## Example of direct sequence CDMA

The most widely adopted CDMA radio system (outside the military), was pioneered by QUALCOM, a California-based company, and is now embodied in the IS-95 standard for cellular telephone applications. This 'air interface' is a direct sequence CDMA design, spreading each user voice or data signal over a 1.25 MHz channel bandwidth. The maximum user data rate per spreading code is 9600 bps, (rates of 1200, 2400 and 4800 are also possible), which is then channel coded up to 19200 bps in the forward channel (base to mobile), and 28800 bps in the reverse channel (mobile to base). In the reverse channel, the 28.8 kps coded data is mapped onto 64 orthogonal symbol states known as WALSH functions, giving a symbol rate of 4800 symbol/second. This narrowband symbol stream is then spread using a unique identifying spreading code for every mobile unit – called the long code, clocking at a rate of 1.2288 million chips/second. The long code is a pseudo-random sequence of length 224 – 1 bits. This means that there are 224 – 1 = 4398046511103 possible different code sequences, enough for all the mobiles in the world!