As Internet usage has increased, so has the need for fast, efficient transmission of Internet data. While there exists a plethora of data transmitting equipment, ranging from analog modems that transmit at 56 kilobits per second (kps) to Cable Modems that transmit at 10-30 megabits per second (mps), conventional systems have their drawbacks. For example, analog modems sacrifice speed for easy installation in a computer and use with an existing phone line. Cable Modems require special cables from a base station before they can be used. Some systems, like Cable Modems, have a further drawback that they are a shared transmission medium so that the rate of transmission decreases with the number of simultaneous users, e.g. neighbors who may also be on-line. Additionally, most conventional systems require a physical transmission line from the customer premise equipment (CPE) to a base station, be it a traditional telephone line or dedicated data line. It is therefore desirable to transmit data without requiring a physical line. However, present wireless devices, such as cellular telephones and pagers have a very limited ability to transmit and receive data.
To increase the capacity of wireless data transmission, Orthogonal Frequency Division Multiplexing (OFDM) may be used. OFDM is a modulation method, which encodes data onto a radio frequency (RF) signal. While conventional RF transmission schemes encode data symbols onto one radio frequency, OFDM encodes data symbols onto multiple frequencies or “sub-carriers.” The high-speed data signal is divided into tens or hundreds of lower speed signals, dividing the data across the available spectrum into a set of sub-carriers. To prevent interference between the sub-carriers, each sub-carrier is orthogonal (independent or unrelated) to all the other sub-carriers, so that guard intervals are not needed around each sub-carrier but, rather, are needed only around a set of sub-carriers (at the edge of the occupied frequency band). Thus, OFDM systems are spectrally efficient and are much less susceptible to data loss due to multipath fading than conventional systems.
An alternative solution is a Multiple Input Multiple Output (MIMO) system which utilizes multiple independent transmitting antennas to communicate with multiple independent receiving antennas. MIMO systems can be used to either increase signal power or increase the data rate transmission. In one configuration, a MIMO system may operate in temporal diversity, i.e. where each transmitting antenna sends a data signal that is correlated to the data signal sent by another antenna. By combining the received signals, a stronger signal can be obtained. In another configuration, the MIMO system operates in spatial diversity, where each transmitting antenna sends a data signal that is independent to the data signal sent by another antenna. For every additional transmitting antenna that is used, the data rate increases proportionality, i.e. using two antennas doubles the data rate, using three antennas triples the data rate, etc.
Each of these methods has previously only been implemented independently. Therefore, designers of data transmission systems had to choose between the benefits of each system and determine which type of modulation method to implement. It would therefore be beneficial to provide the benefits of both OFDM and MIMO in a single system to provide wireless transmission of data.
Such a MIMO OFDM system would probably employ a robust time synchronization method in order to synchronize the multiple OFDM signals received at the multiple receiving stations. Since OFDM has thus far only been utilized in single-input/single-output (SISO) systems, current OFDM synchronization methods are of little value. It would therefore be beneficial to provide a robust time synchronization method for a MIMO OFDM system capable of synchronizing the multiple OFDM signals.