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Friday, 7 January 2022

CDMA - Digital Cellular System

CDMA - DIGITAL CELLULAR SYSTEM


The Digital cellular CDMA system is the main application of the spread spectrum technique. This CDMA digital cellular technology based on Direct Sequence (DS) spread spectrum will be discussed in depth here.


Qualcomm proposed and built this digital cellular communication technology. The Telecommunications Industry Association (TIA) has standardized and labeled it as Interim Standard 95 (IS-95) for usage in the 800 MHz and 1900 MHz frequency bands.


The forward link (or channel) between a base station and mobile receivers has a nominal bandwidth of 1.25 MHz. Signal transmission from mobile receivers to a base station takes place over a separate channel with a bandwidth of 1.25 MHz (reverse link or channel). DS Spread spectrum signals with a chip rate of 1.288 x 106 chips per second (Mchips/s) are used in both the forward and reverse connections.


Forward link or channel


The Forward link or channel refers to the signal transmission from a base station to mobile receivers. The block diagram of the IS-95 forward link is shown in figure 1.


Source coding


A code-excited linear predictive (CELP) coder is used to code speech (source). It produces data at speeds of 9600, 4800, 2400, and 1200 bits per second. The data rate is determined by the user's speaking activity in 20ms frame intervals.


Channel coding


Rate = 1/2, constraint length K = 9 convolutional code is used to encode the data from the speech coder. The output symbols from the convolutional encoder are repeated for reduced speech activity. If the data rate is 4800 bits per second, the output symbols are repeated twice to keep the bit rate constant at 9600 bits per second.

                                                         Figure 1: IS-95 Forward Link

 

Block interleaver:


A block interleaver is used to transmit the encoded bits for each frame. It is important to minimize the effects of burst errors that may arise during channel transmission. The block interleaver's data bits arrive at a rate of 19.2kbits/s.


Symbol scrambler


Multiplication with the output of a long code (period N=242-1) generator scrambles the data bits from the block interleaver. The output of this generator is decimated by a factor of 64 to 19.2 kchips/s, even though it runs at a chip rate of 1.288M chips/s. On the forward and reverse links, the long code is used to uniquely identify a call from a mobile station.


Hadamard Sequence


A Hadamard (or Walsh) sequence of 64 characters is assigned to each channel user. Each base station is allotted 64 orthogonal Hadamard sequences. As a result, there are a total of 64 channels available.


A pilot signal is sent using a single Hadamard sequence. The channel properties, such as signal intensity and carrier phase offset, are measured using the pilot signal. For temporal synchronization, another Hadamard sequence is utilized. A different sequence might be used for paging (messaging). As a result, 61 channels are remaining to assign to different individuals. The data sequence is now multiplied by the Hadamard sequence that each user has been assigned.


Modulator


The resultant binary sequence is then multiplied by two PN sequences of length 215 and rate 1.2288 Mchips/s to spread it out. This technique generates in-phase signal components and quadrature. As a result, the binary data signal becomes a four-phase signal. After that, baseband spectral shaping filters are used to filter both the I and Q signals.


Various offsets of these PN sequences identify different base stations. The signals for all 64 channels are sent at the same time. Finally, a carrier wave is heterodyned using BPSK modulation and QPSK spreading. The CDMA signal is the total of the outputs.


Mobile receiver


A RAKE demodulator is utilized at the receiver to resolve the principal multipath signal components. Then, using the phase and signal strength estimations produced from the pilot signal, they are phase-aligned and weighted according to their signal strength. The Viterbi Soft decision decoder receives all of these components and combines them.


Reverse link or channel


The Reverse link or channel refers to the signal transfer from mobile transmitters to a base station. The block diagram of the IS-95 reverse link is shown in Figure.


Limitations


The signals transmitted from multiple mobile transmitters to the base station are asynchronous in the reverse connection. As a result, there is substantially greater user intervention. Furthermore, because mobile transmitters are often battery-powered, these transmissions are power constrained. To compensate for these two restrictions, we must create the reverse link.

                                                                Figure IS-95 Reverse link

 

Source coding


Data on the reverse link may be sent at rates of 9600, 4800, 2400, and 1200 bits per second. The data rate is determined by the user's speaking activity in 20ms frame intervals.


Channel coding


Rate = 1/3, constraint length K=9 convolutional code is used to encode the data from the speech coder. In a fading channel, this coder has a larger coding gain. This compensates for the constraints described above.


The output bits from the convolutional encoder are repeated two, four, or eight times for decreased speech activity.


Block interleaver


A block interleaver is used to pass the encoded bits for each frame. It is required to mitigate the consequences of burst mistakes. The 576 encoded bits are block-interleaved for each 20ms frame. The coded bit rate, on the other hand, is 28.2 kbits/s.


Hadamard sequence


An M=64 orthogonal signal set with 64-length Hadamard sequences is used to modulate the data. As a result, each of the 64 Hadamard sequences is mapped to a 6-bit block of data. At the modulator's output, this results in a bit (or chip) rate of 307.2 kbits/s.


Symbol scrambler


The time position of the transmitted code symbol repeats is randomized to prevent interference to other users. As a result, at lower speaking activity levels, subsequent bursts are not uniformly separated in time.


The output of the long code generator, which runs at a pace of 1.2288 Mchips/s, likewise spreads the signal. This is done for secrecy, scrambling, and spreading, as well as channelization (addressing).


Modulator


The multiplier's output binary sequence of 1.2288 Mchips/s is then multiplied by two PN sequences of length N=215 at a rate of 1.2288 Mchips/s. This procedure generates in-phase signals and quadrature. Baseband spectral shaping filters are used to filter both the I and Q signals.


Before the baseband filter, the Q-channel signal is one-half PN chip time delayed from the I-channel signal. An offset QPSK signal emerges from the output of the two baseband filters. Finally, quadrature mixers receive the filtered signals. The CDMA signal is the total of the outputs.


Base station Receiver


To receive the signals of each active user in the cell, the base station allocates a different channel. Although the chips are sent as an offset QPSK signal, the base station receiver's demodulator uses non-coherent demodulation. In the demodulation procedure, a quick Hadamard transform is applied to decrease computing complexity. The demodulator's output is then passed into the Viterbi detector, which synthesizes the speech signal.

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