This paper discusses the effects of frequency offset on the performance of orthogonal frequency division multiplexing (OFDM) digital communications. The main problem with frequency offset is that it introduces interference among the multiplicity of carriers in the OFDM signal. It is shown, and confirmed by simulation, that to maintain signal-to-interference ratios of 20 dB or greater for the OFDM carriers, offset is limited to 4% or less of the intercarrier spacing. Next, the paper describes a technique to estimate frequency offset using a repeated data symbol. A maximum likelihood estimation (MLE) algorithm is derived and its performance computed and compared with simulation results. Since the intercarrier interference energy and signal energy both contribute coherently to the estimate, the algorithm generates extremely accurate estimates even when the offset is far too great to demodulate the data values. Also, the estimation error depends only on total symbol energy so it is insensitive to channel spreading and frequency selective fading. A strategy is described for initial acquisition in the event of uncertainty in the initial offset that exceeds 1/2 the carrier spacing, the limit of the MLE algorithm. >

A technique for orthogonal frequency division multiplexing frequency offset correction

This article attempts to identify different issues and to put many of the activities in wireless into a framework that can provide perspective on what is driving them, and perhaps even yield some indication of where they appear to be going in the future The technologies and systems that are currently providing, or are proposed to provide, wireless communications services can be grouped into about seven relatively distinct groups All of the technologies and systems are evolving as technology advances and perceived needs change Some trends are becoming evident in the evolutions The different groups and evolutionary trends are explored along with factors that influence the characteristics of members of the groups The grouping is generally with respect to scale of mobility and communications applications or modes Different design compromises are evident in the different technologies and systems The evidence suggests that the evolutionary trajectories are aimed toward at least three large groups of applications or services, namely, high-tier PCS (current cellular radio), high-speed wireless local-area networks (WLANS), and low-tier PCS (an evolution from several of the current groups) It is not clear to what extent several groups, eg, cordless telephones, paging, and wide area data, will remain after some merging with the three large groups >

Wireless personal communications: what is it?

An explanation of the widely different technological approaches and technologies being proposed for different interpretations of wireless personal communications services (PCS) is presented. Different wireless communications applications and issues surrounding the applications are discussed. Descriptions of the compromises that have been made in developing specific technologies optimized to serve the cordless telephone and the vehicular mobile radio applications are presented. Characteristics of a technology that is aimed at the widespread pedestrian PCS application and environment are outlined. Different wireless technologies that have been aimed at different applications and environments are compared. >

Wireless network access for personal communications

The authors examine microcell radio systems that use subcarrier transmission and spectrum delivery switch (SDS) techniques to meet traffic demands for a large number of portable phones. Radio signals are transmitted over fibre between base stations (BS) and a central station (CS). Relationships between blocking probability and offered traffic are described as a parameter of SDS ports. Optical link performances, such as link gain and carrier-to-noise ratio (CNR), are presented. Moreover, it is found that using FM techniques can improve the dynamic range by 20 dB compared to conventional methods for the uplink from BS to CS. The proposed technique can also increase the number of transmittable carriers compared to conventional methods for the downlink from CS to BS. This network strategy results in more flexible transmission. >

Fiber-optic microcell radio systems with a spectrum delivery scheme

Contemporary wireless communications are based on digital communications technologies. The recent commercial success of mobile cellular communications has been enabled in part by successful designs of digital signal processors with appropriate on-chip memories and specialized accelerators for digital transceiver operations. This article provides an overview of fixed point digital signal processors and ways in which they are used in cellular communications. Directions for future wireless-focused DSP technology developments are discussed.

Digital signal processors in cellular radio communications

The recent growth in the use of digital radio is reviewed. The technology used to implement low-power digital radio in the local exchange loop plant is discussed. The integration of digital radio subscriber loops withnetwork intelligence is explored. The relationship between low-power digital radio loops and broadband fiber loops is briefly examined, and standards and frequency allocation activities are summarized. >

W.S. Gifford

Papers

Low-power digital radio as a ubiquitous subscriber loop