Introduction to CDMA Basic Knowledge

Multiple Address Access Modes of the Mobile Communication System

FDMA (Frequency Division Multiple Access): Divides the designated spectrum into several parts according to certain requirements with each part occupied by a user during communication connection.

TDMA (Time Division Multiple Access): Divides a continuous period of time into several sections (called time slots) with each user occupying some time of the allocated frequency within a certain period.

CDMA (Code Division Multiple Access): Each user occupies all frequencies and time during communication. However, different users have different quadrature codes to distinguish different user information, thus avoid mutual interference between them. Since the information transmission between different users is isolated from each other via quadrature codes, this mode is characteristic of very high spectrum utilization ratio.

SDMA (Space Division Multiple Access): During communication, each user occupies all frequencies, all time, all or some code words but only a specific space (direction).

Characteristics of the CDMA System

1. Multiple types of diversity

To talk about the "Diversity" technology, we have to mention the concept of "Multipath" at first.

On the RF channels of mobile communication, various types of obstructions may exist in the channels' propagation processes. They may cause arrival of signals transmitted from a point to the RX point through different paths due to direct light, reflection and dispersion. The typical obstructions include buildings, trees, vehicles, human and so on. Since obstructions causing multipath are characteristic of mobility, a wireless channel is a time-varying multipath channel.

Multipath has the following three features:

(1) It can disperse signals from time. For example, to transmit a rational pulse over a multipath channel, the received signals will not be a pulse but a pulse stream.

(2) Every multipath channel has a different attenuation factor. For example, if one rational pulse passes a multipath channel, every pulse of the received signal stream will have different intensity.

(3) Every multipath channel has different phases. For example, to transmit a rational pulse over a multipath channel, every received pulse has its respective phase.

The diversity technology is an effective way to cope with multipath attenuation. In this way, the receiver can make decisions on multiple received signals carrying the same information and with independent attenuation characteristics after combining them. Since the frequency, time and space are selective for attenuation, the diversity technology includes frequency diversity, time diversity and space diversity.

Space diversity--The BTS transmits and receive signals with several independent antennas or at different sites to ensure attenuation dependency between signals. Since these signals are in different geographic environments during transmission, the attenuation of each signal is different too. In this case, the selective combination technology is adopted to select an output with powerful signal, thus reducing the influence on signals caused by factors like the landform.

Both the BTS and MS use Rake (multipath) receivers to perform diversity receiving and combine signals with different transmission delay. For a CDMA system with the channel bandwidth of 1.23MHz, when the delay of signals from two different paths is 1us (that is, when the difference between these two paths is about 0.3KM), the Rake receivers can extract the signals without confusing them. (One chip is equivalent to 1/1.26M=0.8us. If two received multipath signals are less than one chip, the CE demodulation chip will combine them into one multipath signal. However, such case is rare.)

Soft handover also serves the space diversity purpose. During soft handover, an MS will contact multiple BTSs simultaneously, select the best signal from them and send it to the switch.

Frequency diversity--Since the frequency is selective for attenuation, when the interval of two frequencies is more than the channel-related bandwidth, the attenuation signals of these two received attenuation signals will be irrelevant. Generally, the relevant bandwidth in urban areas is 50kHz and that in suburban areas is 250kHz. However, the bandwidth of a CDMA channel is 1.23MHz, which far exceeds the relevant bandwidth requirements no matter in urban or in suburban areas. Therefore, the CDMA broadband transmission itself is frequency diversity.

Timer diversity--Methods like symbol interleave, error check and error correction encoding are adopted.

2. Power control

The capacity of the CDMA system is restricted to interference between MSs within the system. If the minimum required S/N ratio is achieved when the signal from every MS reaches the BTS, the system capacity will reach the maximum value. The objective of power control is to maintain high communication quality without affecting other users occupying the same channel. Power control can be divided into forward power control (from the BTS to MSs) and backward power control (from MSs to the BTS). In addition, backward power control can be further divided into open-loop and closed-loop backward power control.

(1) Forward power control

Forward power control is a way for the BTS to assign different traffic channels with different power. Different MSs may be at different distances and in different environments and the transmission loss from the BTS to every MS is different too. Therefore, the BTS should control the TX power and assign the adequate power to each user's forward traffic channel.

(2) Backward power control

Backward power control can be subdivided into open-loop and closed-loop. Open-loop power control is self-adjustment of the TX power by MSs, which is active. Closed-loop power control indicates that an MS performs TX power adjustment according to the command passively received from the BTS. (1) Backward open-loop power control

When the signal received by an MS from the BTS is intense, it indicates that either the MS is close to the BTS or the propagation path is in very sound conditions. In this case, the TX power of the MS can be reduced and the signal can be normally received by the BTS. To the contrary, when the signal received by an MS is very weak, its TX power can be increased. That is open-loop power adjustment.

