Transmit diversity schemes enable increased reliability in the transmission of data through the use of multiple antennas at the transmitter. In a conventional transmit diversity scheme, space time block coding (STBC) and/or space frequency block coding (SFBC) are often used. One of the simplest and most commonly used STBC codes is known as the Alamouti transmission scheme. In the Alamouti scheme, data is sent in groups of two time slots, where the symbols [s0, −s1*] are sent in the first time slot from a two antenna transmitter (i.e., the first symbol s0 is sent through the first antenna, and the second symbol −s1* is sent through second antenna), and the same symbols with a certain phase shift and scaling [s1, s0*] are sent through the respective antennas in the second time slot. At the end of each second slot, the receiver can use a linear combination of the signals received during first and second time slots to decode s0 and s1 with a lower error probability as compared with the single-input-single-output (SISO) case.
One of the requirements of the Alamouti transmission scheme is that the channel conditions must be constant, or as close to constant as possible, during first and second time slots. In systems such as, for example, the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE), in which several modulation symbols may be multiplexed in frequency for transmission, the Alamouti scheme can be used by exploiting the frequency domain instead of the time domain (i.e., the scheme becomes an Alamouti SFBC scheme). In the Alamouti SFBC scheme, the pairs of data symbols [s0, s1] and [−s1*, s0*] are transmitted in two frequency subcarriers instead of in two time slots. In order to comply with the requirement that the channel conditions be as close to constant as possible, the frequency subcarriers are typically selected to be adjacent to one another in frequency.
In one exemplary SFBC transmission scheme, a 10-ms frame consists of 10 subframes, each having a length of 1 ms. Each subframe consists of two slots, each having a length of 0.5 ms. Each slot is configured for communication of seven orthogonal frequency division multiplexed (OFDM) symbols. Each OFDM symbol is communicated on 12 consecutive subcarrier frequencies. The 12 consecutive subcarrier frequencies are referred to as a resource block (RB). Each individual subcarrier within a single OFDM symbol is referred to as a resource element (RE).
When using a SFBC transmission scheme in a system such as 3GPP LTE, an inability to identify two consecutive subcarrier frequencies for the transmission of the pair of data symbols may occur. For example, in some instances, a reference symbol or a muted reference symbol may be designated to be transmitted in a specific subcarrier frequency band.
A new type of reference symbol, known as channel state information reference symbol (CSI-RS) has been introduced in 3GPP LTE Release 10. In some cases, the presence of CSI-RS can lead to SFBC blocks being allocated to non-contiguous subcarriers if the current specifications are followed. Moreover, muting of CSI-RS patterns is also introduced in 3GPP LTE Release 10, which also leads to situations where frequency gaps inside SFBC codes appear when following current specifications. Accordingly, there is a need to address the allocation of SFBC blocks to non-contiguous subcarriers as described above.
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