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Antenna:
Antenna pattern, antenna gain, antenna tilting, and antenna height 6 all affect the cellular system design.
The antenna pattern can be omnidirectional, directional, or any shape in both the vertical and the horizon
planes. Antenna gain compensates for the transmitted power. Different antenna patterns and antenna gains
at the cell site and at the mobile units would affect the system performance and so must be considered in
the system design. The antenna patterns seen in cellular systems are different from the patterns seen in
free space. If a mobile unit travels around a cell site in areas with many buildings, the omnidirectional
antenna will not duplicate the omnipattern. In addition, if the front-to-back ratio of a directional antenna is
found to be 20 dB in free space, it will be only 10 dB at the cell site. Antenna tilting can reduce the interference to the neighboring cells and enhance the weak spots in the cell. Also, the height of the cellsite
antenna can affect the area and shape of the coverage in the system.
Switching Equipment:
The capacity of switching equipment in cellular systems is not based on the number of switch ports but on
the capacity of the processor associated with the switches. In a big cellular system, this processor should
be large. Also, because cellular systems are unlike other systems, it is important to consider when the
switching equipment would reach the maximum capacity. The service life of the switching equipment is
not determined by the life cycle of the equipment but by how long it takes to reach its full capacity. If the
switching equipment is designed in modules, or as distributed switches, more modules can be added to
increase the capacity of the equipment. For decentralized systems, digital switches may be more suitable.
The future trend seems to be the utilization of system handoff. This means that switching equipment can
link to other switching equipment so that a call can be carried from one system to another system without
the call being dropped.
Data Links:
The data links are shown in Fig 1. Although they are not directly affected by the cellular system, they are
important in the system. Each data link can carry multiple channel data (10 kbps data transmitted per
channel) from the cell site to the MTSO. This fast-speed data transmission cannot be passed through a
regular telephone line. Therefore, data bank devices are needed. They can be multiplexed, many-data
channels passing through a wideband T-carrier wire line or going through a microwave radio link where
the frequency is much higher than 850MHz. Leasing T1-carrier wire lines through telephone companies
can be costly. Although the use of microwaves may be a long-term money saver, the availability of the
microwave link has to be considered.
Explain about NMT & NTT Systems.
Answer:
NTT: Nippon Telegraph and Telephone Corporation (NTT) developed an 800-MHz land mobile
telephone system and put it into service in the Tokyo area in 1979. The general system operation is
similar to the AMPS system. It accesses approximately 40,000 subscribers in 500 cities. It covers 75
percent of all Japanese cities, 25 percent of inhabitable areas, and 60 percent of the population. In Japan, 9
automobile switching centers (ASCs), 51 mobile control stations (MCSs), 465 mobile base stations
(MBSs), and 39,000 mobile subscriber stations (MSSs) were in operation as of February 1985.
The Japanese mobile telephone service network configuration is shown in Fig.2. In the metropolitan
Tokyo area, about 30,000 subscribers are being served. The 1985 system operated over a spectrum of 30
MHz. The total number of channels was 600, and the channel bandwidth was 25 kHz. This system
comprised an automobile switching center (ASC), a mobile control station (MCS), a mobile base station
(MBS), and a mobile subscriber station (MSS). At present there is no competitive situation set up by the
government. However, the Japanese Ministry of Post and Telecommunication (MFT) is considering
providing a dual competitive situation similar to that in the United States.
NMT: Nordic System: This system was built mostly by Scandinavian countries (Denmark, Norway,
Sweden, and Finland) in cooperation with Saudi Arabia and Spain and is called the NMT network. It is
currently a 450-MHz system. But an 800-MHz System will be implemented soon since the frequency
transparent concept as the AURORA 800 system is used to convert the 450-MHz system to the 800-MHz
System. The total bandwidth is 10 MHz, which has 200 channels with a bandwidth of 25 kHz per
channel. This system does have handoff and roaming capabilities. It also uses repeaters to increase the
coverage in a low traffic area. The total number of subscribers is around 100,000.
