Pre-coding in OFDM
A pre-coder in a radio transceiver comprises a Discrete Fourier Transform (DFT) spreader that DFT-spreads a block of data symbols to produce DFT-spread data symbols. The spread data symbols are mapped to Orthogonal Frequency Division Multiplexing (OFDM) subcarriers assigned to the radio transceiver for transmission. An OFDM transmitter employs an inverse-DFT to modulate the DFT-spread data symbols onto the OFDM subcarriers to produce a pre-coded OFDM transmission signal. The spreading reduces the OFDM transmission signal's peak to average power.
1. A method employed by a radio transceiver, comprising:
selecting a data symbol block comprising a plurality N of data symbols;
spreading the data symbol block to produce N spread symbols, wherein each spread symbol is expressible by
w
m
=
α
m
∑
n
=
0
N
-
1
s
n
e
-
i
2
π
mn
/
N
wherein w m is an m th one of the N spread symbols, α m is an m th complex-valued scaling factor, s n is an n th one of the plurality N of data symbols, e is natural log, i=√{square root over (−1)}, π is mathematical constant Pi, m and n are indices each having integer values of 0 to N−1, e −i2πmn/N expresses elements of a Discrete Fourier Transform (DFT) spreading matrix, and N is equal to a number of Orthogonal Frequency Division Multiplexing (OFDM) subcarriers assigned to the radio transceiver for transmission, where N>2;
mapping each of the N spread symbols to one of the OFDM subcarriers; and
impressing each of the N spread symbols onto one of the OFDM subcarriers to generate an OFDM transmission signal comprising a superposition of data-bearing subcarriers, wherein the spreading provides the superposition with a reduced peak-to-average-power ratio.
2. The method recited in claim 1 , wherein α m =1.
3. The method recited in claim 1 , wherein α m values are selected to provide for amplitude scaling of the OFDM transmission signal.
4. The method recited in claim 1 , wherein the OFDM subcarriers are assigned to the transceiver based on channel-dependent scheduling.
5. The method recited in claim 1 , wherein the mapping comprises mapping in sequence the spread symbols, starting with w 0 , to the OFDM subcarriers.
6. The method recited in claim 1 , wherein the plurality N of data symbols comprises control information.
7. The method recited in claim 1 , wherein the spreading employs an N-point DFT and the impressing employs an M-point inverse-DFT, wherein M>N.
8. The method recited in claim 1 , wherein at least one of the spreading and the impressing employs a fast Fourier transform.
9. A signal-processing hardware device programmed to perform the method recited in claim 1 .
10. A radio transceiver, comprising:
a spreader configured to:
select for input a data symbol block comprising a plurality N of data symbols; and
spread the data symbol block to produce N spread symbols, wherein each spread symbol is expressible by
w
m
=
α
m
∑
n
=
0
N
-
1
s
n
e
i
2
π
mn
/
N
wherein w m is an m th one of the N spread symbols, α m is an m th complex-valued scaling factor, s n is an n th one of the plurality N of data symbols, e is natural log, i=√{square root over (−1)}, π is mathematical constant Pi, m and n are indices each having integer values of 0 to N−1, e −i2πmn/N expresses values of a Discrete Fourier Transform (DFT) spreading matrix indexed by m and n, and N is equal to a number of Orthogonal Frequency Division Multiplexing (OFDM) subcarriers assigned to the transceiver for transmission, where N>2;
a mapper configured to map the N spread symbols to the OFDM subcarriers; and
an OFDM transmitter configured to impress the N spread symbols onto the OFDM subcarriers to produce an OFDM transmission signal comprising a superposition of data-bearing subcarriers, wherein the spreader provides the superposition with a reduced peak-to-average-power ratio.
11. The radio transceiver recited in claim 10 , wherein α m =1.
12. The radio transceiver recited in claim 10 , wherein α m values are selected to provide for amplitude scaling of the OFDM transmission signal.
13. The radio transceiver recited in claim 10 , wherein the OFDM subcarriers are assigned to the radio transceiver based on channel measurements.
14. The radio transceiver recited in claim 10 , wherein the mapper performs mapping in sequence the spread symbols, starting with w 0 , to the OFDM subcarriers.
15. The radio transceiver recited in claim 10 , wherein the plurality N of data symbols comprises control information.
16. The radio transceiver recited in claim 10 , wherein the spreader employs an N-point DFT and the OFDM transmitter performs an M-point inverse-DFT, wherein M>N.
17. The radio transceiver recited in claim 10 , wherein at least one of the spreader and the OFDM transmitter performs a fast Fourier transform.
18. The radio transceiver recited in claim 10 , wherein at least one of the mapper and the OFDM transmitter are configured to perform frequency hopping.
19. An apparatus, comprising:
a processor; and
a memory coupled to the processor, the memory including a set of instructions stored therein and executable by the processor for:
spreading a data symbol block comprising a plurality N of data symbols to produce N spread symbols, wherein each spread symbol is expressible by
w
m
=
α
m
∑
n
=
0
N
-
1
s
n
e
-
i
2
π
mn
/
N
wherein w m is an m th one of the N spread symbols, α m is an m th complex-valued scaling factor, s n is an n th one of the plurality N of data symbols, e is natural log, i=√{square root over (−1)}, π is mathematical constant Pi, m and n are indices each having integer values of 0 to N−1, e −i2πmn/N expresses elements of a Discrete Fourier Transform (DFT) spreading matrix, and N is equal to a number of Orthogonal Frequency Division Multiplexing (OFDM) subcarriers assigned to a radio transmitter for transmission, where N>2;
mapping each of the N spread symbols to one of the OFDM subcarriers; and
performing an inverse-DFT on the N spread symbols to generate an OFDM transmission signal comprising a superposition of data-bearing subcarriers, wherein the spreading provides the superposition with a reduced peak-to-average-power ratio.
20. The apparatus recited in claim 19 , wherein α m =1.
21. The apparatus recited in claim 19 , wherein α m values are selected to provide for amplitude scaling of the OFDM transmission signal.
22. The apparatus recited in claim 19 , wherein the OFDM subcarriers are assigned to the radio transmitter based on channel-dependent scheduling.
23. The apparatus recited in claim 19 , wherein the mapping comprises mapping in sequence the spread symbols, starting with w 0 , to the OFDM subcarriers.
24. The apparatus recited in claim 19 , wherein the plurality N of data symbols comprises control information.
25. The apparatus recited in claim 19 , wherein the spreading employs an N-point DFT and the performing the inverse-DFT employs an M-point inverse-DFT, wherein M>N.
26. The apparatus recited in claim 19 , wherein at least one of the spreading and the performing the inverse-DFT employs a fast Fourier transform.