipedia.org/wiki/Janpath

http://en.wikipedia.org/wiki/Land_line

http://en.wikipedia.org/wiki/Cellular_phone

http://en.wikipedia.org/wiki/India

http://en.wikipedia.org/wiki/Telecommunication

http://en.wikipedia.org/wiki/State-owned

across all segments of the society. The corporation (then DoT), however, failed miserably to achieve this and India languished among the most poorly connected countries in the world.

BSNL was born in 2000 after the corporatization of DoT. The efficiency of the company has since improved. However, the performance level is nowhere near the private players. The corporation remains heavily unionized and is comparatively slow in decision making and implementation. Though it offers services at lowest tariffs, the private players continue to notch up better numbers in all areas, years after year.

BSNL has been providing connections in both urban and rural areas. Pre-activated Mobile connections are available at many places across India. BSNL has also unveiled cost-effective broadband internet access plans (DataOne) targeted at homes and small businesses. At present BSNL enjoys around 60% of market share of ISP services.

2007 has been declared as "Year of Broadband" in India and BSNL is in the process of providing 5 million Broadband connectivity by the end of 2007. BSNL has upgraded existing Dataone (Broadband) connections for a speed of up to 2 M bit/s without any extra cost. This 2 M bit/s broadband service is being provided by BSNL at a cost of just US$ 11.7 per month (as of 21/07/2008 and at a limit of 2.5GB monthly limit with 0200-0800 hrs as no charge period).

BSNL is rolling out new broadband services such as triple play.BSNL is planning to increase its customer base to 108 million customers by 2010. With the frantic activity in the communication sector in India, the target appears achievable.BSNL is a pioneer of rural telephony in India. BSNL has recently bagged 80% of US$ 580 m (INR 2,500 crores) Rural Telephony project of Government of India.

On the 20th of March, 2009, BSNL advertised the launch of BlackBerry services across its Telecom circles in India. The corporation has also launched 3G services in select cities across the country. Presently, BSNL and MTNL are the only players to provide 3G services, as the Government is still in the process of auctioning the 3G spectrum to private players.

BSNL has also launched a Entertainment Portal called BSNL Hungama Portal from where subscribers could download contents like music, music videos for free and also download or play various games online. Only Tamil, Kannada, Telugu & Hindi are provided at present. Hopes are there that the database could be expanded. BSNL charges a fixed monthly subscription fee for this function.


INTRODUCTION(CDMA)Code Division Multiple Access technology emerged as an alternative to the GSM cellulararchitecture and has shared in the past decade’s explosive growth in the wireless market. CDMA,like GSM, has seen incremental improvements in capacity throughout this period. Now both types of networks are making a transition to third-generation (3G) systems around the globe, offering yet more capacity and data services.This paper will briefly describe the origins of CDMA technology and the emergence of 3G implementations such as CDMA2000 1X and CDMA2000 1x EV-DO. An overview of network topology is included, with a detailed explanation of the role of each element and interface in the network and of protocol testing to address the changing requirements of the network. The paper concludes with a discussion of some of the technical problems that can occur in CDMA networks and some proposed solutions.

BIRTH OF CDMA

At World War II CDMA is a military technology first used during World War II by the English allies to foil German attempts at jamming transmissions. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal.

HISTORY OF CDMA Somewhere close to the Second World War, Hollywood actress-turned-inventor, Hedy Lamarr and co-inventor George Antheil, co-patented a way for controlling torpedoes by sending signals over multiple radio frequencies using random patterns. They called this “frequency hopping”. After some hue and cry, the US Navy discarded their work as architecturally unfeasible. In 1957, Sylvania Electronic System Division, in Buffalo, New York , took up the same idea. After the expiry of the inventor’s patent, they used the same technology to secure communications for the US military. In the mid-80s, the US military declassified what is now called CDMA technology, a technique based on spread-spectrum technology, for use in wireless communication. The spread-spectrum technology works by digitizing multiple conversations, attaching a


code(known only to the sender and receiver), and then breaking the signals into bits and reassembling them. Qualcomm, which patented CDMA, and other telecommunication companies, were attrached to the technology because it enabled many simultaneous conversations, rather than the limited stop-and-go transmissions of analogue technology and the previous digital option.

DIGITAL REVOLUTION AND EVOLUTIONWhen the mobile communications industry began its transition from first-generation analogtechnology to second-generation (2G) digital architecture, manufacturers and operators chose sides: in Europe, frequency-hopping GSM architecture became almost universal, while in the U.S., parts of Asia, and elsewhere, spread-spectrum CDMA technology took a large share of the market. Because spread spectrum uses wide band, noise-like signals, they are hard to detect. They are also difficult to intercept or demodulate. Further, spread spectrum signals are harder to jam (interfere with) than narrowband signals. These Low Probability of Intercept (LPI) and antijam (AJ) features are why the military has used spread spectrum for so many years. Both network implementations, GSM and CDMA, have advanced to keep pace with subscribers’ demands for more bandwidth, features and reliability at lower cost.CDMAOne Helps 2G Mobile Communications Take Off The Telecommunications Industry Association (TIA/EIA) IS-95 CDMA standard published in July 1993 established the ground rules for a complete end-to-end digital wireless communications system. The commercial network system architecture based on this standard is known as CDMAOne. TIA/EIA IS-95 and the subsequent IS-95A revision (published in May 1995) form the basis for most of the commercial 2G CDMA-based networks deployed around the world. From the standpoint of voice services, CDMAOne technology offers important features for mobile

NETWORK OPERATORS:• An 8X to 10X increase in voice capacity increase compared to analog AMPS systems• Simplified network planning, with the same frequency used in every sector of every cellThe early 2G CDMA infrastructure proved its effectiveness in delivering high-quality, low-loss voice traffic to subscribers. But it didn’t take long for mobile users to begin asking for basic data services, such as Internet and Intranet services, multimedia applications or high-speed business transactions, to supplement the voice services on their handsets. The TIA/EIA IS-95A standard answered this demand with its definition of the wideband 1.25 MHz CDMA channels, power control, call processing, hand-offs and registration techniques for system operation. TIA/EIA IS-95A brought true circuit-switched data services to CDMA subscribers; however, these were limited to a maximum speed of 14.4 Kbps per user.A second round of revisions to the original specification produced the TIA/EIA IS-95B standard.This new development gave subscribers packet-switched data services at speeds up to 64 Kbps per subscriber in addition to the existing voice services. With this increased data rate, TIA/EIA IS- 95B-compliant networks qualify as 2.5G CDMA technology.


