In recent times, with the increasing development of information communication technologies, a variety of multimedia services, and a variety of high-quality services have been developed and introduced to the market, so that demands of wireless communication services are rapidly increasing throughout the world. In order to actively cope with the increasing demands, capacity of a communication system must be increased.
A variety of methods for increasing communication capacity under wireless communication have been considered, for example, a method for searching for a new available frequency band in all frequency bands, and a method for increasing efficiency of limited resources. As representative examples of the latter method, a transceiver includes a plurality of antennas to guarantee an additional space utilizing resources so that a diversity gain is acquired, or MIMO communication technologies for increasing transmission capacity by transmitting data via individual antennas in parallel have been developed by many companies or developers.
Particularly, a Multiple-Input Multiple-Output (MIMO) system based on an Orthogonal Frequency Division Multiplexing (OFDM) from among the MIMO communication technologies will hereinafter be described with reference to FIG. 1.
FIG. 1 is a block diagram illustrating an OFDM system equipped with multiple transmission/reception (Tx/Rx) antennas.
Referring to FIG. 1, in a transmission end, a channel encoder 101 attaches a redundant bit to a Tx data bit to reduce a negative influence of a channel or noise. A mapper 103 converts data bit information into data symbol information. A serial-to-parallel (S/P) converter 105 converts the data symbol into a parallel data symbol so that the parallel data symbol can be loaded on several sub-carriers. A MIMO encoder 107 converts the parallel data symbol into space-time signals.
In a reception end, a MIMO decoder 109, a parallel-to-serial (P/S) converter 111, a demapper 113, and a channel decoder 115 have functions opposite to those of the MIMO encoder 107, the S/P converter 105, the mapper 103, and the channel encoder 101 in the transmission end.
Various techniques are required for a MIMO-OFDM system to enhance data transmission reliability. As a scheme for increasing a spatial diversity gain, there is space-time code (STC), cyclic delay diversity (CDD) or the like. As a scheme for increasing a signal to noise ratio (SNR), there is beamforming (BF), precoding or the like. In this case, the space-time code or the cyclic delay diversity scheme is normally employed to provide robustness for an open-loop system in which feedback information is not available at the transmitting end due to fast time update of the channel. In other hand, the beamforming or the precoding is normally employed in a closed-loop system in order to maximize a signal to noise ratio by using feedback information which includes a spatial channel property.
As a scheme for increasing a spatial diversity gain and a scheme for increasing a signal to noise ratio among the above-mentioned schemes, cyclic delay diversity and precoding are explained in detail as follows.
When a system equipped with multiple Tx antennas transmits OFDM signals, the CDD scheme allows the antennas to transmit the OFDM signals having different delays or amplitudes, so that a reception end can acquire a frequency diversity gain.
FIG. 2 is a block diagram illustrating a transmission end of a MIMO system based on the CDD scheme.
Referring to FIG. 2, an OFDM symbol is distributed to individual antennas via the S/P converter and the MIMO encoder, a Cyclic Prefix (CP) for preventing an interference between channels is attached to the OFDM symbol, and then the resultant OFDM symbol with the CP is transmitted to a reception end. In this case, a data sequence transmitted to a first antenna is applied to the reception end without any change, and the other data sequence transmitted to a second antenna is cyclic-delayed by a predetermined number of samples as compared to the first antenna, so that the cyclic-delayed data sequence is transmitted to the second antenna.
In the meantime, if the CDD scheme is implemented in a frequency domain, a cyclic delay may be denoted by a product (or multiplication) of phase sequences. A detailed description thereof will hereinafter be described with reference to FIG. 3.
FIG. 3 is a block diagram illustrating a transmission end of a MIMO system based on a conventional phase shift diversity (PSD) scheme.
Referring to FIG. 3, different phase sequences (Phase Sequence 1˜Phase Sequence M) of individual antennas are multiplied by individual data sequences in a frequency domain, an Inverse Fast Fourier Transform (IFFT) is performed on the multiplied result, and the IFFT-multiplied data is transmitted to a reception end. The above-mentioned method of FIG. 3 is called a phase shift diversity scheme.
In the case of using the phase shift diversity scheme, a flat fading channel may be changed to a frequency-selective channel, a frequency diversity gain may be acquired by a channel encoding process, or a multi-user diversity gain may be acquired by a frequency-selective scheduling process.
In the meantime, if a closed-loop system includes finite feedback information, two precoding schemes may be used, i.e., a codebook-based precoding scheme and a scheme for quantizing channel information and feeding back the quantized channel information. The codebook-based precoding scheme feeds back an index of a precoding matrix, which has been recognized by transmission/reception ends, to the transmission/reception ends, so that it can acquire a SNR gain.
FIG. 4 is a block diagram illustrating the transmission/reception ends of a MIMO system based on the codebook-based precoding.
Referring to FIG. 4, each of the transmission/reception ends has a finite precoding matrix (P1˜PL). The reception end feeds back an optimum precoding matrix index (l) to the transmission end using channel information, and the transmission end applies a precoding matrix corresponding to the feedback index to transmission data (χ1˜χMt). For reference, the following Table 1 shows an exemplary codebook used when feedback information of 3 bits is used in an IEEE 802.16e system equipped with two Tx antennas to support a spatial multiplex rate of 2.
TABLE 1Matrix Index(binary)Column 1Column 20001   0   0   1   0010.7940−0.5801 − j0.1818−0.5801 + j0.1818−0.79400100.79400.0579 − j0.6051  0.0579 + j0.6051−0.79400110.7941−0.2978 + j0.5298−0.2978 − j0.5298−0.79411000.7941  0.6038 − j0.0689  0.6038 + j0.0689−0.79411010.3289  0.6614 − j0.6740  0.6614 + j0.6740−0.32891100.5112  0.4754 + j0.7160  0.4754 − j0.7160−0.51121110.3289−0.8779 + j0.3481−0.8779 − j0.3481−0.3289
The above-mentioned phase-shift diversity scheme can acquire a frequency-selective diversity gain in an open loop, and can acquire a frequency scheduling gain in a closed loop. Due to these advantages of the phase-shift diversity scheme, many developers are conducting intensive research into the phase-shift diversity scheme. However, the phase-shift diversity scheme has the spatial multiplexing rate of 1, so that it cannot acquire a high transfer rate. And, if a resource allocation is fixed, the phase-shift diversity scheme has difficulty in acquiring the frequency-selective diversity gain and the frequency scheduling gain.
The codebook-based precoding scheme can use a high spatial multiplexing rate simultaneously while requiring a small amount of feedback information (i.e., index information), so that it can effectively transmit data. However, since it must guarantee a stable channel for the feedback information, it is inappropriate for a mobile environment having an abruptly-changed channel and can be available for only a closed-loop system.