all handsets transmitting at maximum range, 20m, with a bit error probability o f 5x1 O'

means to observe the aggregated opinions of lecture audiences numbering 30 or more.

8.6 Cost per handset

The handsets built as a ‘one o ff system cost £50 each; this falls to £40 if more than one system is built due to economies of scale (a calculation detailed in full in

appendix 7). The receiver will cost less than £100 to construct. This extrapolates to a cost, for the construction of more than one 249 handset system, of £12550. The design lifetime of the feedback system is 5 years; therefore the annualised cost of the system is some £2500 (less than 10% of the cost of one lecturer).

However, if many systems were constructed the cost of each handset would be substantially reduced. All of the components, bar the transmitter, could be placed on an Application Specific Integrated Circuit (ASIC). The chief costs would then be the radio transmitter, box and the buttons. The cost of each handset falls to

approximately £15,16 (a 249 handset system costing less than £4000).

8.7 Handset complement

The modulation may not have more than 7 identical consecutive bits as the modulation voltage level may not be constant for more than 7.5ps (section 8.3.1). This precludes the use of 7 addresses, as detailed in table 8.1; 249 handsets can be accommodated.

16 £7 for the transmitter, £3 for the box, £4 for the buttons, and £1 for the combined costs o f the ASIC, handset construction and distribution

o o AO A1 A2 A3 A4 A5 A6 A7 DO D1 mch. X 0 0 0 0 0 0 0 0 X X X 1 1 1 1 1 1 1 1 X X 0 0 0 0 0 0 0 0 1 X X X 1 0 0 0 0 0 0 0 0 X X 0 1 0 0 0 0 0 0 0 0 X 1 1 0 0 0 0 0 0 0 0 X 0 1 1 1 1 1 1 1 1 X

Table 8.1 -- The transmitter precludes using certain addresses (with bits designated

AO to A7) as the modulation voltage must not stay constant for 8 bits or longer. The preceding clock synchronisation bit is a low; the succeeding data bits may take the

value 00, 01 or 10 according to which button has been pressed. Irrelevant bits have been marked “X ”

8.8 Probability of transmission failure and misreception

Each handset transmission lasts 40ps. If a full complement of 249 handsets transmit once per second, equation 5.1 (p.36) indicates that the probability of collision is an acceptable 2.0%. This figure indicates the maximum probability of error; a decrease in the number of handsets (and/or the rate of button pressing) will result an almost linear decrease in the probability of collision (for example, for an audience of 62 using their handsets once per second the probability of collision falls to 0.49%).

There is a separate probability of a bit error during the transmission due to the effects of noise. The probability of one or more bit errors in an n bit data word, Pex, where the probability of error in a single bit is pu, is:

If a bit error occurs during the preamble or clock recovery bits it is likely that the transmission would not be recognised by the receiver as the transmission may not meet the checks outlined in section 8.4 or may fail to synchronise the receiver clock. The radio receiver does not recover baseband data accurately during the first few bits of a transmission - the quickest stabilisation observed was 3 bits, the longest 6 bits. The probability of a bit error in any one of the subsequent bits before receiver clock synchronisation, pu, is 5x1 O'4 at the transmitter’s maximum range, 20m. This probability of bit error is used to give the worst case probabilities for the system; quicker stabilisation also results in an increase in the probability of error during preamble and clock recovery (there are more bits in which to have an error). Finally, it is assumed that any errors during this period result in transmission failure, to provide a worst case estimation of this probability. Using equation 8.1, the maximum probability of one or more errors during the maximum 26 bits of stable preamble and clock synchronisation is 1.3%.

This probability can be reduced if the signal to noise ratio is increased. The radio transmitter radiates at its maximum power, so to increase this ratio would require a post-transmitter radio frequency power amplifier - not an attractive option in light of the power limitation. This probability of transmission failure was therefore tolerated.

The total maximum probability of transmission loss in a 249 handset system, each transmitting once per second, is 3.3%. There is also a 0.6% probability of one or more undetected bit errors during the 11 bits of data and address at the maximum, 20m, transmission range, equation 8.1; however, some errors will not be displayed by the software as they could not have been generated by a handset (section 8.5).

8.9 Energy consumption per transmission

Each handset transmission dissipates 1.17mJ of energy (the calculation is conducted in full in appendix 8). Commercial non-renewable “AA” size batteries typically contain 15kJ of energy [DURA]; each handset is powered by two. Assuming constant peak handset usage - each handset being operated once per second -

throughout one hour lectures, the batteries would last some 7100 hours. However, a more realistic assumption of the average rate of handset use, one press every ten seconds, indicates a battery life of tens of thousands of lecture-hours (that is, the shelf life of the battery expires before it is drained). The batteries do not need regular replacement, and the low-maintenance criterion is satisfied.

8.10 Conclusion

System 2 meets the design criteria for radio frequency lecture feedback. Very large class sizes (up to 249 students) can be accommodated with an acceptable probability of collision, 2.0%, under high use conditions (each of these 249 handsets transmitting once in a second). The handset radio transmissions can be reliably detected at a range of 20 metres, sufficient for use in most lecture theatres. The maximum total probability of transmission failure (including the effects of noise) is 3.3% for a handset transmitting at this maximum range; this handset will also have a maximum 0.6% chance of one or more undetected bit errors in the address or data. The

handsets will cost £40 each - indicating a total cost for a 249 handset system of some £12550. The handsets do not require significant maintenance, as the power