CDMA2000 Takes the Next StepThe transition to 3G networks, still underway, began with a profusion of newly proposedstandards. Some were designed to build on GSM infrastructures and others emerged directly from CDMA technology. Ultimately the ITU took a position on the matter, defining an IMT-2000 standard that encompassed five different radio interfaces including CDMA2000. Note that all of the IMT-2000 protocols use spread-spectrum techniques, which has implications about network installation, operation and maintenance.The ITU defines a 3G network as one that delivers, among other capabilities, improved systemcapacity and spectrum efficiency versus 2G systems. It supports data services at transmission rates of at least 144 Kbps in mobile (moving) environments and at least 2 Mbps in fixed (indoor) environments. The CDMA2000 architecture meets these objectives and includes several implementations that an operator can select to best serve a transition strategy based on competitive concerns, existing infrastructures, cost, and other variables.Among these implementations are CDMA2000 1X and CDMA2000 1xEV:• CDMA2000 1X doubles the voice capacity of CDMAOne networks, delivering peak datarates of 307 Kbps per subscriber in a mobile environment.• CDMA2000 1xEV includes two variants, both backward compatible with CDMA2000 1X andCDMAOne technologies. o CDMA2000 1xEV-DO (Data Only), capable of delivering data multimedia services such as MP3 transfers and video-conferencing at peak data rates of 2.4 Mbps per subscriber in a mobile environment;o CDMA2000 1xEV-DV (Data Voice), capable of delivering integrated voice andsimultaneous data multimedia services at peak data rates of 3.09 Mbps persubscriber.

CDMA PRINCIPLE If we change our communication topology from point-to-point to point-to-multipoint, we have hanged the communication environment from single-link to a multiple-access link. The multiple-access scheme in a spread-spectrum system is termed code-division multiple-access (CDMA). Each access to a common channel needs some form of orthogonality. For frequency-division multiple-access (FDMA), we achieve orthogonality in the frequency domain by selecting nonoverlapping unique frequency bands to each user. We achieve orthogonality in the time domain by selection nonoverlapping unique time segments to each user; this process is referred to as time-division multiple-access (TDMA). The spread-spectrum form of multiple access exploits the orthogonality in the code domain and is termed code-division multiple-access (CDMA). The multiuser environment in the spread-spectrum case is set up for each user in assigning each user a unique spreading sequence out of a family of orthogonal sequences. Each user in a CDMA network occupies the same channel bandwidth. A CDMA system is clearly not a collision avoidance system like FDMA and TDMA. The opposite is true and explains the differences in the behavior of CDMA systems compared to FDMA and TDMA. In general, the collisions at the channel is a disadvantage of CDMA system


and can be mitigated by careful selection of the sequence and power control that is close to perfect.

CDMA MODULATION Both the Forward and Reverse Traffic Channels use a similar control structure consisting of 20 millisecond frames. For the system, frames can be sent at either 14400, 9600, 7200, 4800, 3600, 2400, 1800, or 1200 bps. For example, with a Traffic Channel operating at 9600 bps, the rate can vary from frame to frame, and can be 9600, 4800, 2400, or 1200 bps. The receiver detects the rate of the frame and processes it at the correct rate. This technique allows the channel rate to dynamically adapt to the speech or data activity. For speech, when a talker pauses, the transmission rate is reduced to a low rate. When the talker speaks, the system instantaneously shifts to using a higher transmission rate. This technique decreases the interference to other CDMA signals and thus allows an increase in system capacity. CDMA starts with a basic data rate of 9600 bits per second. This is then spread to a transmitted bit rate, or chip rate (the transmitted bits are called chips), of 1.2288 MHz. The spreading process applies digital codes to the data bits, which increases the data rate while adding redundancy to the system. The chips are transmitted using a form of QPSK (Quadrature Phase Shift Keying) modulation which has been filtered to limit the bandwidth of the signal. This is added to the signal of all the other users in that cell. When the signal is received, the coding is removed from the desired signal, returning it to a rate of 9600 bps. When the decoding is applied to the other users' codes, there is no despreading; the signals maintain the 1.2288 MHz bandwidth. The ratio of transmitted bits or chips to data bits is the coding gain. The coding gain for the IS-95 CDMA system is 128, or 21 dB.

CDMA for Cellular When implemented in a cellular telephone system, CDMA technology offers numerous benefits to the cellular operator and their subscribers. These can be summarized as follows:

• Capacity increases: 8 to 10 times that of an AMPS analog system, and 4 to 5 times that of a GSM system.

• Improved call quality: CDMA will provide better and more consistent sound as compared to AMPS. Cellular telephone systems using CDMA should be able to provide higher quality sound and phone calls than systems based on other technologies.

• Simplified system planning: Engineers will no longer have to perform the detailed frequency planning which is necessary in analog and TDMA systems.

• Enhanced privacy: Increased privacy over other cellular systems, both analog and digital, is inherent in CDMA technology.


• Increased talk time and standby time for portables: Because of precise power control and other system characteristics, CDMA subscriber units normally transmit at only a fraction of the power of analog and TDMA phones

• Advanced Features: These include Multiple/High Quality Vocoders, Short Messaging Services, Over-the-Air-Activation, Sleep Mode, and Data/Fax.

A Network Structure Designed for Packetized CommunicationFigure 1 illustrates a simplified CDMA2000 1X network, showing both the telephony(ANSI-41) and data structures. Refer to Figure 1 for the following discussion

Figure 1: The structure of a CDMA network

The Mobile Station (MS)In a CDMA2000 1X network, the mobile station—the subscriber’s handset—functions as a mobile IP client. The mobile station interacts with the Access Network to obtain appropriate radio resources for the exchange of packets, and it keeps track of the status of radio resources (e.g. active, stand-by,dormant). It accepts buffer packets from the mobile host when radio resources are not in place or are insufficient to support the flow to the network.Upon power-up, the mobile station automatically registers with the Home Location Register (HLR) in order to:• Authenticate the mobile for the environment of the accessed network• Provide the HLR with the mobile’s current location• Provide the Serving Mobile Switching Centre (MSC-S) with the mobile’s permitted featureSet after successfully registering with the HLR, the mobile is ready to place voice and data calls.These may take either of two forms, circuit-switched data (CSD) or packet-switched data (PSD),depending on the mobile’s own compliance (or lack thereof) with the IS-2000


standard. This document defines protocols for several critical CDMA interfaces pertaining to packet transmission, namely A1, A7, A9, and A11Mobile Stations must comply with IS-2000 standards to initiate a packet data session using the 1xRTT1 network. Mobile stations having only IS-95 capabilities are limited to CSD, while IS-2000 terminals can select either the PSD or CSD. Parameters forwarded by the terminal over the air link (AL) to the network will determine the type of service requested.Circuit-switched data has a maximum rate of 19.2 Kbps and is delivered over traditional TDMcircuits. This service allows users to select the point of attachment into a data network usingordinary dialled digits.Packet-switched data service has a maximum data rate of 144 Kbps. For each data session aPoint-to-Point Protocol (PPP) session is created between the mobile station and the Packet DataServing Node (PDSN). IP address assignment for each mobile can be provided by either thePDSN or a Dynamic Host Configuration Protocol (DHCP) server via a Home Agent (HA).

The Radio Access Network (RAN)The Radio Access Network is the mobile subscriber’s entry point for communicating either data orvoice content. It consists of:• The air link• The cell site tower/antenna and the cable connection to the Base Station TransceiverSubsystem (Um)• The Base Station Transceiver Subsystem (BTS)• The communications path from the Base Station Transceiver Subsystem to the basestation controller (Abis)• The Base Station Controller (BSC)• The Packet Control Function (PCF)

The RAN has a number of responsibilities that impact the network’s delivery of packet services in particular. The RAN must map the mobile client identifier reference to a unique link layer identifier used to communicate with the PDSN, validate the mobile station for access service, and maintain the established transmission links.