(2) Backward closed-loop power control

The forward and backward channels of the CDMA system occupy different bands and the RX/TX interval is 45MHz. These lead to very low relativity between the attenuation of these two channels. In the entire adjustment process, the average value between the attenuation of these two channels should be the same, which is most likely different at a specific moment. In this case, the BTS needs to order the MS to adjust the TX power (closed-loop adjustment) at any time according to the difference between the currently needed S/N ratio and the actually received S/N ratio. The S/N ratio currently needed by the BTS is adjusted (outloop adjustment) at any time according to the initially set error frame rate.

Outloop adjustment: The selector performs statistics on the received bad frames and the total number of frames periodically to see whether their ratio exceeds 1%. If it exceeds 1%, it indicates that the currently set target Eb/N0 is still insufficient. In this case, an instruction will be sent to the BTS to increase the target Eb/N0 by several steps. Otherwise, it will be reduced.

Closed-loop adjustment: The BTS performs Eb/N0 (S/N ratio) measurement on signals received from MSs. If the measurement result is more than the threshold, it will send a "Reduce" command. Otherwise, it will send an "Increase" command until the optimum status is achieved.

During soft handover, an MS will receive power control commands from multiple BTSs. If there are either "Increase" or "Reduce" commands, only the power reduction command will be executed.

3. Low transmission power

Introduction of power control to the CDMA system leads to reduction of the average TX power. Therefore, since the transmission status is sound and the TX power is low in general cases, the TX power can be automatically increased through power control in case of attenuation. However, since no power control is available for other systems, intense enough power should be transmitted all along to meet the S/N ratio in the worst situations.

4. Variable rate vocoder

One major characteristic of the variable rate vocoder is to determine the needed rate with an adequate threshold. This threshold varies with the background level. In this way, the background noise can be suppressed.

5. Security

The scrambling mode of CDMA signals provides high security, which has advantages incomparable by other systems in respects like cross-talk and stealing. The CDMA digital voice channel also allows direct introduction of the encryption technologies of the data encryption standard (DES) or other standards. 6. Soft handover

In this case, an MS needs not to break its link with the previous BTS when communicating with a new BTS. The advantages of this mode are as follows:

(1) Seamless handover

(2) Reduction of call drop possibility

(3) Reduction of the TX power of an MS in the handover area, which is implemented via diversity receiving. Reduction of the TX power facilitates increase of the backward capacity.

Disadvantages:

(1) More hardware devices (channel boards) have to be added.

(2) The forward capacity is reduced. However, since the forward capacity is more than the backward capacity in the CDMA system, appropriate reduction of the forward capacity will not lead to capacity reduction of the entire system.

7. Voice activation

In typical full duplex two-way calls, the duty ratio of every call is less than 35%. The CDMA system is designed with the voice activation technology. In this way, the transmission rate is reduced when there is no talk, thus reducing inference on other users and nearly doubling the capacity. In other systems, since there is some delay for channel reallocation in case of idleness, it is very difficult to use this technology.

8. Frequency reuse and sectorization

In the CDMA system, every cell can use the same band. When the cell uses a directional antenna (1,200 sectorized antenna), the interference can be reduced by 1/3 and the capacity of the entire system is increased by three times.

9. Low Eb/N0 value and high-redundancy error correction encoding

The Eb/N0 value is the proportion of the energy per bit over the noise power spectrum density, which is the standard measuring the digital modulation and encoding mode quality factors. Since the CDMA channel band is very high, a high-redundancy error correction encoding technology can be adopted. In this way, a lower Eb/N0 value can be used. In the CDMA system, a forward error correction code and very effective digital demodulator are used, which reduces the TX power and increases the system capacity. 10. Soft capacity

The system operator can slightly increase the error frame ratio at the traffic peak to increase the number of available channels. Since the CDMA is a self-interference system, when the load on the adjacent cell is low, the interference on the current cell will reduce accordingly. In this case, the capacity can be appropriately increased.

Another form of soft capacity is cell breathing, which indicates that the coverage area of each cell is dynamic. When the load on two adjacent cells is different from either other, the pilot transmission power of the cell with higher load can be reduced to switch over a subscriber at the border of this cell to the adjacent cell. In this way, load sharing is implemented, which is equivalent to capacity increase. For the time being, the ZTE system does not provide this function.

Several Codes Used in the CDMA System

1. PN long code

The PN long code is a "power of 42-1" m sequence with the length of 2.

On a forward channel, the PN long code is used in scrambling of the traffic channel.

