Forward Channel Structure in CDMA
Here i write down on how Forward Channel works in CDMA ? and its Structure. The following figure shows an example of the code channels transmitted by a base station. Out of the 64 code channels available for use, the example depicts the Pilot Channel (always required), one Sync channel, seven Paging Channels (the maximum allowed), and fifty-five Traffic Channels.
Code channels on the forward link are addressed by different Walsh Codes. Each of these code channels is spread by the appropriate Pseudo-Noise Sequence at a fixed Chip Rate of 1.2288 Mega- Chips per second. The uniqueness of the forward channel structure is the use of the Pilot Channel.It is transmitted by each cell site and is used as a coherent carrier reference for demodulation by all subscriber stations. The pilot signal is unmodulated and uses the zeroth Walsh Code which consists of 64 zeros. Hence, the pilot simply contains the I and Q spreading code.
The choice of this code allows the subscriber to acquire the system faster. The Walsh Codes are generated with a 64 x 64 Hadamard Matrix. Thus, the maximum number of code channels per carrier is 64 which consists of a Pilot Channel, a Sync Channel, a maximum of 7 Paging Channels and a minimum of 55 Traffic Channels (TCH).
In view of the channel structure, a 1.23 MHz CDMA carrier can support up to 55 TCHs if the effect of interference is not considered. Another possible configuration could replace Paging Channels and Sync Channels one for one with TCHs to obtain a maximum of 63 TCHs, 1 Pilot Channel, 0 Paging Channel, and 0 Sync Channel. In practice, due to the intense interference in the spectrum, a satisfactory quality of service in terms of voice quality and FER is difficult to maintain if all 55 traffic channels are implemented in the system.
The SCTM CDMA equipment requires a carrier frequency, a pilot offset, and a Walsh Code to encode/decode the channel. The BSS allocates a traffic channel in response to the Assignment Request message from the MSC. BSS does not allocate traffic channels unless a request from the MSC is acknowledged. The traffic channel will be allocated in the sector with which the call is associated.
The BSS maintains a pool of traffic channels and Walsh Codes in each sector for new call setups and soft/softer handoffs. Traffic channel allocation for new originations and soft handoffs require an assignment of a physical traffic channel and a Walsh Code. Softer handoff requires just the assignment of a Walsh Code, no new traffic channel element has to be assigned. The assignment of Walsh Codes and traffic channels is separated to allow the allocation process to adjust for the different needs of soft and softer handoff. In order to reduce the risk of soft/softer handoff assignment failure during the conversation, the BSS denies assignment of traffic channels and Walsh Codes for new call setups if traffic channels or Walsh Codes are not available or being used for soft/softer handoffs.
The number of traffic channels is defined by the In-Service Hardware in the BSS. It could be less than the number configured if some of the hardware is out of service. The number of Walsh Codes assigned to a sector is set to 64 which is the maximum specified by the EIA/TIA standard. Limiting the number of Walsh Codes in a sector is a method of controlling service quality. Since Walsh Codes are not associated with any hardware, they cannot go out of service. As a result, 64 is the hard limit of the number of code channels per sector according to the protocol specifications.