MSBTE  SYLLABUS 

Subject  Name	Subject  code	Click here
MWC	22533	Download
DTE	22320	Download
EEM	22217	Download
EOE	22213	Download
CND	22634	Download

Draw neat block diagram of transmitter unit of mobile handset. State function of APC loop and duplexer unit in unit.

CELLULAR TRANSMITTER

The Fig. below shows the block diagram of cellular transmitter. It is a low power FM transmitter operating in the frequency range of 825 to 845 MHz. There are 666 3D-KHz transmit channels. Channel 1 is 825.03 MHz, channel 2 is 825.06 MHz and so on up to channel 666 on 844.98 MHz. As shown in the Fig. 15.68, the phase modulator modulates the preamplified voice signal and the carrier input from the frequency synthesizer. It produces a deviation of ± 12 KHz. The modulated output is translated upto final transmitter frequency with the help• of mixer whose second input also comes from the frequency synthesizer.

The mixer output is fed to class C power amplifier stages where the output signal is generated. The final amplifier stage is designed to supply about 500 mW to 3W power to the antenna. A unique feature of the high power transmitter is that its output is controllable by the cell site and MTSO. In such phones, a special control signals picked up by the receiver are sent to an automatic power control (APC) circuit that sets the transmitter to one of 8 power output levels.

The directional coupler samples the transmitter output power and rectifies it into a proportional de signal. This signal is used in the APC circuit and is transmitted back to the cell site permitting the MTSO to know the present power level. This automatic power control feature permits optimum cell site reception with minimal power. It also helps to minimize interference from other stations in the same or adjacent cells.

The transmitter output is then fed to a duplexer or isolator. It allows the transmitter and receiver to share the same antenna. Since cellular telephone units operate in full-duplex mode, the transmitter and receivers operate simultaneously. For proper isolation the transmit and receive frequencies are spaced 45 MHz apart from each other. However, to minimize the interference it is necessary to keep the transmitter power out of the sensitive receiver. To keep proper isolation, duplexer consists of two very sharp bandpass filter, one for the transmitter and one for the receiver.

RECEIVER

In cellular phone, dual-conversion superheterodyne receiver is used. An RF amplifier boosts the level of the received cell signal. The receiver frequency range is 870.03 to 889.98 MHz. The first mixer translates the incoming signal down to first IF of 82.2 MHz. The local oscillator frequency is derived from the frequency synthesizer. The local oscillator frequency sets the receiver channel.

The signal from the output of the mixer is amplified by an IF amplifier and then passed to the second mixer. The second mixer is controlled by a crystal controlled local oscillator. The second IF is usually either 10.7 MHz or 455 kHz. The signal is then demodulated, de-emphasized, filtered, and amplified before being applied to the output speaker in the handset.

The output of the FM demodulator is also sent to band pass filter circuit that selects the control audio tones and digital control data stream. This information is used to set and control both the transmitter and receiver. The demodulator output is amplified and used as a receiver signal strength indicator (RSSI).

The RSSI signal is sent back to the cell site so that the MTSO can monitor the receiver signal from the cell and make decision about switching to another cell if signal from current cell is weak.  

Draw block diagram of frequency synthesizer unit of mobile handset and state its function in cellular handset.

• The synthesizer is used for developing all the signals used by the transmitter and receiver.
   It uses the PLL circuits and a mixer.
   The crystal oscillator provides a reference for the two PLLs.
   The output of VCO-2 is used as a local oscillator frequency for the first mixer in the ...
• 
  

Difference betweem Hard Hand-off &Soft Hand-off Soft Hand-off

Sr No.	Hard Hand-off	Soft Hand-off
1	The definition of a hard-hand off is one where an existing connection must be broken when the new one is established	Soft hand-off is defined as a hand-off where a new connection is established before old one is released
2	It allocates Different frequency	It allocates same frequency
3	Hard hand-off typically used in TDMA and FDMA	Soft hand-off used in CDMA and some TDMA systems
4	Hard hand-off is not very complicated	More complex than hard hand-off
5	In hard hand-off handset always communicated with one BS at a time	Communicate up to three or four radio link at the same time

handoff

Following are the type of Handoff :

