Technical field

The invention relates to digital wireless communication and particularly for a process for canceling interference generated by interference base stations in a synchronized OFDM communication system and a receiver for the same.

Background Art

Digital wireless communication are being widely used throughout the world particularly with the latest development of the Orthogonal Frequency Division Multiplex (OFDM systems) and the last evolution, namely the so-called Long Term Evolution (LTE) systems.

With the advent of those new systems and the development of OFDM, the number of communicating cells and users will tremendously increase over one given area.

This will strongly increase the level of interference of the communications between the UEs and the Cells, and improved techniques will be required for canceling the interference resulting from a continuously increasing number of base stations. Summary of the invention

It is an object of the present invention to carry out a process for canceling the interference in a UE fitted with MIMO antennas.

It is another object of the present invention to provide a process of interference cancellation for a synchronous OFDM cellular system based on the use of MIMO receivers.

It is still another object of the present invention to provide an effective interference cancellation process which is suitable for any synchronous OFDM system and particularly adapted to the Long Term Evolution (LTE) standard.

It is still a further object of the present invention to carry out a receiver for a synchronous OFDM cellular system which achieves efficient OFDM receiving and interference cancellation of the present interfering base stations or antennas.

Those and other objects are achieved by means of the process for canceling the interference generated by a set interference base stations, which comprises the steps of:

- performing an OFDM demodulation of the received signal for each antenna within the User Equipment;

- monitoring, for each antenna within the UE, the pilot signals (for instance the Reference Signal in LTE or the more generic Broadcast Pilot Channel) generated by the present base stations;

- extracting the detected pilot signals and establishes the list of the existing base stations antennas;

- measuring the channel power of the interfering antennas; - selecting the Nrx - N most powerful channels;

- performing a joint detection of the N Useful Data Symbols (UDS), together with the (Nrx - N) Interfering Data Symbols (IDS) demodulated by the demodulation blocks. It has been obsevered that the joint detection of both useful data symbols (USD) and interfering data symbol (IDS) significantly increases the efficiency of the receiver and, therefore, the MIMO structure existing in the UE can be advantageously used for the purpose of interference cancellation.

In one embodiment the detected most powerful Interfering Data Symbols which are generated by the joint decoder are decoded before being re-encoded and introduced in a interference cancellation loop.

In one embodiment, the joint decoder is a sphere decoder.

Alternatively, the joint decoder is a linear equalizer, or a maximum likelihood decoder.

The invention also provides with a receiver for a synchronous OFDM cellular system, which includes:

- a set of Nrx OFDM demodulators associated to a set of Nrx antennas, said OFDM demodulators performing time and frequency synchronization, cyclic prefix discard and IFFT conversion of four data streams; - means for estimating the channels associated to the multiple signals received through said antennas from base stations belonging to said OFDM cellular system;

- a joint decoder for decoding the Nrx demodulated OFDM symbol; Characterized in that it further comprises:

- means for measuring the channel power corresponding to all the interfering base stations and base station antennas;

- means for controlling said joint decoder in order to perform joint detection of N useful data stream and the N-Nrx most powerful interference data streams.

In one embodiment, the receiver further includes re-encoding means for re- generating the OFDM symbol of the detected interference data streams so as to perform interference cancellation.

The invention is particularly useful for carrying out a UE fitted with a receiving complying with the future LTE standard. Description of the drawings

Other features of one or more embodiments of the invention will best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings.

Figure 1A illustrates one OFDM-MIMO communication system comprising three base stations and one UE.

Figure 1 B illustrates the general principle of the OFDM modulation.

Figure 2 illustrates the principle of the OFDM modulation and demodulation process with the processing of the guard band or cyclic prefix.

Figure 3 illustrates one embodiment of the process of the present invention.

Figure 4 illustrates one embodiment of a OFDM - MIMO receiving system based on a 2x2 joint decoder.

Figure 5 illustrates another embodiment of a OFDM - MIMO receiving system based on a 4x4 joint decoder with reencoding of the decoded interfering data symbols.

