Technology Study Of The Feedback Interference Cancellation For WCDMA Repeater

In WCDMA communication system, repeater is a reasonable choice to extend the network coverage in the blind spots and weak signal areas. Stability and output signal quality are two important indicators to evaluate the performance of a repeater. Besides, a high gain is the basic requirement of the repeater. Performance of these three indicators in the repeater is therefore identified as the topic of this paper. Due to the coupling between the transmitting and receiving antennas, and the reflection caused by barriers, the output signal from the transmitter will return back to the receiver again, known as the feedback interference signal. When the repeater's isolation can't meet the requirement of 10-15dB greater than the gain, the feedback interference signal will be amplified and transmitted, reducing the output signal quality.What's worse, it will affect the stability of the repeater, and even lead the repeater to self-oscillation. But considering the network coverage, the gain of the repeater is expected to be high, while enough isolation is not often available.A kind of the feedback interference cancellation scheme, based on LMS algorithm, for WCDMA repeater is proposed in this paper. Four carriers WCDMA signal is the input signal of this repeater. Then the scheme is designed and simulated on Matlab/Simulink, so that the impact of feedback closed loop to the repeater can be described more conveniently and accurately. It is shown that the proposed scheme increases the gain in the absence of sufficient isolation, and ensures the repeater stable and effective work.Firstly, the repeater without cancellation is designed and simulated. The results show that compared to the channel with single path, which is caused by the coupling, multipath channel environment requires a lower gain against the repeater self-oscillation. And the higher the gain is, the easier the repeater goes into self-oscillation with the cycle of feedback closed loop. Delay mitigates this problem but doesn't solve it fundamentally.Secondly, the repeater with the proposed cancellation system is designed and simulated. The results show that the proposed scheme is able to cancel the feedback interference. And in order to ensure the stability, output signal quality, as well as a relatively high gain, the repeater gain G can be slightly higher in a certain range, the analog channel filter order N should try to cover all the delays of feedback channel, the introduced delay d should be as long as possible, additionally, different channel conditions require different N and G.Finally, two kinds of variable gain approaches in the repeater are presented. It effectively increases the relative gain (after offsetting the isolation) on the assumption of the stable work of repeater. Appropriate selection of the gain makes it possible to avoid the output signal quality deterioration. It is also confirmed that stability, signal quality and the gain are mutually restrained to some extent. It gives some clues to the project application of this program.