User guide
8 Tempest®24 00 4-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.