This paper deals with the measurement and modeling of multipath delay on fixed wireless paths at 1.9 GHz in suburban environments. The primary focus is on the delay profile, which is the normalized plot of received power versus delay in response to an RT "impulse." We describe measurement campaigns in the western suburbs of Chicago, IL, and in suburban north-central New Jersey. Our analysis of the data suggests to us that, for directive terminal antennas, the delay profile can be modeled as having a "spike-plus-exponential" shape, i.e., a strong return ("spike") at the lowest delay, plus a set of returns whose mean powers decay exponentially with delay. This delay profile can be characterized by just two parameters (both variable over the terrain), namely, the ratio (K/sub 0/) of the average powers in the "spike" and "exponential" components and the decay time constant (/spl tau//sub 0/) of the "exponential" component. No such simple structure appears to apply for delay profiles using omnidirectional antennas. For a directive antenna with a 32/spl deg/ beamwidth, we find that: (1) the statistical correlation between the profile parameters K/sub 0/ and /spl tau//sub 0/ is negligible; (2) these parameters are relatively insensitive to antenna height and path length; and (3) over each measured region (Illinois and New Jersey), K/sub 0/ and /spl tau//sub 0/ have median values close to 8 dB and just below 0.2 /spl mu/s, respectively. Moreover, we have found simple probability distributions that accurately portray the variability of K/sub 0/ and /spl tau//sub 0/ over the terrain.

Engineers designing and installing outdoor and indoor wireless communications systems need effective and practical tools to help them determine base station antenna locations for adequate signal coverage. Computer-based radio propagation prediction tools are now often used in designing these systems. We assess the performance of such a propagation tool based on ray-tracing and advanced computational methods. We have compared its predictions with outdoor experimental data collected in Manhattan and Boston (at 900 MHz and 2 GHz). The comparisons show that the computer-based propagation tool can predict signal strengths in these environments with very good accuracy. The prediction errors are within 6 dB in both mean and standard deviation. This shows that simulations, rather than costly field measurements, can lead to accurate determination of the coverage area for a given system design.

The general principles, implementation, and performance of a 1-Mb/s time-division-multiple-access (TDMA), slow frequency hopping and coding, 1.5-GHz radio communications system for a medium-sized office building are discussed. TDMA is provided for service flexibility and slow frequency hopping and coding for immunity against multipath fading and interference. Measurements show that, with a hallway-mounted distributed antenna system installed on one floor of the building and with the mobile unit transmitting only 1 mW of peak RF power from anywhere on that floor, only a single 384-b frame out of a total of 200000 transmitted frames suffered an unrecoverable error. This kind of performance is comparable to that of wired data modems. Similar performance was obtained from a central antenna covering the same floor. However, the required transmitted power in this case was 100 mW. Implementation issues, such as spectrum allocation, cellular subdivisions, and spectrum efficiency, are also discussed.>

We have investigated the performance of the times-four method of carrier recovery in a digital radio link transmittingM-level quadrature amplitude modulation (M-QAM), whereMcan be 16, 64, 256, etc.. With the help of a recently built multipath fade simulator, we measured rms phase jitter for various combinations of fade notch frequency and fade depth. This paper reports on the experiment and the associated analysis. The times-four method has the virtues that it does not depend upon accurate data decisions, and it does not interact with the receiver's adaptive equalizer. Our results show that the rms phase jitter using this method, for all but the most severe and improbable fades, could be held to a few degrees. Although the measurements were done only for 16 QAM, the analysis predicts only modest increases for higher numbers of levels. The analysis also predicts, and the measurements verify, that the recovered carrier amplitude shrinks to zero for four distinct combinations of fade notch frequency and fade depth. To our knowledge, this phenomenon has not been reported in the literature heretofore. We show that, from a statistical viewpoint, the damage associated with it (measured in terms of outage time) should be quite small.

The idea of implementing an indoor radio communications system serving an entire building from a single central antenna appears to be an attractive proposition. However, based on various indoor propagation measurements of the signal attenuation and the multipath delay spread, such a centralized approach appears to be limited to small buildings and to narrow-band FDMA-type systems with limited reliability and flexibility. In this paper, we present the results of indoor radio propagation measurements of two signal distribution approaches that improve the picture dramatically. In the first, the building is divided into many small cells, each served from an antenna located in its own center, and with adjacent cells operating in different frequency bands. In the second approach, the building is divided into one or more large cells, each served from a distributed antenna system or a "leaky feeder" that winds its way through the hallways. This approach eliminates the frequency cell handoff problem that is bound to exist in the first approach, while still preserving the dramatic reductions in multipath delay spread and signal attenuation compared to a centralized system. For example, the measurements show that, with either approach, the signal attenuation can be reduced by as much as a few tens of decibels and the rms delay spread becomes limited to 20 to 50 us, even in large buildings. This can make possible the implementation of sophisticated broad-band TDMA-type systems that are flexible, robust, and virtually building-independent.

Three experimental programs at Bell Laboratories, Crawford Hill, NJ, have collected rain attenuation data at three frequencies in the 10-30 GHz range for over 2 years. As expected from a simple geometrical model, rain attenuation statistics scale approximately as the cosecant of the path elevation angle. Thus, greater attenuation is encountered on lower elevation angle paths. The increased rain attenuation at higher frequencies is illustrated by comparing cumulative rain attenuation distributions at three widely separated frequencies. Typical year-to-year variations in these distributions are presented. Periods of severe rain attenuation are shown to occur more frequently during the summer months and during the afternoon hours. Cumulative attenuation distributions are compared for these and other time periods.

A wide-band inverse-sine phase modulator is demonstrated. The modulator has application in a LINC amplifier for linear amplification at microwave frequencies using commonly available saturating amplifiers or injection-locked oscillators. A bandwidth greater than 10 MHz at a carrier frequency of 96 MHz with distortion products 40 dB or greater below desired components has been shown. The distortion behavior as a second-order effect of the loop delay as predicted by analysis has been verified. The output of the inverse-sine phase modulator and its conjugate, generated by a second phase modulator, were subtracted as in a LINC amplifier, to provide at least 40-dB suppression of the zeroth order and even-order components.

A theoretical and experimental comparison of performance has been made between two types of predetection switching space diversity mobile radio systems. This comparison was made at a frequency of 840 MHz using simulated Rayleigh fading for a vehicle speed of about 80 mi/h. The switch diversity system was a conventional receiver antenna switching technique with two simulated physically separated receiving antennas and a single transmitting antenna. The feedback diversity system used a single receiving antenna with two simulated physically separated transmitting antennas. The transmitting antennas were switched remotely from the receiver. The difference in the performance of the two systems was shown to be primarily due to time delay inherent in the remote antenna switching technique.