IL153984A

IL153984A - Synchronizing, in a base station, signals transmitted by several terminals for fdma communication systems

Info

Publication number: IL153984A
Application number: IL153984A
Authority: IL
Original assignee: Telediffusion Fse
Priority date: 2000-07-17
Filing date: 2003-01-16
Publication date: 2008-12-29
Also published as: AU2001220148A1; EP1302016A1; FR2814301A1; IL153984A0; WO2002007375A1; DE60044948D1; EP1302016B1; ATE480917T1; FR2814301B1

Abstract

The invention concerns a FDMA signal, formed by a multiplex of carrier frequencies transmitted by at least two separate terminals and designed to be processed in the form of a single signal in a base station, bearing temporal synchronising data comprising, for at least some of said terminals, a phase deterministic value simultaneously and continuously transmitted on at least two of said carrier frequencies, for a duration not less than two symbol times.

Description

FDMA »1W Synchronising, in a base station, signals transmitted by several terminals, for FDMA communication systems , Telediffusion De France C. 143029 SYNCHRONIZATION OF SIGNALS TRANSMITTED BY SEVERAL TERMINALS IN A BASE STATION FOR FDMA COMMUNICATION SYSTEMS The field of the invention is communication systems using frequency division multiple accesses (FDMA) . More precisely, the invention relates to suc*h systems in which carrier frequencies transmitted by several terminals (or more generally transmitting stations) are received and processed in the form of a single signal (multiplex of carrier frequencies) in a base station (or more generally a receiving station)'.

For example, a system using this type of technique is described in patent application^ FR-93 JJJJL¾9 · ϊη this system, several terminals (called distributed stations) are assigned one or several carrier frequencies chosen among several carrier frequencies.

These carrier frequencies are modulated independently in each terminal and are transmitted in a synchronized manner. The result, seen from a base station (called the central station) is thus a multiplex of orthogonal carrier frequencies that can be treated like a single signal, by performing a single demodulation operation, in particular using a mathematical transformation (for example an FFT) .

This technique is particularly advantageous, since each terminal only transmits one or a small number of carrier frequencies. Thus, it is relatively simple to implement them and the necessary power is limited. Similarly, the base station remains relatively simple since it only receives a single signal.

One articular field of the invention is digital television, in systems involving a feedback from television sets to the television signal transmitter. Feedbacks have been, known for a long time, particularly in cable television networks. They may be used in particular to . transmit data to enable interactivity between the user of the set and the transmitted program (for example participation in games or polls) .

The invention may be applied on all return paths (cable, radio) based on an FDMA system for fixed, portable or mobile . transmitters .

More generally, the invention is applicable in all cases in which an FDMA communication is used, regardless of whether it is a multi-point to point or point-to-point transmission, and whether or not it is symmetrical.

Thus for example, the invention may also be used in local networks in which there are several stations, each, station acting as a terminal, according to the above definition to send, and as ,a base station, also according to the above definition, to receive.

However, the system described in patent application FR-93 10339 mentioned above has fa disadvantage, which is time synchronization of the different terminals.

It is easy to understand that the propagation time from terminals to the base station -(and vice versa) are different depending on the: distance separating them. If all terminals are . synchronized on a synchronization signal broadcast by the base station and transmit without any particular precautions, the base station will not receive a single coherent signal and will be unable to demodulate all carrier frequencies.

This disadvantage is overcome by slaving by a counter-reaction loop done separately for each transmitter; the base station measures the time lag of the signal received from each transmitter, and sends this time lag, or equivalent information, to th'e transmitter. The transmitter, re-adjusts its transmission time accordingly, and so on until it obtains good compensation of the forward/return propagation times between the base station and the transmitter.

However, ' this method has several disadvantages.

Firstly, it increases the complexity of each transmission terminal, although in many applications such as return paths it is desirable for it to be as simple and inexpensive as possible. Similarly, the base station' receiver is made more complex, since it must manage the treatment of time lags.

The time necessary for this counter-reaction slaving process is another disadvantage. In particular, this method is practically incompatible with mobile terminals, and the procedure then needs to be re-iterated almost continuously.

