Method and system for controlling the minimal distance between successive errors in digital radio transmission

Abstract: The invention concerns a method of controlling the minimal distance between successive errors in digital radio transmissions when multilevel modulations are used. With such a method the errors are spaced according to the needs, with an extremely reduced use of hardware with respect to prior art techniques. The method is characterized in that, in transmission a differential delay, i.e. different from flow to flow, is applied to each parallel flow not yet modulated, and in reception a delay also differential is applied to each demodulated flow in such a way that the overall delay applied to each flow, i.e. the sum of the delay applied in transmission and the delay applied in reception,is constant and equal for all the flows.

Description: Background of the invention The present invention relates to a method of controlling the minimal distance between successive errors in digital radio transmission, said distance being the number of correct bits between two successive incorrect bits, the method comprising the steps of: in transmission, splitting up a serial signal into first parallel signal flows; multi-level modulating the said first parallel signal flows and transmitting the modulated signal over a transmission channel; in reception, demodulating the modulated signal received via said transmission channel, so obtaining second parallel signal flows, each of said second parallel signal flows corresponding to one of said first parallel signal flows; reconverting the demodulated signal into a unique serial flow. The invention relates also to a corresponding system for controlling the minimal distance between successive errors in digital radio transmission. Description of prior art High capacity digital radio transmissions use multilevel modulations as a rule. The bit flow at the input of the modulator is converted into n parallel flows, depending on the type of modulation, and is transmitted in the channel in the form of 2 n - ary symbols. An eventual error occurring on the channel affects the 2 n - ary symbol; when parallel flows are re-converted into a unique binary flow, the channel error reveals itself in a set of errors which affect up to n successive bits. Because of the various needs it can be necessary to provide that these errors are spaced out in time and are not left concentrated in n successive bits. In order to oppose these error sets on the serial flow it is usual to use particular error correction codes or interleaving techniques, as described for example in US-A-4677626 where a self-synchronizing interleaver for trellis encoder used in wireline modems is disclosed, having on the transmission side a randomizer, a QAM encoder, a QAM modulator and a convolution encoder. Both error correction codes and interleaving techniques have however a high circuit complexity and need anything but simple decoding techniques. Summary of the invention An object of the present invention is to provide a simple method which allows to space out in time the error on serial flow, thus avoiding groups of close, if not consecutive, errors. The proposed method has the advantage of being easily implementable, with the use of extremely reduced hardware with respect to the known techniques. It is characterized in that, as described in claim 1, it comprises the steps of: in transmission, applying a first differential delay to each of the first parallel signal flows, said first differential delay being different from flow to flow; in reception, applying a second differential delay to the second parallel signal flows, the sum of said second differential delay and said first differential delay being constant and equal for corresponding flows of said first parallel signal flows and second parallel signal flows, the value of said sum controlling said minimal distance. A further object of the present invention is to provide a corresponding simple system, as described in claim 4. General solution The solution according to the invention is based upon the introduction of a different delay on each of the n parallel flows which compose the symbol transmitted into the channel. If Δ = nd is the minimal distance which one wants to obtain between two successive errors on the serial flow, the delays are introduced respectively Δo = 0 d T on the first flow Δ1 = 1 d T on the second flow Δ2 = 2 d T on the third flow Δn - 1 = (n - 1) dT on the nth flow, where T is symbol time . In reception,differentiated delays are introduced in a complementary way with respect to transmission, i.e. Δ n - 1 = (n - 1) dT on the first flow, Δ n - 2 = (n - 2) dT on the second flow , Δo = 0 d T on the nth flow. With this scheme,the possibility of having two errors on the serial flow at a distance less than nd is eliminated. In fact an error-on- the channel event, which as said may reveal itself in a set of at most n successive errors, thanks to differentiated delays will appear on the serial flow with incorrect bits at a distance greater than or equal to Δ = nd. Obviously all above is true if only single errors on the channel are taken into consideration, but the probability of having pairs of successive errors is, in a first approximation, negligible as compared with the probability of having single errors. In order to realize delays Δ k = kdT it is sufficient to use simple delay lines consisting of flip - flops, easily obtainable both in discrete and in integrated logic. The proposed technique can he used for all the multilevel modulations, and by varying the parameter "d" the distances necessary to the requirement of the system can be obtained. An example of use of the proposed technique is given by the transmission of an SDH signal, of the synchronous hierarchy, on a radio link. The SDH frame contains special bits, called pointers, that give information on the synchronization of the frame itself. These bits are organized in two successive bytes of eight bits each and, when disturbed by three or more errors, a loss of synchronism of the system takes place, with catastrophic consequences on the quality of the link.Let us assume to transmit the SDH signal with a 32 - QAM modulation i.e. with n=5. The distance between serial errors is desired to be Δ > 16. Since Δ = nd we obtain d = 4 . The five delays to be introduced on five flows will be then Δ o = 0, Δ 1 = 4T,Δ 2 = 8 T, Δ 3 = 12 T, Δ 4 = 16 T. In this way the effect of single channel errors on the two bytes of pointers is completely eliminated thus improving the quality of the link. The scheme of the proposed solution for a 2n level generic modulation is illustrated in detail (fig. 1). Signal i from the source the series/parallel converter 2 through lines 1. At its output there are n parallel flows Xo, X1, ...., Xn - 1 (lines 3). To each flow Xi, the delay Δi indicated by reference numeral 4 is applied and the signals so obtained are fed to modulator 6 through lines 5. The modulator generates the multilevel signal s and transmits it in channel 8 through line 7. The channel adds the eventual errors by providing signal r which is transmitted to demodulator 10 through line 9. The demodulator provides n flows Yo,.....Yn - (lines in which an eventual error reveals itself at the same time. The delay Δ n-1-i , indicated with 12, complementary to the transmission one, is applied to each flow Yi. Signals zi thus obtained are transmitted through lines 13 to the parallel/series converter 14 which provides the output u on the line 15.

