Implementing CDMA Technology
The following sections describe how a system might implement the steps illustrated in
Figure 1.
Input data
CDMA works on Information data from several possible sources, such as digitized voice
or ISDN channels. Data rates can vary, here are some examples:
Data Source Data Rate
Voice Pulse Code Modulation (PCM) 64 kBits/sec
Adaptive Differential Pulse Code Modulation (ADPCM) 32 kBits/sec
Low Delay Code Excited Linear Prediction (LD-CELP) 16 kBits/sec
ISDN Bearer Channel (B-Channel) 64 kBits/sec
Data Channel (D-Channel) 16 kBits/sec
The system works with 64 kBits/sec data, but can accept input rates of 8, 16, 32, or 64
kBits/sec. Inputs of less than 64 kBits/sec are padded with extra bits to bring them up to
64 kBits/sec.
For inputs of 8, 16, 32, or 64 kBits/sec, the system applies Forward Error Correction
(FEC) coding, which doubles the bit rate, up to 128 kbits/sec. The Complex Modulation
scheme (which we’ll discuss in more detail later), transmits two bits at a time, in two bit
symbols. For inputs of less than 64 kbits/sec, each symbol is repeated to bring the
transmission rate up to 64 kilosymbols/sec. Each component of the complex signal
carries one bit of the two bit symbol, at 64 kBits/sec, as shown below.
