Subject : Mobile Communication

Topic : CDMA

Difficulty : Medium

Data rate transmission in CDMA

• Variable rate data is sent on the reverse CDMA channel.

• Code symbol repetition introduces redundancy when the data rate is less than 9600 bps. A data randomizer is used to transmit certain bits while turning the transmitter off at other times.

• When data rate is 9600 bps all interleaver output bits are transmitted. When data rate is 4800 bps, half of the interleaver output bits are transmitter and so forth.

• Data in each 20 ms frame is divided into 16 power control groups, each with 1.25ms. Some are gated-on some are gated-off.

• The data randomizer ensures that every repeated code symbol is transmitted exactly once. During the gated-off process, the mobile station reduces its EIRP either by at least 20 db with respect to the most recent gated-on period or to the transmitter noise floor, whichever is greater. This reduces interference to other mobile stations operating on the same reverse CDMA channel.

• The data burst randomizer generates a masking pattern of 0’s and 1’s that randomly masks the redundant data generated by code repetition process. A block of 14 bits taken from the long code determines the masking pattern.

• The last 14 bits of the long code used for spreading in the second to last power control group of the previous frame are used to determine the random mask for the gating. These 14 bits are denoted as, b0, b1, b2, ........ , b13 where b0 represents earliest bit and b13 represents latest bit.

• For the data rates is 9600 bps transmission occurs on all 16 power control groups.

• For the data rates is 4800 bps transmission occurs on all 8 power control groups given as,

  b0 ,2 + b1 ,4 + b2 ,6 + b3 ,8 + b4 ,10 + b5 ,12 + b6 ,14 + b7.

• For the data rates is 2400 bps transmission occurs on all 4 power control groups given as,

  • b0 if b8 = 0, or 2 + b1, if b8 = 1

  • 4 + b2 if b9 = 0, or 6 + b3, if b9 = 1

  • 8 + b4 if b10 = 0, or 10 + b5, if b10 = 1

  • 12 + b6 if b11 = 0, or 14 + b7, if b11 = 1

• For the data rates is 1200 bps transmission occurs on all 2 power control groups given as,

  • b0 if ( b8 = 1 and b12 = 0), or 2 + b1 if ( b8 = 1 and b12 = 0), or 4 + b2 if ( b9 = 0 and b12 = 1), or 6 + b3 if ( b9 = 1 and b12 = 1)

  • 8 + b4 if (b10 = 0 and b13 = 0), or 10 + b5 if (b10 = 1 and b13 = 0), or 12 + b6 if (b11 = 0 and b13 = 1), or 14 + b7 if (b11 = 1 and b13 = 1).

Power Control in CDMA

• Since CDMA has near-far effect which is from other users transmitting in the same frequency band at the same time, therefore is is important to implement good power control.
• Near-far effect occurs in the absence of power control that is if all mobiles were to transmit at the same power level, the mobile closest to the base station will overpower all others (since the signal power drops exponentially with the distance).
• Another reason for power control is battery life—if the mobile station were to continuously transmit at a power higher than that needed to maintain an acceptable SIR, the battery lifetime is reduced.
• Using power control, each mobile station may transmit using the minimum power needed for maintaining the required SIR ratio, thus conserving its battery life.
• Using frame error rate for power control decisions is preferred over using signal strength used in other systems.
• Power control is especially important on the Reverse Link where non-coherent detection is employed.

• Two types of power control are implemented:

  a). an Open loop b). a Closed loop.

• Before a traffic channel is assigned, there is no closed power loop control in CDMA because it involves feedback from Base station, and to prevent the sudden fall of signal strength an open loop power control is implemented.

• Here, power transmitted is inversely proportional to received signal strength of pilots of base station.
• Open loop power control starts when the first mobile attempts to communicate with the base station.
• This power control is used to compensate for the slow variables shading effects. This power control fails or too slow for fast Rayleigh fading channels.
• Closed loop power control: The power of closed loop control is used to compensate for the rapid Rayleigh discoloration. This time, the mobile transmit power is controlled by the base station. For this purpose, the base station continuously monitors the reverse link signal quality. If the quality of the connection is low, it tells the mobile to increase its power; and if the quality of the connection is very high, the mobile base station controller reduces its power.