1. Field of the Invention
The present invention relates generally to the field of radio communication receivers. More particularly, the present invention relates to the field of reducing residual phase error (RPE) of orthogonal frequency division multiplexing (OFDM) signals in OFDM communication receivers.
2. Description of the Related Art
An increasing need for broadband mobile/fixed wireless communications services has motivated researchers and radio engineers to search for a variety of feasible radio airlink interface techniques for such wireless communication systems. An airlink interface technique based on the Orthogonal Frequency Division Multiplexing (OFDM) modulation is considered an attractive candidate for a broadband wireless transmission system due to its spectral efficiency, its robustness in different multipath propagation environments and its ability to combat intersymbol interference (ISI).
OFDM is a multi-carrier modulation method. It divides an entire frequency band into many, say N, subchannels or frequency tones, and each subchannel is modulated with a constellation symbol to be transmitted. In its application as a multiple access method for a point-to-multipoint wireless communication system, OFDM arranges these total N subchannels as follows. M adjacent subchannels are grouped together (where M<<N) without overlapping between adjacent groups. Each mobile user is assigned a cluster of M subchannels when it needs to transmit information data between its serving base station and its terminal. In each cluster of M subchannels assigned to an individual user, one or more subchannels may be used to transmit pilot signals and are called “pilot subchannels.” The rest of the subchannels bear information data and are called “information subchannels.” For an available bandwidth of B MHz, there are total of N subchannels with a frequency space of B/N MHz; this band can simultaneously support N/M users.
An OFDM-based wireless system, however, is very sensitive to channel phase errors and the phase noise of the receiver local oscillator (LO). Therefore, an effective fading channel estimation and the transform domain channel compensation become necessary to restore OFDM signal orthogonality, to correct phase error, and to conduct coherent demodulation in the receiver.
Pilot symbol-aided approaches are widely used to estimate the fading channel properties which corrupt the transmitted signal. In an OFDM/TDMA system, an OFDM data block is the block of M constellation symbols to be transmitted within a TDMA time slot. When the transmission channel or phase noise changes significantly from one OFDM data block to the next, channel estimation and transform domain channel compensation must be performed in each individual data block with the pilot symbols inserted in the given data block. For example, the interval between two OFDM data blocks may be on the order of 3-5 milliseconds. In such a relatively long time period, the phase noise effect of the receiver LO may change significantly.
Co-channel interference due to any frequency reuse pattern, multipath fading, and additive white Gaussian noise (AWGN) are primary constraints to an acceptable performance in a cellular/wireless communication system. In addition, intercarrier interference (ICI) due to channel variation and phase noise always exists in an OFDM-based wireless system. Thus, the pilot symbols as well as the information symbols are corrupted by co-channel interference, intercarrier-interference, noise, and other channel impairments. All of these impairments in the received pilot signals significantly affect the accuracy of the channel estimation. A residual phase error (RPE) is the phase error that remains after the received constellation symbols are compensated based on an inaccurate channel estimation.
With conventional techniques, the accuracy of the channel estimation may be improved by either increasing the number of pilot signals and/or increasing their transmission power. On one hand, using a larger number of pilot symbols results in a higher transmission overhead and hence a lower system capacity. On the other hand, a larger transmission power for pilot sub-channels results in larger ICIs for information-bearing tones and hence causes implementation difficulties. Accordingly, there is an existing need to reduce the residual phase error in OFDM communication signals without the deficiencies of the prior art.