Patent GB-574627-A (GB)
Title: Radio relay system
IPC: H04B7/17
Filed: 1944-02-11
Abstract: 574,627. Pulse modulation. STANDARD TELEPHONES & CABLES, Ltd. (International Standard Electric Corporation). Feb. 11, 1944, No. 2544. [Class 40 (v)] In a relay arrangement for a pulse modulation communication system, automatic repeater stations are used, each of which comprises a transmitter operating on a common frequency, a time-delay network, and a receiver. When a pulse has been received at a repeater station, it is retransmitted after such delay that the receiver at that station may be rendered inoperative during the retransmission. Pulses of carrier frequency energy from the transmitter A (Fig. 1) are picked up on an aerial 1 at the repeater station B, and demodulated by a receiver 2. Each pulse is delayed a fixed time by the network 4, being then retransmitted by the transmitter 3, which produces a control voltage on the line 7, whereby the device 6 makes the receiver 2 momentarily inoperative. The transmitter 3 energizes simultaneously the aerial 1 and a directive aerial 8, retransmission occurring only in the direction of the receiving station C, thus preventing re-operation of earlier repeater stations by the retransmitted pulses. Alternatively, Fig. 2 (not shown), it is arranged that the strongest signal arriving at each repeater station is that emitted by its immediate precursor, an amplitude discriminator permitting no other signal to reach the receiver. The signal transmitted comprises initial marker or synchronizing pulses S1, S2, S3, &c. (Fig. 3), and final marker pulses s1, s2, &c. The initial marker pulses S1 &c. are spaced apart a uniform time-interval T, and may be suppressed at the transmitting station and reintroduced at the receiving station, the signal being conveyed by the final marker pulses s1 &c., each of which is spaced a variable time t after its corresponding initial pulse S1 &c. At each repeater station, the pulses s1, s2, &c. are delayed the same time-interval t (Fig. 3a) before retransmission as the pulses s11, s21, &c. respectively. If the times t, t1 be sufficiently great, there is a danger that the moment of retransmission of the one pulse s11 may coincide with. the time of arrival of the next pulse s2, which will be lost by the receiver being inoperative at that instant. This may be avoided by limiting the depth of modulation to less than the timer T /2 and making the time delay at the repeater stations also less than the time T/2. In a two - way communication system, different carrier frequencies may be employed for the two directions of transmission, in which case (Fig. 4, not shown), each repeater station may comprise a separate transmitter, delay networks, and receiver for each of the two directions and a common aerial. Filter networks such as the tuned lines described in Specifications 457,911 and 491,359 may be employed to separate the incoming signals, and additional directive aerials may be arranged to be energized from the appropriate transmitters in order to retransmit the signals in the correct directions. The same carrier frequency may be used for transmission in both directions, if some distinction be made in the respeetive pulses, such as an initial identifying portion of characteristic amplitude (Fig. 5, not shown), or shaping of the pulses such as is described in Specifications 524,671, 529,648 or 528,192. The repeater station, Fig. 6, may comprise a common aerial 1, receiver 2, delay network 4, transmitter 3 and receiver blocking network 6, as previously described. The direction of retransmission of each pulse is determined by the networks 15G and 15J which discriminate between pulses from the stations G and J, respectively, and cause the appropriate directive aerials 8G or 8J to be energized from the transmitter 3. Alternatively, a single receiver may be used; the discriminating networks preceding a pair of transmitters, each of which energizes an aerial for one direction of communication; or the discriminating networks may directly follow the -receiving aerial and completely separate systems be employed for the two directions (Fig. 7, not shown). The two directions of communication may be distinguished by the timing of the pulses. Thus (Fig. 3), the first half of the time-interval T between successive synchronizing pulses may be allocated to pulses travelling in the one direction, and the second half of the interval to those travelling in the opposite direction.. If all pulses are delayed at the repeater station, a time T equal to the interval between synchronizing pulses, the resultant relayed signal is effectively delayed the time T and not otherwise, altered. In Fig. 8, a common aerial 1, receiver 2, delay network 4 and transmitter 3 are employed as previously described, a commutating device 17 causing each of the directive aerials 8K, 8M to be energized from the transmitter 3 for an appropriate half-period T/2. The device 17 may be operated by the synchronizing pulses, which are separated out by a discriminating network 18. The synchronizing pulses may have a greater amplitude than the final marker pulses, in which case 'an amplitude limiter may be employed, or the synchronizing pulses may be characteristically shaped and a pulse shape discriminator employed.