Free space communications system employing line of sight radiation

A signal transmission system comprises a transmitter arranged to transmit a coded signal as a series of pulses of line of sight radiation having a predetermined minimum interval therebetween, a receiver responsive to such radiated pulses received at a corresponding minimum interval, and a plurality of repeater devices. Each repeater device comprises a radiation receiving element, a radiation transmitting element, and means responsive to receipt of a pulse of transmitted energy by said receiving element for causing said transmitting element to emit a corresponding pulse of transmitted energy and for causing said receiving element to be disabled for a predetermined period of time corresponding to at least a multiple of the duration of the pulse transmitted by said transmitting element. A plurality of the repeater devices can thus be arrayed between said transmitter and receiver to provide a plurality of line of sight radiation propagation paths with a plurality of repeaters each in line of sight of the transmitter and/or other repeaters and with the receiver in line of sight of a plurality of said repeaters so that random interruption of the signal propagation paths does not interrupt the transmitted signal. Each repeater may comprise its own transmitter and key pad to provide an audience polling system.

FIELD OF THE INVENTION 
This invention concerns signalling devices employing line of sight 
radiation, and more especially, but not exclusively, to infra red 
transmitters and receivers of the kind that are designed to communicate 
using coded signals. 
BACKGROUND OF THE INVENTION 
Such devices have particular application in circumstances where relatively 
short range line of sight communication is appropriate, and avoid 
disadvantages inherent in, for example, radio communication or the use of 
cables. They do have the disadvantage, however, that signal communication 
is interrupted when there is no direct line of sight between transmitter 
and receiver. Where relatively short range communication is involved, it 
has been proposed to overcome the latter disadvantage by providing infra 
red reflectors or repeaters to enable transmission via an indirect path. 
This presupposes however that the path or paths of transmission can be 
planned in advance and does not allow for possible random interruption of 
signal paths. 
One known communication system, for example, makes use of a communication 
link for polling a number of hand held keypad devices used in gathering 
opinions from delegates at meetings. 
At such a meeting, each delegate is equipped with a hand held keypad. The 
presenter displays a question on a video screen and the delegates enter 
their response on their keypad. Each keypad responds to a different 
address and, as the controlling computer polls the keypad addresses, they 
each return their delegate's response. 
Such a system uses serial bi-directional communications over cables. Most 
conferences are held in venues which do not provide group response systems 
so that such a system would be hired in for the duration and installed on 
a temporary basis. The installation of such a system is labour intensive 
in view of the requirement for the provision of cabling, and thus a system 
that avoided the use of cables would provide a significant advance on the 
state of the art. 
WO-A-91 07028 discloses an arrangement wherein a plurality of repeaters are 
arranged to provide line-of-sight communication between a transmitter and 
a receiver located within a given area. The repeaters act as passive 
repeaters of pulses of radiated energy, and are arranged to incorporate a 
lock-out period wherein each repeater is disabled for a predetermined 
period of time following its response to a received pulse. Such an 
arrangement enables line-of-sight propagation of a pulsed signal via a 
plurality of possible propagation paths, without ambiguity caused by a 
repeater responding to a pulse that it has already transmitted. In such an 
arrangement, however, the repeaters, which are intended to be arranged at 
a fixed spacing, act merely passively to propagate radiation between 
dedicated transmitters and receivers. 
U.S. Pat. No. 5,099,346 describes a system for the propagation of 
line-of-sight radiation between devices that can receive and transmit in 
alternative modes in which they act either as originating transmitters, or 
as passive repeaters of a signal transmitted by another device. In this 
system, however, for a device to act as a repeater it is necessary for a 
predetermined signal path to be established so that the repetition of 
signals is via a defined path through a network. For the device to 
operate, a relatively complicated communications protocol is therefore 
required. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide an improved 
means of line of sight signal transmission that is less susceptible to 
failure as a result of random blockage of signal paths and that is 
therefore usable in systems of a kind that has hitherto required the use 
of radio or direct cabling. 
The present invention provides a polling system employing line-of-sight 
radiation, and a device for use therein, as respectively set out in claim 
5 and claim 1. 
Further preferred features of the invention will become apparent from the 
dependent claims taken in conjunction with the following description.

