Coherent cooperative UWB communication system

1. A distributed space-time coding method in an UWB (ultra-wide band) pulse telecommunication system including a source terminal which transmits a signal to a destination terminal during a transmission interval constituted by K frames, K≧1, each frame being divided into a first and a second half-frame, the signal transmitted in each first half-frame being received, then retransmitted after amplification during a next second half-frame by a distinct relay terminal among K relay terminals of said system, said method comprising:

coding, by said source terminal, 4K data symbols belonging to a PPM (pulse position modulation) modulation alphabet or a composite PPM-PAM (pulse position modulation-pulse amplitude modulation) modulation alphabet including a plurality of time positions, so as to provide a sequence of four transmission symbols per frame, said transmission symbols being obtained from 4K linear combinations of said data symbols using a plurality of coefficients belonging to a real algebraic extension of 2K of a field of rational numbers and, for one of said transmission symbols of predetermined rank in said sequence of each frame, a permutation of its PPM components; and

modulating a UWB pulse signal by the transmission symbols thus obtained.

2. The distributed space-time coding method according to claim 1 , wherein the transmission symbol of each frame having undergone a permutation of its PPM components is also subjected to a sign inversion of one or more of its PPM components.

3. The distributed space-time coding method according to claims 1 or 2 , wherein the four transmission symbols c 1 k , c 2 k , c 3 k , c 4 k of a k th frame of the transmission interval are obtained from the 4K data symbols s 1 , s 2 , . . . , s 4K , as follows:

c

1

k

=

∑

i

=

1

K

⁢

v

i

k

⁢

s

i

+

θ

⁢

∑

i

=

1

K

⁢

v

i

k

⁢

s

K

+

i

c

2

k

=

∑

i

=

1

K

⁢

v

i

k

⁢

s

2

⁢

K

+

i

+

θ

⁢

∑

i

=

1

K

⁢

v

i

k

⁢

s

3

⁢

K

+

i

c

3

k

=

Ω

⁡

(

∑

i

=

1

K

⁢

v

i

k

⁢

s

2

⁢

K

+

i

+

θ

1

⁢

∑

i

=

1

K

⁢

v

i

k

⁢

s

3

⁢

K

+

i

)

c

4

k

=

∑

i

=

1

K

⁢

v

i

k

⁢

s

i

+

θ

1

⁢

∑

i

=

1

K

⁢

v

i

k

⁢

s

K

+

i

where the v i k , θv i k , θ 1 v i k i=1, . . . , K are said coefficients; θ and θ 1 are real conjugated roots of a polynomial of degree 2 irreducible in the field of the rational numbers, Ω is a permutation operation of the time positions of the PPM-PAM alphabet possibly associated with a sign inversion of one or more PPM components;

wherein the sequence transmitted by the source terminal during said k th frame is c 2 k , c 4 k , c 1 k , c 3 k or c 2 k , c 1 k , c 4 k , c 3 k or c 3 k , c 4 k , c 1 k , c 2 k or c 3 k , c 1 k , c 4 k , c 2 k .

4. The distributed space-time coding method according to claim 3 , wherein

θ

=

1

+

5

2

⁢

⁢

and

⁢

⁢

θ

1

=

1

-

5

2

⁢

⁢

or

⁢

⁢

θ

1

=

1

+

5

2

⁢

⁢

and

⁢

⁢

θ

=

1

-

5

2

.

5. The distributed space-time coding method according to claim 4 , wherein K=1 and v 1 1 =1.

6. The distributed space-time coding method according to claim 4 , wherein K=2 and:

v

1

k

=

3

-

ϕ

k

2

⁢

⁢

and

v

2

k

=

3

-

ϕ

k

2

⁢

ϕ

k

⁢

⁢

with

⁢

⁢

ϕ

1

=

1

+

2

⁢

⁢

and

⁢

⁢

ϕ

2

=

1

-

2

and the coefficients v i k are defined up to a permutation on at least one of indices i and k and their values are defined up to a common multiplication coefficient, within ±10%.

7. The distributed space-time coding method according to claim 4 , wherein K=3 and:

v

1

k

=

-

2

+

2

⁢

ϕ

k

+

3

⁢

ϕ

k

2

7

;

v

2

k

=

6

+

ϕ

k

-

2

⁢

ϕ

k

2

7

;

v

3

k

=

3

-

3

⁢

⁢

ϕ

k

-

ϕ

k

2

7

;

with

⁢

⁢

ϕ

k

=

2

⁢

⁢

cos

⁡

(

2

⁢

⁢

π

⁢

⁢

k

7

)

,

wherein the coefficients v i k are defined up to a permutation on at least one of indices i and k and their values are defined up to a common multiplication coefficient, within ±10%.

8. The distributed space-time coding method according to claim 4 , wherein K=4 and:

v

1

k

=

2

+

3

⁢

ϕ

k

-

ϕ

k

2

8

;

v

2

k

=

2

-

ϕ

k

8

;

v

3

k

=

2

-

3

⁢

⁢

ϕ

k

+

ϕ

k

2

8

;

v

4

k

=

2

+

ϕ

k

8

;

with

⁢

⁢

ϕ

k

=

2

⁢

⁢

cos

⁡

(

π

⁢

⁢

k

8

)

,

wherein the coefficients v i k are defined up to a permutation on at least one of indices i and k and their values are defined up to a common multiplication coefficient, within ±10%.

