In the next generation communication system, also known as the 4th Generation (4G) communication system, research efforts are actively in progress to provide users with various services with a Quality of Service (QoS) at a data transfer rate of about 100 megabit per second (Mbps). A representative example of such a communication system is an Institute of Electrical and Electronics Engineers (IEEE) 802.16 system. The IEEE 802.16 system applies an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme to a physical channel to support a broadband network.
In the OFDM/OFDMA-based communication system, a Multiple Input Multiple Output (MIMO) scheme that uses multiple Transmit (Tx) antennas can be applied to improve system reliability or to support a higher data transfer rate. To improve the system reliability, a Space-Time Coding (STC) scheme can be used, and a spatial diversity gain is increased. To improve the data transfer rate, a Spatial Multiplexing (SM) scheme can be used. According to the two methods above, all transmitting ends transmit a plurality of separate streams through separate Tx antennas during the same time period.
FIG. 1 illustrates an example of a Tx frame for each antenna based on a MIMO scheme in a broadband wireless communication system. Two Tx antennas are used in a Tx frame structure of FIG. 1. A downlink subframe is divided into a preamble area 110 that includes one symbol length, a map area 120 in which burst allocation information is carried, and a data burst allocation area 130. An antenna Cyclic Delay Diversity (CDD) scheme is applied to signals transmitted through the respective antennas in the preamble area 110 and the map area 120. The CDD scheme provides a cyclic delay which differs from one antenna to another when one stream is transmitted through a plurality of Tx antennas. A diversity gain is obtained when using the CDD scheme. A signal transmitted using the data burst area 130 includes different data streams in an antenna #0 and an antenna #1. In addition, although not shown, when using four Tx antennas, Tx frames illustrated in FIG. 1 are used for each antenna pair. That is, the four Tx frames are transmitted through the respective antennas. In this situation, the CDD scheme can be used in each antenna pair.
Because the aforementioned MIMO scheme is used to increase a data transfer rate, it is very effective when a traffic amount is great. However, when the traffic amount is small, the MIMO scheme based on multiple antenna transmission may be inefficient. This is because unnecessary power consumption may be caused when signals are transmitted through a plurality of Tx antennas in a situation where a higher data rate is not required in transmission. That is, although a required data rate can be achieved even if the signals are transmitted by using one Tx antenna, the use of a plurality of Tx antennas results in the increase of power consumption caused by an overlapping preamble, MAP, and such, as illustrated in FIG. 1. In this situation, the power consumption increases in proportion to the number of Tx antennas in use. Therefore, when using the MIMO scheme, there is a need for a method for avoiding unnecessary power consumption when a traffic amount decreases.
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