For on-site private systems, e.g. those used in hospitals, hotels and factories, the coverage is limited and usually a single-site system using a low-cost, low-power (up to 10W) compact transmitter will suffice.
The coverage for public paging systems can vary significantly in extent. For a small city or country, one paging transmitter site may be sufficient, but when the area to be covered is extensive it may be necessary to have multiple transmitter sites. When more than one transmitter is used for the purpose of improving coverage, the technique of simultaneous transmission (simulcasting) from all the transmitters usually needs to be employed. This will be discussed in greater detail later.
Also, "network" paging will be examined. This is an arrangement whereby individual systems are linked up or networked such that a subscriber who roams out of range of his "home" system can still be paged. This is made possible by passing the original page request from his "home" paging system through the network to the paging system covering his new location. Networking enables nationwide paging for big countries like the USA and when satellite links are employed in the network, international or global paging facilities can be realized.
An omni-directional antenna radiates an input signal with equal gain in all directions
while a directional antenna provides relatively higher gain in a particular direction. The
appropriate antenna type is chosen and aligned/adjusted to give a pattern consistent with
the desired coverage. Sometimes the coverage pattern is also adjusted with directional
antennas to minimize interference with other communication systems using a frequency
similar to or close to that used by the paging system.
Other contributions to path losses include obstruction by the earth itself, known as
"diffraction" loss due to the signal grazing the earth's surface, and by hills, trees and
buildings.
The severity of multipath fading usually increases as the frequency increases, and it can
be predicted using statistical methods. A reliability margin is often added during
coverage calculation to take care of fading.
To check if a location is within the coverage area of a paging station, the received field
strength at that location is calculated. This is obtained by starting with the transmitted
power, adding gains and losses from the antenna and subtracting all the path losses
mentioned above plus any transmission line, connector or filter losses. A point at the
edge of the coverage area will have the received field strength value greater than the
pager receiver sensitivity by the amount of fading margin.
In simulcasting, the following parameters must be kept within tight tolerances:
The paging transmitters incorporate ultra stable oscillators to ensure that any difference in
carrier frequency of the paging transmitters is kept below specified limits for proper
simulcast operation. At regular intervals the oscillators are re-synchronised, i.e. their
carrier frequencies are reset to within acceptable limits.
The difference in distances from the paging terminal to the various transmitters causes
audio signals to arrive at the transmitters at different times, i.e. they arrive out of phase
with respect to each other. Audio equalizers, which are variable time delay elements are
used at the base stations to take care of this undesirable phase offset.
Precise audio level adjustments are also performed at the transmitters to optimize the
system for simulcasting.
For communication between the networked paging terminals, various communications
protocols have been developed. A communication protocol is defined as a set of rules
designed to facilitate efficient and reliable transfer of information between two stations.
For example, a protocol may specify that the information should be divided into packets
of a certain size and that sending and destination address information should precede data
information. And if there are errors in a received packet, the destination station will ask
for a resend of that packet from the sender.
Proprietary protocols are mostly incompatible with each other and hence paging networks
are usually established using paging terminals from a single supplier.
To allow for interconnection of equipment from different manufacturers into paging
networks, "gateways" must be used. These are essentially bridging devices which
perform protocol conversion, thus acting like language interpreters, allowing networks of
incompatible equipment to "talk" and work together. A better solution is the creation of
an industry standard protocol, such as X.25 and TNPP.
The Telocator Network Paging Protocol (TNPP) is one such protocol widely accepted
and employed for creating networks of paging terminals from different manufacturers. It
can also be used in the case of networks using similar paging terminals. It is a point-to-
point digital communications protocol ensuring reliable delivery of information from one
paging terminal to another. The routing of page requests through the network is not
covered in the TNPP specifications, it being a point-to-point protocol. However, in a
large network of paging terminals, network routing needs to be performed efficiently and,
hence, in most of such networks, routing algorithms are always implemented to move
paging requests between one or more nodes.
The TNPP is not restricted to linking paging terminals through wireline. Page requests
can even be broadcast over a satellite to paging terminals in distant locations. This can
extend paging coverage across international borders. In this satellite network system, the
protocol ensures reliable delivery of data by transmitting it many times from the source,
at the same time ensuring that duplicate information will not be received at the
destination. This multiple re-transmission technique is required because the long
communication distance involved precludes the sending of signals from the destination to
the source, confirming that data has been received correctly. TNPP is being used to
provide features such as electronic mail (E-mail) communications and remote status
monitoring.