A CDMA network consists of the following components: Mobile station. The CDMA mobile station (or mobile phone) communicates with other parts of the system through the base-station system.The Base Station Transceiver Subsystem (BTS) :controls the activities of the air link and acts as the interface between the network and the mobile. RF resources such as frequencyassignments, sector separation and transmit power control are managed at the BTS. In addition,the BTS manages the back-haul from the cell site to the Base Station Controller (BSC) to minimize any delays between these two elements. Normally a BTS


connects to the BSC through un-channelized T1 facilities or direct cables in co-located equipment. The protocols used within this facility are proprietary and are based on High-level Data Link Control (HDLC).

The Base Station Controller (BSC): routes voice- and circuit-switched data messages between the cell sites and the MSC. It also bears responsibility for mobility management: it controls and directs handoffs from one cell site to another as needed. It connects to each MTX using channelized T1 lines for voice and circuit switched data; and to un-channelized T1 lines for signalling and control messages to the PDSN using the 10BaseT Ethernet protocol.

Mobile switching center (MSC): The MSC performs the telephony switching functions of the system. It also performs such functions as toll ticketing, network interfacing, common channel signalling, and others.

The Packet Control Function (PCF) :routes IP packet data between the mobile station within the cell sites and the Packet Data Serving Node (PDSN). During packet data sessions, it will assign available supplemental channels as needed to comply with the services requested by the mobile and paid for by the subscribers. Home location register (HLR): The HLR database is used for storage and management of subscriptions. The home location register stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. Visitor location register (VLR): The VLR database contains temporary information about subscribers that is needed by the mobile services switching center (MSC) in order to service visiting subscribers. When a mobile station roams into a new mobile services switching center (MSC) area, the visitor location register (VLR) connected to that MSC will request data about the mobile station from the HLR, reducing the need for interrogation of the home location register (HLR). Authentication center (AC): The AC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The authentication center (AUC) also protects network operators from fraud. Operation and administration (OAM): The OAM is the functional entity from which the network operator monitors and controls the system. The purpose of operation and support system is to offer support for centralized, regional, and local operational and maintenance activities that are required for a CDMA network.

The PCF maintains a “reachable” state for between the RN and the mobile station, ensuring a consistent link for packets; buffers packets arriving from the PDSN when radio resources are not in place or insufficient to support the flow from the PDSN; and relays packets between the MS and the PDSN.


SPREAD SPECTRUM PRINCIPLE

Originally Spread spectrum radio technology was developed for military use to counter the interference by hostile jamming. The broad spectrum of the transmitted signal gives rise to “ Spread Spectrum”. A Spread Spectrum signal is generated by modulating the radio frequency (RF) signal with a code consisting of different pseudo random binary sequences, which is inherently resistant to noisy signal environment.

A number of Spread spectrum RF signals thus generated share the same frequency spectrum and thus the entire bandwidth available in the band is used by each of the users using same frequency at the same time.

Fig : 10.1 CDMA ACCESS – A CONCEPT

On the receive side only the signal energy with the selected binary sequence code is accepted and original information content (data) is recovered. The other users signals, whose codes do not match contribute only to the noise and are not “despread” back in bandwith (Ref Figure 10.1 ) This transmission and reception of signals differentiated by “codes” using the same frequency simultaneously by a number of users is known as Code Division Multiple Access (CDMA) Technique as opposed to conventional method of Frequency Division Multiple Access and Time Division Multiple Access.

In figure 10.1 it has been tried to explain that how the base band signal of 9.6 Kbps is spread using a Pseudo-random Noise (PN) source to occupy entire bandwidth of 1.25 Mhz. At the receiving end this signal will have interference from signals of other users of the same cell, users of different cells and interference from other noise sources. All these signals get combined with the desired signal but using a correct PN code the original data can be reproduced back. CDMA channel in the trans and receive direction is a FDD (Frequency Division Duplexing) channel. The salient features of a typical CDMA system are as follows:


s Frequency of operation: 824-849Mhz and 869-894 Mhz

s Duplexing Mehtod: Frequency Division Duplexing (FDD)

s Access Channel per carrier: Maximum 61 Channels

s RF Spacing: 1.25 Mhz

s Coverage: 5 Km with hand held telephones and approx. 20 Km with fixed units.

The different types of codes used for identification of traffic channels and users identification etc as follows:

Different Codes

Walsh Code : In addition to the above two codes, another special code, called walsh is also used inCDMA. Walsh codes do not have the properties of m-sequences regarding crossCorrelation.. Is-95 uses 64 walsh codes and these allow the creation of 64 channels from the base station. In other words, a base station can talk to a maximum of 64 (this numberIs actually only 54 because some codes are used for pilot and synch channels) mobiles atThe same time. CDMA 2000 used 256 of these codes.Walsh codes are created out of haddamard matrices and transform. Haddamard is theMatrix type from which walsh created these codes. Walsh codes have just oneOutstanding quality. In a family of walsh codes, all codes are orthogonal to each otherAnd are used to create channelization within the 1.25 mhz band.Here are first four hadamard matrices. The code length is the size of the matrix. EachRow is one walsh code of size n. The first matrix gives us two codes; 00, 01. The secondMatrix gives: 0000, 0101, 0011, 0110 and so on.

In CDMA the traffic channels are separated by uinique “Walsh” code. All such codes are orthogonal to each other. The individual subscriber can start communication using one of these codes. These codes are traffic channel codes and are used for orthogonal spreading of the information in the entire bandwidth. Orthogonality provides nearly perfect isolation between the multiple signals transmitted by the base station.

The basic concept behind creation of the code is as follows:

(a) Repeat the function right

(b) Repeat the function below

(c) Invert function (diagonally)


0 0 0 0 0 0

0 1 0 1 0 1

0 0 1 1

0 1 1 0

Long code : the long pseudo random noise (PN) sequence is based on 242 characteristic polynomial. With this long code the data in the forward direction (Base to Mobile) is scrabled. The PN codes are generated using linear shift registers. The long code is unique for the subscribers and is known as users address mask.

Short Code : The short pseudo random noise (PN) sequence is based on 215 characteristic polynomial. This short code differentiates the cells & the sectors in a cell. It also consists of codes for I & Q channel feeding the modulator.

Advantages CDMA wireless access provides the following unique advantages:

Larger Capacity : let us discuss this issue with the help of Shannon’s Theorem. It states that the channel capacity is related to product of available band width and S/N ratio.

C = W log 2 (1+S/N)

Where C = channel capacity

W = Band width available

S/N = Signal to noise ratio.

It is clear that even if we improve S/N to a great extent the advantage that we are expected to get in terms of channel capacity will not be proportionally increased. But instead if we increase the bandwidth (W), we can achieve more channel capacity even at a lower S/N. That forms the basis of CDMA approach, wherein increased channel capacity is obtained by increasing both W & S/N. The S/N can be increased by devising proper power control methods.