9. The distributed space-time coding method according to claim 4 , wherein K=5 and:

v

1

k

=

4

+

2

⁢

⁢

ϕ

k

+

2

⁢

⁢

ϕ

k

2

-

ϕ

k

3

11

;

v

2

k

=

15

-

2

⁢

⁢

ϕ

k

-

12

⁢

⁢

ϕ

k

2

+

ϕ

k

3

+

3

⁢

ϕ

k

4

11

;

v

3

k

=

6

-

7

⁢

⁢

ϕ

k

+

4

⁢

⁢

ϕ

k

2

+

2

⁢

ϕ

k

3

-

ϕ

k

4

11

;

v

4

k

=

8

+

7

⁢

⁢

ϕ

k

+

5

⁢

⁢

ϕ

k

2

-

3

⁢

ϕ

k

3

-

2

⁢

ϕ

k

4

11

;

v

5

k

=

7

+

ϕ

k

2

+

ϕ

k

3

11

⁢

⁢

with

⁢

⁢

ϕ

k

=

2

⁢

⁢

cos

⁡

(

2

⁢

⁢

π

⁢

⁢

k

11

)

,

wherein the coefficients v i k are defined up to a permutation on at least one of indices i and k and their values are defined up to a common multiplication coefficient, within ±10%.

10. The distributed space-time coding method according to claim 1 , wherein said permutation of the PPM components is a circular permutation.

11. The distributed space-time coding method according to claim 1 , wherein transmission powers of the source terminal and the K relay terminals are chosen so as to be respectively equal to a s P and a r P, k=1, . . . , K where P is a power value complying with a UWB spectral mask and where a s and a r k are coefficients such that 0<a s <1, 0<a r k <1 and

a

s

+

1

2

⁢

∑

k

=

1

K

⁢

a

r

k

=

1.

12. The distributed space-time coding method according to claim 11 , wherein the coefficients a s and a r k are determined according to respective conditions of a propagation channel between the source terminal and the destination terminal, and respective channels between said relay terminals and the destination terminal.

13. The distributed space-time coding method according to claim 11 , wherein the coefficients a s and a r k are controlled by control loops with K+1 return paths from the destination terminal to the source terminal and the K relay terminals, respectively.

14. The distributed space-time coding method according to claim 1 , wherein transmission powers of the source terminal and the relay terminals are each chosen so as to be equal to a power value complying with a UWB spectral mask.

15. The distributed space-time coding method according to claim 1 , wherein said K relay terminals are determined by the source and destination terminals by the following steps:

determining a distance separating the source terminal and the destination terminal;

determining a first set of terminals located at less than said distance from the source terminal;

determining a second set of terminals located at less than said distance from the destination terminal; and

selecting the K relay terminals from terminals common to said first and second sets, called candidate terminals, as those minimising a sum of distances between the source terminal and a candidate terminal, and between the candidate terminal and the destination terminal.

16. The distributed space-time coding method according to claim 1 , wherein said K relay terminals are determined by the source and destination terminals by the following steps:

determining a distance separating the source terminal and the destination terminal;

determining a first set of terminals located at less than said distance from the source terminal;

determining a second set of terminals located at less than said distance from the destination terminal; and

determining terminals common to said first and second sets, called candidate terminals, and sending a sequence of predetermined symbols by the source terminal to said candidate terminals, wherein each candidate terminal detects said sequence with an error rate, and the K relay terminals are selected as candidate terminals detecting said sequence with the K lowest error rates.

17. A coding device for a UWB (ultra-wide band) pulse telecommunication terminal intended to transmit a signal to a destination terminal during a transmission interval (TTI) constituted by K frames, K≧1, said coding device comprising:

first distribution means for providing 4K data symbols belonging to a PPM (pulse position modulation) modulation alphabet or a composite PPM-PAM (pulse position modulation-pulse amplitude modulation) modulation alphabet including M time positions, to K coding modules in parallel, with each coding module corresponding to a frame and operating on said 4K data symbols to provide four transmission symbols;

second distribution means for providing each coding module with a set of 4K coefficients (V K , V 1 K ) belonging to a real algebraic extension of a field of rational numbers;

each coding module being adapted to perform a linear combination of said 4K data symbols with the 4K coefficients that it receives, and to perform a permutation of PPM components of one of the symbols obtained by combination; and

a plurality of delay lines mounted in series, each applying a delay equal to a frame duration and receiving, at its input, an output of a coding module.

18. The coding device according to claim 17 , wherein

each coding module includes four calculation sub-modules,

said first distribution means are adapted to provide a first half (s 1 , . . . , s K , s K+1 , . . . , s 2K ) of said data symbols to a first and a fourth of said sub-modules and a second half (S 2K+1 , . . . , s 3K , s 3K+1 , . . . , s 4K ) of said data symbols to a second and a third of said sub-modules, and

said second distribution means are adapted to provide a first half of said coefficients (V k ) to the first and second sub-modules and a second half of the coefficients (V 1 k ) to the third and fourth sub-modules; and

the coding device further includes,

a permutation sub-module adapted to perform a permutation of the PPM components of the symbols at an output of one of said calculation sub-modules; and

a plurality of other delay lines mounted in series, each applying a delay equal to a duration of a transmission symbol and receiving, at its input, an output of one of said calculation sub-modules or an output of the permutation sub-module.

19. The coding device according to claim 18 , wherein said permutation sub-module is also suitable for reversing a sign of one or more PPM components of the symbols that it receives.

20. The coding device according to claims 18 or 19 , wherein each calculation sub-module includes multiplication means for respectively multiplying each of the 2K data symbols that it receives by a respective coefficient among the 2K coefficients that it receives, and summing means for summing the 2K symbols thus obtained.