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8 Tempest®2400 2-Channel Wireless Intercom System

Theory of Operaon

The above illustraons show the theory of operaon for both the 4-channel (dual antenna) and 2-channel (single antenna) models of

Tempest. The 2-channel model lacks some of the diversity characteriscs found in the 4-channel model. Tempest uses Frequency Hopping

Spread Spectrum (FHSS), Time Division Mulple Access (TDMA), and 2xTX® technology. For example, on Frequency 1 the BaseStaon and

each BeltStaon take turns broadcasng, each with its own me slot to send audio data (TDMA). The total me for one cycle is 5 milliseconds

(1/200 second). The BaseStaon and all BeltStaons hop (FHSS) to a dierent predetermined frequency and the same audio data is sent

again, but on a new frequency and on a dierent BaseStaon antenna (2xTX). Therefore, each receiver has two opportunies to receive the

audio data. Duplicate data is discarded by the receiver and the process starts over with the next packet of audio data.

Tempest is a DSP (Digital Signal Processor) based, full duplex, wireless intercom system. It is a digital, point-to-point

communicaons system, operang in 80MHz of spectrum in the 2.4GHz ISM frequency band. This band allows users to

operate with no license requirements in most locaons. The world-wide acceptance of the 2.4GHz band makes Tempest

ideal for traveling users.

The 2.4GHz band oers many benets. There are, however, a wide variety of technologies and users compeng for clear

spectrum in this limited band. Tempest ulizes patented and proprietary technologies to ensure a robust and reliable RF

link under a wide variety of physical and RF environments. This level of reliability is the most important aribute of any

wireless system.

Tempest ulizes proprietary, Frequency Hopping Spread Spectrum (FHSS) technology that has seven U.S. patents. Mulple

BeltStaon access is achieved by implemenng Time Domain Mulple Access (TDMA) with Frequency Shi Keying (FSK)

modulaon. Tempest uses an Algebraic Code - Excited Linear Predicon (A-CELP) voice compression algorithm to reduce the

bandwidth necessary for transmission and yet maintain the highest possible voice intelligibility. In addion, this algorithm

ulizes advanced lost packet masking technology, which greatly reduces the aect of packet loss, possible in all Digital RF

transmission schemes. The loss of audio packets is managed by the algorithm to reduce the possibility of detecng a loss

of audio.

In the 2.4GHz RF band mul path interference occurs frequently. The Tempest RF scheme is dramacally enhanced with the

addion of proprietary 2xTX technology that transmits each data packet twice. This technique reduces spectral eciency

by half, but greatly increases audio intelligibility by reducing the Eecve Packet Error Rate (EPER) by orders of magnitude.

The system transmits packet data every 5ms and then “hops” or changes frequency. The sequence of frequency changes

is controlled by a predetermined hopping paern. Each hopping paern is opmized such that any new frequency has

a quasi-orthogonal relaonship to the last operang frequency. This quasi-orthogonal frequency relaonship ensures

maximum frequency separaon between any two consecuve hops. Because of this, if an interfering source is present in

one area of the operaonal band, and interferes on a parcular hop, the next hop will be in a clear area of the spectrum

with the best possible chance of transmission success.

Clear-Com TEMPEST 2400 Reference Manual

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