WiMAX的关键技术

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WiMAX的关键技术--OFDM／OFDMA，HARQ，AMC，MIMO，QoS机制，睡眠模式，切换技术等等 u.Tknw-X  
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OFDM（正交频分复用）是一种高速传输技术，是未来无线宽带接入系统／下一代蜂窝移动系统的关键技术之一，3GPP已将OFDM技术作为其LTE研究的主要候选技术。在WiMAX系统中，OFDM技术为物理层技术，主要应用的方式有两种：OFDM物理层和OFDMA物理层。无线城域网OFDM物理层采用OFDM调制方式，OFDM正交载波集由单一用户产生，为单一用户并行传送数据流。支持TDD和FDD双工方式，上行链路采用TDMA多址方式，下行链路采用TDM复用方式，可以采用STC发射分集以及AAS自适应天线系统。无线城域网OFDMA物理层采用OFDMA多址接入方式，支持TDD和FDD双工方式，可以采用STC发射分集以及AAS。OFDMA系统可以支持长度为2048、1024、512和128的FFT点数，通常向下数据流被分为逻辑数据流。这些数据流可以采用不同的调制及编码方式以及以不同信号功率接入不同信道特征的用户端。向上数据流子信道采用多址方式接入，通过下行发送的媒质接入协议（MAP）分配子信道传输上行数据流。虽然OFDM技术对相位噪声非常敏感 .. /[!<rhY  
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Wimax OFDMA sub-channel allocation Y]5spqG  
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In Wimax OFDMA, the entire spectrum is divided into sub-channels, which can be considered the basic unit of allocation. A single sub-channel can be shared by multiple users, and a single user can potentially be allocated multiple sub-channels. Each sub-channel defines a particular combination of sub-carriers (these are the basic narrow band carriers into which the overall spectrum is broken), which may change over time (even from symbol to symbol). However, the ensemble properties of the link remains statistically constant over all. &n#yxv4  
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OFDMA offers three main mechanisms for sub-channel allocation. The three differ in the interleaving density, pilot density and consequently, the radio-environment that they offer.

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Error control scheme can be broadly classified into two types ARQ and FEC. EU>`$M&w-  
In classic ARQ, reliability is achieved by retransmission of lost data. FEC on the other hand adds redundancy to facilitate recovery of the lost information. ARQ techniques offer reliability when channel conditions deteriorate, but throughput suffers due to frequent retransmissions. In comparison, FEC techniques offer a constant throughput but reliability depends upon the code rate. The lower the code rate, the better the performance, but throughput is lower as well. s@5~HyeI  
Proper combination of these two techniques can overcome their respective drawbacks. A hybrid of FEC and ARQ scheme known as Hybrid ARQ can be used to better the throughput and reliability performances. H-ARQ schemes are very adaptive and flexible in nature. The retransmission protocol helps in maximizing the the probability of correct delivery in dynamically changing environment, ensuring improved throughput. Hybrid ARQ involves an encoded forward link for error correction and detection and a feedback link for possible retransmission. At the transmitter parity bits are added to the data block to detect and correct errors. In case the receiver is not able to correct these errors, the data block is transmitted again. For each received data block the receiver either sends an ACK if data block is decoded successfully or a NAK if data block is un-decodable. The transmitter responds to a NAK by re-transmitting the information

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AMC )%MC*Z:^  
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AMC在WiMAX的应用中有其特有的技术要求，由于AMC技术需要根据信道条件来判断将要采用的编码方案和调制方案，所以AMC技术必须根据WiMAX的技术特征来实现AMC功能。与CDMA技术不同的是，由于WiMAX物理层采用的是OFDM技术，所以时延扩展、多普勒频移、PAPR值、小区的干扰等对于OFDM解调性能有重要影响的信道因素必须被考虑到AMC算法中，用于调整系统编码调制方式，达到系统瞬时最优性能。WiMAX标准定义了多种编码调制模式，包括卷积编码、分组Turbo编码（可选）、卷积Turbo码（可选）、零咬尾卷积码（ZeroTailbaitingCC）（可选）和LDPC（可选），并对应不同的码率，主要有1／2、3／5、5／8、2／3、3／4、4／5、5／6等码率。

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