Bruce McNair

A robust timing and frequency offset estimation scheme for orthogonal frequency division multiplexing (OFDM) systems

This paper presents a novel, robust technique to estimate timing and frequency offset in an orthogonal frequency division multiplexing (OFDM) system without the use of pilot signals. We first present an overview of an OFDM system for high-speed wireless data communications, including the particular parameters chosen for our design. We then describe some prior attempts to estimate timing and frequency offset. We finally present our approach and show simulation results in the presence of the variety of severe impairments that are likely to be observed in a typical outdoor PCS environment.

An integrated OFDM receiver for high-speed mobile data communications

In this paper, we present the system design of an integrated receiver for the next generation high-speed mobile data communications based on OFDM. First an overview of the OFDM system suitable for mobile communications is described. The effects of non-ideal transmission conditions of the OFDM system including channel estimation errors, symbol timing offset, carrier and sampling clock offset, phase noise and time-selective fading are analyzed. We then propose an integrated receiver in which all of these issues relevant to the mobile transmission environment are addressed. Novel techniques for symbol timing and frequency synchronization are proposed. The architecture of the receiver and resulting implementation complexity are then analyzed. The overall performance degradation of the proposed receiver is simulated to be within 2-2.5 dB of the ideal receiver in various channel conditions.

Outdoor IEEE 802.11 Cellular Networks: Radio and MAC Design and Their Performance

This paper explores the feasibility of designing an outdoor cellular network based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, which was originally developed for wireless local area networks. Specifically, the performance of the 802.11 radio and medium access control (MAC) protocol in outdoor environments is investigated. For channels typical in cellular networks, we study the radio link power budget and the bit error performance of three kinds of receivers. We also propose a new timing structure for the MAC protocol to handle increased signal propagation delay and analyze its throughput performance in the outdoor network. Our analysis shows that the MAC protocol can handle a cell radius of 6 km without violating the 802.11 standard. However, the link budget reveals that the maximum cell radius in an outdoor 802.11 network ranges from 0.4 to 1.2 km, which is about one third that supported by wideband code-division multiple access and enhanced data rates for global system for mobile communications evolution networks. For a root-mean-square delay spread of 1 mus, which is typical for urban-area cells of this size, our simulation results show that the conventional urban-area cells RAKE receiver can yield a satisfactory performance. Combining these results, we conclude that using ordinary equipment, an 802.11-based cellular network with a cell radius up to 1.2 km is feasible. It is possible to further extend the service range by advanced techniques such as smart antennas.

Comparison of frequency offset estimation techniques for burst OFDM

A major impediment to high-bit-rate wireless transmission is multipath delay spread. Orthogonal frequency division multiplexing (OFDM) addresses this impairment, transmitting several parallel lower bit rate streams. Independent transmit and receive oscillators create frequency offset, generating interchannel interference and degrading performance. Several frequency offset estimation techniques have been proposed but varied test conditions make comparison difficult. Here, we compare these schemes under identical conditions and propose three new offset estimation techniques and include them in our comparison.

Processing of encrypted voice signals

The occurrence of a transmission error which produces a multibit error burst in the decrypted version of a composite speech and synchronization encrypted signal using a one-bit cipher feedback or similar encryption scheme is detected by monitoring the decrypted signal to detect synchronization errors. Upon detection of an error, the decrypted speech output is muted or disabled to avoid the annoying audible click otherwise produced.