Channel Independent Precoding Design For MIMO OFDM With Insufficient CP
In OFDM system, the frequency domain channel is divided into a set of parallel narrow subchannels, therefore eliminating or reducing the innate intersymbol interference (ISI) in the wideband channel. OFDM systems transform from the ISI channel to the ISI free subchannels by resorting to IDFT and CP insertion at the transmitter and DFT and CP removal at the receiver respectively. The DFT based OFDM system also enable the simple one-tap equalization by using sufficient CP as guard interval. The enormous capability of MIMO systems in terms of boosting data transmission rate has been witnessed in the past two decades. Exploiting the so-called spatial multiplexing, MIMO systems allow for an increase of bit rate in the wireless communication link without requiring extra power and bandwidth. Combining MIMO and OFDM is a natural choice for broadband wireless communication system in pursuit of higher data rate.
Either spatial multiplexing or beamforming can be achieved on a per subcarrier basis in the MIMO OFDM system. The concise MIMO OFDM system model relies on the employment of sufficient guard interval, for example, sufficient duration of CP as the guard interval. However, wireless communication standards, which utilize MIMO OFDM as the modulation technique, usually set a length constraint for the CP, since the CP portion is merely a repeated information symbols and thereby a considerably large portion of CP results in a lower bandwidth efficiency. One of the major consequences of constraint on CP is the insufficient CP transmission in the MIMO OFDM system. Moreover, the burgeoning trend of extension coverage in wireless communication system leads to the long delay spread propagation scenario, which alternatively increases the probability of insufficient CP transmission. Insufficient CP destroys the orthogonality of OFDM signal structure and it gives rise to the interblock interference (IBI) and intercarrier interference (ICI).
MIMO OFDM system, or the single input single output OFDM/DMT counterpart, with insufficient CP have been researched extensively in the past, in, for example. In a transmitter based scheme based on the Tomlinson-Harashima precoder is proposed. The precoder for DMT eliminates interference by processing the signals at the transmitter so that the signal is ISI free at the receiver. In essence, this type of processing aims at channel shortening. However, the time varying nature of the wireless channel limits the application of this method in OFDM. proposes structure preventing ICI that utilizes redundancy in the frequency domain at the transmitter and removes ISI with a simple cancellation method at the receiver. proposed a training-based two stage solution to rst shorten the channel and then perform the per-tone equalization. In, a novel and efficient ICI/ISI-aware beamforming solution is proposed based on the optimal steering vector design, whose advantage is this solution uses low complexity one-tap per subcarrier processing at both transmitter and receiver. Yet this solution requires precise channel state information (CSI) at the transmitter.