Vocoder and variable data rates: As the telephone quality speech is band limited to 4 Khz when it is digitized with PCM its bit rate rises to 64Kb/s Vocoding compress it to a lower bit rate to reduce bandwidth. The transmitting vocoder takes voice samples and generates an encoded speech/packet for transmission to the receiving vocoder. The receiving vocoder decodes the received speech packet into voice samples. One of the important feature of the variable rate vocoder is the use of adaptive threshold to determine the required data rate. Vocoders are variable rate vocoders. By operating the vocoder at half rate on some of the frames the capacity of the system can be enhanced without noticeable degradation in the quality of the speech. This phenomenon helps to absorb the occasional heavy requirement of traffic apart from suppression of backgraound noise. Thus the capacity advantage makes spread spectrum an ideal choice for use in areas where the frequency spectrum is congested.

Less (Optimum) Power per cell: Power Control Methods: As we have already seen that in CDMA the entire bandwidth of 1.25Mhz is used by all the subscribers served in that area. Hence they all will be transmitting on the same frequency using the entire bandwidth but separated by different codes. At the receiving end the noise contributed by all the subscribers is added up. To minimize the level of interfering signals in CDMA, very powerful power control methods have been devised and are listed below:

1. Reserve link open loop power control

2. Reserve link closed loop power control

3. Forward link power control

The objective of open loop power control in the reverse link (Mobile to Base) is that the mobile station should adjust its transmit power according to the changes in its received power from the base. Open loop power control attempts to ensure that the received signal strength at the base station from different mobile stations, irrespective of their distances from the base site, should be same.

In Closed loop power control in reverse link, the base satation provides rapid corrections to the mobile stations’ open loop estimates to maintain optimum transmit power by the mobile stations. The base station measures the received signal strength from the mobile connected to it and compares it with a threshold value and a decision is taken by the base every 1.25 ms to either increase or decrease the power of the mobile.

In forward link power control (Base to Mobile) the cell (base) adjusts its power in the forward link for each subscriber, in response to measurements provided by the mobile


station so as to provide more power to the mobile who is relatively far away from the base or is in a location experiencing more difficult environment.

These power control methods attempt to have an environment which permits high quality communication (good S/N) and at the same time the interference to other mobile stations sharing the same CDMA channel is minimum. Thus more numbers of mobile station are able to use the system without degradation in the performance. Apart from the capacity advantage thus gained power control extends the life of the battery used in portables and minimizes the concern of ill effects of RF radiation on the human body.

Seamless Hand-off : CDMA provides soft hand-off feature for the mobile crossing from one cell to another cell by combining the signals from both the cells in the transition areas. This improves the performance of the network at the boundaries of the cells, virtually eliminating the dropped calls.

No Frequency Planning : A CDMA system requires no frequency planning as the adjacent cells use the same common frequency. A typical cellular system (with a repetition rate of 7) and a CDMA system is shown in the following figures which clearly indicates that in a CDMA network no frequency planning is required.

Fig : 10.2 CDMA Frequency


Fig : 10.3 Frequency Reuse of 7 in GSM

High Tolerance to Interference : The primary advantage of spread spectrum is its ability to tolerate a fair amount of interfering signals as compared to other conventional systems. This factor provides a considerable advantage from a system point of view.

Multiple Diversity : Diversity techniques are often employed to counter the effect of fading. The greater the number of diversity techniques employed, the better the performance of the system in a difficult propagation environment.

CDMA has a vastly improved performance as it employs all the three diversity techniques in the form of the following:

A .Frequency Diversity: A wide band RF signal of 1.25 Mhz being used.

B. Space Diversity: Employed by way of multipath rake receiver.

C. Time Diversity: Employed by way of symbol interleaving error detection and correction coding.

Capacity ConsiderationsLet us discuss a typical CDMA wireless in local loop system consisting of a single base station located at the telephone exchange itself, serving a single “cell”. In order to increase the number of subscribers served the cell is further divided into “sectors”. These sectors are served by directional antennas.

The capacity of a cellular system is claimed to be 20-40 active lines per sector per 1.25 MHz for a single CDMA Radio Channel. In WLL environment assuming an average busy hour


traffic of 0.1 Erlang, 400 subscribers can be served per sector over a single 1.25 MHz

channel.

Fig : 10.4 A Typical six sectored cell

Assuming typically six sectors in a cell the total capacity of a CDMA network consisting of 1.25 MHz duplex channels is 2400 (400x6) subscribers.

Capacity can further be increased if we use another frequency on the same base station covering the same geographical area (overlapping cell). Thus in 10 Mhz in the bandwidth we can utilize 5 MHz of bandwidth in the forward link and 5 Mhz in the reverse link. Hence if we have 4 RF carriers in 5 Mhz bandwidth, the network can support 12000 (5x400x6) subscribers per cell. A typical CDMA wireless in local loop system is depicted in the above figure10.4.

Physical and Logical Channels:

In is 95a, in the forward link pilot, sync, paging and traffic channels exist whereas in reverse link access and traffic channel are available. All overhead information is carried on the paging channel. During conversation or in dedicated mode the signaling info is exchanged by either fully or partially clearing the traffic. CDMA2000 technology defines new physical and logical channels for the transport of user data and signaling information.

A physical channel is a communication path between the mobile and the base station, described in terms of the digital coding and rf characteristics.

A logical channel is a communication path within the protocol layers of either the base station or the mobile. Information is grouped onto a logical channel based on criteria such as these:

_ is it for a single user or for multiple users?


_ is it signaling or user data?

_ is the direction of the transfer forward or reverse?

The information on a logical channel is ultimately carried on one or more physical channels. Mappings are defined between physical and logical channels. These mappings may be permanent or may be defined only for the duration of a call.

(e.g) the forward common signaling channel (f-csch) carries information that may

Ultimately be mapped onto the forward sync channel (f-synch), the forward paging channel (f-pch), and the forward broadcast control channel (f-bcch).

CDMA2000 physical channel naming

CDMA2000 logical channel namingA logical channel name consists of three lowercase letters followed by “ch” (channel).

A hyphen is used after the first letter. Logical channel names are differentiated by:

Direction (forward or reverse) Whether the information is shared by all users (common) or specific to an individual

user (dedicated) Whether the information is control information (signaling) or user information

For common signaling channels, the mappings shown assume that all common signaling physical channels are supported (f-bcch, f-ccch, f-pch, r-each, and r-ach). If the base station is configured to support only the tia/eia-95 compatible common channels, then the f-bcch, f-ccch, and r-each channel are not present in the mapping.


For dedicated channels, the mapping is established for each call, as a function of what services are in use (voice, circuit-switched data, packet data).

Radio configurations:

A radio configuration (rc) defines the following characteristics of a forward or reverse traffic channel, viz rate set, spreading rate channel coding (turbo or convolutional), channel coding rate, modulation (qpsk or bpsk)and transmit diversity allowed.

Is-2000 defines radio configurations:

– rc1 and rc2 correspond to is-95 a/b rate set 1 and rate set 2 respectively

– rc3 through rc9 on the forward link

– rc3 through rc6 on the reverse link

Forward link radio configuration:


Reverse link radio configuration:

Variable length walsh codes:walsh code used in is95 is 64 chips long. CDMA20001x can use walsh codes up to 128 chips long. Higher data rate channels use shorter length walsh codes to maintain a constant chip rate. Using one of the shorter walsh codes precludes using all longer codes that contain the bit pattern of the shorter code.