Figure 6 is a flow chart illustrating the significant increase of performance resulting from the joint decoding of both the useful data symbols (UDS) and the interfering data symbols (IDS). Description of the preferred embodiments

There will now be described how one can significant improve the cancellation of the interference generated by the multiple communications established in a synchronous OFDM communication system, comprising a set of base stations or cells , at least one communicating with one user equipment (UE).

The invention will be particularly described in relation with the Long Term Evolution (LTE) standard which is the next step of the evolutation of digital wireless communications, but it should be clear that this is only one particular embodiment and application of the invention and the invention may be used in any cellular system based on a OFDM modulation with synchronous cells or base stations.

Indeed, the OFDM modulation is widely spread in many existing standards (such as DAB, DVB-T, WiMAX, IEEE 802.16, ADSL, WLAN IEEE 802.11a/g, etc..) and expected to be used in many future developing standards. As known by the skilled man, the OFDM converts a broadband frequency-selective channel into a multiplicity of parallel narrow-band single channels. This is achieved by means of the insertion of a guard interval (so-called Cyclic Prefix CP) between the individual symbols, as shown in Figure 1 B. Such guard interval is assumed to be temporally long enough to compensate for jitter, ie the variation of the delays experienced by the transmitted OFDM symbols through the channel. This prevents the appearance of inter-symbol interference (ISI) can thus be prevented.

In addition, the OFDM technique can be advantageously combined with the use of multiple antennas on the sending side as well as the receiving side to increase diversity gain and/or transmission capacity in time-varying and frequency- selective channels.

Further information may be found in the following reference "Implementation and Experimental Results of a Three-Transmitter Three-Receiver OFDM/BLAST Testbed", by Xiang, Waters, Bratt, Barry, Walkenhorst, IEEE Communicatin Magazine 2004. Since the OFDM communication system is also presumed to be synchronous, the OFDM symbols which are exchanged with the base stations are also synchronous.

It should thus be noticed that the invention which will be disclosed below is likely to be used in a wide range of applications and should not be limited to the sole use of LTE.

With respect to figure 1A there is now illustrated the synchronous OFDM system comprising three base stations, respectively 10, 20 and 30, fitted with MIMO capabilities. Base Station 10 is assumed to have two emitting and receiving antennas 11 -12, Base Station 20 (resp. 30 ) has a correspond set of two antennas 21-22 (resp. 31 , 32).

On the other side of the communication channel, one User Equipment (UE)

100, which can be a mobile telephone, a laptop computer, a handheld or any communicating device fitted with MIMO capabilities, has four distinctive antennas for instance, each one receiving a composite signal formed of the combination of the six different signals received from antennas 11-12, 21-22, and 31-33.

In the conventional OFDM - MIMO communication between base station 10 and UE 100, one single data stream is split into several partial data streams and are transmitted through the different antennas forming the MIMO system of User Equipment 100.

The invention deviates from this known technique in the fact that UE 100 now incorporates more antennas - and corresponding OFDM receivers - so as to achieve, not only the detection of the useful OFDM symbols received from base station 10, but also the interference cancellation of the signals received from the interfering base stations 20 and 30.

Therefore, part of the resources of the MIMO system of the User Equiment - ie two antennas - are affected to the communication of the useful data streams, while the remaining part of the resources of the MIMO system - ie the two remaining antennas - can be used for interference cancellation of the base stations.

Figure 2 more particularly illustrates one embodiment of the process of the invention.

It is assumed that the User Equipment is fitted with Nrx antennas, among which N being associated to the communication of useful data for the user. As a consequence, N antennas are dedicated to the OFDM-MIMO modulation while Nrx - N antennas will be assigned to the cancellation of existing interfering antennas.

The process starts with a step 110, wherein there is performed, for each antenna of the User Equipment 100, the OFDM demodulation of the received signal.

In a step 120, the process proceeds with the monitoring, for each antenna of the UE 100, of the pilot signals generated by the different base stations. Such pilot signal may take diverse forms in accordance with the standard and the OFDM modulation being considered. Generally speaking, it is designated as a broadcast pilot channel and, in the particularly case of LTE , known under the designation of Reference Signal.

