4 System Feature Overview
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first to demonstrate very low bit rates while producing toll-quality speech such as traditionally
associated with wireline telephone systems.
Together with the vocoding process, Forward Error Correction (FEC) is also applied. FEC is a
mathematical checksum technique that enables the receiver to both validate the integrity of a
received message and determine which, if any, bits have been corrupted. FEC enables the
receiver to correct bit errors that may have occurred due to radio frequency (RF) channel
impairment. This effectively rejects noise that can distort an analog signal and by comparison
enables more consistent audio performance throughout the coverage area. At this stage, the
vocoder has already compressed the 128,000bps input signal to 3,600bps.
2.1.1.3 Part Three: Framing
In framing, the vocoded speech is formatted for transmission. This includes organizing the voice
and any embedded signaling information (such as color code, group ID, PTT ID, call type, etc.)
into packets. These packets form a header and payload type of structure – the header contains the
call control and ID information, and the payload contains the vocoded speech. This same structure
can also relay Internet Protocol (IP) data packets – the IP packets are simply an alternative form of
payload to the MOTOTRBO radio. The header information is repeated periodically throughout the
transmission, thereby improving the reliability of the signaling information as well as enabling a
receiving radio to join a call that may already be in progress – we refer to this condition as “late
entry”.
2.1.1.4 Part Four: TDMA Transmission
Finally, the signal is encoded for a Frequency Modulation (FM) transmission. The bits contained in
the digital packets are encoded as symbols representing the amplitude and phase of the
modulated carrier frequency, amplified, and then transmitted.
TDMA (Time Division Multiple Access) organizes a channel into 2 time slots: a given radio’s
transmitter is active only for short bursts, which provides longer battery life. By transmitting only on
their alternating time slots, two calls can share the same channel at the same time without
interfering with one another, thereby doubling spectrum efficiency. Using TDMA, a radio transmits
only during its time slot (i.e. it transmits a burst of information, then waits, then transmits the next
burst of information).
2.1.1.5 Standards Compliance
The digital protocols employed in MOTOTRBO (from vocoding and forward error correction to
framing, transmission encoding, and transmission via two-slot TDMA) are fully specified by the
ETSI
1
DMR
2
Tier 2
3
Standard, which is an internationally recognized standard with agreements
among its supporting members. Although formal interoperability testing and verification processes
for this standard have yet to fully mature, Motorola anticipates that MOTOTRBO radio systems will
be interoperable with other solutions that comply to the ETSI DMR Tier 2 standard.
1. European Telecommunications Standards Institute
2. Digital Mobile Radio
3. Tier 2 indicates full power conventional operation in licensed channels for professional and commercial
users.