Kannangara, Shyama Dilrukshi (2006) Adaptive Duplexer for Software Radio. PhD thesis, Victoria University.

Abstract

Different geographies and localities around the world have adopted various wireless interface standards for mobile communications. As a result roaming users will require multiple handsets with multiple standards and multiple band capabilities. Triple-band hand sets are currently offered for high end users. In the future quad-band handsets including GSM 850 will become common in the market. This trend will continue. The addition of third generation functionality to second generation platforms will be even more difficult and complex. The radio handset should be able to use the same hardware for communications anywhere in the world. Therefore users will require small low cost terminals with multimode/ multi-band capability. The software radio concept has been developed to address these challenges. The replacement of fixed frequency components in the front end of the software radio is one of the key architectural changes required. The duplexer is one such component. Since duplexing filters are not normally tuneable, each band requires a separate duplexer in a multi-band system. The duplexers are passive devices (ceramic or SAW) and multiple duplexers lead to a dramatic increase in terminal cost and size. Abstract Adaptive Duplexer for Software Radio iv This thesis proposes a new adaptive duplexer architecture to reduce/eliminate the multiple duplexer problem in software radio. This technique is based on combining a low isolation device with an adaptive double loop cancelling scheme. The proposed double loop cancellation provides the required transmitter leakage and transmitter noise isolation over wideband using a delay element and an adjustable vector attenuator in each cancellation path. This thesis analyses the double loop cancellation technique. The cancellation path delay constraints are derived for coefficients with limited adjustment range in the cancellation paths. A linear relationship between the bandwidth and the achievable cancellation level is obtained. It is shown that the residual signal power is proportional to the square of the duplexing frequency. It is concluded that the delays in the cancellation paths should be chosen to straddle the expected range variation of the delay in the main path, predominantly caused by variations in antenna matching. The new algorithm uses a single cost function to achieve simultaneous cancellation in both the transmit band and the receive band. A direct conversion receiver architecture was chosen for the hardware prototype, since it is more suitable for multi-band systems. Alternate structures are also possible. A prototype of the adaptive duplexer using a 20dB circulator and a single loop cancelling technique was designed and implemented. It achieved a total Tx leakage cancellation of 69dB at 2GHz with 45MHz duplexing frequency. However it was not possible to simultaneously cancel the transmitter noise in the receiver band. The original prototype was extended to include the second loop. The achieved isolation between the transmit and the receive signals and the achieved reduction of the transmitter noise in the receiver band were 66.8dB and 58dB respectively. These results were obtained over 5MHz bandwidth and using a 190MHz duplexing frequency. The performance is more than adequate for W-CDMA applications. Lowering the duplexing frequency improves the cancellation bandwidth and so the scheme performs better with other standards, such as IS-95 (CDMA), using 45MHz duplexing offset.

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