Claims: Method of controlling the minimal distance between successive errors in digital radio transmission, said distance being the number of correct bits between two successive incorrect bits, the method comprising the steps of: in transmission, splitting up a serial signal (i) into first parallel signal flows (X0, ...... Xn-1); multi-level modulating the said first parallel signal flows and transmitting the modulated signal (s) over a transmission channel (8); in reception, demodulating the modulated signal (r) received via said transmission channel, so obtaining second parallel signal flows (Y0, .... Yn-1), each of said second parallel signal flows corresponding to one of said first parallel signal flows; reconverting the demodulated signal into a unique serial flow (u), characterized in that it further comprises the steps of: in transmission, applying a first differential delay (Δ0, .... Δn-1) to each of said first parallel signal flows (X0, ...... Xn-1), said first differential delay being different from flow to flow; in reception, applying a second differential delay (Δn-1, .... Δ0) to said second parallel signal flows (Y0, .... Yn-1), the sum of said second differential delay and said first differential delay being constant and equal for corresponding flows of said first parallel signal flows and second parallel signal flows, the value of said sum controlling said minimal distance. Method of controlling the minimal distance between successive errors in digital radio transmission as in claim 1, characterized in that said first differential delay (Δ0, .... Δn-1) increases progressively from one flow to the following one according to an increasing law, and said second differential delay (Δn-1, .... Δ0) decreases progressively according to a complementary decreasing law. Method of controlling the minimal distance between successive errors in digital radio transmission as in claim 2, characterized in that said increasing law applies delays increasing by the same fixed quantity "dT", being T the symbol time, and said minimal distance Δ = nd, being n the number of parallel flows. System for controlling the minimal distance between successive errors in digital radio transmission, said distance being the number of correct bits between two successive incorrect bits, the system comprising: in transmission, means for converting (2) an input serial signal (i) into first parallel signal flows (X0, ...... Xn-1); means for multi-level modulating (6) the said first parallel signal flows and transmitting the modulated signal (s) over a transmission channel (8); in reception, means for demodulating (10) the modulated signal (r) received via said transmission channel (8), so obtaining second parallel signal flows (Y0, .... Yn-1), each of said second parallel signal flows corresponding to one of said first parallel signal flows; means for reconverting (14) the demodulated signal into a unique serial flow (u), characterized in that it further comprises: in transmission, first delay means (4) for applying a first differential delay (Δ0, .... Δn-1) to each of said first parallel signal flows (X=, .... Xn-1), said first differential delay being different from flow to flow; in reception, second delay means (12) for applying a second differential delay (Δn-1, .... Δ0) to said second parallel signal flows (Y0, .... Yn-1), such that the sum of said second differential delay and said first differential delay is constant and equal for corresponding flows of said first parallel signal flows and second parallel signal flows, the value of said sum controlling said minimal distance. System for controlling the minimal distance between successive errors in digital radio transmission as in claim 4, characterized in that said first delay means (4) apply said first differential delay (Δ0, .... Δn-1) increasing progressively from one flow to the following one according to an increasing law, and said second delay means (12) apply said second differential delay (Δn-1, .... Δ0) decreasing progressively according to a complementary decreasing law. System for controlling the minimal distance between successive errors in digital radio transmission as in claim 5, characterized in that said increasing law applies delays increasing by the same fixed quantity "dT", being T the symbol time, and said minimal distance Δ = nd, being n the number of parallel flows.

Inventors: BOLLA MAURIZIO, PELLIZZONI ROBERTO, SPALVIERI ARNALDO, BOLLA, MAURIZIO, PELLIZZONI, ROBERTO, SPALVIERI, ARNALDO
Applicants: CIT ALCATEL, ALCATEL
Application year: 1993
Main classification: H04L27
IPC codes: H04B7, H04L27, H04L1