With a neat diagram, explain forward CDMA channel

CDMA is a duplex mobile communication technology; CDMA uses frequency division duplex to achieve duplex communication. With FDD, there are separate frequency channels for communication between mobile and base station and base station and mobile. In CDMA, the base station to mobile station transmission path is termed as forward link. Whereas the mobile station to base station transmission path is termed as reverse link.

• FORWARD LINK: 869 MHz TO 894 MHz

• REVERSE LINK: 824 MHz TO 849 MHz

In CDMA channelization is done with spreading codes. The forward link the information is spread used Walsh codes. There are 64 Walsh codes. Each Walsh code is of bits. These 64 channels are divided into different function ones. Some Walsh codes are reserved for synch, paging etc and rest for traffic.

Forward channels

• Traffic channel

• Pilot channel

• Sync channel

• Paging channel

Traffic channel

Forward traffic channel is used for the transmission of user and signalling information to a specific mobile station during a call. The maximum number of forward traffic channels that can be simultaneously supported by a given forward CDMA channel is equal to 63 minus the number of paging and sync channels. The data rate is selected on frame-by-frame basis.

The traffic channel frame is 20ms. The rates that are transmitted are 9600 bps, 4800 bps, 2400 bps and 1200 bps. Base station will reduce power for lower date rates.

Pilot channel

Uses Walsh code 0 all 64 bits are 0. All data into Walsh modulator is 0. Output of Walsh modulator is therefore all 0’s. Pilot channel is just as the short codes. Pilot channel is an unmodulated spread spectrum signal that is used for synchronization by a mobile station operating within the coverage area of the base station. A pilot channel is transmitted at all times by the base station on each active forward channel. Each base station transmits a pilot signal having a different PN (timing) offset, pilot that discriminates cells.

Synchronization channel

Sync channel is defined as the code channel W32, transports synchronization message to the mobile station, used by the mobiles to acquire initial time synchronization. Sync channel will operate at a fixed rate of 1200 bps. A sync channel frame is of 26.666ms, uses the same PN offset as that of pilot. Mobile will normally not reuse sync channel until it powers on again.

Sync channel-Walsh spreading:  has a symbol rate of 4800 ksps. The spreading rate required is 1.2288 Mcps, this means 256 symbols are required per PN chip (1228800/4800) for spreading. Walsh code (32) is repeated 4 times for spreading (64 X 4=256), each sync channel symbol is repeated 256 times, then Xored.

Sync channel time alignment: sync channel frame is of 26.667 ms. Sync channel super frame is formed by 3 frames; it is of 80ms (26.667 X 3), will have 384 mod symbols and 96 bits. Messages transmitted on the sync channel begin only at the start of a sync channel super frame.

Sync channel messages:

• Base station protocol revision

• Min protocol revision supported

• SID, NID of cellular system

• Pilot PN offset of base station

• Long code state

• System time

• Leap seconds from start of system time

• Local time offset from system time

• Daylight savings time flag

• Paging channel data rate

• Channel number

Paging channel

The BS to transmit system overhead information and mobile station specific messages uses paging channel. It shall transmit information at fixed data rate of 9600 bps and frame is of 20ms. All paging channels in a given system (i.e. in the same SID) should transmit information at the same data rate.

Paging channel encoding and interleaving:

For 4.8 kbps channels, symbol repetition is used after convolutional coding and for 9.6 kbps channel, only convolutional coding is used. Interleaving arrays are same as traffic channel.

Paging channel time alignment and frames:

Paging channels use the same PN offset as pilot channel and is divided into 80ms slots. This each 80ms slot is composed of four paging channel frames, each of 20ms; this each 20ms frame is divided into 10ms long paging channel half frames. 80ms paging slots are grouped into cycles of 2048 slots; slots in this cycle are numbered as 0-2047 and will reoccur every 163.84sec. Mobiles will monitor the paging channel in slotted or non-slotted mode.

Paging channel messages:-

• System parameters

• Access parameters

• CDMA channel list

• Order

• Channel assignment

• Data burst

• Authentication

• SSD update

• Feature notification

• Extended system parameters

• Extended neighbour list

• Status request

• Service redirection

• General page

• Global service redirection

• TMSI assignment

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