The fourth generation mobile communication OFDM-core technology review

Paper Keywords: OFDM synchronization pilot channel estimation
Abstract: OFDM technology is considered the 4th generation mobile communications core technology. In this paper, OFDM technology development, basic principles, key technologies, such as 4 in terms of advantages and disadvantages of OFDM technologies are described and analyzed.

1 Introduction
own modern communications, people have been seeking the freedom of communication. Popularity of wireless communications, such as the 1st generation mobile communication (1G), 2nd generation mobile communication (2G) part of this desire to meet people. But with the rise of the Internet and multimedia technology, people have a higher mobile communication requirements. So the 3rd generation mobile communication (3G) lead the bow to go, and 4th generation mobile communication (4G) of the theory and practice reports had been described as warm. In recent years, with the DSP chip technology, Fourier transform / inverse transform, channel adaptive technology, into the protection period to reduce the computational load balance the gradual introduction of mature technologies such as, OFDM (Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiplexing) as a very effective against the interference of the high-speed waveform transmission technology, has attracted wide attention. More and more people began to focus on developing OFDM technology in the field of mobile communications applications, wireless broadband access as well as the 4th generation mobile communications, OFDM technology CDMA technology will become the core technology after another. OFDM using a variety of new technologies, higher spectrum efficiency and good anti-multipath interference, which can not only increase system capacity, more importantly, it can better meet the requirements of multimedia communications, will include voice, data, video and other multimedia services through large amounts of information of high quality broadband channel to send out. It has high bandwidth, high stability, low cost, is called "wireless fiber" technology development prospects for solving the communication "last 1 km access" problem provides a new powerful tool.

2 OFDM
OFDM development process originated in the 20th century, mid 50s, which consists of MCM (Multi-Carrier Modulation, multi-carrier modulation) developed from the first time in January 1970 published on OFDM patent. However, after a long period of time, OFDM theory into practice, the pace slowed. The reason is the various sub-carrier OFDM between orthogonal, using FFT (Fast Fourier Transform) to achieve such a modulation device complexity, transmitter and receiver oscillator stability and high RF power amplifier linearity requirements. Later, after extensive Research, and finally 80 in the 20th century, MCM get a breakthrough, large scale integrated circuit technology allows the realization of FFT is no longer insurmountable obstacles, a number of other difficult and hard to achieve have been resolved, since , OFDM communication onto the stage and gradually into the Modem and high-speed digital mobile communications field. 90 20th century, OFDM has been broadcast channel broadband data communications, digital audio broadcasting (DAB), high-definition digital television (HDTV) and wireless LAN (WLAN) and wired telephone network based on existing copper twisted pair of asymmetric high bit rate digital subscriber line technology (such as ADSL) has been applied. The basic principle
3 OFDM
OFDM technology is actually a kind of MCM. The main idea is: the channel into a number of orthogonal subchannels, each subchannel on the narrowband modulation and transmission, thus reducing interference between subchannels, while improved spectrum utilization. The signal bandwidth of each subchannel is less than the correlation bandwidth of the channel, so each sub-channel frequency selective fading on is flat, substantially eliminating the ISI.

OFDM allows the carrier frequency between the mixing with each other, and using the FFT-based orthogonal modulation carrier frequency. As the center of the carrier frequency in each point, no other components of the carrier spectrum, so to achieve the orthogonality of each carrier.

Or frequency division multiplexing though, but not by a lot of OFDM bandpass filter to achieve, but directly in the baseband processor, which is different from other systems, the advantages of OFDM one. OFDM receiver is actually a modem, it will move to a different carrier frequency of zero, then in a symbol cycle points. As with the other carrier signal quadrature points, it will not affect the results of this integration.

OFDM for high data rate and related to the number of subcarriers to increase the number of subcarriers, can improve the data transmission rate. OFDM modulation for each frequency band may be different, which increases the flexibility of the system. Multi-user OFDM for high flexibility, high availability communication systems. The key
4 OFDM channel estimation techniques
4.1 channel estimation in OFDM
system plays an important role, the design of the channel estimator has two main problems: First, the choice of pilot information. As is often the fading wireless channel, the channel need to keep track, so the pilot must continuously transmit the information; the second is the lower complexity have both good tracking ability of the pilot channel estimator design. Links to Research Papers Download http://www.hi138.com