Abstract
An improved technique with a fractional sampling based on two samples per chip, according to the Nyquist criterion, has been employed by the authors to enhance the performance in the code synchronization of UMTS (or W-CDMA) systems. In this paper, we investigate on the theoretical rationale of such a promising behavior. The performance is analyzed for several wireless channels, in the presence of typical pedestrian and vehicular scenarios of the IMT2000/UMTS cellular systems.
Highlights
Initial cell search in the wireless access of the International Mobile Telecommunications-2000/Universal Mobile Telecommunications System (IMT2000/UMTS) is the process of the mobile station that includes the search for cell and scrambling codes as far as time synchronization [1]
One testing variable is accumulated after correlation with each possible PN code shifted by each code offset
The benefits on the mean acquisition time of using two samples per chip, since the first stage of code synchronization, are twofold: first, the cross-correlation between the received signal and the code’s candidates are better estimated, increasing the testing power of stage 1; second, the timing error provided to the stages 2 and 3 is ideally the half of the conventional technique, being maximized by one fourth of the chip period
Summary
Initial cell search in the wireless access of the International Mobile Telecommunications-2000/Universal Mobile Telecommunications System (IMT2000/UMTS) is the process of the mobile station that includes the search for cell and scrambling codes as far as time synchronization [1]. The average operation of the “scheme 2” in [7] as well as the matched filter correlation by the “averaged” code c (t) in [8] are equivalent to using the filter with impulse response h(t) = [ (t-Tc/2) + (t+Tc/2)] /2, corresponding to the low-pass frequency response H(f) = cos( Tcf), before down-sampling, where Tc is the chip period It is well known [12] that some kind of anti-aliasing filtering is required before decimation to avoid the spectral alias error, at the cost of missing the information at frequencies higher than 1/2Tc (while the spectrum of a raised cosine waveform is as large as (1+R)/2Tc, where R is the roll-off). The results of our computer simulations, assuming a uniform distribution of the timing offset, are going to show that the “middle” case (defined for one given “middle” offset) is representative of such a random jitter of actual timing offsets
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