Research On FPGA Realization Algorithm Of OFDM Synchronization Algorithm

Digital mobile communication of VHF/UHF wave band is all along the hot spot of telecommunications field. From 1980s, digital mobile communication technology became more mature, gradually be applied in all wireless communications, such as wireless paging, digital trunked phone, digital cell phone, mobile data terminals and universal mobile telecommunication system. But mobile channels of VHF/UHF have serious frequency selective fading, fast fading and slow fading, and all kinds of intereference of noise and intersymbol interference under multipath propagation. These issues could seriously affect quality and performance of digital mobile communication system. In order to overcome the limitation to improve the performance of system mentioned above, most digital communication system use FSK of amplitude jitter insensitivity and GFSK with Gause filter modulations and etc. besides, such as decrease code rate and adaptive equalization could be used. But utilization rate of modulation technology above is not so high, and along with frequency resource is becoming more limited, more new high efficient modulations are developed. OFDM is one of excellent modulation technology.In these years, OFDM (Orthogonal Frequency Division Multiplexing) technique has been paid more and more attention because it has strong transmission ability in wireless channels, which comes from its own parallel transmission mechanism, inherited robustness to Inter Symbol Interference (ISI) and the diversity with the multi-path channel. Meanwhile, OFDM system has very high channel utilization ratio because of the orthogonal properties among its sub carriers. There are a lot of wireless LAN standards which are supported by some big companies. However, IEEE802.11a is undoubtedly a very special standard among these wireless LAN standards that is because it transmits data through OFDM. Based on the WALN standard IEEE802.11a, this thesis focus on study and implement of the synchronization issue of OFDM in FPGA.In this system, the data are transmitted in burst frames and there exists a preamble, which is composed of ten repeated short training sequences and two repeated long training sequences in front of each frame. This redundancy provides potential to do time and frequency synchronization in IEEE802.11a OFDM system.Meanwhile it could be realized by DSP technology– FFT【1】【2】, without comb filters, easily be implemented. Along with increase of transmission rate, intersymbol interference due to multipath becomes more distinctFor single carrier system, it needs complex equalization technology to solve this issue, but OFDM system due to its special structure makes it easier to withstand the affects of multipath to signals. Peled and Ruiz introduce Cyclic Prefix mechanism, to periodically extend OFDM signals, as long as CP larger than impulse response of signals; it could efficiently withstand intersymbol interference of multipath.The cost of using CP is lost of some signal power, it is proportional with the length of CP, and however, the advantages of using CP are extremely more than all kinds of power lost. Under broad band wireless applications, advantages of OFDM technology are more outstanding, and it may use efficient technology to fix and compensate its disadvantages. Based on above consideration, this article mainly researches that under IEEE802.11a WLAN environment, timing synchronous and carrier synchronous, and how to realize it in FPGA.In IEEE 802.11a wireless LAN of with OFDM as core sub layer technology, data are sent by burst transmission of frame, for each frame's head, it contains a leader structure of ten repeated short training sequences and two same long training sequences. Generally this article basically realizes the synchronization of OFDM based on IEEE 802.11a protocol on FPGA, in ideally environment could accurately realize synchronization.However, still remains some defaults, for example, FPGA resource is not enough in this project, when select algorithm, the easier ones are adopted, so that performance is affected, this is continuous improving spot in the future. Moreover, in the long term, this article works the first step, which realizes OFDM system synchronization by hardware, in order to have deeply research based on the article, and develops OFDM technology into actual application.This thesis is organized as follows:Chapter 1 introduces this thesis's background and contents. In Chapter 2, The theory of OFDM technique is described here detailedly, its advantages and disadvantages and the physical layer of IEEE 802.11a WLAN system also introduced here. Chapter 3 discusses the synchronization issues and the synchronization algorithms in the OFDM system. Synchronization of OFDM includes timing synchronization and carrier synchronization, and timing synchronization also includes symbol synchronization and clock synchronization. Timing synchronization is divided to coarse timing synchronization and fine timing synchronization. In Chapter 4, Implementation of the synchronization algorithm in FPGA was discussed. Chapter 5 gives the conclusion of the whole thesis.