Orthogonal Frequency Division Multiplexing (OFDM) is a well-known multicarrier modulation method that is used in several wireless system standards. Some of the systems using OFDM include 5 GHz high data rate wireless LANs (IEEE802.11a, HiperLan2, MMAC), digital audio and digital video broadcast in Europe (DAB and DVB-T, respectively), and broadband fixed wireless systems such as IEEE802.16a. An OFDM system divides the available bandwidth into very many narrow frequency bands (subcarriers), with data being transmitted in parallel on the subcarriers. Each subcarrier utilizes a different portion of the occupied frequency band.
Spreading can also be applied to the data in an OFDM system to provide various forms of multicarrier spread spectrum. Such spread-OFDM systems are generally referred to as either Spread OFDM (SOFDM), multicarrier CDMA (MC-CDMA), or Orthogonal Frequency Code Division Multiplexing (OFCDM). For systems employing MC-CDMA, spreading is applied in the frequency dimension and multiple signals (users) can occupy the same set of subcarriers by using different spreading codes. For OFCDM, different users are assigned different mutually orthogonal spreading codes, and the spread signals are combined prior to transmission on the downlink. Spreading can be applied in the frequency dimension, or the time dimension, or a combination of time and frequency spreading can be used. In any case, orthogonal codes such as Walsh codes are used for the spreading function, and multiple data symbols can be code multiplexed onto different Walsh codes (i.e., multi-code transmission).
For an OFCDM system with a spreading factor of SF in the time dimension, in which each symbol is represented by SF chips, up to SF Walsh codes can be active on each subcarrier. For channel estimation, one of these Walsh codes can be assigned as a pilot signal (i.e., in the same way that a pilot signal is created in conventional single-carrier CDMA systems such as IS-95). In order to estimate more than one channel (such as measuring the channels from two transmit antennas), additional Walsh channels can be assigned as pilot channels. However, note that assigning a second Walsh channel as a pilot doubles the pilot overhead of the system, leading to a reduction in the number of Walsh codes available for data transmission. This additional overhead is very significant in systems with a small spreading factor and/or a large number of transmit antennas. Therefore, a need exists for a method and apparatus for transmitting and receiving data from multiple antennas within an OFDM system that eliminates the need for multiple spreading codes being used for pilot channels emanating from multiple antennas.
USPTO Backgrounds
USPTO Backgrounds