Garry C. Hess

Communication load and delay in mobile trunked systems

This paper characterizes the traffic behavior of single channel conventional repeaters and various arrangements of trunked repeaters. The characterizations are based on extensive observations of repeater operation of each type. Key findings are: 1) Trunked system users talk longer than conventional repeater users (21.9 sec. vs. 13.7 sec. average message length); 2) Traffic loads from multiple sources tend to be uncorrelated and hence peak system load results from noise-like rather than direct additions of the individual source load peaks. An equation for estimating peak load under these conditions is presented along with comparisons of estimated and measured peak loads which confirm its utility; and 3) Erlang-C theory is applicable to the problem of quantifying trunked repeater performance. The message length cited in 1) favors the performance of conventional repeaters whereas the load-smoothing aspect noted in 2) favors the performance of trunked repeaters. A performance curve of average first access delay versus system air-time load in Erlangs is generated via Erlang-C theory for a Motorola trunked repeater system. This system utilizes a short time-out arrangement which substantially reduces the effective hold-time but appears message trunked to users due to priority queuing, hence first access delay is the relevant performance parameter. Fine-tuning the hold time to a value slightly greater than the average access duration produces a curve in good agreement with the performance observed on commercial systems.

Estimation of peak load on trunked repeater systems

This paper discusses procedures for estimating the peak loads of trunked repeater systems. In addition to nominal peak load prediction, two aspects of overload are addressed: (1) system-to-system variation and (2) day-to-day variation. The procedures allow one to estimate the maximum communication load to be expected during a specified time interval for a specified fraction of trunked systems with equal mobile counts. One application of these procedures concerns the recent proposal of trunking at the 3 channel level. The results indicate that for most such systems to have viable grades-of-service, appreciably lower loading standards than exist today at the 5 channel level are necessary.

Verification of coverage

This paper addresses the task of proving that some required level of cellular radio system coverage has in fact been provided. For a given design margin m in probability, the sample size required to successfully verify (1-/spl alpha/)100% of the time that the promised coverage has indeed been provided is developed. The impact of measurement errors is quantified. Finally, a special two-tier test plan is considered.

Land-mobile satellite excess path loss measurements

An experiment conducted with the ATS-6 satellite to determine the additional path loss over free-space loss experienced by land-mobile communication links is described. This excess path loss is measured as a function of 1) local environment, 2) vehicle heading, 3) link frequency, 4) satellite elevation angle, and 5) street side. A statistical description of excess loss developed from the data shows that the first two parameters dominate. Excess path loss on the order of 25 dB is typical in urban situations, but decreases to under 10 dB in suburban/rural areas. Spaced antenna selection diversity is found to provide only a slight decrease (4 dB, typically) in the urban excess path loss observed. Level crossing rates are deprsessed in satellite links relative to those of Rayleigh-faded terrestrial links, but increases in average fade durations tend to offset that advantage. The measurements show that the excess path loss difference between 860- MHz links and 1550-MHz links is generally negligible.

Propagation aspects of importance to packet data frequency reuse system simulation

Procedures for simulating frequency reuse, multiple-access packet data systems are discussed. Due to the random access nature typical of packet data protocols, the characterization of interference is more complex than for voice systems. Specifically, angle of arrival difference (AAD) correlations among lognormal shadowing deviations must account for the possibility of multiple interference sources residing in single cells. For collision resolution, the authors relate block error probability to link quality (taken as average signal-to-interference plus noise power ratio, C/(I+N)) through an offset exponential form and the quasi-static approximation. For one particular QPSK trellis-coded transmission scheme, the block error impact of using C/N, rather than C/I, is shown to be conservative, as is the quasi-static approximation.

Adjacent channel interference between 800 MHz trunked FM and multiple site reuse systems

The interference implications of introducing multisite mobile communication systems into the 800-MHz FM trunked spectrum are examined. Emphasis is on talk-in interference into an FM system caused by multisite systems, as this is the worst of four possible interference scenarios. A Monte Carlo simulation which determines the range reduction when the adjacent channel has multiple interferers is described. The method for calculating each interferers power at the FM base is explained and includes a means of accounting for log-normal fades and frequency drifts. Results are presented for a modulation just fitting the US FCC data mask, Part 90.209(g), and an actual modulation which fits the passband of the US FCC data mask but has 15-dB more protection in the stopband. Two different FM receivers are modeled to show the importance of receiver selectivity. Power control is evaluated for its ability to reduce interference.<<ETX>>

Spectrum efficiency potential of 25 kHz offset channel assignments in the 821-824/866-869 MHz public safety bands

The US Federal Communications Commission (FCC) recently allocated the paired bands of 821-824 MHz and 866-869 MHz to the Public Safety Radio Services and solicited comment on service rules and technical standards (Docket 87-112). The author examines the spectrum efficiency attainable through the use of 25-kHz offset channel assignments and contrasts it with that attainable via 12.5-kHz split channel assignments. It is concluded that due to the limited and well-defined coverage requirements typical of public safety applications, the offset approach can offer nearly as much spectrum efficiency as the split channel approach, while avoiding problems with timeliness of application, expansion of existing systems, high-speed data and encrypted voice needs, and mutual aid communication.<<ETX>>

Further sharing of UHF television by private land mobile radio services

General Docket No. 85-172 proposes rules for further sharing of the UHF Television Band by Private Land Mobile Radio services. This paper describes methodologies for evaluating the interference potential to television reception based on the proposed rules. Two general situations are considered: (1) co-channel interference caused by land mobile operation outside the television coverage area, and (2) non co-channel interference caused by land mobile operation inside (or nearly so) the television coverage area. The interference probabilities are contrasted with those related to self-imposed UHF television interference.

Trunked simulcast system stimulator

This paper discusses a traffic loader/performance monitor developed for the Motorola trunked simulcast system. It has the capability of fully loading systems sized up to 20 channels at 11 sites with mixed dispatch and interconnect calls. It does this without the need for any actual repeater equipment or mobile radios, in contrast to a real system which could involve up to 220 repeaters and about 3000 mobiles. Each simulated call is checked in real time for proper sequencing and histograms of critical time periods are generated.