Chapter2 GSM Air Interface

Wireless Network Planning Table of contents

Table of Contents

Chapter 2 GSM Air Interface...................................................................................................1-12.1 GSM System Introduction............................................................................................1-12.2 Wireless Channel Structure..........................................................................................1-3

2.2.1 Time Slot and Frame Structure..........................................................................1-3

2.2.2 Physical Channel...............................................................................................1-5

2.2.3 Logic Channel....................................................................................................1-6

2.2.4 Allowed Channel Combination Type...................................................................1-8

2.2.5 The Frame Structure of the Logic Channel.........................................................1-9

2.2.6 The Use of the Common Control Channel..........................................................1-9

2.2.7 The Early Timing of Uplink and Downlink as well as MS.....................................1-9

i

Wireless Network Planning Chapter 2 GSM Air Interface

Chapter 1 GSM Air Interface

1.1 GSM System Introduction

GSM system structure is shown as in Figure 2-1.

MS

BTS

BTSBSC

BSS

OMC

MSC/VLR

NMC

EI R

HLR/AUC

DPPS PCS SEMC

OSS

PSTNI SDNPDN

NSS

OSS: Operation and Maintenance Subsystem

BSS: Base Station Subsystem

NSS: Network Subsystem

NMC: Network Management Center

DPPS: Data Post Processing System

SEMC: Security Management Center

PCS: SIM Card Personalization Center

OMC: Operation and Maintenance Center

MSC: Mobile Switching Center

VLR: Visitor Location Register

HLR: Home Location Register

AUC: Authentication Center

EIR: Equipment Identification Register

BSC: Base Station Controller

BTS: Base Transceiver Station

MS: Mobile Station

PDN: Public Date Network

1


PSTN: Public Switched Telephon Network

ISDN: Integrated Services Digital Network

Figure 2-1 GSM System Structure

Figure 2-1 illustrates that a GSM system consists of three subsystems: Operation and Maintenance Subsystem (OSS), Base Station Subsystem (BSS), and Network Subsystem (NSS). BSS is the basic part of the GSM system which deals directly with the wireless cellular aspect. Connected to the mobile station via wireless interface, it takes charge of transceiving wireless signals and managing wireless resources. NSS is the core part of the whole GSM system. It has such functions as switching, connection, and management between different GSM subscribers as well as between GSM sbscribers and subscribers of other communications networks.Its major responsibilities include call processing, communication management, mobility management, part of the wireless resource management, security management, subscriber data and equipment management, billing record processing, common channel and signaling processing, and local opreation and maintenance.BSS is mainly responsible for transceiving wireless signals and managing wirelsss resource. At the same time, it connects to NSS to realize the connection beteen different mobile subscribers as well as between mobile subscribers and subscribers of the fixed network to send system information and subscriber information. Of course this requires the interworking with OSS.

The actual GSM network can be divided into several different areas. To be specific, it include the following aspects:

Service Area is the area where the service is available for mobile stations. Within this area, subcribers of different communications networks can communicate with a mobile station without knowing its exact position.

PLMN Area refers to the whole area covered by the Public Land Mobile Network (PLMN), which is a network independent from other communications networks such as ISDN and PSTN.

MSC Area refers to a part of the PLMN network covered by all the cells controlled by one MSC.One MSC area may consist of several location areas.

Location Area refers to the area where a mobile station can move freely without location update. One location area may consists of several cells or base station areas. In order to call a mobile station, the calling can be initiated from all the base stations in one location area at the same time.

Base Station Area refers to the area of all the cells within the range of one or more BTSs placed in the same area.

Cell refers to the radio coverage area labelled by base station identity code or global cell identity code. When the omnidirectional antenna is used, the cell is equivalent to the base station area. In design, a specific cellular area, is a cell.

2


Figure 2-2 GSM Area Classification

1.2 Wireless Channel Structure

In PLMN, MS connects to the fixed part of the GSM system via wireless channel so as to enable subscribers to access communications services. In order to realize the interconnection of MS and BTS, the signal transmission via the wirless channel has to undergo a series of regulations and a set of standards has to be established. This set of regulations for signal transmission via the wireless channel is the so-called Air Interface, also named Um interface.

1.2.1 Time Slot and Frame Structure

The Um interface integrates such technologies as Frequency Dividion Multiple Access (FDMA), Time Dividion Multiple Access, and frequency hopping technology.

The transmission unit through the Um interface is the Burst composed of about 100 modulated bits. Burst occupies 200kHz frequency band width, and it last 0.577ms(15/26ms). As is shown in Figure 2-3, the time and frequency window it occupies is called slot. The time a slot occupies is called a time slot, while the frequency bandwidth it occupies is called a frequency slot. The frequency slot is equivalent to the radio frequency channel in the GSM norm.

