Cooperative Space-Time Coded OFDM with Timing Errors and Carrier Frequency Offsets F. S´ anchez ∗ , T. Zemen ∗ , G. Matz † , F. Kaltenberger ‡ , N. Czink ∗ ∗ FTW Telecommunications Research Center, Vienna, Austria † Inst. of Communications and RF Engineering, Vienna University of Technology, Vienna, Austria ‡ Institute Eurecom, Sophia Antipolis, France Abstract—The use of distributed space-time codes in coopera- tive communications promises to increase the rate and reliability of data transmission. These gains were mostly demonstrated for ideal scenarios, where all the nodes are perfectly synchronized. Considering a cooperative uplink scenario with asynchronous nodes, the system suffers from two effects: timing errors and individual carrier frequency offsets. In effect, timing errors can completely cancel the advantages introduced by space-time codes, while individual carrier frequency offsets provide a great challenge to receivers. Indeed, in cooperative communications, frequency offsets are perceived as a time-variant channel, even if the individual links are static. We show that using OFDM, space-time codes (STCs) become robust to timing errors. Channel estimation and tracking takes care of frequency offsets. Our simulations demonstrate that the bit error rate (BER) performance improves by an order of magnitude, when using a cooperative system design, which takes these two effects into account. Index Terms—OFDM, Block codes, distributed space-time code, virtual MIMO, diversity methods, cooperative systems, time-varying channels, delay effects, channel estimation, spatial diversity, transmit diversity, Slepian basis expansion. I. I NTRODUCTION Cooperative communications can increase both data rate and link reliability. Large gains can be expected when nodes are acting as a virtual array. Popular schemes to utilize multi- node diversity are based on STCs. Many publications study the advantageous effects of STCs in distributed cooperative communications but assuming perfectly synchronized nodes, i.e. their signals have no carrier frequency offsets, and arrive at the same time at the receiver [1]. The performance of STCs is deteriorated by synchronization errors [2]. Timing offsets in the order of the duration of a symbol can destroy the code structure, if the STC is not designed to be delay tolerant. Damen et al. [3] and Sarkiss et al. [4] adapted STCs to be delay tolerant, however the authors did not investigate the effect of non-sample-spaced timing offsets. Li and Xia [5] proposed to combine space-time trellis codes with the stack construction concept from [6], but their codes require high complexity for optimal decoding. This work was supported by the project PUCCO funded by the Vienna Science and Technology Fund (WWTF). The work was carried out within the framework of COST 2100 and in the FP7 Network of Excellence NEWCOM++. The Telecommunications Research Center Vienna (FTW) is supported by the Austrian Government and the City of Vienna within the competence center program COMET. removed at Rx CP Tx 1 Tx i . . . Rx . . . . . . + Time of the signal Useful part Relative delay CP OFDM Sym. CP OFDM Sym. CP OFDM Sym. CP OFDM Sym. Channel Channel Desynchronization is perceived as a cyclic rotation in the OFDM symbols δ1 foff,1 foff,i δi foff,1 H1(k) Hi(k) δi foff,i Fig. 1. OFDM distributed cooperative transmission. Many current communication systems are based on or- thogonal frequency division multiplexing (OFDM). Prominent examples are LTE [7], WiFi [8], and DVB-T [9]. DVB-T single frequency networks use distributed OFDM transmission, hence several television stations transmit simultaneously the same signal. OFDM uses the cyclic prefix (CP) to prevent inter- symbol interference. It turns out that the CP provides also the means to make cooperative communication systems inherently robust against timing synchronization errors. In this contribution we combine OFDM with STCs and time-variant channel estimation for distributed cooperative communications scenarios. The system presented successfully compensates the main effects of real implementations i.e. timing errors and frequency offsets between the distributed nodes. We consider a scenario where at least two nodes join up to transmit a common message to a distant base station as outlined in Fig. 1. Using STCs, the nodes increase the diversity and the overall uplink SNR, leading to a lower BER. We show that the combination of OFDM and STC is advan- tageous when using a proper receiver architecture. We consider two STCs: delay diversity and the Alamouti code. Carrier frequency offsets introduced by the individual transmitters (TXs) lead to an effective time-variant channel at the receiver, this effect can be compensated by proper time variant channel estimation. We employ a channel estimator based on discrete prolate spheroidal sequences (DPSs), which tracks the channel in the time-frequency domain simultaneously. The paper is organized as follows: Sec. II discusses the sys- tem model of our cooperative communication system. Section III analyses the system impairments. In Sec. IV, we detail the channel estimators used. Simulation results are presented in Sec. V. Finally, in Sec. VI, we draw conclusions. 978-1-61284-233-2/11/$26.00 ©2011 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE ICC 2011 proceedings