EP1550234A1

EP1550234A1 - Method of receiving signals in a spread-spectrum telecommunications system with terrestrial repeaters, comprising a complementary source

Info

Publication number: EP1550234A1
Application number: EP03776948A
Authority: EP
Inventors: Michel-Guy Francon; Nicolas Chuberre; Steven Bouchired; Christophe Nussli
Original assignee: Alcatel CIT SA; Alcatel SA; Alcatel Lucent SAS
Current assignee: Alcatel Lucent SAS
Priority date: 2002-10-03
Filing date: 2003-10-01
Publication date: 2005-07-06
Also published as: FR2845540A1; US20050255816A1; AU2003286207A1; WO2004032360A1; JP2006501732A; CN1711703A; FR2845540B1

Abstract

The invention relates to a method of receiving signals in a spread-spectrum telecommunications system with terrestrial repeaters, comprising a complementary source. More specifically, the invention relates to a third-generation terminal for a code division multiple access telecommunications system, which comprises two rake receivers (16, 18), each receiver having a set of demodulation channels (20-1, 20-n, 24-1, 24-n) and a combiner (22, 26). A third combiner (28) receives the signals provided by the combiners belonging to the two above-mentioned receivers. In this way, signals arriving from a diffusion system comprising terrestrial receivers and a complementary source can be received, in spite of the time differences between the different paths of the signals. The terminal can be used in a telecommunications system with numerous multiple paths.

Description

METHOD FOR RECEIVING SIGNALS IN A SPREAD SPECTRUM TELECOMMUNICATIONS SYSTEM WITH LAND REPEATERS HAVING A COMPLEMENTARY SOURCE

The present invention relates to telecommunications systems and more precisely to multiple access telecommunication systems with broadband code distribution (W-CDMA or "wideband code division multiple access ¹ 'in English).

The code division multiple access (CDMA) technique is based on the principle of spreading the signals to be transmitted by one or more codes reserved for a communication. The codes consist of a set of bits (or "chips") of a duration much less than the unit duration of an item of information to be transmitted. The codes are orthogonal so that each user receives the signals intended for him by despreading using the code or codes which are assigned to him. The principle of CDMA is described in CDMA: Principles of Spread Spectrum Communication (Addison-Wesley Wireless Communications), by AJ. Viterbi, published by Prentice Hall PT; ISBN: 0201 633744, I sf edition - Jυne 1995.

One of the problems encountered with this technique is that of multiple paths, caused for example by reflections on obstacles such as buildings. The result of multipath is that a terminal or user equipment receives different copies of the signal intended for it, which are time-shifted. Because of this offset, these different copies can undergo destructive interference which weakens the signal. The problem is identified and a solution proposed in the state of the art consists in proposing rake receivers or "ra e" receivers (from the expression "rake receiver" used in English). Such receptors are described for example in the part of WO-A-01 47133 relating to the state of the art, in WO-A-00 25439 or also in EP-A-1 154 584. A rake receptor is formed a set of demodulation channels and a combiner; the information provided by the different channels is affected by respective delays before being combined to optimize the identification of the signal. In this context, a receiver's "finger" is called a demodulation channel. A rake receiver uses the same de-spread code for all fingers.

A third generation terminal baseband processor is sold under the reference CDMAx by Sirius Communications. This baseband processor is intended to be used in a telecommunications system of the type defined by the UMTS (Universal Mobile Telecommunications System) standards. This baseband processor has two rake receivers; the first receiver is used for receiving the signal in a cell; the second receiver is used, near the border of the cell, for the reception of the signal coming from the neighboring cell. In this case, the first receiver operates with a first spreading code applied to all of its fingers while the second receiver operates with a second spreading code different from the first, on different channels. This use of the two receivers allows the terminal to pass from one cell to another without interrupting the communication; the corresponding technique of handover is called "hand-off". Each receiver in this document has eight fingers; the demodulator of a finger is capable of demodulating six physical channels and applying a delay which can reach forty bits, ie a typical maximum offset of 10 μs. This maximum offset between the signals received on the different fingers or channels of the rake receiver is called the recombination window.

