Cellular Mobile Radio Systems
 
     Introduction

    The early mobile radio telephone systems all used one high powered base station per city
     or service area to blanket the area. This meant that the limited number of channels could
     not be reused in neighboring cities or towns because the signals would interfer with each
     other. Furthermore, the frequency spectrum is, and it will always be , a scarce national
     resource.  The limited number of channel is the main abstacle which prevented mobile
     radio systems designers to achieve a truly high capacity system. Even utilizing channel
     frequencies in the microwave band does not give the ultimate solution. However, cellular
     mobile radio systems are able to popularize the service by offering expandable high capacity
     systems.
 
     In cellular radio system, frequency reuse techniques are employed to achieve high capacity
     mobile radio systems . Furthermore, cell splitting techniques are employed to expand
     these systems to meet the ever increasing public demand.

     Cellular mobile radio systems were developed and actually installed in several countries
     and are showing success over the past few years and is one of the fastest growing and most
     demanded telecommunication applications ever. In the long term perspective, cellular radio
     using digital technology will become the universal way of communication for virtually
     everybody.
 
 
 
 
 
 
 
      Cellular Concepts

     Cellular radio makes better use of the limited frequency spectrum available for mobile radio
     by re-using the same frequencies many times over . Frequency re-use is achieved by dividing
     a large geographical area into a number of small, nominally hexagonal areas, known as cells,
     over the whole country. The transmitted power level of each base station is limited to
     restrict the coverage area of that base station. Frequencies are assigned in such a way that the
     same frequency can be used for different voice transmissions only a few cells away.

     The cells are arranged in clusters and the allocated bandwidth is divided between the cells
     in each cluster. Three-, four- and seven-cell clusters are shown in Fig. 35  and 12- and 21-
     cell clusters are also sometimes employed. Regular patterns of clusters then give total
     coverage of the geographical area. Fig. 36  shows how coverage of an area is achieved
     using a large number of seven-cell clusters.

  
Fig.35
 
Fig36
     Cellular radio uses multitudinous access points sited according to local traffic demands.
     The physical size of a cell is limited by radio wave propagation characteristics. At VHF and
     UHF , propagation is  'line-of-sight' and the coverage area is influenced by buildings and the
     local terrain. In a town or city it is necessary to place some base station aerials at the top
     of tall buildings and to position others lower down, on the top of lamp posts for example.
     This means that in the center of a town, the size of a cell may be as small as 1 km in diameter
     and in such cases a cell is then known as a microcell. Within a high office block it is often
     necessary to use even smaller picocells. Fig. 37 shows a mixed-cell cellular radio system.
 
Fig.37
 
     Co-channel interference can be reduced by the use of a sectored aerial at the base station.
     A three-sectored aerial has a coverage angle of 120 degree and its use effectively divides
     a cell into three sectors, each of which can be regarded as a new cell with its own set
     of channel frequencies. Each of the new cells is excited at one corner; this is shown by
     Fig. 38 which shows
 
Fig. 38
          (a) a sectored three-cell cluster,
          (b) a sectored four-cell cluster and
          (c) a sectored seven-cell cluster.

     The number of cells provided are ;
           (a) 9
           (b) 12
           (c) 21.
 
 
 
 

      Cellular Radio Systems

     The original cellular radio system employs analogue technology, and a large number of
     incompatible systems have been installed in different countries. The UK system is known
     as the Total Access Communication System (TACS) . It occupies the 900 MHz frequency
     band and has an r.f. channel spacing of 25 kHz.  The main specifications are :

    The base station is known as the mobile switching centre (MSC) or the mobile telephone
     switching office (MTSO) , and it automatically controls and maintains all calls inititiated by ,
     or incoming to , a mobile in its cell . The MTSO also swtiches, bills and administers
     telephone traffic , Each MTSO is connected to the PSTN by a local switching office (LSO).

     When a mobile is turned on, it searches for both a dedicated control channel and a paging
     channel in the cell in which the mobile currently is located, and then it goes into its idle state
     in which it continuously monitors the paging channel . If at any time the amplitude of the
     paging signal falls below a set value , the mobile will search for another, stronger, paging
     signal . At all times a mobile is automatically listening for an incoming call. As the mobile
     moves its position, it must register its whereabouts with the nearest base station so that
     its location is up-dated whenever it moves into another cell .

     When a mobile wishes to initiate a call , the wanted telephone number is keyed and this
     information is transmitted over the control channel to the base station.  If a speech channel
     is free, the MTSO allocates a channel to the mobile and sets up the required connection via
     the PSTN. Should there be no free channels at that time , the mobile will automatically try
     again after a random short interval of time.  When the calls is terminated, the mobile
     sends an  8 kHz tone for 1.8 seconds to the base station to signal end-of-call before it
     returns to its idle state.

     When there is an incoming call for a mobile , the LSO pages all base stations near the
     last known location of the wanted mobile by sensing a paging signal on the paging
     channel of each base station .  When the wanted mobile receives the paging signal, it
     automatically accesses the network . The mobile is then allocated a free speech channel
     by the nearest base station and the mobile automatically tunes to that channel frequency .
     The base station then transmits an 8 kHz tone to the mobile to indicate that there is
     an incoming call.  When the mobile answers, this tone is turned off and the connection is set
     up.

     If , during the progress of a call, the mobile travels from one cell to another, the received
     signal level will fall and this reduction in amplitude will start an in-call hand-over . The
     base station notifies the LSO and this then tells all base stations to measure the signal
     level from that mobile , and then the call is handed over to the base station that has
     the strongest signal. The mobile transceiver is automatically tuned to the new carrier
     frequency . The hand-over process is illustrated by Fig. 39 . There are rarely more than
     one or two hand-overs in a sigle call. To reduce co-channel interference, adaptive power
     control is used. This means that the power transmitted by a mobile  is controlled by the
     base station to just above the minimum level needed to give an acceptable signal-to-noise
     ratio.
 
 

Fig. 39
Hand-over as mobile goes
from one cell to another
 
 
 
 
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