Characterization of Wireless Feedback Channels near a Highway Based on Sounding Measurements

This paper presents the method of measuring the feedback channel, which is developed between the transmit and receive antennas of a wireless repeater by receiving the transmit singal from its tranmit antenna at the receive antenna of the identical repeater, and experiment results otained by anlyzing the measurements. This experiment uses 2GHz W-CDMA signal and is carried out near a highway. The high-speed mobiles on highways cause feedback signals with high Doppler frequencies and large energy. In order to characterize the feedback channel, the power delay profile and the scattering function are estimated to identify the delay spread, the Doppler spread, the number of fingers, and the attenuation with delay. Since the feedback channel is constructed between the fixed TX and RX antennas, which is dependent upon the multipaths developed due to moving or fixed objects around the antennas, the channel has different properties comparing to the conventional channel between the base station and the mobile station. Therefore, the results presented in the paper will provide guidelines for designing and evaluating the wireless repeater systems. 2. Measurement Setup Recently, the wireless communications have been developed with high-speed and large-capacity transmissions. For this reason, the cell sizes become smaller and many repeaters are required to improve service quality. A cochannel repeater is very efficient solution to this problem. Comparing to the existing repeater using optical cables, the co-channel repeater cost less expensive in maintenance and its installation and maintenance is easy. The co-channel repeater usually has feedback channels, through which the transmitted signal from the transmit antenna is received at its receive antenna. The feedback channels consist of the direct path between the transmitter (TX) and receiver (RX) antennas and multi-paths due to surrounding various objects. The signal from this feedback channel operates as interference to degrade the performance of the co-channel repeater. In general, the co-channel repeater uses the finite impulse response (FIR) filter to remove or mitigate the feedback interference from the TX antenna to the RX antenna and employs a channel estimation technique. For the purpose of designing a interference canceller and furthermore constructing efficient communication system, we need to measure and analyze the propagation properties of the feedback channels including the number of the multi-paths, attenuation according to the delays, Doppler shift, delay spread and so on. The existing researches on mobile channels mainly focused on the channels between a base station and a user equipment while the study on the feedback channels is scarce. The sounding measurements were carried out at a side of the highway at Namhae, Kyoungnam, Korea, where fast frequency-selective fading due to high-speed vehicles has been observed, because co-channel repeaters malfunctioned due to high interference and fast frequency selective fading. Therefore, the purpose of this paper is to characterize the feedback interference channel that is built up between the TX and RX antennas of the co-channel repeater. This paper is organized as follows. Section 2 describes the measurement setups and approaches for probing feedback channels. Section 3 provides the channel analysis results from the measurement data. Finally, section 4 concludes this paper. 2. Measurement Setup The channel considered in this paper represents the multipath, through which the signal generated by a signal generator is transmited from the TX antenna and received by the RX antenna. In order to measure the feedback channel, we use a W-CDMA signal generator to generate a W-CDMA signal, which has 2 GHz carrier frequency, 3.84 Mcps chip rate, 5 MHz bandwidth, and 10 ms period for repeatition. Since this period is longer than the maximum delay spread of the feedback channel, it is immaterial to the sounding experiment [1]-[4]. The 23rd International Technical Conference on Circuits/Systems, Computers and Communications (ITC-CSCC 2008)