On a backward channel, the PN long code is directly used in spreading. Every subscriber is assigned with a m sequence phase, which is calculated from the subscriber's ESN. These m sequence phases are distributed randomly, which will not be repeated with each other. Since the m sequence is characteristic of self-relativity between two values, the backward channels of these subscribers are basically quadrature.

2. PN short code

It is a "power of 15-1" m sequence with the length of 2.

On a forward channel, the PN short code is used in quadrature modulation of the forward channel. Different BTSs use m sequences with different phases for modulation and the phase difference is 64 bits at least. Therefore, in one system, the maximum number of sectors is 512 (215/64).

On a backward channel, the PN short code is used in quadrature modulation of the backward channel. Since the backward channel needs no BTS identification, all MSs use the m sequence with the same phase.

3. Walsh code

In the CDMA system, every forward code division channel uses a walsh code for spreading so that each pair of forward and backward code division channels are mutually quadrature.

Introduction to CDMA 2000 1X Voice Service

Voice Service

Here, the voice service especially refers to the voice service in the CDMA system. In the call process, it includes processing flows like call, handover and release.

Voice Call

The voice call refers to calls in the voice service process, including Call Origination and Paging. Now, we will take the MS 's call origination for example. The appropriate flow is as follows: The major flow in normal cases is as follows:

(1) The MS initiates a call origination message to the BS to request for service.

(2) After receiving this message, the BS sends an acknowledgement message to the MS.

(3) The BS sends a configuration request message to the MSC and starts to allocate the related wireless resources.

(4) The MSC sends an assignment request message to the BS.

(5) The BS sends a channel assignment message to the MS to ask for wireless traffic channel setup.

(6) The MS sends a traffic channel probe frame.

(7) After receiving the probe frame, the BS sends a BS acknowledgement instruction to the MS.

(8) The MS sends a reply MS acknowledgement message to the BS and sends a vacant traffic channel frame.

(9) The BS sends a service connection message to the MS to specify the call service option configuration. The MS starts to process services according to the specified service configuration.

(10) The MS sends a message to the BS, showing that the service connection is successful.

(11) The BS sends an assignment complete message to the MSC.

(12) The MSC sends a ringback tone to the MS.

Voice Handover

Voice handover refers to handover in the voice service process, which can be divided into multiple Handover Types, including Soft Handover, Softer Handover ,Hard Handover and Semi-soft Handover.

Now, we will take the soft/softer handover flow for example. The appropriate flow is as follows:

The major flow in normal cases is as follows:

(1) The source BS determines one or more cells of the target BS to perform soft/softer handover and support this call. The source BS sends a handover request message to the target BS.

(2) The target BS sends a connection message to the source BS to initiate connection.

(3) The source BS sends a connection acknowledgement message as response to finish the connection.

(4) The source BS sends forward frames to the target BS.

(5) After receiving the first forward frame from the source BS, the target BS starts to send vacant backward frames.

(6) The target BS sends a handover request acknowledgement message to the source BS, indicating successful participation of the cell.

(7) The target BS returns a traffic channel state message to the source BS.

(8) The source BS sends a handover indication message to the MS to add the new cell into the effective set to boot the MS to perform handover operation.

(9) After receiving the handover indication message, the MS sends an acknowledgement command to the source BS.

(10) The MS sends a handover complete message, indicating that the handover indication message has been successfully processed.

(11) The source BS sends a BTS acknowledgement command as a reply to the received handover complete message.

(12) The source BS sends a handover complete message to the MSC.

Voice Release

Voice release refers to release initiated in the voice service, including MS-initiated and MSC-initiated release.

Now, we will take MS-initiated release for example. The appropriate flow is as follows:

The major flow in normal cases is as follows:

(1) The MS initiates a call release message to request for traffic channel release.

(2) The BS sends a release message to the MS.

(3) The BS clears the related resources and sends a release request message to the MSC.

(4) The MSC sends a release command to the BS. (5) The BS sends a release complete message to the MSC.

Introduction to CDMA 2000 1X Data Service

Data Service

The data service refers to the packet data service in the CDMA system, which includes processing flows like the call, handover and release.

Several States of an MS in the Data Service

In the CDMA system, an MS can be in three states, which are Null, Dormant and Active.

Null state: Is the state that the MS has not set up connection with the PCF and PDSN.

Dormant state: Indicates that an MS has set up PPP connection with the PDSN through the PCF but the MS is in the dormant state.

Active state: Indicates that an MS is in the data transmission state, that is, it has set up PPP connection with the PDSN through the PCF and the MS is active.

