Precoding In Wireless Communications

Modern communication systems require higher data transmition rate than ever, this promote the application and study of new and sophisticated signal processing methods. Precoding is one of the signal processing methods which requires channel state information at transmitter. Precoding is widely studied now.Precoding is developed to increase the data transmission rate of wireless communications. It has been included in IEEE 802.16e with both open-loop and closed-loop techniques. And the 3GPP standard uses a closed-loop beamforming technique, based on quantized feedback of channel phase and amplitude. In the case of symmetric time-division-duplex systems, where each receiver in turn also acts as transmitter, the estimates from the receiving direction can be used for the subsequent transmission if the "ping-pong time" is short relative to the fading coherence time. For other applications, channel state information has to be made available to the transmitter via some backward channel.MIMO is one of the promising techniques in B3G systems. MIMO systems enable to grow linearly in transmission rate with the minimum of the number of antennas at either end. And OFDM is widely studied because of its high speactrum efficiency and capababilty to combat multipath fadind. So, the precoding for MIMO and OFDM systems are studied in this thesis. To summarize, the research includes the following aspects:Firstly, a precoder design method was proposed for OFDM systems with insufficient cyclic prefix (CP). CP is used as guard interval to combat multipath fading. But the length of CP may less than the maximum multipath delay in practical systems. ICI and ISI would degrade the performance. In order to eliminate the rest ISI and ICI, a new optimum precoder design method was given with the consideration of channel estimation error and changes caused by time deday of feeback channel.Secondly, another precoder design method based on Bayes criterion was proposed for point-to-point MIMO systems with imperfect known channel. Jakes channel model was adopted in the design and channel estimation error and time delay of feedback channel were considered. An optimum precoder was designed under MMSE criterion.Then, precoding techniques based on lattice reduction was studied. A new precoder which combined beamforming and lattice reduction was provided. In this method, the multi-user channel was decomposed into independent single user channel on which lattice reduction precoding can be used. And each user can be equiped with several antennas.Finally, recognition scheme using Hilbert transform for MQAM signal was given. Carrier frequency was not required in this scheme. Baud rate and timing was estimated using FFT, Chirp-Z transform and maximum average power timing estimation method separately. Then, constellation was extracted based Hilbert transform for clustering and decision. Modulation scheme recoginition is an important issue for non-cooperative communication study.