1) Hard handoff - During the handoff process, if the old connection is terminated before making the new connection, it is called a hard handoff .it can be further divided into intrafrequency and interfrequency handoff.
Interfrequency handoffs may occur between two different radio access networks, for example, between GSM and Universal Mobile Telecommunications Services. And In the 2G TDMA systems, the majority of handoffs are intra frequency hard handoffs

2) Soft handoff - The handoff is referred to as soft handoff if the new connection is established before the old connection is released. In the 3G systems, the majority of handoffs are intrafrequency soft handoffs.

3) Softer handoff - In softer handoff, the BS transmits through one sector but receives from more than one sector

Frequency reuse

Frequency reuse

1. Cellular phone networks use cellular frequency reuse. In the cellular reuse concept, frequencies allocated to the service are reused in a regular pattern of areas, called "cells", each covered by one base station.
2. In mobile-telephone nets these cells are usually hexagonal. To ensure that the mutual interference between users remains below a harmful level, adjacent cells use different frequencies. However in cells that are separated further away, frequencies can be reused.
3. Particularly in the United States, the term "cell phone" is often used by the public when a wireless phone is meant. The cellular approach was proposed and developed predominantly by the Bell System, in the U.S. in the early 70's, after the regulatory agency FCC has asked for an upgrade of the existing radio telephone service.

Typical frequency reuse plan for 3 different radio frequencies, based on hexagonal cells. Radio channels are indicated by color. In fact some problems in cellular frequency assignment are solved using map coloring theory.

The FCC had the foresight to require:

1. a large subscriber capacity
2. efficient use of spectrum resources
3. nationwide coverage
4. adaptability to traffic density
5. telephone service to both vehicle and portable user terminals
6. telephony but also other services including closed user groups with voice dispatch operations
7. toll quality

Co-channel cells: Frequency reuse implies that in a given coverage area, there are several cells that use the same set of frequencies. These cells are called co-channel cells.

Causes:

1. Reduction of D/R ratio, which reduce distance between two co-channels.

2. Use of omnidirectional antennas at the base station.

3. Increasing the antenna height at the base station.

Effects of co-channel interference on system capacity:

The parameter Q, called the co-channel reuse ratio, is related to cluster size N,

Q=D/R=3N−−−√$Q=D/R=\sqrt{3N}$

A small value of Q provides larger capacity since the cluster size N is small, whereas a large value of Q implies smaller level of co-channel interference. Thus with reduction in cp-channel interference there will reduction in system capacity.

Mobile Phone Unit

Transmitter: It is low power FM unit operating in the frequency range of 825 to 845MHz. There are 666, 30 KHz transmit channel. The carrier is furnished by a frequency synthesizer is a phase modulated by voice signal.

Receiver: The receiver is a dual conversion super heterodyne. The incoming signal frequency is down converted twice to frequency of 455KHz or 10.7MHMz with the help of mixer and IF amplifier stages. The signal is then demodulated deemphasized and filtered and given to loud speaker.

Frequency Synthesizer: This block generates all the signals used by transmitter and receivers. It uses standard PLL circuits and a mixer.

Logic Unit: This unit contains master control circuit for a cellular radio. It is made up of microprocessor with RAM and ROM and additional circuit used for interpreting signals from MSC and BS and generates control signal for the transmitter and receiver.

Control unit: The control unit contains the handset with speaker and microphone. The control unit is operated by a separate microprocessor that drives the LCD display and other indicators.

Features:

1) Typical o/p power is 3 W if mobile unit is mounted on vehicle

2) o/p power is only 500Mw if it is a handheld unit

3) transmitter is a low power FM unit operating in frequency range of 825 to 845 Mhz

4) It has 666 transmit channels which are spaced 30 Khz apart