System for synchronizing mutually interfering signals in digital radio transmissions with frequency re-use

Abstract: The present invention concerns a method and a system for demodulating mutually interfering signals according to a suitable synchronism bond between said signals. With such a system a complete decoupling of the demodulators (8,8',10,10') of said signals is obtained. The system is characterized in that on each polarization there is at least a pair of demodulators (8,8',10,10'), a first pair (8,8') operate a baseband conversion of both signal H and V using the carrier recovery circuit (21) driven by data of one of the two signals (e.g. V) and a second pair (10,10') operates still a baseband conversion of both signals H and V but using said carrier recovery circuit (21') now driven by data of the other signal (e.g. H).

Description: The present invention relates to a system for demodulating mutually interfering (H and V) radio signals in which system a demodulator, a cancellation circuit, an equalizer, a carrier recovery circuit and a decision circuit are used on each polarization. In particular the invention relates to a system for demodulating mutually interfering signals in digital radio transmissions with frequency re-use. Prior Art In microware radio link digital transmission there is the need of increasing, as far as possible, the information transmitted per unit of occupied band. The microware transmission is typically carried out by transmitting two distinct signals on two right-angled "polarizations" of the electromagnetic field, hereinafter indicated by H and V. Because of non-ideality of the hardware and transmissive medium, signals H and V tend to interfere each other considerably. There are two conventional ways for making negligible the effects of such interference. The first way consists in transmitting signals H and V on bands which are only partially overlapped, as indicated in fig. 1. In this way the portion of (e.g.) signal H which really interferes with V is small and the interference is tolerable. The interference zones are comprised between points I1-I2, I3-I4, etc. The more efficient second way consists in the so-called re-use of frequency. In this instance the channeling of signals H and V may be represented as in fig. 2 from which it appears that the transmissive capacity is doubled with respect to fig. 1, but the two overlapped spectra (e.g. H1 and V1, H2 and V2 etc.) interfere completely. In this instance the interference of (e.g.) H on V is much more sensible and is opposed through a suitable cancellation circuit which allows the cancellation of the interference itself. The operation principle of the cancellation circuit, e.g. cancellation of the interference of H on V, is well known: the receiver of the V signal has also the received H signal; the cancellation circuit processes the H signal in such a way as to obtain a replica Hr which is as true as possible to said interference of H present on V. The cancellation effect is obtained by subtracting the replica Hr (processed by the cancellation circuit) from signal V. DE-A-3 406 621 discloses such a scheme. The usual hardware implementation of this system contemplates that receiver of V signal has at its disposal the received H signal downstream of the demodulator of H itself. What described up to now is also true if V is replaced by H and H is replaced by V. One of the drawbacks arising from the use of such a system is that when one of the two demodulators does not operate correctly (because of failure, adverse propagation or any other reason), the other receiver does not have the signal necessary for cancellation and, therefore, it is subjected to a sensible, no longer cancelable interference. In other words, if one of the demodulators fails, it not only looses its useful signal (e.g. H) but, worse still, it eliminates the possibility of operation also from the second demodulator (e.g. V). After all one has the complete loss both of H and V, i.e. of all transmission on that carrier. Summary of the invention It is an object of the present invention to provide a system for synchronizing the received signals H and V in such a way as to allow the correct operation of the cancellation circuit irrespectively of the operation of the demodulator which processes the signal on the other polarization. By means of the system according to the invention the malfunction of one of the two demodulators does not compromise the operation of the cancellation circuit of the other demodulator. The system according to the invention is characterized in that at least a pair of demodulators are on each polarization, a first pair operates by converting in base band both H and V signals using said carrier recovery circuit driven through data of one of the two signals (e.g. V), a second pair operates by converting still in base band both H and V signals but using said carrier recovery circuit now driven by data of the other signal (e.g. H). Other remarkable features of the invention are set forth in the dependent claim. In any case the different aspects and advantages of the invention