Another problem encountered in telecommunications systems is that of increasing traffic and bandwidth demands. To satisfy this increase, it has been proposed to combine a transmission by satellite or by high altitude platform (HAPS, acronym for "high altitude plafform system") and a repetition by terrestrial repeaters. The term "high altitude platform" is defined in the Asia-Pacific Telecommunity Standardization Program (ASTAP) Expert Group on HAPS specification. The High Altitude Platform Stations (HAPS) are unmanned geostationary air vessels that perform long-term flights in the stratosphere at an altitude of approximately 20 km. The term "high altitude" can therefore cover altitudes between 20 and 30 km. We then add to point-to-point broadcasting of conventional telecommunications networks a selective distribution distribution layer; in terms of sources, the telecommunications network presents terrestrial repeaters and a complementary source. This solution is notably proposed in the S-DMB system (acronym of "digital satellite multimedia broadcast"), which plans to use a component emitted by a geostationary satellite, with repeaters for urban or suburban areas; this allows direct transmission to some of the point-to-multipoint traffic. This satellite component uses the IMT2000 frequency extension band allocated to MSS (mobile satellite systems) as well as the W-CDMA terrestrial waveform standardized by 3GPP (acronym for "third generation partnership project" or project partnership for the third generation). These choices allow optimal use of UMTS technologies for user equipment.

These solutions could nevertheless lead to an increase in the number of copies of the signals received, due to the repeaters and multipaths. Figure 1 shows an example of the possible shape of the signals received in a configuration with a satellite or a high altitude transmission system and terrestrial repeaters. On the abscissa axis is plotted the relative time offset between the different copies of the signal, with respect to the signal received directly from the satellite; the axis is graduated in micro-seconds. The relative reception level, in dB, is plotted on the ordinate axis. The figure shows under reference 2 the signal received directly from the satellite or the platform at high altitude, at a level of the order of -7 dB. The figure also shows, under references 4, 6, 8 and 10, copies of the signal, received on multiple paths or from repeaters, with a time offset. We also see in the figure background noise, notably caused by scattering. As shown in the figure, the time range over which copies of signals are received can reach 27 μs, while the level range extends from about -5 dB to -30 dB.

There is therefore a problem with the size of the recombination window; this problem is described above in the example of a telecommunications system with a satellite broadcasting layer, but more generally arises when the number of copies of the received signals increases, even for purely terrestrial systems. It would be interesting to be able to provide a solution to this problem of size of the recombination window while using existing technologies as much as possible.

WO-A-01 47133 provides a method of receiving spread spectrum signals. A rake receiver has two antennas, the signals of which are time-shifted by at least the duration of a chip and combined before being applied to the fingers of the receiver. This solution allows you to benefit from the advantages of antenna diversity while preventing the signals from the two antennas from interfering. This document describes the possible structure of rake receivers.

WO-A-00 25439 also relates to rake receivers; The aim of this document is to allow the simultaneous demodulation of signals with as small arrival time differences as possible. It is proposed to use only one symbol accumulator, placed downstream of the combiner. This solution would reduce the hardware and software complexity, compared to a solution in which each finger of the receiver has an accumulator.

EP-A-1 154 584 proposes to group into two "baskets" the channels of a rake receiver; one or more tracking mechanisms are used for each basket. This technique is applied to the fingers of the receiver, before any combination of signals.

Other receivers are described in US-B-6,381,229, US-A-5,867,527, US-A-2002/0006158, or even DE-A-199 37 247. In one embodiment, the invention therefore proposes a receiver for a spread spectrum telecommunications system, having: a first receiver with at least two demodulation channels and a first combiner receiving the demodulated signals supplied by the demodulation channels; - a second receiver with at least two demodulation channels and a second combiner receiving the demodulated signals supplied by the demodulation channels; a third combiner receiving the signals supplied by the first and second combiners. In one embodiment, the time difference between the recombination window of the first receiver and the recombination window of the second receiver is greater than 30 μs. In one embodiment, the recombination window of the first receiver and the recombination window of the second receiver extend over a time range of at least 50 μs. The invention further provides a telecommunications system having

- terrestrial repeaters and a complementary source;

- such a terminal. Finally, it provides a method for receiving coded signals by spread spectrum in a telecommunications system having terrestrial repeaters and a complementary source, the method comprising: providing a terminal with a first rake receiver and a second rake receiver ; receiving at least the signals coming directly from the complementary source using the first rake receiver; and receiving signals from at least one terrestrial repeater using the second rake receiver. Advantageously, reception using the first receiver and reception using the second receiver is effected by de-spreading with the same code.

