## Saturday, 20 September 2014

### The mobile phone system IB QUESTIONS

1).        (a).       Explain what is meant by 'base-station'.

(b).       How do the base-stations communicate with:-

(i)         the mobile telephone

(ii)        other base stations?

(iii)       with land telephones?

(c).       What are the four essential sub systems in a mobile telephone

2).        (a).       Explain what is meant by a 'cell'.

(b).       (i)         State the typical range of sizes of a 'cell'.

(ii)        What factors determine the size of a cell?

(c).       What are the radio frequencies bands used for mobile telephones?
..

(d).      What properties of the radio waves of these frequency bands enables a large number
of mobile telephones to be used within a restricted frequency allocation?

(e).       How is frequency multiplexing used to maximise the number of available mobile                            telephone channels within the frequency allocation?

(f).       How is time division multiplexing used to maximise the number of available                                   telephone channels within the frequency allocation?

(g).       Explain why there is a restriction in the range of frequencies that can be used in                             adjacent cells.

(i).        Draw a diagram to show how the frequency channels are allocated within a block of                     16 adjacent cells.

(j).        What commercially limits the number of mobile telephones that a telephone network                     operator can connect within a given cell?

3).        (a)        State the bandwidth allocated for the Uplink for a GSM telephone.

(b).       If this allocation is divided into 200kHz frequency channels, explain why there are
only 124 possible frequency channels available.

(c).       How many data channels are fitted into each frequency channels?

(d).      A mobile telephone operator is allocated 40 non-adjacent frequency channels for the
entire network.

(i)         State, with a reason, the maximum number of frequency channels that he can
allocate to each cell in his network?

(ii)        Calculate the maximum number of subscribers that can be connected
simultaneously in a cell.

(e).       The network operator finds that there are insufficient channels available for the
number of subscribers wishing to use the network.  What are the options that he has   for
increasing the capacity?

(f).       (i)         Explain what is meant by frequency re-use.

(ii)        Explain why frequency re-use is essential with the present mobile telephone
system.

4).        Mobile communication systems have brought with them many advantages and issues.
These include:-
The cost of calls.
The environmental impact of mobile phone aerials and masts.
The speed with which people can be contacted.
The benefits of always being able to contact people.
Phone related crime.
Pressures caused by mobile phones on employees working in industry and
commerce.
Mobile phone technologies are also continuing to evolve.

Prepare a presentation in which you consider the current uses of mobile communication systems   and how these are likely to evolve in the future so bringing new advantages and issues.
SOLUTIONS:

1).        (a)        The name given to the fixed transmitters and receivers with which a mobile phone
communicates.
(ii)        cable, fibre cable, direct microwave link, satellite
(iii)       cable, fibre cable
(c)        transmitter, receiver, microcontroller/computer, battery power management system

2).        (a)        A geographical region served by a base station.
(b)        (i)         from a few tens of metres to 30km
(ii)        number of users, physical barriers e.g. buildings, hills etc
(c)        900MHz; 1800MHz and 2GHz
(d)       Only propagate in line of sight; readily attenuated by the air, buildings etc
(e)        For GSM/DCS1800 the allocation is divided into 200kHz channels.
(f)        For GSM/DCS1800 each frequency channel is divided into eight time division slots, so                 allowing eight phones per frequency channel.
(g)        The digital nature of the transmitted signal means that there is the possibly some                            adjacent channel interference.  So there is a restriction that adjacent frequency channels  are not used in adjacent cells.
(i)         The different colours represent the different frequency channels

(j)         A base station needs separate transmitter and receiver for each phone connected.  The                   costs of this equipment commercially limits the number of phones that can connected  within a given cell.

3).        (a)        890 – 915MHz
(b)        Need to allow 100kHz at the band edges to reduce interference to the radio users each                  side of the GSM allocation.
(c)        8

(d)       (i)         40 / 3 = 13
(ii)        13 x 8 = 104
(e)        Purchase channels from other network operators,
reduce the size of the cells by building more base stations etc
(f)        (i)         Because of the attenuation and line of sight properties of microwaves, the same                             frequencies can be used in different geographical areas without interference
(ii)        Frequency re-use – the whole principle on which the mobile phone system                          functions, since there is inadequate frequency channels available for the number    of phones in existence.

4).        Greater use of phones for entertainment (OLED screens, laser projection, head up display on        glasses etc)
Computers, PDAs and phones merging into one entity,
Wearable phones – built into clothes etc