IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 11, NO. 11, NOVEMBER 2012 4161 Linear Receiver for the Uplink in Distributed Antenna Systems Jun Yang, Member, IEEE, Il-Min Kim, Senior Member, IEEE, Dong In Kim, Senior Member, IEEE, and Francois Chan, Member, IEEE Abstract—We consider the uplink of a distributed antenna system (DAS) in the presence of interference and unknown oscillator offsets, which lead to synchronization errors. For this scenario, we develop a linear receiver which maximizes the output signal-to-interference-plus-noise ratio (SINR). Specifically, we first propose a new structured generalized sidelobe canceller (SGSC) formulation for the commonly used minimum-variance receiver, and derive a general framework to improve the robust- ness of a linear receiver when oscillator offsets exist. Then a new linear receiver is proposed in closed-form based on the proposed robust SGSC and a ridge regression technique. It is shown by simulations that the proposed linear receiver can provide very robust SINR performance and excellent symbol error rate (SER) performance in the existence of interference and unknown oscillator offsets. Index Terms—Distributed antenna systems, linear receiver, oscillator offsets, synchronization error. I. I NTRODUCTION W ITH increasing demands for various multimedia ser- vices, the need for reliable data transmission at high data rate arises in the design of future cellular wireless networks. In order to reliably transmit data at higher data rates, one of the main challenges is to mitigate the adverse effects of interference. One promising strategy to address the interfer- ence issue is using a distributed antenna systems (DAS) [1]– [7]. A DAS, which was originally introduced and implemented to cover dead spots in state-of-the-art cellular communication systems, comprises a home base station (BS) and several distributed antenna elements (AEs) that are connected offline to the BS, typically via dedicated wires such as optical fibers. Since the DAS can reduce inter-cell interference, it can provide potential advantages of increased capacity for both the uplink [8]–[11] and the downlink [4]–[7]. In spite of extensive studies on the capacity of DASs in the literature [4]–[11], less attention has been paid to Manuscript received March 20, 2012; revised July 4, 2012; accepted August 17, 2012. The associate editor coordinating the review of this paper and approving it for publication was Y. Li. This work was supported in part by the Natural Sciences and Engineering Research Council (NSERC), and in part by the Ministry of Knowledge Economy (MKE), Korea, under the Information Technology Research Center (ITRC) support program supervised by the National IT Industry Promotion Agency (NIPA) (NIPA-2012-(H0301-12-1005)). J. Yang and I.-M. Kim are with the Department of Electrical and Computer Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada (e-mail: {jun.yang, ilmin.kim}@queensu.ca). D. I. Kim is with the School of Information and Communication Engineering, Sungkyunkwan University (SKKU), Suwon, Korea (e-mail: dikim@skku.ac.kr). F. Chan is with the Department of Electrical and Computer Engineering, Royal Military College of Canada, Kingston, ON, K7K 7B4, Canada (e-mail: chan-f@rmc.ca). Digital Object Identifier 10.1109/TWC.2012.092112.120405 design of actual transmit/receive algorithms for DASs, which is also a practically and theoretically important issue. In fact, a DAS poses many challenges in practical system design and implementations due to its distributed nature. Specifi- cally, effective cooperation of the distributed AEs, such as cooperative transmit/receive beamforming, requires accurate synchronization across different AEs [12], [13]. However, perfect synchronization among the signals from different AEs requires the perfect knowledge of timing offsets, frequency offsets, and oscillator offsets, which might be very difficult to accurately estimate. For example, in the downlink, the signals transmitted from AEs should be frequency synchronized and phase adjusted so that the signals can coherently arrive at the desired user. However, the channel from the multiple AEs to the desired user is a typical multi-access channel so that multiple timing offsets, frequency offsets, and oscillator offsets have to be estimated simultaneously, which is very difficult. These synchronization issues for the downlink have been studied very recently in [14]–[18]. On the other hand, in the uplink of a DAS, one might assume (almost) perfect timing offsets and frequency offsets which are caused by Doppler effect, which is different from the downlink. The reason is that, in the uplink, the channel from the desired user to multiple AEs is typically a broadcast channel. Thus, the transmission from the desired user to each antenna of the DAS is a simple point-to-point coherent com- munication, which makes it possible to accurately estimate the timing offset and frequency offset at the AE side. However, the synchronization problem caused by distinct local oscillators with different offsets of multiple AEs is still a challenging issue, which has been reported as one of the main sources of synchronization errors in distributed systems [12]. This is because in a distributed setting, different AEs have different RF carrier signals generated by their own distinct local os- cillator circuits, which are not synchronized a priori. Thus, the offsets in practical oscillators cause the carrier signals to drift out of synchronization [12]. In this way, although the timing offset and frequency offset caused by Doppler effect for each channel in each AE might be accurately estimated, the different oscillator offsets across different AEs may cause a significant performance loss (e.g., signal-to-noise ratio loss for linear receiver) when the signals from the different AEs are combined at the BS. Note that such synchronization issue in a DAS does not exist in non-distributed systems such as the conventional collocated multiple antenna systems using a single oscillator. In a DAS, to keep the synchronization errors small, one may try to re-synchronize the phases of the signals with a (very) short period for each AE. 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