Maxime Flament

Virtual cellular networks for 60 GHz wireless infrastructure

This paper gives a broad description of a broadband communication system using the 60 GHz frequency band. We describe the channel and its inherent problems such as interference situations and body shadowing. We introduce the use of virtual cellular networks (VNC) and multiple receiving antennas (MRA) in order to solve these problems resulting in an overlapping cellular architecture. We explain how the diversity of the channel can be used to increase reliability and performance. Finally, we simulate the ideal link performance of a VCN using two receiving antennas for a particular set of OFDM parameters.

Performance of 60 GHz Virtual Cellular Networks Using Multiple Receiving Antennas

During the past years, research covering propagation, channelcharacterization and wireless systems performance have yield asubstantial knowledge of the 60 GHz channel. The unlicensed 60 GHzfrequency band presents many attractive properties for wirelesscommunications. The environments in which the 60 GHzinfrastructure are to be designed are typically propagation- andcoverage-limited. This paper describes the important factors thatmust be taken into account when designing a wireless local areanetwork (WLAN) architecture operating in this frequency band.Therefore, we motivate the reasons of using distributedtransmitting antennas and multiple receiving antennas (MRA) inorder to mitigate the poor Direction of Arrival (DoA) diversityand to exploit the spatial diversity at the receiver. Such asystem can be considered as a multiple input multiple output(MIMO) system. We investigate the advantages of combining aVirtual Cellular Network (VCN) (using single frequency network anddistributed antennas) and MRA for the downlink. Several ways tocombine the signals with different levels of complexity arepresented. In the most complex case using Singular ValueDecomposition (SVD), it is possible to add coherently thecontribution of each antenna in a virtual cell while retaining thepath diversity inherent to the VCN infrastructure. The schemesyield several advantages: symbol diversity is improved, pathdiversity is still present, antenna gain using multiplebeamformers is increased and the multipath can be reduced. Theconcept is applicable to most types of single frequency networksbut it is especially well appropriate for the 60 GHz VCN/WLANusing orthogonal frequency division multiplexing (OFDM).Simulations give a realistic performance for QPSK, 8-PSK, and16-QAM baseband modulations with a 256-subcarrier OFDM using arate 1/2–convolutional code for a 2 ×2 VCN system.Results show a Eb/N0 improvement of up to 6.2 dB usingthesingular value decomposition method with 16-QAM compared to thesingle input single output (SISO) coded reference.

Joint channel estimation and turbo decoding for OFDM-based systems

In this paper, we propose a joint channel estimation and decoding scheme using soft bit output information in OFDM-based systems. In order to estimate the complex OFDM channel amplitudes, the estimation algorithms usually rely on the pilot signal assisted modulation (PSAM). We enhanced this method by using the soft code bits after each turbo decoding iteration in a feedback loop. This way, the quality of the initial channel estimation can be significantly increased without the need of using additional pilot symbols. The proposed algorithm is tested on a selection of different time and frequency selective fading channels based on Rayleigh fading discrete taps of an exponentially decaying average power delay profile. We compare the performance of our algorithm with other channel estimation techniques for two different sizes of the data frame and demonstrate gains in order of 1-2 dB. We also show that our algorithm is better suited for fast changing, highly frequency-selective channels.

Frequency reuse and coding for GPRS multi-slot operation

Due to the growing interest in mobile data applications, the Global System for Mobile communications (GSM) has been extended by the General Packet Radio Service (GPRS). It allows the transmission of packet data at moderate bit rates using random access channels within the existing GSM infrastructure. This paper analyses the impact of using multiple channels in a GSM system for GPRS packet transmission. Different combinations of frequency reuse and channel coding are evaluated for 1, 4 and 8 packet data channels (PDCH) per cell. A tight frequency reuse of 1, together with a strong error coding scheme was found to provide the highest capacity. The increased trunking efficiency for a system with 8 PDCH yields a capacity gain of about 64% over a system with only 1 channel. The results indicate that mixing circuit and packet switched traffic causes inefficient use of system resources. Since GSM voice services usually require a frequency reuse of 3 or 7, a separate overlaying network for GPRS provides better resource utilization and maximizes the number of users fulfilling the specified quality of service (QoS) requirements.