Subject: [NEXTEL-1] More On Adaptive Power Control-

From: Marcel <[email-masked]>
Date: Tue, 14 Jan 2003 14:35:14 -0500

Jianhua reiterates the benefits of APC previously discussed here. The level of
interference reduction is notable in a CDMA and a GSM environment. The same techniques
employed in GSM can easily transposed to iDEN. Research continues at Moto and at a major
university. When? That is the big question Moto won't commit to a exact date but
"Insiders" say the troops are marching forward and some solid results should be seen by
Q2.

Marcel
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Adaptive Power Control Module in Cellular Radio System
Jianhua Gan

Abstract

Adaptive power control scheme based on the optimization of transmitter power
and receiver filter coefficients is shown. The scheme is mapped to control logic, digital
filter and adaptive amplifier.

Accurate power control can reduce the interference in both Global System Mobile
(GSM) and CDMA system. When the mobile is close to the base station, it can use lower
power since the signal loss is smaller. For GSM, interference to other cells using the
same frequency is reduced when the power control is accurate. For CDMA, accurate
power control is more important because all CDMA signals interfere with each other.
Lack of accurate power control reduces the capacity for CDMA.

Moreover, power control can reduce battery drain and increase possible talk time. The
major objective of
power control is to alleviate the co-channel and cross-channel interference [1]. Due to
the
effects of fast fading, shadowing and distance loss, an adaptive power control (APC)
scheme is needed. In order to get a better estimation of the received power, the received
power at the mobile can be estimated by using RAKE receiver [2] as shown in Figure 1.
The outputs of the taps of the RAKE receiver are correlated with the de-spreading
sequence c(t) over one bit time, then squared and summed to generate X(k) at time kT.
X(k) is averaged over m bits to form an estimate of the received power.

These processes?are typical DSP processes. By adaptively adjust the transmitter power
level and receiver
filter, we can minimize the interference that users create to each other while maintaining

the desired signal to interference ratio (SIR) [3, 4].

Figure 1. Power estimation diagram using RAKE [2]
The simple power control approach was concerned with assigning the transmitter
powers to the users optimally with the assumption that the receiver filters of the users
were fixed to single user detector [3]. Multi-user detection was concerned with designing
receiver filters to suppress the interference seen by each user with the assumption that
the
received powers of the users were fixed. If the transmitter powers and receiver filters of

the users are optimized jointly, the performance of a CDMA system can be improved
greatly. This can be implemented in a distributed manner using local measurement. Each
mobile can adapt its receiver filter coefficients to interference and perform iterative
transmitter power level adjustment for fixed filter coefficients.
The block diagram of the adaptive power control scheme is shown in Figure 2. This scheme
can be mapped to
control logic, digital filter and adaptive amplifier. The power control algorithm will be
reflected in control logic block.

Receiver
filter
Control
block
Transmitter
amplifier
Power
measurement

Figure 2. Block diagram of the adaptive power control scheme
References

[1] Z. Rosberg and J. Zander, “Towards a framework for power control in cellular
systems,” Wireless Networks, vol. 4, no. 3, pp. 215-222, Apr. 1998.

[2] A. Chockalingam, L. B. Milstein, “Open-loop power control performance in DS-CDMA
networks with frequency selective fading and non-stationary base stations”,
Wireless Networks, vol. 4, no. 3, pp. 249-261, Apr. 1998.

[3] S. Ulukus and R. D. Yates, “Adaptive power control and MMSE interference
suppression”, Wireless Networks, vol. 4, no. 6, pp. 489-496, Nov. 1998.

[4] P. -R. Chang, B. -C. Wang, “Adaptive fuzzy power control for CDMA mobile radio
systems”, IEEE Transactions on Vehicular Technology, vol. 45, no. 2, pp. 225- 236, May
1996.