Table of Contents
Advertisement
CDMA and Cellular Fundamentals
CDMA reverse link physical layer
The CDMA reverse link uses a different coding scheme to transmit data. Unlike
the forward link, the reverse link does not support a pilot channel for
synchronous demodulation (since each mobile station would need its own pilot
channel). The lack of a pilot channel is partially responsible for the reverse link's
lower capacity than the forward link. In addition, Walsh codes cannot be used
for channelization since the varying time delays from each mobile to the base
station destroys the orthogonality of the Walsh codes. (Varying arrival time
makes the Walsh codes non-orthogonal.)
Since the reverse link does not benefit from non-interfering channels, this
reduces the capacity of the reverse link when compared to the forward link (all
mobiles transmitting interfere with each other). To aid reverse link performance,
the 9600 bps voice data uses a one-third (1/3) rate convolutional code for more
powerful error correction. For the 14,400 bps vocoder, the convolutional
encoder is only a half rate encoder that doubles the data rate. Thus the data rate
coming out of the convolutional encoder is the same for either the 9.6 or
14.4 Kbps voice channels. Then, six data bits at a time are taken to point at one
of the 64 available Walsh codes. The data, which is at 307.2 Kbps, is then
XOR'ed with the long code to reach the full 1.2288 Mbps data rate. This unique
long code is the channelization for the reverse link.
Reverse error protection
To improve the performance of the reverse link, a more powerful convolution
encoder is used. The third-rate encoder used in the reverse link outputs three
9600 bps data streams when driven with a single 9600 data stream. This provides
increased error correction capability, but also increases the data rate to 28,800
bps.
64-ary modulation
Walsh codes are not used to provide the channelization in the reverse link. In the
reverse link they are used to randomize the encoded voice data with a
modulation format that is easy to recover. Each six serial data bits output from
the convolutional encoder are used to point to one of the 64 available Walsh
codes (26 = 64). This modulation has the effect of increasing the data rate 10.67
times to 307 Kbps. As the incoming voice data changes, a different Walsh code is
selected. Since this type of modulation can output one of 64 possible codes, it is
referred to as 64-ary modulation.
Reverse channel long code spreading
The channelization in the reverse link must provide for unique code assignments
for each operational phone. Walsh codes could not be used for the reverse
channelization, since they would not provide enough unique channels. Since the
long code is 42 bits in length, this allows 242 (4.3 billion) unique channel
assignments. Thus the long code imprinted with your unique mask is used to
12
Kyocera 200 Module Data Book
82-B7907-1 Rev. 005
Kyocera Proprietary
Advertisement
Table of Contents
