Multi-rate receiver design with IF sampling and digital ... - CiteSeerX

Multi-rate receiver design with IF sampling and digital timing correction Henk Wymeersch and Marc Moeneclaey DIGCOM research group, TELIN Dept., Ghent University Sint-Pietersnieuwstraat 41, 9000 GENT, BELGIUM E-mail: {hwymeers,mm}@telin.ugent.be Tel.: +32-9-264.89.00 Fax.: +32-9-264.42.95 Abstract— This contribution deals with a fully digital multirate radio receiver suitable for vehicular applications. Timing correction and sample rate conversion are performed by a polynomial interpolator. Three different receiver configurations are considered in terms of computational complexity and BER performance. Careful selection of the intermediate frequency turns out to play a crucial role. System parameters are provided yielding good BER performance for all considered symbol rates. Results are verified by computation of the BER degradation as compared to an analog receiver with synchronized symbol-rate sampling.

I. I NTRODUCTION In packet-based digital bandpass communication, the transmitted data symbols are recovered from samples of the filtered complex envelope of the received signal. To eliminate the need for identical analog in-phase and quadrature branches, the IF signal (the signal after bandpass to IF conversion) is sampled directly by means of a fixed freerunning clock. synchronized samples are obtained by interpolating between the available non-synchronized samples [1]. The synchronized instants at which an interpolant is needed are determined by a synchronizer and forwarded to the (polynomial) interpolator. Because of its finite memory the interpolator introduces distortion. The intermediate frequency and the sampling rate must be carefully selected to avoid aliasing from the high-frequency terms in the matched filter bandwidth [2]. As an added difficulty, when multi-rate transmission is employed, the symbol rate can vary from packet to packet. Consequently, we must take into account effects of sample rate conversion (SRC) [3]. Ideally, we wish to keep the more computationally complex filters independent of the symbol rate, while permitting simple filters to change. We show that this can be achieved by performing timing correction and SRC prior to matched filtering, allowing the matched filter to be independent of the symbol rate. Such a structure was first proposed in [4] for baseband signals. We investigate the computational complexity and BER performance of three receiver configurations, operating on bandpass signals. We show that even in the least complex configuration low degradations are achievable when the IF is carefully chosen as a function of the considered symbol rates fT g, the sample rate 1=Ts and N , the ratio of the interpolator output rate and the symbol rate. If we

wish more freedom in the selection of the receiver design parameters, the use of a digital anti-aliasing filter (AAF) is mandatory in order to avoid high BER degradations. II. S YSTEM

DESCRIPTION

We consider multirate burst transmission where the symbol rate during a burst is constant, but can change from one burst to the next. We denote the signal after RF to IF downconversion by r (t) = s (t) + n (t), where n (t) is real additive gaussian noise (AWGN) with power spectral density equal to N0 =2 and s (t) is the IF signal, assuming M -PSK signalling:

s (t) =

p

E

2 s