CDMA

Reverse Channel Long Code Spreading The channelization in the reverse link must provide for unique code assignments for every operational phone. Long Code Spreading  Provides Unique Mobile Channelization Mobiles are Uncorrelated but not Orthogonal with Each Other Since the long code is 42 bits in length, this allows 2^42 (4.3 billion) unique channel assignments. Thus … Read more

There are two key reasons that Walsh codes are not used for the channelization in the reverse link: Mobile transmission are not time aligned and therefore cannot be orthogonal, and Walsh codes do not provide enough unique channels (causing more network overhead to manage Walsh code handoffs). All Walsh Codes Arrive Together in Time to … Read more

Pilot Channel Physical Layer The Pilot channel is essentially the short codes operating on their assigned PN offset. This is accomplished by selecting Walsh code 0 which is 64 zeroes (remember, the first code in any Walsh code set is always composed of all zeroes) as the Walsh code modulation data. In addition, the channel … Read more

The Base Station transmitter signal is, however, the composite of many channels (with a minimum of four). The Pilot channel is unmodulated (Walsh code 0); it consists of only the final spreading sequence (short sequences). The Pilot channel is used by all mobiles linked to a cell as a coherent phase reference and also provides … Read more

Auto-correlation is simply a comparison of a signal against itself. For a digital sequence, such as the short codes used in EIA/TIA-95-B, this comparison is a measure of the number of bits that match relative to the number of bits that do not match. Good pseudo-random patterns (such as the short codes) are designed to … Read more

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 … Read more

Once the data has been scrambled with the user specific long code, the closed loop power control data is then punctured into the data stream. Remember that the power control bits are sent every 1.25 milli-seconds – once in every power control group (a CDMA frame is 20 milli-seconds with each frame having 16 1.25 … Read more

All base stations digital transmissions are referenced to a common CDMA system-wide time scale that uses the GPS time scale, which is traceable to and synchronous with Universal Coordianted Time (UTC). GPS and UTC difer by an integer number of seconds, specifically the number of leap second corrections added to UTC since January 6, 1980. … Read more

For both mobiles and base station in direct sequence CDMA must be synchronized. In the IS-95A system, synchronization is based on the Global Positioning Satellite system time. The CDMA system time is the time reference used by all IS-95 / IS-2000 and IS-856 systems. It is synchronous to UTC time (Universal Coordinated Time) except for … Read more

In an effort to provide CDMA with even greater voice quality, the CDG (CDMA Development Group) has proposed and implemented a new vocoder. This new vocoder uses and improved, higher data rate of approximately 13 kbps to digitized voice signals. After adding bits used to support the traffic channel, the final traffic channel data rate … Read more