Method and apparatus for OFDM spectral diversity using guard bands
An OFDM modulation process and means for transmitting multiple carriers through a communication channel having a bandwidth arranged ion chunks with further subcarriers being allocated to form at least one guard band, comprising automatically and periodically shifting said chunks to the left or to the side in order to overlap said guard band and increase spectral diversity.
1. An Orthogonal Frequency Division Multiplexing (OFDM) process for transmitting data using multiple carriers through a communication channel having a bandwidth W split in N subcarriers arranged in K chunks of M subcarriers each, with N−K×M subcarriers=2×Ms being allocated to form at least a first guard band and a second guard band located at the left side and at the right side of said bandwidth, respectively, said process comprising:
automatically and periodically shifting, for each OFDM symbol being transmitted, said chunks to left or right of the subcarriers allocated to each chunk in order to overlap said guard bands and increase spectral diversity in short-time transmissions;
receiving a sequence of OFDM symbol N-length vector s=(s 1 . . . s N ) in the frequency domain;
applying an inverse Discrete Fourier Transform (IDFT) in order to generate a vector x based on the following formula:
x
=
F
N
,
α
H
s
=
[
F
N
diag
(
1
,
ⅇ
ⅈ
α
1
N
,
…
,
ⅇ
ⅈα
N
-
1
N
)
]
H
s
where F N corresponds to the N-size Discrete Fast Transform (DFT) matrix with entries
F
m
,
n
=
ⅇ
-
2
π
ⅈ
nm
N
copying and multiplying the N L first symbols of x by e iα in order to constitute a cyclic-prefix preventing inter-symbol interference;
concatenating the said cyclic-prefix with the said vector x in order to form an extended vector:
x ext =[e iα x N−NL+1 ,e iα x N−NL+2 , . . . ,e iα x N ,x 1 , . . . ,x N ] T
transmitting said extended vector through said communication channel.
2. The OFDM process in accordance with claim 1 , further comprising, for each symbol being transmitted,
a first shift towards left of the subcarriers allocated to each chunk;
a second shift towards right of the subcarriers allocated to the chunks, so as to increase spectral diversity.
3. The OFDM process according to claim 1 wherein three distinctive values α 1 , α 2 , α 3 are successively applied for three consecutive OFDM symbol vectors.
4. The OFDM process according to claim 3 wherein the three distinct values are (−α, 0, α) and are applied in order to alternatively cause a shift to right and a shift to left of the allocation of the subcarriers to said chunks.
5. The OFDM process according to claim 1 , wherein a shifting pattern is defined as:
=
{
0
,
ⅇ
2
π
ⅈ
1
M
,
…
,
ⅇ
2
π
ⅈ
M
-
1
M
}
.
6. An Orthogonal Frequency Division Multiplexing (OFDM) process for transmitting data using multiple carriers through a communication channel having a bandwidth W split in N subcarriers arranged in K chunks of M subcarriers each, with N−K×M subcarriers=2×Ms being allocated to form at least one a first guard band and a second guard band located at the left side and at the right side of said bandwidth, respectively, said process comprising:
automatically and periodically shifting, for each OFDM symbol being transmitted, said chunks to left or right of the subcarriers allocated to each chunk in order to overlap said guard band bands and increase spectral diversity in short-time transmissions;
receiving N L symbols and suppressing the said cyclic prefix in order to generate a vector y;
generating a length-N vector z in accordance with the formula:
x
=
F
N
,
α
y
=
F
N
diag
(
1
,
ⅇ
ⅈ
α
1
N
,
…
,
ⅇ
ⅈα
N
-
1
N
)
y
where F N corresponds to the N-size Discrete Fast Transform (DFT) matrix with
F
m
,
n
=
ⅇ
-
2
π
ⅈ
nm
N
forwarding said z vector to a receiver unit.
7. Method of Orthogonal Frequency Division Multiplexing (OFDM) Data-Communication, comprising:
transforming data symbols into a frequency domain by Fourier Transformation turning a frequency-selective channel into N flat fading parallel transmission channels using a cyclic prefix (CP) to suppress inter-symbol interference (ISI) wherein a received signal r is identified from a sent signal s by r=HF −1 s+n, wherein n is Additive White Gaussian Noise with variance σ 2 , and H is a circulant matrix which is diagonalized in the Fourier basis F, with diagonal elements the discrete Fourier transform of the first column [h 0 , . . . , h L−1 ,0, . . . ,0] T by multiplying r in with F with
F
·
r
=
diag
(
H
(
0
)
,
…
,
H
(
N
-
1
N
)
)
s
+
n
,
wherein H(.) is the discrete Fourier transformation (DFT) of the first column of H
H
(
m
N
)
=
∑
j
=
0
L
-
1
h
j
ⅇ
-
2
π
ⅈ
j
m
n
;
multiplying samples of the cyclic prefix by z=ρe iα where ρ is constrained to equal 1; and
multiplying the OFDM signal by (1, e −iα/N , . . . , e −iα(N−1)/N ) after the inverse DFT (IDFT), leading to α-OFDM.