Forward link code channels:

The forward pilot, sync, and paging channels are compatible with tia/eia-95a/b. In radio configurations 1 and 2, the fundamental and supplemental channels are backward compatible. In these configurations, the maximum number of supplemental channels is seven, which allows the transmission rate to reach up to 115.2 kbps.As in tia/eia-95a/b, the power control sub channel is associated with the fundamental channel for radio configurations 1 and 2.

The forward link channels are assigned as follows:

W0 64 reserved for forward pilot channel W32 64 reserved for sync channel


W1 64 to w7 64 reserved for paging channels wn 64 may be used for radio configurations 1 and 2 fundamental and

supplemental channels, for 0 < n < 64, except for those channels used for sync and paging channels

New common channels:

CDMA2000 introduces several new forward link common channels:

Pilot channels – if transmit diversity is supported; one or more pilots may be used.the auxiliary pilot channels may be used for smart antenna applications.

Quick paging channel – this channel provides for improved slotted mode operation and improved battery life for the mobile. Walsh codes w80, w48 and w112 are reserved for quick paging channels, if the base station supports quick paging channels.

Common control channel – this channel carries mobile-directed messages for CDMA2000 compatible mobiles.

Broadcast channel – this channel carries broadcast messages for CDMA2000 compatible mobiles, including overhead messages and broadcast short message service (sms) messages.

Common power control channel – this channel is used with enhanced access channel procedures (reservation mode), to send power control bits to the mobile so that access channel messages may be sent under power control.

Common assignment channel – this channel is used with enhanced access channel procedures (reservation mode) to assign the reverse common control channel (r-ccch) and common power control sub channel.

New dedicated channels:

CDMA2000 introduces several new forward link dedicated channels:

Forward fundamental channel – this channel is used for the transmission of user and signaling information to a specific mobile during a call. Each forward traffic channel may contain one forward fundamental channel.

Forward dedicated control channel – this channel is used for transmission of user and signaling information to a specific mobile during a call. Each forward traffic channel may contain one forward dedicated control channel.

Forward supplemental channel (valid for radio configurations 3 thro 9) this channel is used for the transmission of user information to a specific mobile during a call. This is


typically used for high-speed data applications. Each forward traffic channel may contain up to two supplemental channels.

Power control subchannel – this subchannel is typically associated with the fundamental channel, but if the f-fch is not used for a given call, then it is associated with the dedicated control channel (f-dcch).

All of the CDMA2000 dedicated channels can be established using the tia/eia paging (f-pch) and access (r-ach) channels.

Reverse link channels:

The CDMA2000 reverse link channels are:

_ access channel (r-ach)

_ reverse pilot channel (r-pich)

_ enhanced access channel (r-each)

_ reverse common control channel (r-ccch)

_ reversed dedicated control channel (r-dcch)

_ reverse fundamental channel (r-fch)

_ reverse supplemental channel (r-sch)

_ reverse supplemental code channel (r-scch)

The access channel and reverse supplemental channel are retained for backward compatibility with tia/eia-95a/b. For radio configurations 1 and 2, the channel structure for the reverse fundamental channel and reverse supplemental channel is the same as the channel structure of rate set 1 and rate set 2 used in tia/eia-95a/b.

Reverse common and dedicated channels

Reverse link common channels are used by multiple mobiles primarily for a brief exchange of information between a mobile and a base station. The reverse link common channels are:

_ access channel (r-ach)

_ enhanced access channel (r-each)

_ reverse common control channel (r-ccch)

Reverse link dedicated channels are assigned to a single mobile for the duration of a call. The reverse link dedicated channels include:


_ reverse dedicated control channel (r-dcch)

_ reverse fundamental channel (r-fch)

_ reverse supplemental channel (r-sch)

_ reverse supplemental code channel (r-scch)

The reverse pilot channel is used with both Common and Dedicated Channels.

Data Multiplexing

CDMA2000 can multiplex data from multiple sources (e.g., signaling, voice, and data) onto one or more Physical Channels. Data can be multiplexed in one or two Supplemental Channels.


Tia/eia-95 a/b compatible access channel proceduresIf the mobile monitors the paging channel (f-pch), then its access attempts are made on the access channel (r-ach). These procedures are identical to tia/eia-95 a/b access procedures.

CDMA2000 enhanced access channel proceduresIf the mobile monitors the forward common control channel (f-ccch) and broadcast control channel (f-bcch), then its access attempts are made on the enhanced access channel (r-each) using the CDMA2000 enhanced access procedure

The Core Network’s Role in the CDMA InfrastructureThe Packet Data Serving Node / Foreign Agent (PDSN/FA)The PDSN/FA is the gateway from the RAN into the public and/or private packet networks. In a simple IP network, the PDSN acts as a standalone Network Access Server (NAS), while in a mobile IP network it can be configured as a Home Agent (HA) or a Foreign Agent (FA).The PDSN does the following activities:• Manage the radio-packet interface between the BSS (Base Station Subsystem = BTS +


BSC) and the IP network by establishing, maintaining and terminating link layer to themobile client• Terminate the PPP session initiated by the subscriber• Provide an IP address for the subscriber (either from an internal pool or through a DHCPserver or through an AAA server; see below)• Perform packet routing to external packet data networks or packet routing to the HAwhich optionally can be via secure tunnels• Collect and forward packet billing data• Actively manage subscriber services based on the profile information received from theSCS server of the AAA server• Authenticate users locally, or forward authentication requests to the AAA serverThe AAA ServerThe AAA (Authentication, Authorization, and Accounting) server is used to authenticate andauthorize users for network access and to store subscriber usage statistics for billing andinvoicing.The Home AgentThe Home Agent (HA) supports seamless data roaming into other networks that support 1xRTT. The HA provides an anchor IP address for the mobile and forwards any mobile-bound traffic to the appropriate network for delivery to the handset. It also maintains user registration, redirects packets to the PDSN and (optionally) tunnels securely to the PDSN. Lastly, the HA supports dynamic assignment of users from the AAA and (again optionally) assigns dynamic homeaddresses.

Detecting and Solving Some Common Problems inCDMA20001XNetworksAll of the features and capacities embodied in the modern 3G mobile network make for a complex system with many modes, nodes, elements, interfaces, and protocols. Problems, when they arise may have their origins in either hardware or software. As mobile Internet connectivity becomes common, the challenge of maintaining uninterrupted data transactions will require new, morepowerful monitoring solutions and procedures, among other things. In this section, we willexamine some common problems that can occur in CDMA2000 1X networks.Failure in Mobile Initiated Packet Data Call Set-up and Mobile IP RegistrationIn order to obtain packet data services, the mobile performs registration with the serving wireless network on the A1 interface and then with the packet network on the A10/A11 interface. The mobile sends an Origination Message to the BS that includes the packet data service option. This results in assignment of the traffic channel, establishment of the A10 connection, establishment of the link layer (PPP) and for the case where Mobile IP is used by the terminal, Mobile IP registration with the serving packet network.


User data traffic can now be passed over the A10 connection encapsulated within GRE frames.The PCF periodically re-registers with the selected PDSN by sending the A11-Registration Request message before the A10 connection Lifetime expires.

Figure 2: Setting up a CDMA2000 1X mobile data call

A successful call set-up scenario is illustrated in Figure 2. This standard message sequence chart outlines a series of steps, summarized in items 1-12 to follow. Note that this explanation bypasses the radio reception/transmission activities of the BTS, concentrating instead on the protocol functions that begin with the Origination dialogue between the mobile and the BSC.

