Philippe Sehier

Quantitative analysis of split base station processing and determination of advantageous architectures for LTE

Centralization of the baseband processing in radio access networks may reduce radio site operations costs, reduce capital costs, and ease implementation of multi-site coordination mechanisms such as coordinated multipoint transmission and reception (CoMP). However, the initial architecture proposals for a centralized Long Term Evolution (LTE) deployment using transport of radio samples require a high-bandwidth, low-latency interconnection network. This may be uneconomical, or it may only be cost effective for a limited number of sites. To mitigate that deficiency without sacrificing the benefits of centralized processing, we identified alternative interfacing options between central and remote units. To do so we analyzed possible splits of the Long Term Evolution (LTE) baseband processing chain for their bandwidth and latency requirements. Next, we analyzed the operational impacts of potential splits based on a number of criteria including ease of CoMP introduction, the possibility of realizing pooling gains, and the ability to update the system and introduce new features. Based on our results, we propose architectures that can leverage the benefits of centralization at a much-reduced cost.

Reduced complexity frequency estimator for burst transmission

In burst transmission, carrier recovery is a critical point for synchronization systems. With a feedforward carrier phase recovery algorithm, a small frequency offset can significantly increase the cycle slips rate and then the phase error variance. Hence, in order to obtain an accurate carrier phase estimation a precise frequency correction is required. For M-PSK modulated signals an unbiased feedforward reduced complexity frequency estimator (RCFE), operating in the non-data aided mode (NDA) is derived from the maximum likelihood (ML) principle. A compromise is realized between noise filtering and estimation slip probability by minimizing the estimator variance. It is optimized for operation at low signal to noise ratio and with short bursts. Its performance is compared to the ML estimator. The estimator is applied to all feedforward synchronization architecture with QPSK modulated signals. The global performance of the modem synchronization unit is presented.

A novel approach for full digital modems implementing asynchronous sampling techniques

This article deals with full digital modem architectures, and more specifically with multirate demodulators. We aim at an overall complexity minimization of the receiver structure, by implementing a completely asynchronous sampling scheme, in conjunction with a single optimized filtering module, which carries out the intersymbol interference cancellation along with a specific interpolation algorithm. This allows one to decrease the oversampling ratio down to the theoretical bound, thereby restraining computational requirements, while reducing dramatically the traditional analog circuitry. Concretely, such a multirate demodulator should ultimately appear as a low-cost A/D converter operating at a unique sampling frequency, followed by an ASIC.

A new frequency estimator applied to burst transmission

In TDMA communications systems using all feedforward synchronization techniques, the quality of data decoding strictly depends on the estimation accuracy of the synchronization parameters (timing, carrier phase/frequency and preamble detection) extracted from the received signal. The frequency offset estimation is the most critical point. Indeed, an inaccurate frequency estimation can cause cycle slips and then errors during decoding. In this paper, we propose an new frequency estimator, analytically derived from the maximum likelihood principle and optimized thanks to variance simulations. Its performance is compared to the Cramer Rao bound.

Strategic and technological challenges for wireless communications beyond 3G

In order to facilitate and to meet the ever increasing demands for the wireless communication in the next decade and also to satisfy the high data rates for the new services, the future development of 3G (IMT2000) and systems beyond 3G (B3G) are foreseen. This article presents the need for these developments and motivations for systems beyond 3G taking into account both strategic and technological challenges to be faced. Moreover, the concepts and potential technologies involved including multiple air-interfaces, efficient spectrum allocation and utilisation for the success of systems B3G are briefly reviewed.