iDEN F2581A, Подробное руководство по обслуживанию

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OVERVIEW: Voice Compression Technology
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Adding error-correction bits to the coded-voice bits increases the required RF-
transmission bit rate to approximately 7.4 kbps for the 4.2 kbps voice coder and
14.8 kbps for the 8.0 kbps voice coder.
RF Transmission Bursts
All RF transmissions within an iDEN system occur in 15-millisecond blocks
called
bursts . An iDEN base station transmits bursts continuously on each RF fre-
quency it uses. Transmission bursts from the stations are synchronized in time by
signals received from the global positioning satellite (GPS) system. Each burst is
numbered; the number is referred to as the
slot number. All bursts occurring at a
given time carry the same slot number.
Inbound transmission bursts (sent from the unit) are offset 19 milliseconds from
the outbound burst; the inbound burst begins 4 milliseconds after the end of the
outbound burst (see Figure 2 on page 3). This offset in time allows the unit to
appear capable of transmitting and receiving at the same time (it actually is
switching rapidly back and forth between receiving and transmitting).
In current iDEN systems, outbound transmissions range from 851-866 MHz;
inbound transmissions are 45 MHz lower in frequency.
An iDEN channel is created by grouping bursts so that their slot numbers differ by
a number referred to as the repetition rate. The portable uses two repetition rates
for interconnect voice calls: 6:1 and 3:1. A single frequency can handle six calls
using a 6:1 repetition rate with the 4.2 kbps coder. Dispatch calls always use this
rate. However, the audio quality of the 3:1 repetition rate with the 8.0 kbps coder
is superior to the 6:1 rate with the 4.2 kbps coder. Most service providers config-
ure the system and unit to allow only the 3:1 rate for interconnect calls.
Calls are setup on a primary control channel (PCCH) in response to a request from
a unit or a regular telephone connected to the Public Switch Telephone Network
(PSTN). The PCCH is established by the base station equipment; typically, it is a
6:1 channel.
The outbound PCCH carries system information and “pages” to specific sub-
scriber units and asks them to respond if they are available for a call. Inbound
PCCH transmissions carry responses to the “pages” and requests by subscriber
units to set up calls. Inbound transmissions on the primary control channel only
take 7.5 milliseconds and may be timed to occur in either half of a 15-millisecond
burst.
Using the above techniques, the iDEN system can operate six voice channels
simultaneously in dispatch mode or three voice channels simultaneously in inter-
connect mode on a single RF carrier at 25 kHz bandwidth.