Subject : Mobile Communication

Topic : CDMA

Difficulty : Medium

The forward logical channels are of four types - the pilot, the sync, the paging, and the traffic channels. The reverse channels are either access channels or traffic channels. The forward traffic channels carries user data (either encoded data bits or encoded voice) at 9600, 4800, 2400, or 1200 bps in Rate Set 1 and 14400, 7200, 3600, or 1800 bps in RS2. The forward traffic frame is 20 ms long.

Frame contents for forward traffic channels

The sync channel provides the MS info about the system indentification (SID), the network id (NID), PN, short sequence offset, the PN long code state o and the system time among other things. Such messages can be long and are fragmented into synch frames of 32 bits.

Three of sync channels are combined into sync channel superframe of 96 bits. The start of message is 1 for the first sync channel frame and zero for the subsequent ones the belong to the same message. The message itself consists of the message length, the data, and error checking code and some padding. Padding to ensure that every new message starts in a new superframe.

The paging channel announces a no. of parameters to MS that includes the traffic channel information, the temporary mobile subscription identity, response to access request and list of neighbouring base station and their parameters.

Paging can be slotted or unslotted. In the former cases, which enables the MS to save on battery power, the channel is divided into 80 ms slots. The paging channel msg is similar in structure to the sync channel message. Because it is too long for transmission in one slot, it is fragmented into 47 or 95 bits (data rate of 9600 or 4800 bps) and transmitted over a paging channel half frame (10ms).

The half frame has one bit called the synchronisation capsule indicator (SCI) that has functionality similar to the SOM bit. In this case however, a message can start anywhere (not necessarily in a half frame) and a zero value for the SCI could indicate that one paging message ends and another starts within the same half frame. Eight paging half frames are combined into one paging slot of 80 ms.

The no. of bits depends on the data rate. If data rate is 9600 bps, a 10ms half frame will carry a 96 bits (one bit in a SCI) and 48 bits if the data rate is 4800 bps.

The access channel data rate is 4800 bps each access channel message is composed of several access channel frames lasting 20ms. Thus an access frame is 96 bits long. An access channel message and it consists of several 96 bits frames with all bits in the frame equal to Zero. The actual message itself is fragmented into 96-bit frames that have 88 bits of data and 8 tail bits set to zero.

The reverse traffic channel is once again broken into 20 ms traffic channel frames. The frame is further divided into 1.25ms power control groups (PCGs). There are thus 16 PCGs in one frame. A data burst randomizer randomly masks out individual PCGs depending on the data rate that results in less interference on the reverse channel.

For instance, at 4.8kbps (half the data rate), eight PCGs are masked. In addition to voice traffic, the traffic channel can also be used to transfer signalling or secondary data. In the blank and burst case, the entire frame carries data. The aim of burst case, part of frame carries voice and part of it data. The frame structure for the reverse traffic channels are very similar to that of the forward traffic channel.