CDMA Reference Blockset

Fundamentals of the IS-95A CDMA System

This section describes the important features of the IS-95A CDMA system specifications. The CDMA Reference Blockset is designed to help you develop models to simulate different components of the IS-95A CDMA system.

Channel Schematics

The following figure illustrates an IS-95A forward channel. The transmitter section includes channel coding, modulation and spreading, and filtering. The receiver section includes filtering, despreading and demodulation, and channel decoding.

IS-95A Forward Channel Diagram

The following figure illustrates an IS-95A reverse channel. It includes many of the same operations that are in the forward channel, but the functionalities of the blocks correspond to the reverse channel specifications.

IS-95A Reverse Channel Diagram

Forward and Reverse Channels

The primary features of the forward and reverse channels are described below in these sections:

• Channel Assignment in IS-95A
• Forward Channel Coding
• Base Station Modulation and Spreading
• Base Station Transmitter Interface
• Coherent Rake Receiver
• Reverse Channel Coding
• Mobile Station Walsh Modulation and Spreading
• Noncoherent Rake Receiver

Channel Assignment in IS-95A

The CDMA system requires spreading of the spectrum using a PN sequence. In IS-95A, the rate of this PN sequence (called the chip rate) is 1.2288 Mchips/s. This causes the resulting bandwidth of the spread signals to be about 1.25MHz, which is about one-tenth of the total bandwidth allocated to one cellular service carrier.

The IS-95A system uses two PN codes:

• The short PN code is a pair of periodic binary PN sequences with a period of 2¹⁵. These sequences are used for spreading and despreading signals into in-phase and quadrature components. The same short PN code is used by multiple base stations using the same frequency band by using different timing offsets in the code cycle.
• The long PN code is a sequence with a period of 2⁴²-1, and this is used for spreading on the reverse link, as well as for data scrambling and power control burst randomization.

In addition to the PN codes, there is a set of length-64 mutually orthogonal codes called the Walsh code, which is used for ensuring orthogonality between the signals for different users receiving from the same base station. The Walsh code is also used for modulation for the reverse channel of IS-95A. Thus, the logical channel on the forward link is determined by the short PN code offset, the Walsh code assigned, and the assigned frequency of operation. On the reverse link, the logical channel is determined by the short PN code offset, the long code offset, and the assigned frequency of operation.

The IS-95A forward link uses several logical channels:

• The Pilot channel modulates a constant symbol and is used for channel estimation, which allows for coherent demodulation of the other channels that carry information bits.
• The Sync channel is used for providing synchronization and configuration information to the mobile stations.
• The Paging channels are used for control information and sending paging messages to the mobile station.
• The Traffic channel carries the speech or data.

Similarly, the reverse link has the Traffic and Access logical channels:

• The Access channel is meant for control information, and is used for originating requests, responding to paging and other messages, or providing other data to the base station.
• The Traffic channel carries the speech or data. The Traffic channel supports variable data rate operation. According to the J-STD-008 requirement, there are two sets of Traffic channel data rates. Rate Set I has a maximum data rate of 9.6 kbps, whereas Rate Set II has a maximum data rate of 14.4 kbps. Both rate sets support full, half, quarter, and eighth rates with respect to the maximum data rate.

Forward Channel Coding

The IS-95A system uses the forward channels Sync, Paging, and Traffic to carry information from the base station to the mobile units. The channel coding operations in the forward CDMA operations use 20 ms frames for all channels, except for the Sync channel, which is coded using 26.666 ms frames. For error protection, a 1/2-rate convolutional code is used for all information channels. The receiver can use the Viterbi algorithm for optimal decoding of the encoded data.

Because the system uses variable data rates, the number of bits generated by the vocoder in one frame changes depending on the voice activity. To ensure that the symbol rate at the modulation stage is kept constant, the symbols are repeated for the lower data rate frames. For protection against bursts of errors, the frame data is interleaved prior to modulation. These error protection measures improve the overall bit error rate (BER) on the link.

Base Station Modulation and Spreading

The IS-95A forward CDMA channel consists of the Pilot channel for coherent demodulation and the information channels. These channels are spread orthogonally by using a set of codes of length 64 called the Walsh codes. The combined signal is spread in quadrature by a pair of PN sequences with a fixed spreading rate.

The base station transmitter performs the encoding, the repetition, the interleaving, and the scrambling prior to spreading. The generated modulation symbols also need to carry power control commands to correct the mobile station transmit power. For this purpose, some of the symbols are replaced with command bits known as power control bits. The locations of these bits are randomized using a scheme based on a decimated long PN code. The details of these are in the IS-95A standard. The spread signal is filtered at baseband before transmission.

