Motorola i760, Field Service Manual

Page: 15 / 112

68P80400P94-O
1-1
CHAPTER 1
OVERVIEW
To achieve a high spectrum efficiency, the i850/i760 digital multi-service, data-capable portable
uses a unique modulation technology and sophisticated voice-compression algorithm. The voice of
the person speaking into the microphone is converted into a digital bit stream consisting of zeros (0)
and ones (1). The stream then is modulated into a radio-frequency (RF) signal, which is transmitted
over the air to another unit. This process is called digital modulation.
1.1 iDEN Digital Modulation Technology
The i850/i760 digital portable is a dual band 806-870 MHz and 896-940 MHz unit that can operate
in multiple modes: phone, private, data, and group. It uses three digital modulation technologies:
Quad QAM, Quadrature Phase Shift Keying (QPSK), and Time Division Multiple Access (TDMA).
Quadrature Amplitude Modulation (QAM) is a modulation technique that transmits information by
altering the amplitude and phase of the radio frequency (RF) signal. Data is converted into complex
symbols, which alter the RF signal and transmit the information. When the signal is received, the
amplitude and phase are converted back into symbols and then into the original data.
There are three different QAM modes that are used for different applications: 64QAM, 16QAM, and
4QAM. In voice applications, 16QAM is used, while in data applications, 64QAM, 16QAM, and
4QAM are used.
NOTE: The 900 MHz band operates only in 16QAM modulation.
In 64QAM, 64 possible combinations of 6 bits are converted into a unique amplitude and phase. In
16QAM, there are 16 possible combinations of 4 bits, while in 4QAM, there are 4 possible
combinations of 2 bits.
The traditional 25 kHz channel used for two-way radios is split into four QAM signals (subcarriers)
that are transmitted simultaneously. This technique can transmit 64 Kbps in a single 25 kHz
channel.
The iDEN system requires approximately 10 Kbps to transmit a compressed voice; therefore, 64
Kbps can accommodate 6 voice channels or 3 voice channels in enhanced systems.
The signal spectrum of the Quad 16QAM is shown in Figure 1-1.