3. Explain the phenomena of severe fading?
Answer:
Severe Fading: If the antenna height of the mobile unit is lower than its typical surroundings, and the
carrier frequency wavelength is much less than the sizes of the surrounding structures, multipath waves
are generated. At the mobile unit, the sum of the multipath waves causes a signal-fading phenomenon.
The signal fluctuates in a range of about 40 dB (10 dB above and 30 dB below the average signal). We
can visualize the nulls of the fluctuation at the baseband at about every half wavelength in space, but all
nulls do not occur at the same level, as Fig.3 shows. If the mobile unit moves fast, the rate of fluctuation
is fast. For instance, at 850 MHz, the wavelength is roughly 0.35 m (1 ft). If the speed of the mobile unit
is 24 km/h (15 mi/h), or 6.7 m/s, the rate of fluctuation of the signal reception at a 10-dB level below the
average power of a fading signal is 15 nulls per second.
4. Distinguish between the permanent splitting and dynamic splitting?
Answer: There are two kinds of cell-splitting techniques:
1. Permanent splitting: The installation of every new split cell has to be planned ahead of time; the
number of channels, the transmitted power, the assigned frequencies, the choosing of the cell-site
selection, and the traffic load consideration should all be considered. When ready, the actual service cutover
should be set at the lowest traffic point, usually at midnight on a weekend. Hopefully, only a few
calls will be dropped because of this cut-over, assuming that the downtime of the system is within 2 h.
2. Dynamic splitting: This scheme is based on using the allocated spectrum efficiency in real time. The
algorithm for dynamically splitting cell sites is a tedious job, as we cannot afford to have one single cell
unused during cell splitting at heavy traffic hours.
5. What are the limitations of conventional mobile systems and how are they overcome by
cellular mobile systems?
Answer:
Limitations of conventional mobile telephone systems: One of many reasons for developing a cellular
mobile telephone system and deploying it in many cities is the operational limitations of conventional
mobile telephone systems: limited service capability, poor service performance, and inefficient frequency
spectrum utilization.
1. Limited service capability: A conventional mobile telephone system is usually designed by selecting
one or more channels from a specific frequency allocation for use in autonomous geographic zones, as
shown in Fig.5. The communications coverage area of each zone is normally planned to be as large as
possible, which means that the transmitted power should be as high as the federal specification allows.
The user who starts a call in one zone has to reinitiate the call when moving into a new zone because the
call will be dropped. This is an undesirable radio telephone system since there is no guarantee that a call
can be completed without a handoff capability. The handoff is a process of automatically changing
frequencies as the mobile unit moves into a different frequency zone so that the conversation can be
continued in a new frequency zone without redialing. Another disadvantage of the conventional system is
that the number of active users is limited to the number of channels assigned to a particular frequency
zone.
CELLULAR AND MOBILE COMMUNICATIONS Questions & Answers
Poor Service Performance: In the past, a total of 33 channels were all allocated to three mobile
telephone systems: Mobile Telephone Service (MTS), Improved Mobile Telephone Service (IMTS) MJ
systems, and Improved Mobile Telephone Service (IMTS) MK systems. MTS operates around 40 MHz
and MJ operates at 150 MHs; both provide 11 channels; IMTS MK operates at 450 MHz and provides 12
channels. These 33 channels must cover an area 50 mi in diameter. In 1976, New York City had 6
channels of( MJ serving 320 customers, with another 2400 customers on a waiting list. New York City
also had 6 channels of MK serving 225 customers, with another 1300 customers on a waiting list. The
large number of subscribers created a high blocking probability during busy hours. Although service
performance was undesirable, the demand was still great. A high-capacity system for mobile telephones
was needed.
Inefficient Frequency Spectrum Utilization: In a conventional mobile telephone system, the frequency
utilization measurement Mo, is defined as the maximum number of customers that could be served by one
channel at the busy hour.