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Tempest 2400 Wireless Intercom System

Theory of Operation

Tempest uses Frequency Hopping Spread Spectrum (FHSS), Time Division Multiple Access (TDMA), and

2xTX technology. For example, on Frequency 1 the base station and each beltstation take turns

broadcasting, each with its own time slot to send audio data (TDMA). The total time for one cycle is 5

milliseconds (1/200 second). The base station and all beltstations hop (FHSS) to a different

predetermined frequency and the same audio data is sent again, but on a new frequency and on a

different base station antenna (2xTX). Therefore, each receiver has two opportunities 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 communications system, operating in 80MHz of spectrum in the 2.4GHz ISM frequency

band. This band allows users to operate with no license requirements in most locations. The world-wide

acceptance of the 2.4GHz band makes Tempest ideal for traveling users.

The 2.4GHz band offers many benefits. There are, however, a wide variety of technologies and users

competing for clear spectrum in this limited band. Tempest utilizes 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 attribute of any wireless system.

Tempest utilizes proprietary, Frequency Hopping Spread Spectrum (FHSS) technology that has seven

U.S. patents. Multiple beltstation access is achieved by implementing Time Domain Multiple Access

(TDMA) with Frequency Shift Keying (FSK) modulation. Tempest uses an Algebraic Code - Excited Linear

Prediction (A-CELP) voice compression algorithm to reduce the bandwidth necessary for transmission

and yet maintain the highest possible voice intelligibility. In addition, this algorithm utilizes advanced lost

packet masking technology, which greatly reduces the affect 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

detecting a loss of audio.

In the 2.4GHz RF band multipath interference occurs frequently. The Tempest RF scheme is dramatically

enhanced with the addition of proprietary 2xTX™ technology that transmits each data packet twice. This

technique reduces spectral efficiency by half, but greatly increases audio intelligibility by reducing the

Effective 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 pattern. Each hopping pattern is optimized

such that any new frequency has a quasi-orthogonal relationship to the last operating frequency. This

quasi-orthogonal frequency relationship ensures maximum frequency separation between any two

consecutive hops. Because of this, if an interfering source is present in one area of the operational band,

and interferes on a particular hop, the next hop will be in a clear area of the spectrum with the best

possible chance of transmission success.

With 2xTX technology, each audio packet is transmitted twice, each on one of two consecutive hops. The

loss of one packet transmission in a harsh RF environment is common. However, because of the quasi-

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Clear-Com TEMPEST 2400 Dokumentacja Strona 17