Another solution to the time synchronization, problem is proposed , in patent application FR-96 10670. According to. this approach, the base station comprises at least two parallel . demodulation lines, each of the said demodulation lines, in particular applying the following on the FDMA signal: - a temporal symbol reception window beginning with a line delay greater than or equal to zero compared with a synchronization reference, the said line delays all being different; and preferably, a Nyquist multi-carrier filter.

Thus, according to the invention, a very precise synchronization of the terminals is not imposed. Selective demodulation is done on reception as function of propagation, delays .

More precisely, the received signal is considered as a whole 'with, several (at least two) different delays. Therefore each carrier frequency can be correctly demodulated in one or the other of the two demodulation lines, regardless of its delay.

Obviously, one disadvantage of this technique is that it makes the processing to be done in the base station more .complex. It also requires the use of shaping (Nyquist type filter) of signals, sufficiently robust to reduce inter-carrier interference due to non-synchronization of adjacent carriers.

In particular, one purpose of the invention is to overcome the various disadvantages of the state of the art .

More precisely, another objective of the invention is to provide a technique implementing efficient and low cost demodulation of an FDMA multi-carrier signal made up from the' contributions of several remote terminals to eliminate the need for temporal slaving by counter-reaction (synchronization) between . terminals and the base station. ' Thus, another purpose of the invention is to provide such a technique that does not require' complex means and processing in the terminals, so that the terminals can remain simple and inexpensive. - In particular,, one particular objective of the invention is to provide a technique that does not involve complex filtering for shaping, or more generally any special means of eliminating inter-carrier interference.

Another purpose of the invention is to provide such a technique that can be used on any type of transmission channel (cable, radio transmission, etc.) and for any type of fixed, portable or mobile terminal.

These objectives and others that will become clear later, are achieved using an FDMA signal formed from a multiplex of carrier, frequencies transmitted from at least two distinct terminals and that will be processed in the form of a single signal in a base station. According to the invention, this signal carries temporal synchronization information, at least for some of the said. terminals, comprising a deterministic phase value transmitted simultaneously and continuously on at least two of the said carrier frequencies, for a duration greater than or equal to two symbol times.

Preferably, the said duration is equal to (Ns+1) Ts, where: · Ts is the duration of one symbol in the said signal; Ns is. a non null integer corresponding to the number of symbols necessary to cover a predetermined spread that is to be eliminated.

Thus,, according to the invention, it is checked that each synchronization data occupies at least one complete symbol, time, in reception (i.e. that it is complete during at least one demodulation window) . In other words, according to the approach used in the invention, it is always possible to find an. analysis window common to all received, signals in which the synchronization information can be recovered, in. other words in the sense that there, is no inter-symbol interference on each carrier (within the carrier) .

Thus, when the invention is applied to a robust FDMA signal (authorizing . some multi-carrier interference) , ; a simple rectangular window is sufficient.

Advantageously, the said predetermined duration takes account of at least one of the following three spreads, or combinations of them: - the maximum spread of the forward propagation time of signals between the said base station and the said terminals; the maximum spread of the .signals return propagation time between the said terminals and the said base station; the maximum internal latency of the said terminals . . . .

. Advantageously, the said signal comprises at least one null symbol on the carrier frequency concerned, before and / or after the said deterministic value.

■ Preferably, it comprises at least Ns+1 null symbols before and / or after the said deterministic value.

According to . one advantageous aspect of the invention, the carrier frequencies carrying the said deterministic values belong to a coherent range of the transmission channel in which the said signal is transmitted. In particular, the said carrier frequencies carrying the said deterministic values can advantageously be adjacent.

In particular, the signal according to the invention ' can . be modulated using the OFDMA or . SFDMA technique.

According to one particular embodiment of the invention, the . said carriers carry useful data as well as the said phase deterministic value, in the form of a modulation that keeps the phase and shaping properties necessary for synchronization.

The invention also relates to base stations designed to receive a signal like that presented above. This base station advantageously comprises means of calculating information representing a delay (positive, negative or zero) to be applied by each of the said terminals, starting from an analysis of the said deterministic values received by the said base station, and means of transmitting the said information representative of a delay to the corresponding terminal .

