Mumbai University > Electronics and Telecommunication > Sem8 > Wireless Networks

Marks: 5M, 10 M

• To minimize the BER for each user, CDMA tries to force each user to provide the power level at the base station receiver.

• The base station reverse traffic channel receiver estimates and responds to the signal strength and interference. For a particular mobile station. Since both the signals and interference are continually varying, power control updates are sent by the base station every 1.25ms.

• Power control commands are sent to each subscriber unit on the forward control subchannel which instruct the mobile to raise or lower its transmitted power in 1 db steps.

• If the received signal is low 0 is sent over power control subchannel instructing the mobile station to increase its mean output power level. If mobile’s power level is high 1 is sent to indicate that the mobile station should decrease the power level.

• The power control is bit corresponds to two modulation symbols on the forward traffic channel. Power control bits are inserted after data scrambling.

• Power control bits are transmitted by using puncturing techniques. During a 1.25ms period. 24 data symbols are transmitted and IS-95 specifies 16 possible power control positions for the power control bit.

• Each position corresponds to one of the first 16 modulation symbols. 24 bits from the long code decimator are used for data scrambling in a period of 1.25ms. Only the last four bits of the 24 bits are used to determine the position of the power control bits.

A. Open loop power control: It is implemented on reverse link in which the quality of the signal coming from reference channel of BS is measured by MS. If the strength of the signal coming from BS is above some threshold then the transaction power of the MS is reduced by MS. If the quality of signal reaching to the MS is not good then MS increases its transmission power. Increasing or decreasing the transmission power is done by measuring the quality strength of the signal coming from BS to MS on forward channel.

Limitations: In this forward and reverse channels are not correlated. Ms does not know that whether it has minimized the transmission power efficiency or not. There may be delay between signal transmission and reception.

B. Close loop power control: Open loop power control has some limitations and to overcome them the closed loop power control is implemented. Is uses feedback between BS and MS, in this the power of the signal coming from MS is measured by BS and BS send a control signal to tell Ms that what action the MS should perform.