Motorola i860 manual Overview, DEN Digital Modulation Technology

CHAPTER 1

OVERVIEW

To achieve a high spectrum efficiency, the i860 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.

1.1i DEN Digital Modulation Technology

The i860 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.

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