why the data needs to be scrambled again at the same rate
After all of the coding used so far in the forward channel, you’re probably wondering why the data needs to be scrambled again at the same rate. If all cells used the same 64 Walsh codes without another layer of scrambling, the resulting interference would severely limit system capacity.
Since all cells can use the same frequency (frequency domain), and all cells use the same Walsh codes (code domain); the only other means to allow cells to reuse the same Walsh codes is by using time offsets (time domain).
This final layer of scrambling uses another code called the short code to allow reuse of the Walsh codes and to provide a unique identifier to each cell. The short sequence is 32768 bits long and runs at the 1.2288 Mbps rate (repeats every 26.667 ms).
Since everything in CDMA is synchronized to system time, it is possible to have each cell site identified by using a time offset in the short sequence. These “PN Offsets” are separated by multiples of sixty-four 1.2288 Mbps clock chips. This allows for 512 unique time offsets for cell identification (32768 bits / 64 bits = 512 offsets).
By scrambling the Walsh encoded channels with the short code, each base station can reuse all 64 Walsh codes and be uniquely identified from other adjacent cells using the same frequency.
- Provides a Cover to Hide the 64 Walsh Codes
- Each Base Station is Assigned a Time Offset in its Short Sequences
- Time Offsets Allow Mobiles to Distinguish Between Adjacent Cells
- Also Allows Reuse of All Walsh Codes in Each Cell