A multiple-input multiple-output (MIMO) technology is one of key technologies of the 802.11n standard protocol. Currently, to further increase a system throughput, the Institute of Electrical and Electronic Engineers (Institute of Electrical and Electronic Engineers, IEEE) is designing the 802.11ac standard. In the 802.11ac standard, a downlink (Multi User Multiple Input Multiple Output, MU-MIMO) technology is introduced, to resolve a problem of asymmetry between quantities of antennas on two sides of a link, and to use a degree of freedom in MIMO more effectively.
To further increase a throughput of a wireless local area network, introduction of an uplink MU-MIMO technology into a wireless local area network (for example, WIFI) system may be considered in the future. The introduction of the uplink MU-MIMO technology is to resolve the problem of asymmetry between quantities of antennas on two sides of a link. In the uplink MU-MIMO technology, multiple stations can simultaneously transmit data to an access point (Access Point, AP). In this case, time synchronization and frequency synchronization need to be ensured for users that simultaneously transmit data. For an orthogonal frequency division multiplexing (OFDM) system, a system bandwidth may be divided into some subcarriers. In this case, frequency synchronization is particularly important. Frequency synchronization can ensure that the subcarriers are orthogonal to each other, so that pieces of data transmitted on the subcarriers do not interfere with each other.
However, stations that simultaneously transmit data are generally not synchronous in frequencies, and the stations and the AP are not synchronous in frequencies. Therefore, when decoding data that is sent on a subcarrier by a station, the AP receives interference imposed both by data that is sent on another subcarrier by the station and by data that is sent on subcarriers by another station. In this case, if a frequency offset between each station and the AP is not estimated and compensated, a bit error rate of the system increases greatly. In another aspect, to correctly decode the data, the AP needs to perform channel estimation by using a long training sequence. However, if the stations are not synchronous in the frequency offsets, a possibility of erroneous channel estimation increases, which causes an increasing probability of errors during subsequent data decoding. Therefore, one of key problems that need to be overcome in uplink MU-MIMO is how to estimate and compensate the frequency offset between each station and the AP. However, currently, in the uplink MU-MIMO technology, there is no method for determining a frequency offset between each station and an AP.