A communication system, such as radiotelephone system, generally includes a transmitter communicating with a receiver over a communication channel. Characteristics of well-designed communication systems include being inexpensive, simple, small, easy to manufacture, and power efficient, while meeting performance specifications and customer expectations.
Communication systems may communicate signals using single or multiple carrier modulation, depending on the particular application. One type of multiple carrier modulation (MCM) is orthogonal frequency division multiplex (OFDM). OFDM is a modulation method that transmits a data stream in parallel using many orthogonal sub-carriers to transmit a signal. An inverse Fourier transform can be used to efficiently split the transmitted signal into the many sub-carriers, and a Fourier transform can be used to demodulate the received signal to recover the data stream. OFDM is more robust than the single carrier modulation because it protects the signal against inter-symbol interference, delay spread, frequency selective fading, and narrowband interference. Each sub-carrier is represented by complex in-phase (I) and quadrature-phase (Q) signals.
I/Q distortion of OFDM signals is caused by distortion on the communication channel, and in each of the modulator and the demodulator, by non-ideal quadrature conversion of the OFDM signals, by non-identical amplitude and phase responses in the I and Q branches, and non-linear phase characteristics in the low pass filters in each of the I and Q branches. The distortion on the communication channel is caused by factors such as inter-symbol interference, delay spread, frequency selective fading, narrowband interference, co-channel interference, and adjacent channel interference. Non-ideal quadrature conversion includes both non-ideal quadrature up-conversion in the quadrature modulator in the transmitter and/or non-ideal quadrature down-conversion in the quadrature demodulator in the receiver. The non-identical amplitude and phase responses in the I and Q paths in the modulator and the demodulator, and the non-linear phase characteristics in the low pass filters in each of the I and Q branches in the modulator and the demodulator may be caused by inaccuracies in analog components used in the modulator and the demodulator.
Traditionally, a receiver has a two-stage front-end circuit that receives an analog radio frequency (RF) signal. At the first stage, the front-end circuit converts the analog RF signal to an analog intermediate frequency (IF) signal having a lower frequency. At the second stage, the front-end circuit converts the analog IF signal to an analog baseband signal having the lowest frequency. The second stage is coupled to inexpensive analog to digital (AD) converters that convert the analog baseband signal to a digital baseband signal for subsequent digital baseband processing.
A direct conversion receiver, otherwise known as a direct down-converter or zero-intermediate frequency (IF) receiver, eliminates the IF conversion circuitry. In this case, the direct conversion receiver receives the analog RF signal, and then converts the analog RF signal directly to the analog baseband signal having the lowest frequency. Advantages of the direct conversion receiver include having fewer parts, which minimizes its cost, and having low current drain.
However, a problem with the direct conversion receiver is that the analog baseband signal is distorted by amplitude and phase imbalances in each of the I and Q branches in the demodulator and by the non-linear phase characteristics in the low pass filters in each of the I and Q branches in the demodulator. Spectral leakage is one type of I/Q distortion, wherein the positive frequency components of the base-band OFDM signal interfere with the negative frequency components of the base-band OFDM signal and vice versa. Attempts have been made to improve the performance of the direct conversion receiver by using components with tighter tolerances, by calibrating or tuning the components or circuitry in the factory before shipping the direct conversion receiver, by using phase equalizers to compensate the low pass filters, and by using frequency domain equalizers. However, components with tighter tolerances are more expensive. Factory calibration or tuning is time consuming. Phase equalizers increase cost, are not stable over time and temperature, and not satisfactory for high, more complex, levels of signal modulation. Frequency domain equalizers use adaptation algorithms that are slow to converge over the received signal. Analogous advantages and disadvantages also apply to an analog quadrature modulator for use in a direct conversion transmitter.
Accordingly, there is a need for I/Q distortion compensation for the reception of OFDM signals, and particularly for OFDM signals received by direct conversion receivers.