Combining scheme of multiple differential filter outputs for direct conversion receiver

In wireless communications for industrial automation, industrial equipments frequently communicate each other through the transmission of a small amount of data. For a random access system, a frame format that multiplexes a data signal to IEEE 802.11 short preamble sequence has been proposed to improve a spectrum efficiency. This frame format enables a receiver to realize timing synchronization, channel estimation, frequency offset compensation, and data transmission simultaneously within a preamble period. In receivers for wireless LAN communication systems, a direct conversion architecture has been widely employed because of its low production cost and low power consumption. However, this architecture suffers from a DC offset mainly caused by the leakage of a local oscillator (LO) signal. In order to eliminate the DC offset, a high-pass filter (HPF) is often inserted before an A/D convertor. When the gain of a low noise amplifier (LNA) changes in the presence of the DC offset, the level of the DC offset varies in the middle of the short preamble signal. With the time-varying DC offset, the received signal suffers from the long transient response of the HPF. In the use of the proposed frame format, the transient response causes severe deterioration on a bit error rate (BER) performance. In the previous research, a differential filter has been applied in a digital domain to eliminate the residual DC offset and estimate a frequency offset. In order to obtain better BER performance with time-varying DC offset elimination, the differential filter outputs with different delay values are summed together. It is shown that the set of differential filters with specific delay values can reduce the BER. The BER performance for two differential filter branches with the delays, d_l = {4, 5}, improves the error floor by a half than that with the single branch only with the delay, d_l = 4, when the number of the receive antennas is 3 on an Indoor Residential A channel model.