The frame is often represented in the form of n consecutively occurred slots. In the GSM system, the value of n is set to be 8, and this kind of frame is called TDMA frame that is each TDMA frame is composed of eight consecutive slots.

3


Figure 2-3 The concept of time slot

One physical channel is the burst sent in specific and cyclic slots. In the GSM system, the cycle is eight, which is a TDMA frame. It is qualified to say that each radio frequency channel consists of eight physical channels. A physical channel can be identified and differentiated from others according to the number of one of its slots in the TDMA frame, and this number is called time slot number.

If a radio frequency channel is not frequency hopping, then its core frequency is constant. Otherwise, its core frequency is changing, and the changing unit is 200kHz.

The complete TDMA frame structure is shown as in Figure 2-4, while the Burst structure in Figure 2-5.

Figure 2-4 The TDMA frame structure of the channel

The explanation for the above figure is as follows:

A TDMA frame lasts 4.615ms(120/26ms), composed of eight time slots.

Several TDMA frames constitute a multiframe, whose structures have two kinds: 26 multiframe and 51 multiframe. The cycle of the 26 multiframe structure is 120ms, and it contains 26 TDMA frames, which are used as service channels and associated control channels. The cycle of the 51 multiframe structure is 3060/13ms, and it contains 51 TDMA frames, which are used as control channels.

4


Several multiframes constitute a super frame, which contains 51×26＝1326 TDMA frames. Each super frame may contain 51 26-multiframes or 26 51-multiframes. The cycle of super frames is 6.12s.

Several super frames constitute a hyper frame, which contains 2048 super frames. Its cycle is 12533.76s, that is three hours twenty-eight minutes fifty-three seconds and 760 milliseconds.In each cycle, a hyper frame contains 2715648 TDMA frames, which are numbered from zero to 2715647.

Figure 2-5 Several kinds of Burst structure

1.2.2 Physical Channel

The physical channel is the combination of frequency division and time division. It is composed of the slot flow between BTS and MS.Therefore, any physical channel has to be described from two dimensions: frequency and time.

5


I. 1. Frequency domain description

(1) The working frequency band of the GSM system

Uplink (MS→BTS): 890～915MHz；1710～1785MHz

Downlink (BTS→MS): 935～960MHz； 1805～1880MHz

Duplex interval: 45MHz(900M)；95MHz(1800M)

Carrier frequency interval: 200kHz

(2)Frequency hopping

(3)Cell (frequency point) distribution and mobile station (frequency point) distribution

(4) Mobility distribution deviation and frequency hopping serial generating number

II. Time domain description

TN----time slot number

FN----TDMA frame number

1.2.3 Logic Channel

The logic channel results from the time complexing on the physical channel. Different logic channels are used for different kinds of information transmission between BTS and MS. The explanation for the logic channel defined in the GSM norm is as follows:

I. Traffic Channel(TCH)

TCH carries speech or subscriber data, and the full rate TCH carries the information with the rate of 22.8kbit/s. TCH include the following kinds of traffic channels:

Enhanced full rate speech TCH (TCH/EFS)

Full rate speech TCH (TCH/FS)

9.6kbit/s full rate data TCH (TCH/F9.6)

4.8kbit/s full rate data TCH (TCH/F4.8)

≤2.4kbit/s full rate data TCH (TCH/F2.4)

II. Control Channel (CCH)

The control channel mainly carries signaling or synchronous data. It can be divided into four kinds according to the different tasks processed:

(1) Broadcasting Channel (BCH)

BCH is the one-point-to-many-points unidirectional control channel from BTS to MS, which is used to broadcast all kinds of information to MS. BCH can be divided into three kinds:

FCCH: Frequency Correction Channel, which is used to correct the MS frequency;

6


SCH: Synchronous Channel, which is used in the MS frame synchronization and BTS identification;

BCCH: Broadcasting Control Channel, which is used to broadcast cell informaiton.

(2) Common Control Channel (CCCH)

CCCH is the one-point-to-many-points bidirectional control channel, which is mainly used to carry signaling information necessary for the access management function, and it can also carry other kinds of signaling. CCCH is commonly used by all MSs of the network. It includes three parts:

PCH: Paging channel, which is used by BTS to page MS;

RACH: Random Access Channel, which is used by MS to randomly access the uplink channel of the network;

AGCH: Access Grant Channel, which is used to assign the special control channel to the connection with the successful access.

