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FP5

ANTIUM Résumé de rapport

Project ID: IST-2000-26222
Financé au titre de: FP5-IST
Pays: France

Space-time algorithms for synchronisation, channel estimation and demodulation of the broadcast channel of UMTS/FDD base stations

Operators of UMTS/FDD cellular communications systems will require efficient network planning to distribute the valuable available resources or frequencies. To do so, the ability to identify node-Bs in the field is necessary. A classical trace mobile does not have sufficient sensitivity or signal separation capacity to identify low interfering signals. Space-time processing based on smart antenna techniques is used to suppress co-channel interference caused by other base stations and significantly improve sensitivity. Identification of the interfering node-Bs first includes the synchronization with all the surrounding base stations and then the demodulation and the interpretation of the Primary Common Control Physical Channel (P-CCPCH) providing valuable information such as the Cell Identity (CI), the Mobile Country Code (MCC) or the Mobile Network Code (MNC).

Since UMTS-FDD mode provides synchronization channels containing a known primary synchronization, we can use a detection approach to obtain synchronization. The aim of such a method is to compute a decision statistic for every possible time instant, which can be processed in further stages to decide whether or not a synchronisation sequence is present. The initial objective of ANTIUM was to detect BTS having a P-CPICH Ec/I0 as low as -25dB, since it was shown that such BTS could have an impact on the mobile performance when they are fully loaded. If we consider P-CPICH Ec/I0 between -20dB and -15dB, a mono-channel processing allows detecting only about 61% of the BTS detected with a 5-channel one. This percentage drops to 17% if we consider P-CPICH Ec/I0 between -25dB and -20dB. Therefore a multi-channel processing improves significantly the detection performance and is required to reach the ANTIUM objectives. A 3 or a 4-channel ANTIUM equipment seems to be a good trade-off between performance / cost / complexity, since it allows to detect respectively 64% and 78% of the BTS detected with 5 channels in the range [-25dB; -20dB].

For identification, several multi-sensor demodulation algorithms were developed. As the spreading factors and the spreading codes allocated to the channels transmitted by the detected node-Bs are unknown, it is impossible to use joint detection algorithms in order to detect the symbols transmitted by the P-CCPCH. We therefore study a family of sub-optimal receivers, which only require the knowledge of the spreading code allocated to the P-CCPCH. Roughly speaking, each of them consists in estimating the chip sequence by a spatio-temporal filter. This filter can be interpreted as a multi-channel equalizer because, ideally, it allows both to compensate the effect of the propagation and to cancel signals due to interfering node-Bs. The studied algorithms are the following ones: 2D Rake, Spatial Wiener Filter, Spatial Wiener Filter with Filtered Reference, Spatial Rake with Spatial Whitening, Space-Time Wiener Filter. The Spatial Rake with Spatial Whitening (SRSW) presents the best trade-off in terms of complexity / performance. With a 5-channel processing, the SRSW allows to demodulate the BCH information with a good CRC rate better than 90% when the P-CCPCH Eb/I0 is greater than -1dB (which corresponds to a P-CPICH Ec/I0 greater than -20dB if the P-CCPCH level is 2dB below the P-CPICH one). In comparison, the loss of performance by using a 1-channel processing is equal to 11dB.

Moreover, the UMTS-FDD standard foresees the use of transmit diversity on different downlink physical channel types. Therefore, all the developed algorithms were extended to the Space-Time Transmit Diversity mode that is used on the P-CCPCH. In a first stage, the algorithms have been developed and tested in simulation using a C environment. The results have been published in several conference papers and a doctoral dissertation. Then they have been implemented on a demonstrator and validated on the field on real UMTS/FDD networks. The performance describe above comes from these field trial measurements. The algorithmic study is detailed in D3011b and the validation on the field in D5011.

Reported by

Thales Communications
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