Charles L. Despins

Radio-channel characterization of an underground mine at 2.4 GHz

This paper presents comprehensive experimental results obtained from narrowband and wideband radio-channel measurements in an underground mine with narrow veins at 2.4 GHz. From continuous-wave (CW) measurement data, large-scale distance-power curves and path-loss exponents of the environment are determined. Other relevant parameters, such as the mean excess delay, the maximum excess delay, the root-mean-square (rms) delay spread, and the coherence bandwidth are extracted from the wideband-measurement data. Results show a propagation behavior that is specific for these underground environments with rough surfaces. The rms delay spread does not follow a dual-slope relation with respect to distance, as in environments with smooth surfaces. Moreover, the dependence of the rms delay spread on the bidimensional position of the user is found to be very significant. For the majority of locations, the rms delay-spread values are less than 60 ns.

Radio Wave Characterization and Modeling in Underground Mine Tunnels

Results are presented on wideband radio propagation measurements and statistical modeling at 2.4 GHz and 5.8 GHz in real underground mine tunnels. This peculiar type of confined environment is characterized by very rough surfaces and a frequent absence of a line-of-sight between transmitting and receiving antennas. The resulting propagation characteristics differ from those frequently encountered in more typical indoor environments such as offices and corridors. Indeed, the rms delay spread shows little or no correlation with respect to transmitter-receiver distance and, in addition, no impulse response path arrival clustering effect is observed. However, the path amplitude distribution does tend to follow a Rice distribution in the line-of-sight case, and a Rayleigh distribution otherwise.

Compound strategies of coding, equalization, and space diversity for wide-band TDMA indoor wireless channels

Compound strategies of equalization and space diversity in the form of an optimum baseband combiner are attractive for wideband time division multiple access (TDMA) portable communication radio links in order to combat dispersive fading and cochannel interference. The authors investigate the performance of such a scheme in conjunction with convolutional coding and soft-decision Viterbi decoding via a semianalytical technique based on the method of moments. Such an approach avoids a Gaussian characterization of interference and yields results for both ideal interleaving and no interleaving. With dual space diversity, three taps per forward filter, and a data rate of 10 Mb/s, it is shown that, although a third space diversity branch remains preferable in terms of performance, channel coding can be a viable alternative, particularly in terms of outage rate, to increasing the space diversity order, even in the absence of interleaving, provided the signal-to-interference ratio is sufficiently high. >

Medium access control in distributed cognitive radio networks

Since cognitive radio technology can significantly boost spectrum utilization by exploiting radio spectrum unused by licensed users, it is rapidly gaining popularity and inspiring numerous applications. However, many technical issues still need to be addressed for successful deployment of CR networks, especially in the MAC layer. We focus on CR networks that have distributed architecture because they offer ease of deployment, self-organizing capability, and flexibility in design, and are believed to be more practical for future deployments compared to their centralized counterparts. The MAC protocols for distributed CR networks should consider the key features of these networks such as lack of a central unit to coordinate the communication, dynamic topology, requirements to keep interference to primary users minimal, and variation of spectrum availability with time and location. To clarify the relevant research challenges and issues, we provide a detailed study of the critical design issues and an overview of current state-of-the-art MAC protocols proposed for DCRNs. A classification of existing proposals is provided, and their salient features, advantages, and limitations are discussed. We then introduce and study a novel MAC protocol that addresses some of the research issues better than existing solutions. We also highlight important research challenges that could drive future research in this area.

Temporal variations of the indoor wireless millimeter-wave channel

This paper reports experiments conducted near 30 GHz to characterize human-induced variations of an indoor radio channel for which both terminals are stationary. The dependence of envelope characteristics on various parameters (degree and localization of movement, terminal configuration, and spatial short-range fading level) is examined. In particular, it is found that the received envelopes may be considered realizations of only locally stationary processes, even when movement in the propagation environment is homogeneous. The results presented may be used in the design of broad-band wireless LANs and serve as a basis for the development of a theoretical model capable of predicting channel temporal variation characteristics under a variety of conditions.

Wideband measurements of channel characteristics at 2.4 and 5.8 GHz in underground mining environments

This paper analyzes the results of wideband radio channel measurements conducted in an underground mining environment at center frequencies of 2.4 GHz and 5.8 GHz using a vector network analyzer. Relevant impulse response parameters such as the rms delay spread and the relative multipath total power are presented and compared for the two bands. The measurements suggest that in such an underground gallery and in the two frequency bands, random reflections have the effect of flattening the relationship between the rms delay spread and distance. In the 2.4 GHz band, the rms delay spread is less than or equal to 6.34 nanoseconds for 50% of all measurement locations. The corresponding value for the 5.8 GHz band is 4.98 nanoseconds. In general, it has been observed that underground radio channel characteristics are influenced by the configuration of this peculiar environment.

MC-CDMA performance evaluation over a multipath fading channel using the characteristic function method

This letter presents an efficient and accurate performance evaluation method for binary MC-CDMA systems with deterministic signature sequences. This method is based on a formulation of the characteristic function, and does not resort to any assumption on the statistical or spectral behavior of the interference. The accuracy of the Gaussian approximation method is also considered.

Narrowband propagation characteristics at 2.45 and 18 GHz in underground mining environments

We present the propagation characteristics at 2.45 and 18 GHz resulting from the statistical analysis of a narrowband measurement campaign in an underground mining environment. Parameters, such as the amplitude distribution of the envelope, level crossing rate, average fade duration and coherence time of the channel, have been examined. An analysis by linear regression has also been used to study the variations of the received power according to the transmitter-receiver distance.

Fractional-delay sequential blind beamforming for wireless multipath communications in confined areas

This paper presents a new adaptive antenna-array beamforming receiver for multipath correlated signals in hostile environments, such as indoor confined or underground areas, where intersymbol interference (ISI) and intrasymbol interference (isi) are predominant. The proposed receiver uses a new synchronization approach for multipath propagation and is based on sequential blind CMA and nested LMS beamformings with adaptive fractional-time-delay estimation. This approach uses on one hand CMA and LMS to recover any time path arrival (TPA) that is an integer multiple of the sampling interval, and nested adaptive fractional time delay estimation with LMS on the other hand to recover any TPA that is a non-integer multiple of the sampling interval. Coherent Maximum Ratio Combining (MRC) with hard Decision Feedback Identification (DPI) is also proposed for optimal and constructive combination of these different extracted paths. Analysis and results show the promising performance of the new adaptive antenna-array structure for different operating conditions and modulation schemes.

Power Loading for Multicarrier Cognitive Radio with MIMO Antennas

We propose a power loading scheme for multicarrier cognitive radio (CR) network employing multiple-input multiple-output (MIMO) antennas.We allocate power to different transmitting antennas within different subcarriers maximizing the total system capacity while maintaining the interference to primary user within a prescribed limit. We formulate this as a convex optimization problem and use numerical methods to solve for optimal power allocation. We also compare the performance of power loading scheme proposed in this paper to conventional uniform power loading scheme with and without interference constraint. Simulation results show that the capacity of MIMO CR using our proposed power loading scheme increases significantly with increase in interference temperature limit and the number of transmit antennas. However, we also confirm that if we include interference constraint in the conventional uniform power loading schemes, the capacity does not increase proportionally with increase in number of transmit antennas as those schemes do not take the instantaneous channel condition between cognitive user transmitter and primary user receiver into account.