Interference Cancellation in Software-Defined CMOS Receivers

Mohyee Mikhemar
Advisor: Asad Abidi

Wednesday, November 18, 2009 at 9:30am
Engr IV Room 67-124

Abstract:
The wide deployment of wireless applications has created an interference-limited wireless environment. Strong unwanted signals create in-band distortion which degrades the receiver (RX) performance and may lead to complete desensitization. Dedicated radio receivers filter out the interference using off-chip narrow-band RF filters. In Software-Defined Radios (SDR), however, the inherent programmability prohibits the use of fixed narrow-band RF filters. Therefore, there is a need to replace off-chip fixed filtering with on-chip tunable filters and to mitigate distortion effects. This work tackles the problem on two fronts: RF, and baseband.

First on the RF level: the strongest interferer to any RX is its own TX. In an FDD system the TX and the RX are concurrently active. A duplexer is needed to isolate the TX and the RX and to provide matching at all ports. A good RF duplexer should provide 50dB of RF filtering. Modern RF duplexers rely on frequency-selective filters for isolation. The limited Quality factor of CMOS components prohibits the implementation of frequency-selective duplexers in CMOS. In this work, we introduce a new filtering approach that does not rely on frequency-selectivity but rather on electrical balance. To prove the concept, a wideband integrated RF duplexer which supports 3G/4G bands I, II, III, IV, and IX was implemented. It achieves a TX-to-RX isolation of more than 55dB in the transmit-band, and greater than 45dB in the corresponding receive-band across 200MHz of bandwidth. A 65nm CMOS duplexer/LNA achieves a transmit insertion loss of 2.5dB, and a cascaded receiver noise figure of 5dB with more than 27dB of gain, exceeding the commercial external duplexers performance at considerably lower cost and area. The implemented integrated duplexer is a crucial enabling technology for FDD cognitive radio receivers.

Second on the baseband level: a novel hardware-efficient mixed-signal technique for the cancellation of AM-distortion is presented. The proposed technique does not need any information about the interferer frequency or modulation. It cancels the AM-distortion created by both second-order and third-order nonlinearities. The cancellation scheme can be efficiently implemented as an extension to the widely-used I/Q correction circuit. Measurement results show an SNR improvement of up to 7dB.

Biography:
Mohyee Mikhemar received the B.S. and M.S. degrees with honors in EE from Ain Shams University, Egypt, in 2000 and 2004, respectively. Since 2004, he has been with UCLA where he is working towards the Ph.D. degree. He is currently a Sr. Staff RFIC designer with Broadcom Corporation, Irvine, CA.