B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

l l .8 Modern Practical DSSS CDMA Systems 645
Fi g ure 11. 1 8
RF bandwidth
requirements for
IS-95 uplink and
downlink.
◄
Uplink
"..,,, f
1.25 MHz
Mobile station
•
Downlink
1.25 MHz
f
Base station
Fi g ure 11.19
Forward link
modulation and
Walsh code
spreading of
cdmaOne
(IS-95).
QCELP
vocoder
Rate 1/2
FEC
encoder
9.6 kbit/s I 9.2 kbit/s
Interleaver Walsh
and
code
scrambler spreading
19.2 kbit/s 1.2288 Mbit/s
8-----+- 1.2288
M s
1.2288 Mbit/s ,
B---. modulator
reuse efficiency. In addition, CDMA provides better overall capacity when the data traffic load
is dynamic. This is because users in a lightly loaded CDMA system would have a lower interference
level and better performance, whereas TDMA users with fixed channel bandwidth do
not enjoy such benefit.
CDMA Cellular System: cdmaOne (IS-95)
The first commercially successful CDMA system in cellular applications was developed by the
Electronic Industries Association (EIA) as interim standard-95 (IS-95). Now under the official
name of cdmaOne, it employs DSSS by adopting 1.2288-Mchip/s spreading sequences on
both uplink and downlink. The uplink and downlink transmissions both occupy 1.25 MHz of
RF bandwidth, as illustrated in Fig. 11. 18.
The QCELP (Qualcomm code-excited linear prediction) vocoder is used for voice encoding.
Since the voice coder exploits gaps and pauses in speech, the data rate is variable from
1.2 to 9.6 kbit/s. To keep the symbol rate constant, whenever the bit rate falls below the peak
bit rate of 9.6 kbit/s, repetition code is used to fill the gaps. For example, if the output of
the voice coder (and subsequently the convolutional coder) falls to 2.4 kbit/s, the output is
repeated three more times before it reaches the interleaver. The transmitter of cdmaOne takes
advantage of this repetition time by reducing the output power during three out of the four
identical symbols by at least 20 dB. In this way, the multiple-access interference is diminished.
This "voice activity gating" reduces MAI and increases overall system capacity.
The modulation of cdmaOne uses QPSK on the downlink, and the uplink uses a variant of
QPSK known as the offset QPSK ( or OQPSK). There are other important differences between
the forward and reverse links. Figure 11. 19 outlines the basic operations of spreading and
modulation on the forward link. After a rate 1/2 convolutional error correction code, the voice
data becomes 19.2 kbit/s. Interleaving then shuffles the data to alleviate burst error effects,
and long-code scrambling provides some nominal privacy protection. The data rate remains
at 19.2 kbit/s before being spread by a length 64 Walsh-Hadamard short-code to result in a
sequence of rate 1.2288 Mbit/s. Because forward link uses synchronous transmissions, in the
absence of channel distortions, there can be as many as 64 orthogonal data channels, each using
a distinct Walsh-Hadamard code. Both the in-phase (I) and the quadrature (Q) components of
the QPSK modulations carry the same data over the 1.25 MHz bandwidth, although different
masking codes are applied to I and Q.
The performance of the reverse link is of greater concern for two reasons. First, as discussed
earlier, the reverse link is subject to near-far effects. Second, since all transmissions
on the forward link originate at the same base station, it uses the orthogonal Walsh-Hadamard