©
Copyright 1995, 1996, Motorola, Inc. All Rights Reserved.
FACTORS AFFECTING COVERAGE
The coverage of a paging station is dependent on the following factors:
Transmitter Power
Increasing transmitter RF power output is an obvious way to increase coverage area but
very often a more effective method to improve coverage is to increase base station
antenna height. In most situations, analysis shows that doubling the transmitter power
results in only a 1.4 times increase in field strength at the receiving point, while doubling
antenna height nearly doubles this field strength.
Receiver Sensitivity
Receiver sensitivity is defined as the field strength required for a paging receiver to
respond to a specified percentage of calls transmitted. A pager with a high receiver
sensitivity is desirable as it will mean that a lower field strength is sufficient for it to
respond correctly at the fringe of the coverage area. Therefore, using pagers with a
higher sensitivity will result in wider coverage as compared with the case where less
sensitive pagers are used. (Receiver sensitivity in analog voice applications is also
commonly defined in terms of signal input level required to produce an audio noise drop
of 20 dB at the receiver output; receiver sensitivity for digital systems is specified in
terms of the signal level required to attain a BER (bit error rate) not exceeding 1%.)
Antenna Gain
The antenna gain is a measure of the directivity of the signal as it is sent or received
through the antenna. For example, an antenna with unity gain will neither magnify nor
attenuate the signal. It is a means by which coverage of a paging system can be
improved. The design engineer has to weigh the penalty of using an antenna along with
gain versus using a unity gain antenna with a transmitter of higher power. Directivity in
antennas comes at the price of narrowed bandwidth.
Antenna Height
As was discussed earlier, increasing antenna height is an attractive alternative to
increasing transmitter output power for better coverage. Increasing the height of an
antenna usually means using a longer feed line with additional line losses, so a complete
system trade-off is required. Typically, transmitters are located at the top of tall
buildings or hills to gain additional height, and therefore coverage. If no buildings are
available in the desired location, a tower is sometimes constructed with a protected
enclosure at the base with the antenna mounted as high as possible.
Frequency of Transmission
Frequencies widely used for paging are the VHF low (40 MHz) and high (150 MHz)
bands, UHF (450 MHz) and 900 MHz. In some countries, 280 MHz is also available.
Generally, penetration of the transmitted signal through buildings is better at higher
frequencies. Trees and foliage tend to attenuate more at higher frequencies. Path loss due
to spreading of the wave as it propagates through the atmosphere is the same at all
frequencies between antennas having a constant aperture with frequency.
Path Loss
We refer here to the attenuation of the signal as it propagates from the transmitting
antenna to the paging receiver. One component of this path loss arises from the spreading
of the wave as it propagates through the air. When the path is long or the antenna height
is low such that transmission is close to the ground, the presence of the ground modifies
the generation and propagation of the radio waves so that the received field intensity is
normally less than what would be expected for propagation in free space.
Fading
Fading is a phenomenon whereby signal level varies within short distances due to
multipath propagation. Additionally, as atmospheric conditions vary, the transmission
path will be altered (bent up or down) and this may increase or decrease the effective path
clearance.
SIMULCASTING
"Simulcast" is a reliable method of achieving wide-area coverage. It involves sending the
paging signal from multiple paging transmitters at precisely the same time. This
technique has the obvious advantage of a larger coverage area arising from combining the
individual transmitter coverage. There are other advantages:
Operational Advantages of Simulcasting
Operational Requirements
A page is initiated by the user using an encoder and/or terminal. The controller could be integrated into the paging terminal or it could be a separate unit as shown in the figure. Paging signals are sent to a distribution system which can either be wireline or radio link.
NETWORK PAGING
This is an arrangement in which paging subscribers can be alerted even when they move
from city to city, state to state or even across national borders. In this arrangement,
individual paging terminals are connected to form a network of paging terminals. Each paging terminal controls its own coverage area, but when a subscriber
moves beyond a coverage area, the page request is passed across the network to the
paging terminal which covers the subscriber's new location. This would require the
paging subscriber to keep the system updated with regards to his movement and
whereabouts.
Goto
Table of Contents ||
Chapter 1 ||
Chapter 2 ||
Chapter 3 ||
Chapter 5 ||
Chapter 6 ||
Chapter 7 ||
Appendix A ||
Appendix B ||
Appendix C ||
Glossary
Last updated: June 5, 1996