1. To register for packet data services, the mobile sends an Origination Message over theAccess Channel to the BSS2. The BS acknowledges the receipt of the Origination Message, returning a Base StationAck Order to the mobile

3. The BS constructs a CM Service Request message and sends the message to the MSC.4. The MSC sends an Assignment Request message to the BSS requesting assignment ofradio resources. No terrestrial circuit between the MSC and the BS is assigned to thepacket data call.5. The BS and the mobile perform radio resource set-up procedures.The PCF recognizes that no A10 connection associated with this mobile is available andselects a PDSN for this data call.6. The PCF sends an A11-Registration Request message to the selected PDSN.7. The A11-Registration Request is validated and the PDSN accepts the connection byreturning an A11-Registration Reply message.Both the PDSN and the PCF create a binding record for the A10 connection.


8. After the radio link and A10 connection are set-up, the BS sends an AssignmentComplete message to the MSC9. The mobile and the PDSN establish the link layer (PPP) connection and then perform theMIP registration procedures over the link layer (PPP) connection.10. After completion of MIP registration, the mobile can send/receive data via GRE framingover the A10 connection.11. The PCF periodically sends an A11-Registration Request message for refreshingregistration for the A10 connection.12. For a validated A11-Registration Request, the PDSN returns an A11-Registration Replymessage.

Both the PDSN and the PCF update the A10 connection binding record.This necessarily complex process can be the source of some problems that affect service andquality. A rigorous monitoring scheme involving simultaneous observation of the A1 interface and the A10/A11 interface is the best way to detect and correct errors early. Here, a multi-interface call-trace application is especially productive, since it can trace and group all of the procedures related to the activity of each single subscriber in a CDMA network, even as the procedures evolve over multiple interfaces.Within the call set-up process, an error in any element or procedural step can inhibit theremaining steps. For example, suppose that the MSC does not respond to the CM ServiceRequest message (Step 3 in Figure 2) sent by the BSC/PCF over the A1 interface. This issometimes caused by internal MSC problems. If this prevents the CM Service Request fromreaching completion, the BSC/PCF cannot assign radio resources to the mobile station, in turn preventing establishment of the connection. The user finds it impossible to make a data call—a service for which he or she has paid a premium.Before a specific timer expires, the PCF sends periodically A11-Registration Request message(Step 11) to refresh the registration for the A10 connection. For a validated A11-Registration Request, the PDSN returns an A11-Registration Reply message (Step 12). Here again, internal problems in the PDSN can cause it to respond late or not at all. As a result the process of establishing or maintaining the connection cannot continue. The user is once again unable to make a data call.In both cases, a protocol analyzer connected to the A1 and A10/A11 interfaces can help track down the problem. The call trace application can distinguish the origin of messages and detect any failure to respond. This makes it easy to pinpoint the MSC and the PDSN, respectively in these examples.

Inefficiency in User Data Packet TransmissionFrequently in a CDMA2000 network the TCP user-plane packets have a small Window Size. This implies that end-to-end TCP connections are not stable. The more TCP packets lost in the network and not acknowledged, the smaller the Window Size, with the result that more TCP connections are dropped and re-established. The small TCP Window Size is a by-product of the soft-start mechanism built into the TCP protocol.


To characterize this problem, it is necessary to capture the TCP/IP user plane packets flowing on the GRE tunnels on the A10 interface. Protocol filtering allows the tool to home in on just the dataor interest. By applying different types of filtering with increasing level of details, it is possible to“drill down” and isolate the root cause of the shrinking TCP packet Window Size.Routing Loops of User Data Packets in the Core Network “Tunnel router loops” are another class of CDMA2000 network problems that can degrade thequality of service for subscribers. The problem is caused by misconfiguration in the PDSNrouters. It can be detected by acquiring and analyzing IP traffic on the P-H interface (see Figure1). To understand tunnel router loops, imagine a subscriber surfing the Web (WWW) with a laptop connected to a CDMA2000 handset. Packets addressed to go to a specific HTTP proxy are routed (after passing through the PCF) from the PDSN/FA (Foreign Agent) to the Home Agent (HA) for de-tunneling. With certain incorrect internal routing configurations, packets destined for Port 80 WWW are not de-tunnelled by the HA. Instead, they are sent back downstream toward the PDSN/FA. As a result, multiple packets travel on the same network segment with the same packet ID, wasting precious bandwidth—and not reaching the intended destination. In addition, for each repetitive hop a packet takes between the PDSN/FA and HA nodes, the IP Time To Live (TTL) field is decremented. If the packet is stuck in a router loop, the TTL eventually decrements all the way to zero and the packet is discarded by the network nodes. “Lost” packets must be retransmitted,leading to excessive packet retransmission overhead and reduced throughput.As in the earlier examples, the solution is to use protocol filtering to capture IP packets on the PH interface. Browsing through the captured data by applying increasingly fine levels of filtering, it is possible to see the repeating packets and resolve the problem.

Duplication of IP trafficPDSN configuration problems can give rise to other types of problems in addition to tunnel loops.One common issue is associating the PDSN´s logical IP addresses with more than one physical MAC address. When this occurs, more than one hardware card has the same IP address. All traffic sent to that IP address goes to two different hardware entities and receives responses from both. This effectively doubles the amount of IP traffic associated with that single IP address on that segment. Once again, protocol filtering capabilities are required for effective troubleshooting. A protocol analyzer should capture IP packets travelling to a specific IP destination address via the P-H interface. Browsing through the data and using filtering to successively narrow down the inquiry, the nature of the problem (the duplicated address) soon becomes apparent.

Routing problems in the Core NetworkSometimes internal problems can cause PDSN routers to go offline and come back online after a period of time. This can happen frequently and continuously in a CDMA2000 core data network.When a router first comes online its routing table are not optimized. It takes


time for the built-in OSPF (Open Shortest Path First) routing algorithm to learn the best way to route packets depending on adjacent available routers. Until the routing tables are optimized, there will be degradation in quality of service.By capturing IP packets on the P-H interface with a protocol analyzer and applying filters on the OSPF routing messages, changes in designated router and changes in neighbours of a router can be easily identified. Using intelligent and detailed filtering capability on OSPF messages and information elements within these messages identifying routing problems on an IP network becomes an easy task.

FEATURES OF CDMA

CDMA and WiLL For many years now, India has been a GSM subscriber. In 1999, when MTNL decided to provide the CDMA-based WiLL(Wireless in Local Loop) service in India, quite a few eyebrows were raised. The biggest reason why mobile operators opposed the entry of WiLL is that it is uncertain to allow mobility in the local loop. CDMA is restricted to a short distance charging area(SDCA). Currently, there are 2600 SDCAs within the country. A CDMA-based phone can thus ‘roam’ only within its SDCA. This is NOT a technological restriction. In India, Reliance Infocom and Tata Indicom use CDMA technology to provide WiLL services. In remote rural areas, where installing cables is difficult as well as expensive, CDMA-based WiLL networks can be deployed quickly.

