Why power control required in CDMA system

CDMA is an interference limited system based on the number of user. Here each user is a noise source on shared cannel. Due to this CDMA system practically has a limit of users who can sustain, this is called soft capacity limit.

Near Far effect is basic feature of CDMA. If we assume that mobile transmit power is same for all user then mobile user near a cell jams a user that is distant from the cell. This problem may be present despite high processing gain. So an effective method to eliminate the near-far effect is necessary.

CDMA system uses power control technique to keep each MS at absolute minimum power level necessary to ensure acceptable service quality. Power control is essential for adjusting BTS and MS transmit power instantly according to communication distance (to overcome near far effect).  Ideally the power received at the base station from each mobile station should be same (Minimum signal to interference).

Physical channels in CDMA and their Description

In CDMA IS95 we have some physical channel to transmit codes. BTS and mobile communicate via these channels. Physical channel is known by fundamental channel or logical channels. There are several channels in both forward and reverse direction.

Forward Channel: Pilot channel, Paging Channel, Sync Channel, Forward Traffic channel

Reverse Chanel: Access Channel, Reverse traffic Channel

Pilot Channel: It obtains a phase offset by short PN sequence to identify different base station. It has timing information. It is used to assist handoff. Mobile station use pilot strength comparison between pilots of base station to identify and perform handoff.

Sync Channel: It carries a data stream of essential system identification and parameter information used by mobiles during system acquisition stage like Pilot number, System time, Long code and paging rate (PRAT).

Paging Channel: it notify mobile station that they are receiving a call. It transmits information at a fixed rate 9600 or 4800 bps, as specified by the “PRAT” parameter sent in sync channel message. It carries system configuration parameter message, access parameters message and CDMA channel list message.

Forward Traffic Channel: It is used to transmit user and signaling information to a specific mobile station during a call.

Access Channel: Initiate communication with the base station not yet in a Call (such as transmit registration requests, call setup requests/origination message) in reverse direction. It is used to respond paging channel message.

Reverse Traffic Channel: It is used when call is in progress state to send voice traffic from subscriber, response to command from base station, request to base station.

What is difference between dBi and dBd

In CDMA, GSM technology we use antenna system to transmit and receive signals from mobile and from BTS. One of the characteristic of antenna is antenna gain. dBi and dBd is a unit to measure antenna gain with respect to isotropic antenna and dipole antenna respectively.

Dipole has 2.15 dB gain compared to an isotropic antenna, hence

dBi = 2.15 dB + dBd

For example an omni antenna has antenna gain 12.15 dBi or 10 dBd

What is difference between dB, dBm and dBW

We need to express power levels in some terms weather it is CDMA technology or GSM technology. for that we use dB, dBm and dBW as unit of power levels which are differentiate with reference power level.

dB is a relative measurement of two different power levels.

dB = 10 Log (P1/P2)

If we take reference power is 1W then it is dBW

dBW = 10 log(P1/1 W)

If We take reference power is 1mw then it is dBm

dBm = 10 Log(P1/1 mw)

Normally we use dB when expressing the ratio between two power values and dBm when expressing an absolute value of power.