It is also possible to provide a step of combining the signals received using the first rake receiver and the signals received using the second rake receiver. Other characteristics and advantages of the invention will appear on reading the following description of embodiments, given by way of example and with reference to the drawings, which show: FIG. 1, a graph of the signals received by the terminal. - Figure 2, a block diagram of a terminal according to an embodiment of the invention; FIG. 3, a schematic view of the recombination window obtained in an embodiment of the invention.

The invention proposes, in one embodiment, to use two separate rake receivers to receive copies of the same signals; unlike the solution proposed in the prior art, the two receivers are not used to receive different signals from neighboring cells in a cell transfer procedure. This use of two receivers has the advantage of being able to use for systems with a satellite broadcasting layer existing solutions - chipset (components). This use also has the advantage of being able to adapt the size of the recombination window as required, as explained below with reference to FIG. 3. In particular, this solution makes it possible to combine the signals coming from heterogeneous sources, such as signals from a satellite and signals from repeaters.

FIG. 2 shows a block diagram of a terminal according to an embodiment of the invention; only the elements of the terminal necessary for understanding the invention have been shown in the figure. The terminal has a reception stage 14 which receives the radiofrequency signals and makes them undergo conversion to lower frequencies, in a manner known per se. The terminal also has two rake receivers 16 and 18. The first receiver has a plurality of fingers 20-1 to 20-n and a combiner 22. Each finger of the receiver demodulates a copy of the signals received and the combiner 22 combines the versions demodulated different signals received, according to the operating principle known per se of the rake receiver. The figure does not show the means for applying the delays to the different fingers of the receiver to select the copy of the signal which is demodulated, nor the possible tracking means which can be used for a finger. The second receiver has a similar structure, with fingers 24-1 to 24-n - in the example the number of fingers is identical, but this is by no means essential - and a second combiner 26. The terminal still has a third combiner 28 which receives the signals supplied by the first and second combiners 22 and 26 and provides a combined signal representative of all the copies processed in the fingers of the first and second rake receivers. A possible delay applied to the signals coming from one of the combiners 22, 26 has not been shown in the figure before the combination at 28.

In the terminal of the state of the art mentioned above, there is a first rake receiver and a second rake receiver; however, these receptors are used during a handover; one of the receivers receives the signals from one cell and the other receives the signals from the other cell. The terminal does not have a combiner making it possible to combine the signals received on the two rake receivers: on the contrary, the terminal alternately uses the signals received on one or the other of the receivers, depending on the progress of the intercell transfer .

In addition, in the handover scenario proposed in the prior art, the two receivers use different codes: in fact, the signals received by the terminal on a cell and on the neighboring cell are spread out with codes different. Conversely, in the proposed solution, the two rake receivers use the same de-spreading code.

The operation of the terminal in FIG. 2 is explained, with reference to FIG. 3. This shows the recombation windows 30 and 32 of the two rake receivers; typically, each window has a width of 10 μs, which corresponds to 40 flanges. A time difference 34 can be applied between the two windows which can typically vary between 0 and 33 μs. The lower limit of 0 μs corresponds to the case where the recombination windows are adjacent, which leads to a total width ΔT of reception of the signal of 20 μs. The upper limit of 33 μs corresponds to a value deduced from UMTS third generation standards, more specifically from technical specifications 3GPP TS 25.21 1 and 3GPP 25.922. In the scenario of a handover on node B during an active connection, the user terminal receives the signals transmitted by the nodes N of the two neighboring cells on the dedicated transport channels (DCH). The same information is transmitted by the two nodes, but with separate spreading codes. The time difference between the signals transmitted by one node and by the other node depends on the synchronization between the two nodes; this synchronization takes place at node B of the new cell, from synchronization information transmitted by the terminal. In fact, the terminal periodically transmits to the network information relating to the difference measured between the signals coming from the two nodes. The temporal adjustment of the transmission at the level of node B is carried out in 256-bit steps; in this way, the maximum time difference between the signals from the two nodes B is 128 bits, or a duration of 33 μs. Third generation terminals meeting the technical specifications of 3GPP can therefore track with both rake receivers signals shifted by 33 μs. This explains the upper limit of 33 μs shown in Figure 3.