(3) Dedicated Control Channel (DCCH)

DCCH is point-to-point bidirectional control channel. Based on the requirement for the communications control process, DCCH is assigned to MS to enable it to conduct point-to-point signaling transmission with BTS. It can be divided into the following kinds:

SDCCH/8: Separate Dedicated Control Channel;

SACCH/C8: Slow Associated Control Channel associated with SDCCH/8;

SACCH/TF: Slow Associated Control Channel associated with TCH/F;

FACCH/F: Fast Associated Control Channel/Full Rate;

SDCCH/4: Separate Dedicated Control Channel combined with BCCH/CCCH;

SACCH/C4: Slow Associated Control Channel associated with SDCCH/4.

(4) Cell Broadcasting Channel (CBCH)

CBCH is used to broadcast cell short message bit/s only with the downlink direction. It carries cell broadcasting short message service information, and it uses the same physical channel with SDCCH.

The summary for the logic channel supported by M900 BTS is shown as in Figure 2-6.

7


Figure 2-6 Logic Channel Classification

1.2.4 Allowed Channel Combination Type

The logic channel is projected to the physical channel based on a specific rule. The channel combination types allowed in GSM PHASE 2 Norm are as follows:

(1) TCH/F + FACCH/F + SACCH/TF

(2) TCH/H (0,1)+ FACCH/H(0,1)+ SACCH/TH(0,1)

(3) TCH/H (0,0)+ FACCH/H(0,1)+ SACCH/TH(0,1)+ TCH/H(1,1)

(4) FCCH + SCH + BCCH + CCCH (main BCCH)

(5) FCCH + SCH + BCCH + CCCH + SDCCH/4(0..3)+ SACCH/C4 (0..3)(Combined BCCH)

(6) BCCH + CCCH (Extended BCCH)

(7)SDCCH/8(0. .7)+ SACCH/C8 (0. .7)

Note:

1. If the system supports SMSCB, the SDCCH (1/8) in the combination (5) and (7) used as CBCH；

2. The combination (5)(Combined CCCH) can only be adopted when there is no other CCCH in the cell.

8


3. CCCH = PCH + RACH + AGCH + NCH.

1.2.5 The Frame Structure of the Logic Channel

Different logic channels have different frame structures. However, the frame structure of logical channels is always cyclic in time, although different logic channels have different cycles. In order to realize high-performance system, the GSM Norm works hard on the frame structure of the logical channel, which results in the complicated frame structure system of the GSM system.

1.2.6 The Use of the Common Control Channel

The following explanation is for the use of the common control channel:

Among all the carrier frequencies in a cell, there is only one carrier frequency can support BCCH. Of course, it also supports CCCH.The cell is uniquely labeled by this carrier frequency, which is called C0 in the protocol.

C0 carrier frequency must transmit at the constant frequency with the constant power. That is to say, C0 carrier frequency is not allowed for frequency hopping. The frequency used by C0 carrier frequency is called Scaling Frequency.

TN0 with and only with C0 can support channel combination (4) and (5), that is main BCCH and combined BCCH.

Parameter BS_CC_CHANS and extended BCCH.

Parameter BS_CCCH_SDCCH_COMB and combined BCCH.

Parameter BS_AG_BLKS_RES.

Parameter BS_PA_MFRMS.

The CCCH_GROUP and PAGING_GROUP of the MS as well as the paging and discontinuous reception.

1.2.7 The Early Timing of Uplink and Downlink as well as MS

In the GSM system, uplinks refer to the links from MS to BTS; while downlinks refer to those from BTS to MS.

As far as MS is concerned, if the requirement for simultaneous transmission and reception can be avoided, it will be great, because there is no need for the MS to protect its transmitter while it is receiving, which can reduce the volume of the MS to a large extent. In order to realize this point, in the GSM system, the TDMA frame of the uplink is always three BPs (about 1730μs) behind that of the downlink. Form BTS perspective, this delay is constant, but for MS, things are different. Due to the mobility of MS, there always exists a transmission delay from MS to the BTS in its service cell, and this delay is usually not constant. In order to compensate for the transmission delay to and from BTS, the transmission of MS must take place earlier,

and this is called the early timing of MS.The range for the early time is between 0～233μs (this limitation comes from protection bit time domain feature of Access Burst).

9


Therefore, from the MS perspective, the accurate deviation between the uplink and downlink is three BPs less the early timing value.

The MS in the special mode must transmit using proper early timing value at any time; otherwise, it will lose the synchronization with BTS. In the GSM system, the method of self-adaptive timing adjustment is employed to ensure that the MS in the special mode always uses proper early timing value.

10

Chapter2 GSM Air Interface

Documents

Transcript of Chapter2 GSM Air Interface