3G (3rd Generation)

3G, as it is popularly called, refers to the 3rd generation of wireless networks. The 3rd

generation provides higher frequency bands (of 2Ghz and more) and a bandwidth of around 5 MHz. The Bandwidth and frequency is matched by speeds of 384 Kbps in a mobile environment. Will CDMA be the path towards 3G” The world seems to be divided on this. While the standard choosen by Reliance-CDMA2000 1x-is the 3G avatar of CDMA, the restrictions imposed by the TRAI(Telecom Regulatory Authority of India) doesn’t let it explore the 3G realms. Plus, some Wide CDMA supporters(W-CDMA) aren’t helping the situation by claiming CDMA 1x is not 3G. Third-generation applications includes WCDMA, 1x and High Data Rate (HDR).


CDMA Vs GSM (The Old Horse) In India, clutching a cell phone is still sometimes a status symbol. And if the phone uses technology that is said to be far superior to the one used in America, well, that calls for a celebration. That is how it was with GSM technology. But just as the drinks were being served, Reliance entered the party, bringing with it ‘outclassed technology’. Suddenly, the phones stopped ringing. Because the ‘outclassed technology’ had become ‘the’ technology. The same people who had said CDMA had no takers were suddenly fascinated with it. What happened? Why did GSM lose its appeal? Or, did it?

The Basics Let’s begin by learning what these two acronyms stand for. TDMA stands for "Time Division Multiple Access", while CDMA stands for "Code Division Multiple Access". Three of the four words in each acronym are identical, since each technology essentially achieves the same goal, but by using different methods. Each strives to better utilize the radio spectrum by allowing multiple users to share the same physical channel. You heard that right. More than one person can carry on a conversation on the same frequency without causing interference. This is the magic of digital technology. Where the two competing technologies differ is in the manner in which users share the common resource. TDMA does it by chopping up the channel into sequential time slices. Each user of the channel takes turns transmitting and receiving in a round-robin fashion. In reality, only one person is actually using the channel at any given moment, but he only uses it for short bursts. He then gives up the channel momentarily to allow the other users to have their turn. This is very similar to how a computer with just one processor can seem to run multiple applications simultaneously. CDMA on he hand really does let everyone transmit at the same time. Conventional wisdom would lead you to believe that this is simply not possible. Using conventional modulation techniques, it most certainly is impossible. What makes CDMA work is a special type of digital modulation called "Spread Spectrum". This form of modulation takes the user's stream of bits and splatters them across a very wide channel in a pseudo-random fashion. The "pseudo" part is very important here, since the receiver must be able to undo the randomization in order to collect the bits together in a coherent order. If you are still having trouble understanding the differences though, perhaps this analogy will help you. This my own version of an excellent analogy provided by Qualcomm: Imagine a room full of people, all trying to carry on one-on-one conversations. In VDMA each couple takes turns talking. They keep their turns short by saying only one sentence at a time. As there is never more than one person speaking in the room at any given moment, no one has to worry about being heard over the background din. In CDMA, each couple talk at the same time, but they all use a different language. Because none of the listeners understand any language other than that of the individual to whom they are listening, the background din doesn't cause any real problems.

Voice Encoding At this point many people confuse two distinctly different issues involved in the transmission of digital audio. The first is the WAY in which the stream of bits is delivered from one end to the other. This part of the "air interface" is what makes one technology


different from another. The second is the compression algorithm used to squeeze the audio into as small a stream of bits as possible. This latter component is known at the "Voice Coder", or Vocoder for short. Another term commonly used is CODEC, which is a similar word to modem. It combines the terms "COder" and "DECoder". Although each technology has chosen their own unique CODECs, there is no rule saying that one transmission method needs to use a specific CODEC. People often lump a technology's transmission method with its CODEC as though they were single entities. We will discuss CODECs in greater detail later on in this article. Voice encoding schemes differ slightly in their approach to the problem. Because of this, certain types of human voice work better with some CODECs than they do with others. The point to remember is that all PCS CODECs are compromises of some sort. Since human voices have such a fantastic range of pitch and tonal depth, one cannot expect any single compromise to handle each one equally well. This inability to cope with all types of voice at the same level does lead some people to choose one technology over another. All of the PCS technologies try to minimize battery consumption during calls by keeping the transmission of unnecessary data to a minimum. The phone decides whether or not you are presently speaking, or if the sound it hears is just background noise. If the phone determines that there is no intelligent data to transmit, it blanks the audio and reduces the transmitter duty cycle (in the case of TDMA) or the number of transmitted bits (in the case of CDMA). When the audio is blanked, your caller would suddenly find themselves listening to "dead air", and this may cause them to think the call has dropped. To avoid this psychological problem, many service providers insert what is known as "Comfort Noise" during the blanked periods. Comfort Noise is synthesized white noise that tries to mimic the volume and structure of the real background noise. This fake background noise assures the caller that the connection is alive and well. However, in newer CODECs such as EVRC (used exclusively on CDMA systems), background noise is generally suppressed even while the user is talking. This piece of magic makes it sound as though the cell phone user is not in a noisy environment at all. Under these conditions, Comfort Noise is neither necessary, nor desirable. You can read my article on EVRC by clicking here.

Spectral Efficiency Channel capacity in a TDMA system is fixed and indisputable. Each channel carries a finite number of "slots", and you can never accommodate a new caller once each of those slots is filled. Spectral efficiency varies from one technology to another, but computing a precise number is still a contentious issue. For example, GSM provides 8 slots in a channel 200 kHz wide, while IS-136 provides 3 slots in a channel only 30 kHz wide. GSM therefore consumes 25 kHz per user, while IS-136 consumes only 10 kHz per user. One would be sorely tempted to proclaim that IS-136 has 2.5 times the capacity of GSM. In a one-cell system this is certainly true, but once we start deploying multiple cells and channel reuse, the situation becomes more complex. Due to GSM's better error management and frequency hopping, the interference of a co-channel site is greatly reduced. This allows


frequencies to be reused at closer range without a degradation in the overall quality of the service. Capacity is measured in "calls per cell per MHz". An IS-136 system using N=7 reuse (this means you have 7 different sets of frequencies to spread out around town) the figure is 7.0. In GSM we get figures of 5.0 for N=4 and 6.6 for N=3. It was hoped that IS-136 could use tighter reuse than N=7, but its inability to cope with interference made this impossible. Computing this figure for CDMA requires that certain assumptions are made. Formulas have been devised, and using very optimistic assumptions, CDMA can provide a whopping 45 users per cell per MHz. However, when using more pessimistic (and perhaps more realistic) assumptions, the value is 12. That still gives CDMA an almost 2:1 advantage over the TDMA competition.