Thus, the use of the chipset of third generation UMTS terminals makes it possible to obtain a recombination window ranging from 20 μs to 53 μs. These values correspond to a recombination window of 10 μs for each rake receiver and to an offset between the windows ranging from 0 to 33 μs. At least, for two adjacent windows, there is a joint window width of 20 μs and at most, for separate windows of 33 μs, there is a width total of 1 window + 33 μs + 1 window, i.e. 53 μs. We can scan all possible values between 20 and 53 μs. The fact that the time difference between the recombination window of the first receiver and the recombination window of the second receiver is at least 30 μs makes it possible to provide a joint recombination window sufficient for an S-DMB system. The total width covered by the two recombination windows is advantageously at least 50 μs; this makes it possible to cover the reception offsets envisaged in the S-DMB system.

This joint window is advantageously used for receiving signals from multiple sources, for example from a satellite and from one or more repeaters. If the signals coming directly from the satellite and the signals coming from one or more terrestrial repeaters are of a similar power - to within a few dB - the signals coming directly from the satellite are advantageously followed on a finger 20-1 of a repeater 16 in rake. The other fingers of the receiver can be used to track signals from the satellite over multiple paths; these signals are typically shifted by less than 10 μs and can be followed by the same receiver 16. It is also possible to follow, with the fingers of this receiver 16, the signals coming from one or more repeaters - if these signals are in the recombination window containing the signals coming directly from the satellite. The second rake receiver can then be used for at least one terrestrial receiver, outside the recombination window of the first receiver. One can also follow with the fingers of the second receiver a plurality of repeaters or even a high altitude platform.

The two receivers can also be used with a variable offset between the recombination windows, so as to scan all of the possible copies of the signals spread with the same spectrum.

The solution of the invention therefore makes it possible to receive signals from a telecommunications system, having terrestrial repeaters and a complementary source - satellite or high altitude platform. It makes it possible to use the same chipsets and in particular the same receivers as the third generation terminals.

Claims

CLAIMS:

1. A receiver for spread spectrum telecommunications system, having: - a first rake receiver (l 6) with at least two demodulation channels

(20-1, 20-n) and a first combiner (22) receiving the demodulated signals supplied by the demodulation channels;

- a second rake receiver (1 8) with at least two demodulation channels (24-1, 24-n) and a second combiner (26) receiving the demodulated signals supplied by the demodulation channels;

- the first and second rake receivers using the same de-spreading code for the same communication;

- a third combiner (28) receiving the signals supplied by the first and second combiners.

2. The receiver of claim 1, characterized in that the time difference between the recombination window of the first receiver and the recombination window of the second receiver is greater than 30 μs.

3. The receiver of claim 1 or 2, characterized in that the recombination window of the first receiver and the recombination window of the second receiver extend over a time range of at least 50 μs.

4. A telecommunications system presenting

- terrestrial repeaters and a complementary source;

- A receiver according to one of claims 1 to 3.

5. A method for receiving coded signals by spread spectrum in a telecommunications system having terrestrial repeaters and a complementary source, the method comprising:

- the supply of a terminal with a first rake receiver (16) and a second rake receiver (18);

- the reception of at least the signals (2) coming directly from the complementary source using the first rake receiver (1 6); and

- receiving the signals (4, 6, 8, 10) from at least one terrestrial repeater using the second rake receiver (18), reception using the first receiver and reception using the second receiver is effected by de-spreading with the same code for the same communication.

6. The method of claim 5, characterized by a step of combining the signals received using the first rake receiver (1 6) and the signals received using the second rake receiver (1 8).