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
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.