Robust Detection for the Uplink of Broadband Wireless Systems with Strong ACI Levels Rui Dinis (1) and David Falconer (2) (1) ISR-IST, Tech. Univ. of Lisbon, Portugal, rdinis@ist.utl.pt (2) BCWS Centre, Carleton Univ., Ottawa, Canada, ddf@sce.carleton.ca Abstract - This paper considers the uplink of wireless sys- tems using SC-FDE schemes (Single-Carrier with Frequency- Domain Equalization) with OQPSK (Offset Quaternary Phase Shift Keying) modulations. We have an asynchronous scenario where only a coarse synchronization between mobile terminals and the base station is required. We present an iterative frequency-domain receiver for joint equalization, residual carrier synchronization and ACI (Ad- jacent Channel Interference) suppression. Our performance results show that the proposed receiver is able to cope with strong ACI levels, even with only a coarse synchronization between users. 1 I. Introduction Future broadband wireless systems will have very high transmission rates, combined with high power and spectral efficiencies. This presents a considerable challenge since it is desirable to have low-complexity MTs (Mobile Terminals) and the complexity of the BS (Base Station) should not be too high. For this reason, block transmission techniques combined with frequency-domain processing have been proposed for these systems [1], [2]. It is generally accepted that OFDM schemes are recommendable for the downlink. Since an efficient and low-cost power amplification is mandatory at the MT, SC- FDE (Single-Carrier with Frequency Domain Equalization) preferable for the uplink. For this reason they were proposed for the uplink of 3GPP LTE (Long Term Evolution). To allow high spectral efficiencies, the transmitted signals are usually designed to have compact spectrum, with band- widths just slightly above the symbol rate (e.g., by employing a square-root raised-cosine filtering with small roll-off fac- tor). However, the adoption of very compact spectra leads to implementation difficulties, since it is difficult to design the filters for channel separation and the synchronization requirements are stricter. Moreover, the transmitted signals have higher envelope fluctuations when the roll-off factor is very low, leading to amplification difficulties. Therefore, the transmitted signals usually employed have bandwidth higher than the symbol rate and the different frequency channels are separated accordingly to avoid ACI (Adjacent Channel Interference), decreasing the overall spectral efficiency of the 1 This work was partially supported by FCT (pluriannual funding and project POSI/CPS/46701/2002 - MC-CDMA) and C-MOBILE project IST- 2005-27423. system. A promising technique was proposed in [3] where the separation between frequency channels is equal to the symbol rate, regardless of the transmission bandwidth asso- ciated to each channel. To cope with the high interference levels that can result form having significant overlap between adjacent channels an iterative frequency-domain receiver with joint equalization and ACI suppression was proposed in [3]. That receiver takes advantage of spectral correlations for the separation of cyclostationary signals [4], [5], [6] and allows excellent performance. However, perfect synchronization be- tween users was required. Since it is very difficult to maintain a good synchronization between different users, some time and, especially, frequency misalignments are almost unavoidable. especially if we want to reduce the cost of the MT. For very-low-cost MT it is desirable to have grossly nonlin- ear power amplifiers, which are simpler to implement and have higher amplification efficiency and output power. However, these amplifiers are only recommendable if the signal at its input has an almost constant envelope. An interesting option is to employ OQPSK-type signals (Offset Quaternary Phase Shift Keying), which can have reduced envelope fluctuations or even a constant envelope in the case of MSK signals (Minimum Shift Keying). In this paper, we consider the uplink of broadband wire- less systems where SC-FDE schemes (Single-Carrier with Frequency-Domain Equalization) are employed by low-cost MT. We consider the use of OQPSK signals and different frequency channels are separated by the symbol rate, allow- ing very high spectral efficiency. Moreover, system is quasi- asynchronous, since only a coarse synchronization between MT is required. We present an iterative frequency-domain receiver for joint equalization, residual carrier synchronization and ACI. II. System Description and Transmitted Signals We consider an FDMA (Frequency Division Multiple Ac- cess) scheme where an SC-FDE modulation is employed for each frequency channel. We have P channels, with the carrier frequency of the pth frequency channel denoted by f p , p = 1; 2;:::;P , with f p ¡ f p¡1 = ±f , i.e., the frequency channels are equally spaced. It is assumed that the blocks transmitted by each frequency channel have the same dimensions. The length-M data block to be transmitted by the pth channel is fs m;p ; m =0; 1;:::;M ¡ 1, where s m;p = §1 § j