Process for decoding ALAMOUTI block code in an OFDM system, and receiver for the same
View Patent ↗A process for decoding a signal being representative of a Space Time or Frequency Block coding during two signaling periods (STBC) or two parallel channels (SFBC) is provided. The process receives an OFDM signal received from at least one antenna. The process also performs an OFDM demodulation in order to generate N frequency domain representations of the received signal. Then the process performs a decoding process on said OFDM demodulated signal and groups the received signal in word code, Y=(y 1 , y 2 ), to represent the signal that was received during two signaling periods (STBC) or two parallel channels (SFBC). The word code is then decoded into a matrix after which a lattice reduction algorithm is applied to the matrix in order to transform the matrix into a reduced matrix having a near orthogonal vector. Then it performs a detection process on the reduced matrix to improve noise and interference immunity.
1. A process for decoding a signal being representative of a Space Time Block coding (STBC) or Space Frequency Block coding (SFBC) based on a transmission, firstly, of a pair of finite-alphabet complex symbols x 1 and x 2 , and secondly, of symbols −x 2 * and x 1 *, wherein * is a conjugate operation, during two parallel channels (SFBC), wherein the process comprises:
receiving an OFDM signal, from at least one antenna, by a receiver comprising one single antenna;
performing an OFDM demodulation in order to generate N frequency domain representations of the received OFDM signal, each N frequency domain representation being associated to one carrier;
performing a decoding process applied on the demodulated OFDM signal, in order to group the received OFDM signal in word code, Y=(y 1 , y 2 ), being representative of the received OFDM signal received during two signaling periods (STBC) or two parallel channels (SFBC);
decoding the word code, Y=(y 1 , y 2 ), in order to compute transmitted symbols x 1 and x 2 in accordance with a formulation,
[
y
1
y
2
*
]
=
[
h
1
h
2
h
~
2
*
-
h
~
1
*
]
[
x
1
x
2
]
+
[
n
1
n
2
*
]
Or
y
=
Hx
+
n
wherein H is a matrix, h 1 and h 2 are representative of the channel applicable to the transmission of x 1 and x 2 while {tilde over (h)} 1 ′ and {tilde over (h)} 2 are representative of the channel applicable to the transmission of −x 2 * and x 1 *, and wherein n 1 and n 2 are noise,
wherein decoding the word code comprises:
applying a lattice reduction algorithm on the matrix H, H=(b 1 , b 2 ), in order to transform the matrix H into a reduced matrix Hred=(b′ 1 , b′ 2 ) comprising vectors that are substantially orthogonal, wherein the lattice reduction algorithm on each carrier k is based on an iterative algorithm that was initialized based on values (b 1 k-1 , b 2 k-2 ) computed at a preceding iteration; wherein the lattice reduction algorithm comprises:
checking the correlation by testing whether
I Re {b 1 , b 2 >}1≦½Il b 1 Il 2 and
I Im {<b 1 , b 2 >}1≦½Il b 1 Il 2 , and, if not,
replacing b 2 with
b
2
-
⌊
(
b
1
,
b
2
)
b
1
2
⌉
b
1
and repeating the checking step again; and
performing a detection process on the reduced matrix Hred in order to minimize noise and interference immunity.
2. The process according to claim 1 , wherein the coding is STBC coding such that the transmission of symbol x 1 (resp. x 2 ) and symbol −x 2 *(resp. x 1 *) occur during two consecutive OFDM frames.
3. The process according to claim 1 , wherein the coding is a SFBC coding such that the transmission of symbol x 1 (resp. x 2 ) and symbol −x 2 *(resp. x 1 *) occur through two consecutive carriers in one OFDM frame.
4. The process according to claim 1 , wherein the lattice reduction algorithm further comprises an optional step of testing the length of b 1 and b 2 .
5. A receiver for decoding a signal being representative of a Space Time or Frequency Block coding based on a transmission, firstly, of a pair of finite-alphabet complex symbols x 1 and x 2 with, secondly, the symbols −x 2 * and x 1 * (* being the conjugate operation) during two signaling periods (STBC) or two parallel channels (SFBC), the receiver comprising:
one antenna;
a receive RF front end circuit adapted to receive an OFDM signal via the one antenna;
an OFDM demodulator adapted to generate a demodulated OFDM signal comprising N frequency domain representations of the received OFDM signal, wherein each N frequency domain representation is associated to one carrier;
a decoder adapted to perform a decoding process on the demodulated OFDM signal, wherein the decoding process groups the received demodulated OFDM signal in word code, Y=(y 1 , y 2 ), being representative of the received OFDM signal during two signaling periods (STBC) or two parallel channels (SFBC);
wherein the decoder is further adapted to decode the word code, Y=(y 1 , y 2 ), in order to compute the transmitted symbols x 1 and x 2 in accordance with a formulation,
[
y
1
y
2
*
]
=
[
h
1
h
2
h
~
2
*
-
h
~
1
*
]
[
x
1
x
2
]
+
[
n
1
n
2
*
]
Or
y
=
Hx
+
n
where H is a matrix, h 1 and h 2 are representative of the channel applicable to the transmission of x 1 and x 2 while {tilde over (h)} 1 ′ and {tilde over (h)} 2 are representative of the channel applicable to the transmission of −x 2 * and x 1 *, and wherein n 1 and n 2 are noise;
wherein the decoder is further adapted to apply a lattice reduction algorithm on said matrix H, H=(b 1 , b 2 ), in order to transform said matrix H in a reduced matrix Hred, Hred=(b′ 1 , b′ 2 ), comprising vectors that are substantially orthogonal, wherein the lattice reduction algorithm on each carrier k is based on an iterative algorithm that was initialized based on values (b 1 k-1 , b 2 k-2 ) computed at a preceding iteration; and
wherein when the decoder applies the lattice reduction algorithm, the decoder is further configured to check a correlation by determining whether:
|Re {<b 1 , b 2 >}|≦½∥b 1 ∥ 2 and
|Im {<b 1 , b 2 >}|≦½∥b 1 ∥ 2. , and, if the correlation is not true, then
replace b 2 with
b
2
-
⌊
(
b
1
,
b
2
)
b
1
2
⌉
b
1
and check the correlation again; and
a detector adapted to detect received symbols using the reduced matrix Hred.
6. The receiver of claim 5 , wherein the coding is a STBC coding such that the transmission of symbol x 1 (resp. x 2 ) and symbol −x 2 *(resp. x 1 *) occur during two consecutive OFDM frames.
7. The receiver of claim 5 , wherein the coding is a SFBC coding such that the transmission of symbol x 1 (resp. x 2 ) and symbol −x 2 *(resp. x 1 *) occur through two consecutive carriers in one OFDM frame.
8. The receiver of claim 5 , wherein the decoder is further adapted to test the length of b 1 and b 2 .
9. The receiver of claim 8 , wherein the decoder, when applying the lattice reduction algorithm, is applying the lattice reduction algorithm in combination with a ZF, MF, DF or PDF detector.
10. The receiver of claim 5 , wherein the receiver is comprised within a mobile communication device adapted for use in an OFDM communication network.