Wooncheol Hwang

Approximation of SNR degradation due to carrier frequency offset for OFDM in shadowed multipath channels

We derive an approximate expression for signal-to-noise ratio (SNR) degradation of orthogonal frequency division multiplexing due to carrier frequency offset over a shadowed multipath channel, to explicitly show the effects of system and channel parameters on the degradation of the received SNR. The results show that, for small frequency offset, the SNR degradation is proportional to the square of the frequency offset and the square of the number of subcarriers. It is also shown that, if E/sub s//N/sub 0/ is reasonably large, the SNR degradation becomes insensitive to E/sub s//N/sub 0/, which is contrary to the case of the additive white Gaussian noise channel.

Performance analysis of OFDM on the shadowed multipath channels

Orthogonal frequency division multiplexing (OFDM) is a very popular scheme for signaling at high data rates over the time-dispersive multipath channel. We analyze the symbol error probability of OFDM on the shadowed multipath channels, and present the numerical results using the typical millimeter-wave channel parameter values. The two-ray model is used as a multipath channel model. Two types of OFDM systems are considered: OFDM with and without the guard interval. We show that channel shadowing is a dominant factor that deteriorates the performance of OFDM and the guard interval is not a great contributor to multipath compensation.

OFDM systems with subchannel power control under the two-ray multipath channel

We propose the OFDM system with a subchannel power control, which transmits data with a variable power for each subchannel based on the received SNR. The OFDM system, employing a subchannel power control with a grouping coefficient equal to 3, gives about 3 dB gain in E/sub b//N/sub 0/ compared with the OFDM system without the subchannel power control, for either coherent or noncoherent QPSK modulation under the two-ray multipath channel with the mean value of the second-rays attenuation coefficient equal to 0.3, for the required BER equal to 10/sup -5/.

A multiple guard interval based frequency error detector for OFDM systems

We deal with the carrier frequency synchronization of the orthogonal frequency division multiplexing (OFDM) system and propose a multiple guard interval based frequency error detector (GIB FED) using a set of consecutive guard interval data for OFDM system. We compare the performance of a multiple GIB FED with two conventional GIB FEDs. Under two-ray multipath fading channel, it is shown that the normalized frequency jitter of the proposed multiple GIB FED is about 2.1/spl sim/4.9 times lower than that of GIB-I FED and 1.2/spl sim/2.1 times lower than that of GIB-II FED. The acquisition time of multiple GIB FED is also about 1.8 and 3.2 times faster than those of GIB-I FED and GIB-II FED, respectively, regardless of the channel condition.

Performance analysis of the OFDM system with one tap equalizer against the two-ray multipath channel

Orthogonal frequency division multiplexing (OFDM) is a suitable transmission scheme for high-speed transmissions in a multipath environment. It is well known that a guard interval (GI) reduces the inter-symbol interference (ISI) and inter-channel interference (ICI) and improves the BER performance. In the OFDM system with a GI, the one-tap equalizer compensates the channel effect successfully and improves the BER performance significantly. We analyze the BER performance of OFDM with a one tap equalizer against the two-ray multipath environment, and show some numerical results. The simple one tap equalizer gives a 7 dB gain in E/sub b//N/sub o/ comparing with the no equalizer case when the mean of the second-ray attenuation coefficient is 0.2.

Practical subchannel power controlled OFDM system under time-varying multipath channel

We evaluate the BER performance of the subchannel power controlled OFDM system considering more practical cases including the limitation in word length in subchannel power control tap-weights and the time-varying multipath channel. According to simulation results, the total required number of bits is 5 for the optimal subchannel power control and the main source of the performance degradation of the subchannel power controlled OFDM system is the time-varying multipath channel.

BER performance of the OFDM system with the polynomial cancellation coding under the frequency offset

It is well known that OFDM transmission is weak against the frequency offset, and one remedy is the polynomial cancellation coding proposed by Armstrong et al. (1998). We derive the BER performance of the OFDM system with polynomial cancellation coding. At the required BER=10/sup -5/, it is shown the coding gain of the polynomial cancellation coding is about 8 dB comparing with the conventional OFDM system under the AWGN channel with the normalized frequency offset =0.1.

Effects of correlated shadowing and multipath fading on OFDM systems with postdetection-combining-based macrodiversity

The performance of orthogonal frequency division multiplexing (OFDM) using postdetection-combining-based macrodiversity is analyzed on a shadowed multipath channel, and the effects of correlated shadowing are investigated. We show that the postdetection-combining-based macrodiversity provides larger performance gain than the selection-based one, especially for the channel characterized by random multipath parameters. It is also shown that the gain obtained by using the postdetection-combining-based macrodiversity is reduced more and more as the correlation among the macrodiversity ports increases. Even for the fully correlated shadowing situation, however, the postdetection-combining-based macrodiversity still provides a considerable gain compared with no diversity case.

An improved frequency synchronization scheme using a modified OFDM burst format for wireless LAN systems

This paper deals with the synchronization scheme for wireless LAN systems using OFDM. We propose an improved frequency synchronization scheme which uses a modified burst format and a dual window to find the frame timing and frequency offset. We compare the performance of the proposed scheme with that of a conventional one under AWGN and frequency selective fading channel. Computer simulation results show that the proposed method requires about 3 dB lower Es/N0 than the conventional one to compensate the residual frequency offset under frequency selective fading channel. The proposed method gives lower failure probability of the correct integer frequency offset under frequency selective fading channel.

Subchannel power controlled COFDM system for the wireless-LAN

We propose a subchannel power controlled COFDM system for wireless-LAN to support multimedia services. The proposed system gives about 3.1 dB gain in E/sub b//N/sub 0/ to support the required packet error rate, 10/sup -2/, compared with the conventional COFDM system under the multipath channel.