In a step 130. the process extracts the pilots signals and establishes the list of the existing base stations antennas.

Once the complete list of the interfering antennas has been drawn, the process proceeds with a step 140 which measures the channel power of the interfering antennas. The measurement of the channel power is an operation with is well known to a skilled man and will not be further developed.

Then, in a step 150, the process selects the Nrx - N most powerful channels, where Nrx corresponds to the number of antennas of UE 100 and N the number of antennas for the useful data streams as mentioned above. Then, in a step 160. for each subcarrier, the process performs, a joint detection of the N Useful Data Symbols (UDS), together with the (Nrx - N) Interfering Data Symbols (IDS) demodulated by the demodulation blocks.

It should be noticed that any joint decoder may be used for that purpose.

In one embodiment, the joint decoder is a so-called Sphere Decoder, but clearly other embodiments may be considered, such as, for instance a simple linear equalizer or the more complex Maximum Likelyhood (ML) decoder.

The key point of the invention is that the joint decoding of the Nrx streams provides, on one hand, the N useful data symbols (UDS) corresponding to the data stream which can be processed in any conventional fashion and forwarded to the upper layer application (wireless communication, Digital TV etc..) and, on the other hand, Nrx - N interfering data symbols (IDS) resulting from the presence of the base stations showing the higher channel power.

The process described above thus achieves an aggregate decoding, by means of the existing MIMO structure, of the N useful data streams and the Nrx - N most disturbing interfering data generated by the interfering Base stations.

The invention thus basically deviates from the known MIMO technique where the decoder is used for the sole detection of the useful data. With the process which is proposed, a selection is made in the detection process of the interfering signals so as to focus the detection process on the most powerful interfering data.

It has been observed, and this is a very advantageous effect of the invention, that such joint decoding of the useful and interfering data symbols significantly increases the overall performance of the receiver, with a increase of the Signal to Noise Ratio (SNR) which can reach many dB (up to 10 dB), as illustrated in the flow chart of figure 5.

Steps 170 and 180 of figure 4 are optional steps which can be used for still improving the efficiency of the whole process. For that purpose, IDS which were detected in step 160 are then decoded by means of any convention decoder, such as a turbo decoder for instance, in a step 170.

Then, in a step 180, the decoded interference data Symbol (IDS) is reinjected in the cancellation loop for the purpose of achieving interfering cancellation.

It should be noticed that the process of joint decoding of Useful Data Symbols (UDS) together with Interfering Data Symbols (IDS) is applicable even in the case of a 2x2 joint decoder, for instance, which is the case illustrated in figure 3.

Figure 3 shows one receiver comprising four OFDM demodulation blocks, respectively 210, 220, 230 and 240 which are used for performing the basic functions of time and frequency synchronization, Cyclic Prefix suppression and IFFT conversion. Block 210 receives via a lead 211 the signal received from the first antenna of UE 100, while block 220 receives via a lead 221 the signal received from the second antenna of UE 100. The signal received from the third antenna (resp. the fourth) of UE 100 is forwarded via a lead 231 (resp. 241) to OFDM block 230 (resp. 240).

A 2x2 joint decoder 200 provides the decoding of two data symbols, comprises two inputs and an output. The first input of joint decoder 200 receives the data symbol forwarded by OFDM demodulator 210 (assumed to receive the useful data symbol) while the second input receives the output of a selector block

250 having three inputs.

Selector block 250 has a first input which receives the output of an adder/subtractor 222, which positive input receives the output of demodulation block 220, and which negative input receives a first output of a reencoder block 280.

Similarly, selector block 250 has a second input (resp. third input) which receives the output of an adder/subtractor 232 (resp. 242), which positive input receives the output of demodulation block 230 (resp. 240) , and the negative input receives the second (resp. the third) output of reencoder block 280.

Joint decoder 200 has two outputs which generates the data and interfering symbols which are respectively forwarded to two turbo decoders, respectively 260 and 270).