Base Station Transmitter Interface

The base station transmitter interface combines the various channels in the CDMA forward channel. The forward channel contains the Pilot channel, the Sync channel, the Paging channels, and multiple Traffic channels. Each Traffic channel has a unique Walsh code assigned to it. These Walsh codes are orthogonal to each other, and the different symbol streams are spread by their respective Walsh code (in bipolar form) and added together in a manner such that the weight given to each corresponds to the intended power in that channel. The Traffic channel has a variable data rate. In the case of lower data rate frames, the symbols are repeated and the Traffic channel power is reduced by the same repetition factor. This reduction ensures that the power transmitted for each information data bit (before coding and repetition) is the same.

Coherent Rake Receiver

The coherent rake receiver demodulates the desired channel in the input signal by despreading it with the corresponding Walsh code and the short PN code. The mobile user receives the signal transmitted from the serving CDMA base station through several paths with different propagation delays. The received signal, in addition to being corrupted by noise, is also distorted by the channel fading. For a basic receiver design, the delay-spread results in a loss of performance.

The rake receiver, on the other hand, uses the direct-sequence spreading of the coded signal to separate the components of the received signal corresponding to different propagation-delay paths. You can almost say that the rake receiver derives diversity gain from a potentially poor channel. After rake receiver despreading, a demodulation routine detects the transmitted data from each delayed-path component and combines the results.

Reverse Channel Coding

On the reverse CDMA channel in IS-95A, all the transmission uses 20 ms frames. The channels used in the reverse link are:

• The Traffic Channel and Access Channel. The Traffic channel carries data and speech. The data rate on the Traffic channel is variable; hence each frame may have a different rate.
• The Access channel carries control messages and requests to the base station.

The mobile station convolutionally encodes the data transmitted on the reverse channel. The station uses a 1/3-rate code when the Traffic channel uses Rate Set I, and a 1/2-rate code when the Traffic channel uses Rate Set II. The Access channel uses the 1/3-rate code. As in the case of the forward channel, the Viterbi algorithm provides optimal decoding at the receiver. Also, prior to modulation, symbols are repeated in the case of lower data rates, and the frame data is interleaved for protection against burst errors.

Mobile Station Walsh Modulation and Spreading

The reverse CDMA channel is composed of the Access and Traffic channels. These channels share the same assigned CDMA frequency using direct-sequence CDMA techniques. Each of these channels is assigned a distinct user long PN code sequence, which is used for spreading the spectrum of the signal.

All transmission on the reverse CDMA channel in IS-95A uses 20-ms frames. The data frames are convolutionally encoded and interleaved for error protection. The modulation used is a 64-ary orthogonal modulation. Direct sequence spreading and filtering are used to obtain the modulation signal for transmission. Because the reverse traffic channel has a variable rate, randomized gating is used in the case of lower data rate frames to control the transmit power. Because the power control is done by gating, the power level of the transmitted symbols is kept constant, unlike the forward channel. The details of these transmitter tasks are in the IS-95A standard.

Noncoherent Rake Receiver

There is no pilot available for the reverse link transmission, as there is for the forward link. A pilot is valuable for obtaining good carrier channel estimation, making it possible to perform coherent detection and combining of multipath components. The absence of carrier phase and amplitude estimation necessitates either noncoherent or differential coherent detection. Timing of all paths must also be acquired and tracked. This discussion assumes that timing is available, but phase and amplitude estimates are not.

The Walsh modulator collects log₂64 = 6 data bits and transmits one of 64 orthogonal Walsh functions. The demodulator tracks L independent paths. Assume that each path has a separate demodulator, but that their outputs are noncoherently combined. The optimum noncoherent demodulator of Walsh modulation is a bank of 64 orthogonal noncoherent correlators. Each of the 64 noncoherent correlators squares each of the two quadrature components and adds the results. For a single path, this demodulator makes its decision by selecting the largest of the 64 output magnitudes. In the case of L paths, the demodulator adds the individual noncoherent correlator outputs for each of the L independent paths before making a decision.

In practice, it is possible to make a decision for each bit separately, rather than making decisions for all six bits at once. This may be done by finding the difference between the highest correlation values that correspond to the two possible values (0 and 1) of the bit of interest. Such an approach may be applied bit by bit and used to arrive at soft decisions (decisions whose difference value serves as a metric for decision reliability). Soft decisions can be used in the Viterbi decoder for the convolutional code to improve performance.

Overview of CDMA

Main Library Structure