Relay technology has been a hot area of research in wireless communication community in recent years. It is widely accepted that relay or multi-hop networks will become essential for beyond 3G mobile radio systems due to the range problem that appears there. As higher carrier frequencies or center frequencies can be envisaged for future mobile radio communication systems, where the expected center frequencies range up to 5-10 GHz and with bandwidth requirements of up to 100 MHz that can be foreseen, a significantly increased pathloss and noise power level has to be expected, which translates into a significantly reduced area a base station can cover.
Using relaying technology, signals can be transmitted much farther away, R. Pabst, B. Walke, D. Schultz, P. Herhold, H. Yanikomeroglu, S. Mukherjee, H. Viswanathan, M. Lott, W. Zirwas, M. Dohler, H. Aghvami, D. Falconer, and G. Fettweis, “Relay-based deployment concepts for wireless and mobile broadband radio,” IEEE Commun. Mag., vol. 42, no. 9, pp. 80-89, September 2004, J. Zhao, I. Hammerstroem, M. Kuhn, A. Wittneben, M. Herdin, and G. Bauch, “Coverage analysis for cellular systems with multiple antennas using decode-and-forward relays,” in Proc. 65th IEEE Veh. Tech. Conf., Dublin, Ireland, Apr. 22-25, 2007. Furthermore relay signals provide additional diversity and improve the received signal quality, J. N. Laneman, D. N. Tse, and G. W. Wornell, “Cooperative diversity in wireless networks: Efficient protocols and outage behavior,” IEEE Trans. Inform. Theory, vol. 50, no. 12, pp. 3062-3080, December 2004.
In order to circumvent the introduction of a denser grid of base stations (BS), the basic idea is to introduce relay stations (RS), which forward data packets to a mobile station (MS) that is out of reach of the base station. Such relay stations can be realized utilizing additional dedicated infrastructure relay stations having fixed power supplies, or they could be built into other mobile stations. Two main concepts of relaying were identified in the past: Amplify-and-Forward (AF) and Decode-and-Forward (DF). While AF has the advantage of being transparent to modulation and coding since a sampled version of the received signal is stored and retransmitted by the relay station without performing any decoding, DF allows for a separate adaptation to both links and avoids also the effect of noise enhancement since DF means that the relay station decodes and re-encodes the signal.
Current relays cannot transmit and receive signals using the same time and frequency channel (half-duplex relays). Half-duplex generally means that communication is possible in both directions, but only one direction at a time (not simultaneously). Hence, traditional relaying schemes suffer from the spectral efficiency loss due to two channel uses.
Recently, a spectral efficient relaying scheme called two-way relaying has been proposed in B. Rankov and A. Wittneben, “Spectral efficient protocols for half-duplex fading relay channels,” IEEE J. Select. Areas Commun., vol. 25, no. 2, pp. 379-389, February 2007 and in P. Larsson, N. Johansson, and K.-E. Sunell, “Coded bidirectional relaying,” in IEEE Veh. Tech. Conf., vol. 2, Melbourne, Australia, May 7-10, 2006, pp. 851-855. In such a scheme, the source and destination terminals transmit simultaneously in a first time interval. After receiving data signals from the source and the destination terminals in the first time interval, the relay station terminal retransmits the combined or superimposed source and destination data signals in a second time interval. Since both the source and the destination know its own data, respectively, both sides can cancel the so called self-interference, i.e. the own useful data transmitted towards the relay station in the first time interval, and decode the useful data from the other side. Two-way relaying achieves bi-directional transmission between the source and destination in two time intervals or time slots by using half-duplex terminals. Thus it avoids the spectral efficiency loss due to the use of half-duplex relays. Up to now, all the research is focusing on canceling the self-interference instead of utilizing it.
The authors of “Spectral efficient protocols for half-duplex fading relay channels” used a so called superposition coding scheme to re-encode data symbols to be transmitted in the second time interval, while the authors of “Coded bidirectional relaying” applied an XOR-operation on bit level to re-encode the data symbols to be transmitted in the second time interval at the relay station. The comparison of the two re-encoding schemes has been made in I. Hammerstroem, M. Kuhn, C. Esli, J. Zhao, A. Wittneben, and G. Bauch, “MIMO two-way relaying with transmit CSI at the relay,” in Proc. SPAWC, Helsinki, Finland, Jun. 17-20, 2007 and T. J. Oechtering, I. Bjelakovic, C. Schnurr, and H. Boche, “Broadcast capacity region of two-phase bidirectional relaying,” March 2007, submitted to IEEE Transactions on Information Theory.
MIMO (Multiple-Input-Multiple-Output) technology has been a major breakthrough in the area of wireless communication in recent years. The use of multiple antennas can provide significant increase in capacity, I. E. Telatar, “Capacity of multi-antenna Gaussian channels,” Europ. Trans. Telecommun., vol. 10, no. 6, pp. 585-595, November 1999. It is almost certain that most communication systems will be equipped with multiple antennas in the years to come. Accurate channel knowledge may be used in order to take the advantage the MIMO system can offer. However, estimating the channel in a MIMO system is difficult. Traditional channel estimation schemes transmit dedicated, predetermined pilot or training sequences comprising known data symbols on different transmit antennas. Thus the spectrum efficiency is reduced in the traditional channel estimation schemes.
MIMO channel estimation has been a research topic along with MIMO transmission technology. One of the most popular and widely used approaches to estimate a MIMO channel is to transmit orthogonal training sequences from different transmit antennas. The received signal is correlated with the training sequences at the receiver. Based on the received data and the knowledge of the predetermined training sequences, the channel matrices can be calculated. In M. Biguesh and A. B. Gershman, “Training-based MIMO channel estimation: A study of estimator tradeoffs and optimal training signals,” IEEE Trans. Signal Processing, vol. 54, no. 3, pp. 884-893, March 2006, the authors considered linear least squares (LS) and minimum mean-squared-error (MMSE) channel estimation approaches and investigated the optimal choice of training sequences. They showed that transmitting orthogonal training sequences from different antennas is optimal for the LS approach.
Instead of multiplexing the training sequences with data symbols, H. Zhu, B. Farhang-Boroujeny, and C. Schlegel, “Pilot embedding for joint channel estimation and data detection in MIMO communication systems,” IEEE Commun. Lett., vol. 7, no. 1, pp. 30-32, January 2003, investigated the performance of embedding the training sequences with data to estimate the channel. Unlike conventional methods where predetermined pilot symbols are time-multiplexed with useful information data symbols, a pilot-embedding method was proposed in H. Zhu, B. Farhang-Boroujeny, and C. Schlegel, “Pilot embedding for joint channel estimation and data detection in MIMO communication systems,” IEEE Commun. Lett., vol. 7, no. 1, pp. 30-32, January 2003, where low power level predetermined pilots are transmitted concurrently with the useful data, and they are used to obtain an initial estimate of the channel such that a turbo decoding process can be started. Soft information obtained from the turbo decoder is subsequently used to improve channel estimates.
A decision directed iterative channel estimation method was proposed in X. Deng, A. M. Haimovich, and J. Garcia-Frias, “Decision directed iterative channel estimation for MIMO systems,” in Proc. IEEE Int. Conf. on Communications, vol. 4, Anchorage, Ak., May 11-15, 2003, pp. 2326-2329.
All the proposed channel estimation schemes rely on an initial estimation of the MIMO channel based on the transmission of pure predetermined training sequences.