will become more apparent from the description of the general solution in combination with a preferred but not limiting embodiment (fig. 3). General solution The solution according to the invention is based on the use of a critical combination of demodulators (e.g. four instead of two) such that, maintaining unchanged all the other functions (e.g. a sole carrier recovery circuit, a sole adaptive equalization circuit, a sole cancellation circuit for the interfering signal, a sole decision circuit for data etc),one succeeds in realizing the complete independence of a cancellation circuit operation (e.g. the one of V) with respect to the other cancellation circuit (e.g. the one of H) when one of the two is subjected to a damage or a failure. According to an advantageous aspect of the invention the two signals V and H are now translated from radio frequency RF to intermediate frequency IF by using a sole local oscillator. The two IF signals are then provided to the demodulator pairs of said critical combination, which in the preferred case is just two (which corresponds to a total of four demodulators). The first pair of demodulators operate by operating a baseband conversion of both signals H and V using the carrier recovery circuit driven by data of one of the two signals, e.g. V. The second pair of demodulators act according to the same principle but is driven by data of the other signal e.g. H. The two demodulators driven e.g. by the carrier recovery circuit driven in turn by data V, provide two baseband signals V and H which are subsequently processed as described previously as far as the cancellation of the interference produced by H on V is reached. In this way the loss of H data (e.g. for unfavourable propagation) in no way affects the features of V data receiver, since the demodulator of H signal that is used for the cancellation of the interference of H on V is driven by V data. The same is true for the receiver driven by H data. THe block diagram of the complete receiver in a preferred but not limiting embodiment is illustrated in fig. 3. Therein two signals H and V received by antenna 1 on the two ortogonal polarizations are applied through lines 2, 2' to RF/IF sections indicated by numeral references 3, 3'. According to a first aspect of the invention such RF/IF sections are driven by a sole RF local oscillator through lines 5, 5'. The signal at the output of section 3 through lines 6 and 6' and lines 7 and 9' is demodulated by demodulators 8 and 8' while signal at the output of section 3' through lines 6 and 6' and lines 9 and 7' is demodulated by demodulators 10 and 10'. The output of demodulator 8 is fed through line 11 to the equalizer 12, while the output of demodulator 8' is fed to cancellation circuit 12' through line 11'. Likewise the output of demodulator 10 is fed to the equalizer 14 through line 13 while the output of the demodulator 10' is fed to the cancellation circuit 14' through line 13'. The outputs of equalizer 12 and cancellation circuit 12' are fed respectively through lines 15 and 15' to the summing node 17 whose output is fed through line 18 to the decision circuit 19 of V data. Likewise the outputs of equalizer 14 and cancellation circuit 14' are fed respectively through lines 16 and 16' to the summing node 17' whose output is fed through line 18' to the decision circuit 19 of data H. Data decided by decision circuit 19 are applied through line 20 to the V carrier recovery 21 which, through lines 22 and then 23 and 23', drives respectively demodulators 8 and 8'. Similarly, data decided by the decision circuit 19' are applied through line 20' to the H carrier recovery 21' which, through lines 22' and then 24 and 24', drives demodulators 10 and 10' respectively. The invention is applicable in all transmissions in which there is interference, and it is susceptible to modifications, variations, replacements and the like, which fall within the scope of the appended claims.

Claims: System for demodulating mutually interfering (H and V) radio signals in which system on each polarization there are used a demodulator (8, 8', 10, 10'), a cancellation circuit (12'), an equalizer (12), a carrier recovery circuit (21, 21') and a decision circuit (19, 20), characterized in that on each polarization there is at least a pair of demodulators (8, 8', 10, 10'), a first pair (8, 8') thereof operates a basehand conversion of both signals H and V using said carrier recovery circuit (21) driven through data of one of the two signals (e.g. V), a second pair (10, 10') thereof operates a basehand conversion of both signals H and V too, but using said carrier recovery circuit (21') now driven by data of the other signal (e.g. H). System according to claim 1, characterized in that mutually interfering signals are those used in digital radio transmission with re-use of the carrier frequency.

Inventors: BOLLA MAURIZIO, ROSSI LEONARDO, SPALVIERI ARNALDO, BOLLA, MAURIZIO, ROSSI, LEONARDO, SPALVIERI, ARNALDO
Applicants: ALCATEL ITALIA, ALCATEL ITALIA S.P.A.
Application year: 1993
Main classification: H04B7
IPC codes: H04J11, H04B7, H04B1, H04L27