According to one preferred embodiment of the invention, the said calculation means determine the phase difference between the carrier frequencies carrying . the said deterministic value, and associate the said information representative of a delay with' it.

Advantageously, the association between the said phase difference and the . said information representing the delay is made using a correspondence table stored in memory.

For example, the said information representing a delay may include an angular phase value and / or a time delay.

Preferably, the base station comprises means of clearing ambiguities due to the periodicity of the function , considered, by the analysis of transitions between the said deterministic values and the preceding and / or following null symbol,, between the said deterministic values .

According to one particular embodiment, the base station comprises prior means of sending a' :time reference to the said terminals.

The invention also relates to terminals of a multiple access system comprising means of modulation and transmission of at least two carrier frequencies of a signal like that presented above.

The invention also relates to multiple access systems for the transmission of data between several terminals and a .base station, in which the said terminals and the said base station respectively are designed to generate and receive a signal described above, and the process for synchronization of at least two distinct terminals of a multiple access system producing a signal ■ processed as a single , entity in a. base station. In particular, this process comprises a step for the transmission of a signal like that described above ' by the said terminals, and for its reception by the said base station.

Other advantages and specificities of the invention will become clear upon reading the following description of a preferred embodiment given as a simple illustrative.. and non-limitative example, and the appended drawings, wherein: - Figure 1 illustrates the spatial environment of the base station - . terminals - user in a multiple access system according to the invention; Figure 2 illustrates the time - frequency space of a receiver in an FDMA system like that presented in Figure 1; Figure 3 illustrates an example of a., signal comprising time synchronization information according to the invention; Figure 4 illustrates reception of two signals like those illustrated in Figure 3, in a base station.

Therefore, the invention relates to the reception of an FDMA signal formed from a multiplex of carrier frequencies transmitted by several terminals. As already mentioned, the invention may be applied to all types of transmission channels, and all types . of transmitting and. / or receiving stations.' The example described in the following more precisely, . but not limitatively, relates to the creation of a return path. This type of system comprises several remote terminals, or slave stations, that transmit to a base station.

This technique can be used to make, low cost slave stations, each of them modulating and transmitting a low throughput carrier (obviously, it would be possible for each slave station to modulate, several carrier frequencies) .

In general, the slave stations are distributed on a service area around the base station, at distances between the terminal and the base station equal to Di (where Di < R, where R is the service radius of the broadcasting system around the base station) . The distances Di may vary with time (mobile stations) . As shown in Figure 2, these signals cannot be processed in the base station without time synchronization.

Therefore, the purpose of the technique according, to the invention is to evaluate the distance between a terminal - user and a base station to adjust its time synchronization and finally to synchronize all signals received on the base station.

Thus, the invention makes an estimate, of a relative time synchronization error (i.e. spreading of synchronizations between different terminals) of a terminal - user within the interval ]0, Ns.Ts], in a time division' multiple access environment, for. example OFDMA (Orthogonal; Frequency Division Multiple Access) or SFDMA · (Synchronous Frequency Division Multiple Access), where : Ts=ts+A, duration of the modulation symbol in an OFDMA type multiple1 access system, where ts the useful part of the modulation symbol,. Δ its guard interval where T3 = (l + roii_off)/c3 duration of the modulation symbol in an SFDMA type multiple access system, where Cs the inter-carrier difference equal to 1/ts, roll-off, the "roll-off" factor of the modulation filter in an SFDMA type multiple access system, and: Ns . the maximum , number of modulation symbols representing the maximum spread of synchronizations received on the base station to be . covered.

In one particular embodiment, this spreading must support the maximum spreading of forward - return propagation times between the base station and the different terminals in the service area, and spreading of their internal latencies,. In other cases, for example when the synchronization reference is output by a satellite channel or by a station other than the base station, only the latency of a single propagation direction can be considered. The first case described above is then applicable.

