Nowadays, a lot of effort is spent on developing inexpensive orthogonal frequency-division multiplexing (OFDM) receivers. Especially, zero intermediate frequency (zero-IF) receivers are very appealing, because they avoid costly IF filters. However, zero-IF front-ends also introduce significant additional front-end distortion, such as IQ imbalance. Moreover, zero-IF does not solve the phase noise problem. Unfortunately, OFDM is very sensitive to the receiver nonidealities IQ imbalance and phase noise. Therefore, we developed a new estimation/compensation scheme to jointly combat the IQ imbalance and phase noise at baseband. In this letter, we describe the algorithms and present the performance results. Our compensation scheme eliminates the IQ imbalance based on one OFDM symbol and performs well in the presence of phase noise. The compensation scheme has a fast convergence and a small residual degradation: even for large IQ imbalance, the overall system performance for an OFDM-wireless local area network (WLAN) case study is within 0.6 dB of the optimal case. As such, our approach greatly relaxes the mismatch specifications and thus enables low-cost zero-IF receivers.

Boris Come, Nadia Khaled IMEC vzw., DESICS, WISE AEISTRACI Channel reciprocity is needed in SDMA or MIMO downlink pre-filtering when the channel knowledge is acquired in the n link We first show analytically that the non-reci roeity ofthe . &e station anal hardware which is part of the channel, introduces a v e q x g h level ok multiuser interference and quantify the effect of the nonreciproeit by means of simulations We then p r o p a novel caliiratinn technique at the base station that enables to compensate for the non-reciprocity and reduce the M U to a negligible value while having a low implementation c a t I. INTRODUIXIONOFDM-SDMA is an attractive technique to enhance the capacity of future wireless LANs since it allows mitigating the frequency selective channel fading (OFWM) and increasing the spectral efficiency by accommodating several users in the same time-frequency slots (SDMA). In the downlink, pre-filtering at the base station (BS) side allows to pre-compensate the phase (and amplitude) of the channel in such a manner that all simultaneous users receive their own signal free of MLn 01, [4l.It is in fact this interference Cancellation property that makes SDMA possible in the downlink. Since the terminals have only one antenna, they have no means IO mitigate the spatial MUI.When the channel is estimated in the uplink, the downlink channel matrix is just the transpose of the uplink matrix, assuming the channel is reciprocal. However, the "channel" is actually made up of the propagation channel (the medium between the antennas), the antennas and the transceiver RF, IF and baseband circuits at both sides of the link. The transceiver circuits are usually not reciprml (the TX and RX frequency responses are different) and this can jeopardise the performance of the SDMA system.In this paper, we depart from the reciprocity assumption, analyse the impact of channel non-reciprocity and propose a mitigation method. The document is organised as fdlows. Section 2 introduces the OFDM-SDMA Uplink and Downlink model, including the impact of the composite channel. In section 3, the effects of non-reciprocities are estimated by simulations. Seaion 4 proposes a simple yet effective calibration method with minimal additional hardware requirement. Then, the conclusions are drawn. SYSTEM MODELWe consider an OFDWSDMA system with a multi-antenna base station and single antenna terminals. Kapeldreef 75, B-3M)l Leuven A. UPLINKIn the uplink, U mobile user terminals (MT) transmit simultaneously to a BS using A antennas. Each user U employs conventional OFWM modulation Lz1. The following linear frequency domain model results on each sub-camer n: where x"[n] is the column vector of the U frequency domain symbols at subcarrier n transmitted by the MTs, y"fn] is the column vector of the A signals received by the BS antenna branches, and H" is the mmposite uplink channel: In the sequel, we drop the explicit dependency on fn] for clarity.Including the hardware, H"[nl can be expressed as:where bs and JhLm are complex diagonal matrices...

No abstract

No abstract

A bstmct-In recent years, wireless indoor networks have received a lot of scientific and industrial attention. Most systems rely on the use of Orthogonal Frequency Division Multiplexing (OFDM) because of its capability t o elegantly cope with multi-path interference. However, while OFDM provides a nice solution for the digital modem, its front-end requirements should be investigated as well. To that goal, we have set up a simulation environment which comprises both the digital modem and the most important front-end non-idealities. We show that for the same data rate, bandwidth and transmit power constraints Single-Carrier with Cyclic Prefix (SC-CP) allows the design of a more power efficient modem than OFDM and is therefore a better candidate for portable wireless terminals.