With the advent of personal wireless communication devices, there is a need for small portable units that have low power dissipation, and can be provided at relatively low cost. To meet these requirements, there is interest in a transceiver with direct conversion architecture (DCA). Different from conventional heterodyne receivers, the receiver of a DCA recovers base band information directly from the carrier signal without passing through an intermediate frequency stage and image signal rejection. This is accomplished by translating the radio frequency signal directly to zero frequency, and then employing a low pass filter to suppress any high frequency interference. The DCA receiver has several advantages over the heterodyne receiver, and meet the requirements of small form factor, low power dissipation, and low cost. However, a disadvantage is the down converted base band extends to zero frequency, and extraneous offset voltages, referred to as direct current (DC) offset, can corrupt the base band signal, and saturate subsequent base band processing stages. In addition, a DCA receiver also suffers I/Q mismatch, a result of mismatches between amplitude of the I and Q signal during quadrature down conversion. There are also concerns with even-order distortion, flicker noise and local oscillator leakage that adversely affect the performance of a DCA receiver.
Orthogonal frequency division multiplexing (OFDM) is a bandwidth efficient modulation technique that is used for high-speed wireless data transmissions. OFDM transmits data on densely packed orthogonal sub-carriers that are spaced in frequency exactly at the reciprocal of symbol interval. A primary advantage of OFDM is it is utilized for transmission at relatively low complexity in multipath channels. A disadvantage of OFDM systems is its sensitivity to frequency offset. Frequency offset can cause a reduction of signal amplitude at the output of OFDM demodulator, and also introduces inter-carrier interference (ICI) from the other sub-carriers, thus destroying the orthogonality of OFDM.
Combining the DCA and OFDM provides a transceiver that has a small form factor, has low power dissipation, is low cost and is bandwidth efficient. However, the DC offset and the frequency offset must be addressed. Typically, the approach that is taken is to estimate the DC and frequency offsets and then apply compensation based on the estimated offsets. In data transmission, offset estimation is typically performed during the reception of the preamble of each data packet, because in a packet switched system each data packet can be treated independently.
With reference to FIG. 1 a preamble 100 is shown with a time line t, where the preamble reception starts at t0, and ends at t3. During a first time interval t0 to t1 a first portion 105 of the preamble is used for DC offset estimation; during the second time interval t1 to t2 a second portion 110 of the preamble is used for DC offset compensation; and during a third time interval t2 to t3 a third portion 115 is used for frequency offset estimation. Here we assume that the receiver amplifier is not saturated by the presence of the DC offset. This conventional method sequentially estimates the DC offset, compensates for the DC offset, and then estimates the frequency offset. Of course it assumes that the DC offset estimation method is based on the initial reception of the first portions 105 of the preamble 100, and that the information from the first portion 105 is available.
Due to the limited time available to process the preamble 100, t0 to t1 interval for DC offset estimation, and t1 to t2 interval for DC offset compensation, one disadvantage is that residual DC offset may still be present when the frequency offset is later estimated in the time interval t2 to t3. Consequently, the residual DC may result in inaccurate frequency offset estimation. In addition, since part of the reception time of the preamble 100, interval t0 to t2, is used for DC offset estimation and compensation, the available time for frequency offset estimation, the interval t2 to t3, is reduced. Hence, a further disadvantage of this method is the short time during which frequency offset estimation is performed, and the consequent inaccuracy of the frequency offset estimation.