As illustrated in Figure 1, the first step is broadcasting of a time reference from the base station so that user terminals can synchronize themselves blind initially, in other words without any pre-judgement about the different propagation delays, or internal processing delays that would modify the homogeneity of synchronizations.

The time, synchronization signal transmitted by the user transmission terminal is illustrated in Figure 3. It is based on the transmission of a signal composed of at least two adjacent carriers with a phase shift from the origin equal to a deterministic value and then continuous over a duration of (Ns+1) Ts symbols, from the user terminal.

The deterministic phase shift between . thes.e tw'o order (k+1) and (k) carriers is denoted: The corresponding mathematical justification is presented in the appendix.

The transmission of these continuous carriers must preferably be preceded by at least the duration of one symbol Ts without emission (1 Null Symbol) and must be followed by a duration of at least (Ns+1) symbols without transmission (Ns+1 Null Symbols) .

The location of time synchronization carriers is defined by carrier allocation rules in the O/SFDMA multiplex - it may possibly be chosen at random from among a set of carrier pairs reserved for this purpose.

The corresponding mechanism for reception of the synchronization signal at the base station is illustrated in Figure 4, in the case of reception of synchronization signals, in this case transmitted from two different terminals.

. In reception, the additional phase difference between the two transmitted carriers, Δφ' extra, measured at time Ί" , chosen optimally so, that it. can simultaneously receive and demodulate all the different, synchronization signals (for example FFT) , is- · directly proportional to the time At elapsed since the reception of the origin T' 0,useri (transition from Null Symbol -> continuously - transmitted carrier) of the time synchronization signal and therefore, a function' that is linear, (except for an. ambiguity due to the periodicity of. this, function) with the sum . of the delays originating from the forward-return propagation time between the . base station and the user, terminal, an'd from internal .delays in the terminal. Αφ' extra = = ΦΛ+Ι' - Λ' s.

Except for the ambiguity, a table associating the complex expression of the phase variation (for example after multiplication of the received signal carried, by carrier k+1 by the conjugate of the signal of the carrier k) as input, with the angular value of the phase and / or At. as output, enables a fairly simple embodiment of this measurement.

The ambiguity to be cleared is +/- n.ts.

If the carriers are not adjacent (carriers k and k+K) , we would have: Δφ' extra = φχ+κ' -' φ*' - = φκ+ι' - φχ' - (φ·Λ-ι-φ ) = 2nAt/ts, In this case, the ambiguity to be cleared is +/- hanism to . clear this ambiguity includes a measurement of powers received by successive FFTs, with a time lag of Ts, in order to detect' a coarse Null Symbol -> modulated Carrier transition and then a. modulated Carrier -> Null Symbol transition.

If necessary, the same synchronization signal must be retransmitted . several times : to reduce the measurement noise.

Finally, in order to complete the time synchronization mechanism, a reframing signal for individual terminal synchros must be downloaded through a down channel.

The same signal generation method may be broadened by the transmission of signal sequences (consecutively on the time axis or simultaneously on adjacent carriers) by introducing a modulation (differential or coherent) in order to transmit data; for the purposes of improving the synchronization technique (for example by identification of the user) or data transmission without attempting to adjust synchronizations or spectral efficiency.

Therefore, in particular the invention proposes: 1) Definition of the synchronization signal and adaptation of its length\ to the constraints of the multiple access system; 2) Transmission of a synchronization signal on adjacent carriers to achieve a "coherent" channel environment in different carriers; 3) A means, of clearing the ambiguity of the measurement by detection of Null symbol <-> modulated carrier transitions; ■·. 4). An optimised synchronization; ) Transmission of data despite differences in • propagation and processing times by attempting to find a temporal or non temporal adjustment; . 6) Materially making the measurement at the base station,, starting from a correspondence table relating the complex rotation value between the two adjacent carriers and the angular value or its time equivalent.

