| With the development of wireless communication technologies, in order to meet the needs of new requirements ,mobile communications in the next generation (4G or B3G) wille use new technology - orthogonal frequency division multiplexing (OFDM) and multi-antenna technology ( MIMO). The main features of OFDM is its channel has been divided into a number of subchannels, different subcarriers is usd to transmit for each subchannel,which is overlapping and orthogonal. So when bandwidth is fixed ,OFDM will be able to transmit more data. MIMO means the transmitter and receiver are placed multi-antenna at the same time, the antenna is non-interfering each other, and use space to transmit more data. The combination of these two technologies improve the wireless communications system on the transmission quality and transmission speed greatly, and can fight for interference. One of the important reasons that they are used is,with the development of related technologies, the realization of these two technologies has become more simple, the applications is more easily . The data in the form of 2-band should be changed into the form which is suitable for transmission in wireless system,this is called modulation.In OFDM the traditional modulation is modulation method fixed in all sub-carriers. the system bit error rate is decided by the worst channel conditions. The disadvantage of this method is , even if there is only one such channel which is in the poor condition , the bit error rate of system will be high.It will be a great waste of resources, so people began to study adaptive modulation method:basing on the different channel conditions ,all subcarriers choose adaptive modulation means by thenselves,when the system error no longer depend on the worst channel. Channel in the same conditions can improve the system bit error rate through this.This paper mainly study the adaptive modulation algorithm in MIMO/OFDM system, the existing OFDM modulation algorithm must be changed before it used in the MIMO/OFDM system to adapt difference.Meantime simple MIMO decoding- STBC coding and adaptive modulation algorithm can apply to the new system together, which will take great advantages for the system.This paper is divided into two aspects, the first part is MIMO and OFDM theoretical knowledge: first, introduce the development of mobile communications, including the main technical characteristics in each stage of development .From 1G that use analog technology to 2G use digital technology,then 3G which is using and the developing 4G,all are researched to meet the needs of people.4G will give us better quality of transmission and higher speed.OFDM and MIMO are more fitable to the developping of communication than others.MIMO/OFDM use orthogonal and multi-antenna at the same time,can transmit more data on the same frequency band,and do not need to consider the inter-symbol interference. Even the antenna channel capacity can grow with the number of antenna linearly.Capacity and quality of communication we can observe improved.MIMO and OFDM use different ways to improve the system. Combination of the two technology made their advantages added, and bring the system performance to a higher level.The second part is the adaptive modulation algorithm: Firstly, introduce the adaptive technology chiefly and its application in MIMO/OFDM system.Basic idea of adaptive algorithm is filling water– subcarriers under good channel condition is used to transmit more information, under poor channel condition transmit less information or do not be used.For the systems which ask the same error rate ,this can increase the transmission capacity and improve system performance. Adaptive modulation is to select the appropriate modulation method in accordance with the channel to ensure the transmission bit error rate. OFDM system has a number of classic algorithms,such as Greedy algorithm,Chow algorithm,Simple Block Loading Algorithm(SBLA) and so on. Greedy algorithm is optimal adaptive modulation algorithm, but it is very difficult to achieve. Chow algorithm is sub-optimal way which is easy to come true and has practical value.SBLA is easy to achieve, but accuracy is not high.The others based on above methods,change some parameters to meet different requests.For MIMO/OFDM system, it is necessary to change the original modulation algorithm to adapt the difference of conditions.The difference includes some aspects. At the beginning ,in MIMO system the number of subcarriers grows with the increasment of the number of antenna.In order to reduce the computational requirements of the system,the algorithm is not carried out according to each sub-carrier , but to several sub-carriers which is called a group. Secondly, the modulation that can be chosen to use in the practical system is fixed.So each sub_carrier should know how many bits it can tranmit once,which should be one of the modulation methods that system can carry out.A premise of these acts is the assumption that the receiver and transmitter know channel conditions.Basing on this we can get the result of algorithm simulation,which is ideal. STBC is a encoding method of MIMO system, its encoding and decoding is simpler than others,so it can also reduce the time that data is kept on the transmitter and the receiver, improve responsive rate of system. Improved algorithm for adaptive modulation and STBC applied to the MIMO/OFDM system together, Under this situation system can be analysed through time, space, frequency .Upon requests, under certain conditions system can achieve the optimum. In the practical applications it is very important to fully utilize the system when it is used to the different situations. |
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