In-building Coverage Now let's deal with another issue involving CDMA and TDMA. In-building coverage is something that many people talk about, but few people properly understand. Although CDMA has a slight edge in this department, due to a marginally greater tolerance for weak signals, all the technologies fair about the same. This is because the few dB advantage CDMA has is often "used up" when the provider detunes the sites to take advantage of this process gain. Buildings come in many configurations, but the most important aspect to their construction is the materials used. Steel frame buildings, or those with metal siding, shield their interiors more thoroughly than building made of wood. Large window openings allow signals to penetrate more deeply into buildings, so malls with glass roofs will generally provide better service than fully enclosed ones. More important than the type of building however, is the proximity of the nearest site. When a site is located just outside a building, it can penetrate just about any building material. When a site is much further away however, the signals have a much harder time of getting past the walls of a structure when it comes to distance, remember that signals are subject to the "distance squared law". This means that signals decrease by the square of the distance. A site at 0.25 kilometers away will have 4 times the signal strength of a site at 0.50 kilometers away, and 16 times that of a site 1.0 kilometers away. Distance squared however, is the rate of signal reduction in free space. Recent studies have shown that terrestrial communications are usually subject to rates as high as "Distance cubed", or even "Distance to the 4th". If the latter is true, then a site 1.0 kilometers away will actually be 256 times weaker than a site 0.25 kilometers away. In-building penetration is therefore less a technology issue than it is an implementation issue. Service providers who have sites close to the buildings you commonly visit will inevitably look better those who don't. Never use someone else's in-building experiences unless you expect to go in the same buildings as they do. You cannot make useful generalizations about in-building coverage based upon one person's experience. CDMA does however have one peculiarity concerning in-building penetration that does not affect TDMA. When the number of users on a channel goes up, the general level of signal pollution goes up in tandem. To compensate for this the CDMA system directs each phone to transmit with slightly more power. However, if a phone is already at its limit (such as might be the case inside a building) it cannot do anything to "keep up with the pack". This condition is known as "the shrinking coverage phenomenon" or "site breathing". During


slow periods of the day you might find coverage inside a specific building quite good. During rush hour however, you might find it exceedingly poor (or non-existent).

Some Final Observations CDMA really comes into its element when you are out in the countryside with few sites covering large expanses of land. Under these conditions CDMA provides extremely stable audio with few frame errors to mess things up. This is because Channel Pollution is almost unknown in these situations. Under similar conditions TDMA suffers too readily from interference and it will often blank the audio. Many people who use CDMA systems in sparsely populated areas have given this technology extremely high marks. TDMA systems also have great difficulties in open regions just outside densely populated areas. In this situation your phone is exposed to signals coming from countless sites in the densely populated areas, but there are no dominant signals from a close-by site. CDMA can suffer under these conditions too (due to channel pollution), but not quite so badly. Valleys don't present a big problem for TDMA, but high ground is a killer. You can experience choppiness in the audio even when your signal indicator is reading 2 or 3 bars. So in the end, can we really proclaim a winner in the CDMA Vs TDMA war? For the time being I think not. Perhaps in the future when newer technologies built around the W-CDMA standard (wideband CDMA) come into existence, the issue will warrant another look. By that time, even GSM will have moved to CDMA as its air interface of choice, but don't let that fool you into believing that they think the current TDMA air interface is inadequate for its purpose. Future standards are being built around high speed data

ADVANTAGES OF CDMA♦No SIM card is required. ♦Improved call quality: CDMA provides better and more consistent sound quality than systems based on other technologies. ♦Enhanced privacy when compared to systems using other technologies. ♦Increased talk time and standby time for mobiles. ♦They are difficult to intercept for an unauthorized person. ♦They are easily hidden. For an unauthorized person, it is difficult to ever detect their presence in many cases. ♦They are resistant to jamming. ♦Capacity increases of 8 to 10 times that of an AMPS Analog system, and 4 to 5 times GSM , because of CDMA’s unique spread spectrum technology. ♦Many users can share the same carrier frequency, and without time-sharing. This means that mobile phone service providers can handle more customers on a CDMA network than on a GSM network. ♦Improved call quality, with better and more consistent sound , CDMA systems use precise power control—that is, the base station sends commands to every mobile phone currently involved in a call, turning down the power on the nearby ones, and increasing the power of


those further away. The result is a nice, even noise level across the carrier, with lower overall power levels and no spiky interference. ♦In this civilized atmosphere, each station can easily pick out its own coded data frames, decode them and deliver a clean end result. Dropped calls are minimized by CDMA's unique ability to keep every sector of every cell on the same frequency, so handoffs are "soft" as the mobile phone moves from one area to the next. (There is no hole in the signal as one cell is dropped and another is acquired.) ♦CDMA decoders interpret constant sounds, such as road noise, as having no useful content, and ignore them as much as possible.

Simplified system planning through the use of the same frequency in every sector of♦ every cell. Other types of systems (analog, GSM, etc.) need to break up their frequency spectrum allotments so that each cell uses a different frequency. And since no two adjoining cells can use the same frequency, a given cell has to be surrounded by a circle of six other cells, all of which have to be on different frequencies. This translates to frequency re-use of only 1 in 7, and if you change one (by adding a cell for example), the effects ripple through the system.

To an eavesdropper, the call looks like unintelligible noise. CDMA was originally♦ developed by the military for this very reason.

CDMA providers have no such planning earaches, since every sector of every cell uses the♦ same frequency. Enhanced Privacy is inherent in the way CDMA works. Each call is spread over the entire 1.25 MHz carrier—much wider bandwidth than is needed for a single call.

The data bits used to convey real information are mixed with digital coding that is known♦ only to the base station and the individual mobile phone.

Improved coverage characteristics, allowing for the possibility of fewer cell sites This♦ comes from the accurate power control of all mobile phones using the site, and the fact that individual sites don't interfere with each other, since they are all on the same frequency.

DISADVANTAGES Collision : In general, the collisions at the channel is a disadvantage of CDMA system and can be mitigated by careful selection of the sequence and power control that is close to perfect. Roaming : Since most countries have chosen the GSM standard, “roaming” on CDMA is limited. M-commerce : A CDMA doesn’t have a SIM card, which makes m-commerce difficult.


CONCLUSION

CDMA infrastructure is widespread and sure to form the basis for broad penetration of CDMA networks. CDMA2000 and other 3G technologies bring telecommunications into the packetswitched domain, adding a host of new services and network complexities in the process. Troubleshooting activities now require an understanding of both traditional “telecom” concepts related to the circuit-switched domain and new “datacom” concepts related to the packet switched-domain. Network operation and maintenance personnel must refine their processes to meet complex new troubleshooting challenges. These range from misconfiguration problems to duplicated IP addresses and more. Protocol analysis tools can play a bigger role than ever in keeping a network running efficiently. Features such as multi-interface call tracing and protocol filtering will become critical to the job of maintenance.


BIBLIOGRAPHY

Books Being Referred:-

1.Wireless Communication Principles and Practice, Theodore S. Rappaport , Second

Edition, Pearson Education , 2002.

2.Mobile Communication, Jochen H. Schiller, Pearson Education., 2000.

3.Digital Communications, Bernard Sklar, 2nd Edition, Pearson Education, 2001.

4.Mobile Cellular Telecommunications, Lee, 2nd Edition, McGraw Hill, 1995.

5.CRESPO, P.M., HONIG, M.L., and SALEHI, J.A. : “Spread-Time Code-Division

6.Multiple Access” IEEE Trans. On Commun., vol 43, pp. 2139-2148, June 1995.

7.COMPUTER NETWORKS, 3rd Ed. - By Andrew S. Tanenbaum

8.VITERBI, A.J.: CDMA Principle of Spread Spectrum Communications, Reading ,MA: Addison-Wesley, 1995.

9.walsh code in code division multiple access by ,FROZEN


07ddec159 CDMA Concepts (1)

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