APPENDIX 1. reminder: . ■ property of Laplace transforms: translation of the time axis L[f(t-a)] = e-asL[f(t)] ■ 2. application to the invention We have the following equations for each of the two carriers considered: ^jM t t+a) ] _ Q2jitka/ ts ^Q2jnkt / tsj Lj-e2j* <k+l) ta <t+a) j _ e2jit (ka+a) / ts Lj-Q2jn (k+1 ) t / tsj The difference in the additional phase rotations due to the delay ( + /-) is expressed by the complex value: gidtfiextra _ ^2jn (ka+a) /ts ^ g -2jnka/ts _ e2jnal/ts The real part Re ( . ) and the imaginary part Im(.) are extracted from this value and are input into a table that, for example, corresponds to a value At, the delay to be applied for the terminal considered.

Claims

.1. .. FDMA signal formed from a multiplex of carrier frequencies transmitted from at least two distinct terminals and that will be processed in the form of. a single signal in a base station, characterised in that it carries temporal synchronization information, at least for some of the said terminals, comprising a deterministic phase value transmitted simultaneously and continuously on at least two of the . said carrier frequencies, for a duration greater than or equal to two symbol times.
2. Signal according to claim 1, characterised in that the said duration is equal to (Ns+l)Ts, where: Ts is the duration of one symbol in the said signal; Ns is a non null integer corresponding to the number of. symbols necessary to cover a predetermined spread that is to be eliminated.
3. Signal according to either of claim 1 or 2, characterised in that the said predetermined duration takes . account of at least one of the following three spreads, or combinations of them: the maximum spread of the forward propagation time. of signals between the said base station and the said terminals; - the maximum spread of the return propagation time of signals between. the said terminals and the said base station;. the maximum internal latency of the said terminals.
4. Signal according to any one of claims 1 to 3, characterised in that it comprises at least one Null Symbol before and ./ or after the said deterministic value, on the carrier frequency concerned.
5. Signal according to . claims 3 and 4, characterised in that it comprises at least Ns+ 1 null symbols before and / or after the said deterministic value.
6. Signal according to any one of claims 1 to 5, characterised in that the carrier frequencies carrying the said deterministic values, belong to a coherent range of the transmission channel in which the said signal is transmitted..
7. Signal according to claim 6, characterised in that the said carrier frequencies, carrying the said deterministic values are adjacent.
8. Signal according to any one of claims 1 to 7, characterised in that it is modulated using the OFDMA technique or the SFDMA technique..
9. Signal according to any one of claims 1 to 8, characterised in that the said carriers carry useful data at the same time as the said deterministic phase value, in the form of a modulation conserving the phase and shaping properties necessary for synchronization.
10. Base station designed to receive a signal according to any one of claims 1 to 9, characterised in that it comprises means of calculating information representative of a delay (that may be positive, negative or zero) to be applied by each of the said terminals, starting from an analysis of the said deterministic values as received by the base station, and means of transmitting the said information representative of a delay to the corresponding terminal .
11. Base station, according to, claim . 10, characterised in that the said calculation means determine the phase difference between the carrier frequencies carrying the said deterministic value, and associate the said information representing'.a delay with this value.
12. Base station according to claim 11, characterised in that the association between the said phase difference and the said information representing a dela is made using a correspondence table stored in memory.
13. Base station according to any one of claims 11 and 12, characterised in that the said information representative of a delay comprises an angular phase and / or time delay value.
14. Base station according to any one of claims 10 to 13, characterised in that it comprises means of clearing the ambiguity due to the periodicity of the function considered, by analysis of transitions between the said. deterministic values and the previous and / or next null symbol between the said deterministic values.
15. Base station according to any one of claims 10 to 14, characterised in that it comprises prior means of transmission of a time reference to the said terminals.
16. Terminal of. a multiple access system, characterised in that it comprises modulation and transmission means of at least two frequencies carrying a signal according to any one . of . claims 1 to 9.
17. Multiple . access system enabling data transmission between several terminals and a base station, characterised in that the said terminals are designed to generate a signal and the said base station is designed to receive this signal according to any one. of claims 1 to 9.
18. Process for synchronization of at least two distinct terminals in a multiple access system, producing a signal processed as ah entity in a bas'e station, characterised in that it comprises a step in which the said terminals transmit a signal, and the said base station receives it, according to any one of claims l.to 9.