DETAILS 
FIG. 1 shows a typical seating plan of a room containing a polling system 
of the kind described above and embodying a signal transmission system 
according to the invention utilising infra red links to eliminate the 
cabling of the known system. Keypads are shown in four regions labelled B, 
C, D, and E. A transceiver A for polling the keypads is shown in the top 
right hand corner of the room and the regions indicate the general 
progression of the communications using the proposed repeater method. The 
polling command and keypad address first emanate from A and act directly 
with region B. Region B repeats the signals to region C and so on to 
region D and then region E. The keypad that is being polled, lets say in 
region E then radiates its signal to region D which repeats it to region C 
and so on back to the transceiver A. 
The key factors to be considered, to make this method successful, are 
feedback loops and propagation delays. 
FIG. 2 shows a timing chart with the transceiver signal A being 
successively delayed through each region B1 in region B, C1 in region C, 
D1 in region D, E1 in region E at times t1, t2, t3, t4 and t5. 
To prevent feedback loops, each repeater has a lockout period B2, C2, D2, 
E2 which is initiated by a received pulse. Looking at FIG. 2 keypads in 
region B could be retriggered by keypads in region D. To prevent this the 
receiving circuit in B is disabled for the lockout period B2. 
This strategy works provided that there are not so many propagation stages 
that keypads could be producing pulses after the lockout period as shown 
by X1 in FIG. 2. 
However as such a key pad would be out of range of keypads that have ended 
their lockout period this would not be a problem. The lockout period may, 
for example, be only 4 to 10 times the propagation delay. 
FIG. 3 shows how the repeater is implemented using standard electronics 
techniques. 
The received signal at the diode detector A is amplified and differentiated 
B. The signal is passed through a comparator C with its threshold Cl set 
above noise levels. The output pulse is then retimed by monostable D to 
produce a signal for repetition. The lockout monostable E is also 
triggered by monostable D. Monostable E disables monostable D during the 
lockout period. 
The lockout period is less than or at most equal to the interval between 
consecutive pulses of a coded signal. As shown in FIG. 2 the lockout 
period B2 ends at time t6 before the next pulse of transceiver signal A at 
time t7. Each keypad thus recognises the individual pulses originating 
from the transceiver whilst ignoring repetitions of the same pulse. 
Whilst repeating the transmitted pulses each keypad also decodes the signal 
via the microprocessor shown in FIG. 3 to determine whether its own 
address has been transmitted and it should transmit a response to the 
poll. Such a response includes data input manually via the keypad and 
stored by the microprocessor as is conventional in the known systems 
referred to above. 
When a keypad is transmitting its delegate's response it changes from 
repeater mode to originator mode using the mode control line and mode 
switch F. After it has completed its transmission the keypad returns to 
repeater mode. 
In the illustrated embodiment the normal non-return-to-zero (NRZ) serial 
data format of the microprocessor is converted to return-to-zero (RZ) 
serial data format for transmission. This improves the integrity of the 
signal once it is converted into pulses. The RZ signal is likewise 
converted to NRZ at the input to the microprocessor. 
Advantageously each keypad has IR detectors and emitters on its sides as 
well as the front edge. Such an arrangement is shown in FIG. 4, wherein IR 
detectors are illustrated at 10 and emitters at 11. This allows the signal 
to pass across a row of delegates sitting next to each other. In addition 
to keys 14 for manual input, the keypad may also have a microphone 12 
which transmits using infra red and preferably also the repeater method. 
In the latter case the mode switch F will be set to originator mode upon 
use of the microphone, for example, in response to actuation of a 
transmission button 13 associated with the microphone. The keypad is held 
up to the speaker's mouth when it is being used, hence the importance of 
sidelooking transceivers which retain their general sideways view when the 
keypad is held like this. It will be appreciated that alternative means 
may be provided for the reception of microphone signals, such as dedicated 
IR receivers at the ends of each row of seating.