It should be noticed that 2x2 joint decoder achieves the decoding of the two symbols X1 and 11 , with X1 being the useful data symbol (UDS) and 11 being the Interfering data Symbol (IDS) corresponding to the particular one among the three interfering signals provided by the three OFDM demodulation blocks 220-230-240 which shows the higher channel power.

By decoding the higher channel power, that is to say symbol X1 , the latter can be reinjected into reencoder 280 so as to compute the OFDM symbol which is then substracted to adder/subractor 222-232-242 forming the cancellation loop.

With respect to figure 4, there will be now described another embodiment of the invention, based on the use of a 4x4 joint decoder which allows the possibility to jointly decode two streams of Useful Data Symbol (UDS) together with two high power interfering data symbol (IDS). This embodiment is particularly suitable for receiving the two data streams exchanged for base station 10 and also to cancel the interference of the more powerful interfering base station 20, for instance.

The receiver is based on the use of four distinctive OFDM demodulation blocks 311 , 312, 313 and 314, respectively receiving the signal from a corresponding antenna, 310, 320, 330 and 340. As above, each OFDM block achieves timing and frequency synchronization, cyclic prefix discard as well as the IFFT transform.

The receiver further includes a 4x4 joint decoder 350, for instance a sphere decoder, for providing the joint decoding of four distinctive OFDM streams, with four inputs and four outputs. Again, any joint decoder can be used, such as a linear equalizer, a serial interference canceller or a ML decoder.

- io - OFDM block 311 has an output which is coupled to a positive input of an adder/substractor 312 having two negative inputs and an output which is forwarded to a first input of 4x4 joint decoder 350.

OFDM block 321 has an output which is coupled to a positive input of an adder/substractor 322 having two negative inputs and an output which is forwarded to a second input of 4x4 joint decoder 350.

Similarly, OFDM block 331 has an output which is coupled to a positive input of an adder/substractor 332 having two negative inputs and an output which is forwarded to a third input of 4x4 joint decoder 350.

At last, OFDM block 341 has an output which is coupled to a positive input of an adder/substractor 342 having two negative inputs and an output which is forwarded to a fourth input of 4x4 joint decoder 350.

Joint decoder 350 has four outputs which generates the four decoded data streams, respectfully forwarded to four corresponding turbo decoders, 313-323-333 and 343 respectively.

Turbo decoders 313 and 323 are allocated to the decoding of the two useful data streams carried by the UDS symbols, while turbo decoders 333 and 343 are dedicate to decode two interfering Data Symbols (IDS).

In order to achieve efficient interfering cancellation, the two IDS generated by

Turbo decoder 333 and 343 are respectively forwarded to two re-encoders, respectively 344 and 334, are used for the purpose of re-generating the OFDM symbols corresponding the interfering signal.

Re-encoder 344 has four outputs which are respectively forwarded to the first negative input of adder/subractors 312-342, while re-encoder 334 has four outputs which are respectively forwarded to the second negative input of adder/substractors 312-342.

- i i - The receiver further includes four Channel Estimation blocks, 315, 325, 335 and 345 respectively receiving the signal through the four antennas, and which are coupled to a control unit 360 providing the main control signals used for performing the process which was described above.

Control unit 360 achieves control of the 4x4 joint decoder 350 (via control line 351), turbo decoders 313-343 (via control line 361) as well as re-encoder 334 (via line 306) and re-encoder 344 (via line 362).

Control unit 360 particularly achieves the selection process of step 150 of figure 2 so as to control 4x4 joint decoder 460 for detecting the two useful data streams with the two most powerful interfering data streams detected by channel estimation blocks 315-345.

Thanks to the re-encoding of the two interfering data symbols (IDS) by re- encoders 334 and 344 , the corresponding contributions can be suppressed from the received signals and cancelled before the detected again by Sphere Decoder , or more generally by the 4x4 joint decoder in order to improve the detection process.

It can be seen that the invention is highly useful for using MIMO structure of the UE 100 for the purpose of canceling the interference from the other present base stations or antennas.