My PhD thesis: Pilot Assisted Block Transmission
English abstract:
Pilot Assisted Block Transmission (PABT)
JIAO Xianjun (Communication and Information system)
Directed by Prof. XIANG Haige
Abstract:
Pilot Assisted Block Transmission (PABT) is a transmission scheme suitable for broadband wireless communication. In this scheme, data are grouped into separated blocks, in which pilot are inserted. Receiver gets channel state information (CSI) based on observation of received pilot in real time and makes demodulation according to CSI. Considering the time variety and frequency selective characters of broadband wireless channel in modern wireless broadcasting, mobile and wireless network system, PABT is a reasonable choice. In this thesis, a uniform signal framework of PABT is summarized based on investigations of existing broadband systems, and two kinds of PABT system are presented under the framework: pilot and data are in time division multiplexing form; pilot and data are in frequency division multiplexing form. Researches on key technologies and systems of PABT are carried out. This thesis include:
-
A uniform signal framework of PABT is summarized, and many signal formats are presented under the framework. Those formats are classified into two kinds: pilot and data are in time division multiplexing form; pilot and data are in frequency division multiplexing form. In the first kind, there are four basic formats: “PN (Pseudo Noise) pilot + SC (Single Carrier) data”, “PN pilot + OFDM data”, “OFDM pilot + SC data”, “OFDM pilot + OFDM data”. The second kind is referred to traditional OFDM systems with frequency domain pilot in fact. In addition, with different form of guard interval, for example CP (Cyclic Prefix), ZP (Zero Padding)…, more signal formats can be created. The formats include not only most existing systems but also new systems. Key technologies of PABT are discussed under the framework, which are: Channel Estimation (CE), Euqalization, Iterative Interference Canceling and guard interval signal design for higher efficency.
-
All kinds of CE algorithm are studied and summarized. The procedure of CE is separated into three steps: pre-process, channel estimate, post-process. Two algorithms of estimating time domain channel impulse response (CIR) are studied, which are move correlation and circular correlation. Two algorithms of estimating channel frequency response are also studied, which are frequency domain estimation after “cut-and-add” and zero padding frequency domain estimation. A new CE algorithm based on PN pilot with circular correlation is proposed. The new CE algorithm has almost the same performance with existing algorithm, but the complexity of new algorithm is much lower than that of existing algorithm. A new post-process algorithm is also proposed, which leads to visible performance gain compared to CE without post-process. The idea of new post-process algorithm is that certain number of strong path is reserved and other paths are removed as noise. The performance of new post-process algorithm has no “floor” phenomenon, which is a drawback of exsisting algorithm.
-
Time domain equalization (TDE) and frequency domain equalization (FDE) of CP/ZP block are summarized and studied. TDE and FDE are proved to be equivalent. Two MMSE (Minimum Mean Square Error) FDE algorithms are studied: direct FDE and FDE after “cut-and-add” pre-process. Quasi-MMSE FDE alogrithms is studied. The complexity of quasi-MMSE algorithm is much lower than that of strict-MMSE algorithm, and performance degradation is only about 0.1dB, when 7, [171 133], 1/2 convolution code and random interleaver are employed.
-
Iterative interference canceling algorithm is studied in the situation that pilot and data are in time division multiplexing form and without guard interval. A simplified algorithm is proposed for situation where pilots are invariant. As applications of iterative interference canceling, “OFDM pilot + SC data” and “OFDM pilot + OFDM data” PABT system is studied. Simulation shows that two systems are suitable for broadband wireless communication with high mobility (moving speed 130km/h; doppler frequency 100Hz in 800MHz band). When CIR is short, for example DVB-T portable reception channel, performance of new systems can approach that of reference system with guard, which needn’t interference canceling, and new systems have higher bit rate.
-
In the situation that pilot and data are in time division multiplexing form, a new PP-OFDM (Pilot Postfixing - OFDM) scheme is proposed as a modification of traditional CP-OFDM. Replacing CP signal with PP signal in guard interval ensures that PP-OFDM has higher efficiency of utilizing pilot power than CP-OFDM. ZP-OFDM can be derived from PP-OFDM. Pilot power allocation is studied in PP-OFDM, and analytic result is derived. When 1/4 guard interval is adopted, PP-OFDM has about 1dB performance gain compared with CP-OFDM.
Key words:
PABT (Pilot Assisted Block Transmission); OFDM (Orthogonal Frequency Division Multiplexing); Channel Estimation; Frequency Domain Equalization; Iterative Interference Canceling