Data Service Status Whether MS Is Connected with PCF Whether PCF Is Connected with PDSN

Null state

Dormant state

Active state

In this table, "" indicates connectionless while "" indicates connection.

Data Call

The data call refers to a call in the data service process, which includes call origination and paging. Now, we will take the MS call origination for example. The appropriate flow is as follows: The major flow in normal cases is as follows:

(1) The MS initiates a call origination message to the BS to request for service.

(2) The BS sends an acknowledgement message to the MS after receiving the message.

(3) The BS sends a configuration request message to the MSC and starts wireless resource allocation.

(4) The MSC sends an assignment request message to the BS.

(5) The BS sends a channel assignment message to the MS to request for wireless traffic channel setup.

(6) The BS assigns a PCF and sends a connection setup message.

(7) The PCF sends a registration request message to the PDSN.

(8) The PDSN returns a message to the PCF and set up connection.

(9) The PCF returns a message to the BS, showing that the connection setup is successful.

(10) The BS sends a BS acknowledgement message and basic traffic channel configuration message to the MS.

(11) The MS returns an acknowledgement message and sends a service connection completed message to the BS.

(12) The BS sends an assignment completed message to the MSC.

(13) The MS and PDSN set up PPP connection and mobile IP registration to start transmission of low-speed packet data.

Data Handover

Data handover refers to the handover in the data service process. Since the handover flow on a basic traffic channel is the same as that of the voice service, we will only describe the handover flow on the PCF, including MS handover in the Dormant and Active states (simply called Dormant handover and Active handover.

1. Normal flow of Dormant handover

(1) When the MS location changes (from the coverage range of the source BSC system to that of the target BSC system), it sends a call origination message to the target BSC. After receiving this message, the target BSC system sends an acknowledgement message to the MS.

(2) The target BSC system constructs a CM service request message, sends it to the MSC and start wireless resource allocation. After receiving this message, the MSC sends an assignment request message to the target BSC.

(3) The target BSC sends a link setup message to the target PCF. The target PCF and PDSN set up a connection and removes the connection set up between the source PCF and PDSN. After setup is finished, the target PCF sends a message showing setup success to the target BSC.

(4) The target BSC sends a message to the MSC. After receiving this message, the MSC sends a clear command to the source BSC. The call is still in the Dormant state.

2. Normal flow of Active handover

(1) When the MS location changes (from the coverage range of the source BSC system to that of the target BSC system), the source BSC sends a handover request to the MSC. Then, the MSC sends a handover request message with hard handover indication to the target BSC.

(2) The target BSC sends a link setup message to the target PCF. After the link between the target BSC and target PCF is set up, the target BSC sends a handover request acknowledgement message to the MSC. The MSC prepares handover from the source BSC to the destination BSC and sends a handover command to the source BSC.

(3) The source BSC disconnects with the source PCF. The source PCF stops data transmission to the MS. The source BSC sends a common handover direction message to the MS via the air interface. After receiving this message, the MS sends an acknowledgement message.

(4) The source BSC sends a handover start message to the MSC and reminds the MS of handover to the target BSC channel.

(5) The MS sends a handover complete message to the target BSC. The destination BSC sends a BSC Ack sequence number to the MS via the air interface. The destination BSC sends a connection request to the destination PCF. The target PCF sets up connection with the PDSN and disconnect the source PCF and PDSN. After connection setup, the target BSC sends a handover complete message to the MSC, indicating that the MS's hard handover is successful.

(6) After receiving this message, the MSC sends a clear command to the source BSC and the source BSC disconnects with the source PCF. After disconnection, the source BSC sends a clear complete message to the MSC, indicating that the clear operation has been completed.

Data Release

Data release refers to release initiated in the data service, which can categorized as follows according to the MS state:

1. Call release from the Active to Dormant state. The call flow can be divided into release initiated from t he MS and that initiated from the BS.

2. Call release from the Active to Null state. The call flow can be further divided into release initiated from the MS and that initiated from the PDSN.

3. Call release from the Dormant to Null state, which especially refers to the PDSN-initiated release.

Now, we will take the MS-initiated call release flow from the Active to Dormant state for example. The appropriate flow is as follows:

The major flow in normal cases is as follows:

(1) The MS initiates a call release message to request for traffic channel release.

(2) The BS sends a clear request message to the MSC.

(3) The MSC sends a clear message to the BS, asking the BS to clear the related resources.

(4) The BS initiates the traffic channel release process.

(5) The BS sends a release message to the PCF to change the state of the packet data call to Dormant and ask the PCE to release the related resources.

(6) The PCF returns an acknowledgement message.

(7) The BS returns a message to the MSC, showing that the clear is successful. The MSC release the connection. The MS changes to the Dormant state.