Clear-Com Tempest2400 - Manual (Page 14)

8 Tempest®24 00 2 -Channel Wireless Intercom System
Theory of Operati on
The above illustrati ons show the theory of operati 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 characteristi cs found in the 4-channel model. Tempest uses Frequency Hopping
Spread Spectrum (FHSS), Time Division Multi ple Access (TDMA), and 2xTX technology. For example, on Frequency 1 the BaseStati on and each
BeltStati on take turns broadcasti ng, each with its own ti me slot to send audio data (TDMA). The total ti me for one cycle is 5 milliseconds
(1/200 second). The BaseStati on and all BeltStati ons hop (FHSS) to a diff erent predetermined frequency and the same audio data is sent
again, but on a new frequency and on a diff erent BaseStati on antenna (2xTX). Therefore, each receiver has two opportuniti 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
communicati ons system, operati ng in 80MHz of spectrum in the 2.4GHz ISM frequency band. This band allows users to
operate with no license requirements in most locati ons. The world-wide acceptance of the 2.4GHz band makes Tempest
ideal for traveling users.
The 2.4GHz band off ers many benefi ts. There are, however, a wide variety of technologies and users competi ng for clear
spectrum in this limited band. Tempest uti 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 att ribute of any
wireless system.
Tempest uti lizes proprietary, Frequency Hopping Spread Spectrum (FHSS) technology that has seven U.S. patents. Multi ple
BeltStati on access is achieved by implementi ng Time Domain Multi ple Access (TDMA) with Frequency Shift Keying (FSK)
modulati on. Tempest uses an Algebraic Code - Excited Linear Predicti on (A-CELP) voice compression algorithm to reduce the
bandwidth necessary for transmission and yet maintain the highest possible voice intelligibility. In additi on, this algorithm
uti lizes advanced lost packet masking technology, which greatly reduces the aff 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 detecti ng a loss
of audio.
In the 2.4GHz RF band multi path interference occurs frequently. The Tempest RF scheme is dramati cally enhanced with the
additi on of proprietary 2xTX™ technology that transmits each data packet twice. This technique reduces spectral effi ciency
by half, but greatly increases audio intelligibility by reducing the Eff ecti 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 patt ern. Each hopping patt ern is opti mized such that any new frequency has
a quasi-orthogonal relati onship to the last operati ng frequency. This quasi-orthogonal frequency relati onship ensures
maximum frequency separati on between any two consecuti ve hops. Because of this, if an interfering source is present in
one area of the operati onal band, and interferes on a parti cular hop, the next hop will be in a clear area of the spectrum
with the best possible chance of transmission success.