# US 8,570,939 B2 · Methods and systems for choosing cyclic delays in multiple antenna OFDM systems

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- Number: US 8,570,939 B2
- Document ID: US8570939B2
- Status: Granted
- Kind: B2
- Application number: 12/357,935
- Filed: 2009-01-22
- Published: 2009-09-10
- Granted: 2013-10-29
- Earliest claimed: 2008-03-14
- Assignees: QUALCOMM Incorporated
- Applicants: Pranav Dayal; Ayman Fawzy Naguib
- Inventors: Pranav Dayal; Ayman Fawzy Naguib
- Web page: https://patentgrep.com/patent/US8570939B2

## CPC

- H04B 7/0671: {using different delays between antennas}
- H04B: TRANSMISSION

## Abstract

Certain embodiments of the present disclosure relate to a method to determine appropriate values of cyclic delays applied at a transmitter with multiple antennas in order to provide accurate estimation of channel gains in a multiple-input single-output (MISO) system or multiple-input multiple-output (MIMO) system.

## Claims

### Claim 1

A method of transmitting pilots in a wireless communication system, comprising:
generating a first pilot for a first transmit antenna based on a first cyclic delay; and
generating a second pilot for a second transmit antenna based on a second cyclic delay larger than the first cyclic delay by at least a cyclic prefix length,
wherein generating the first pilot and the second pilot comprises generating OFDM symbols that comprise pilot symbols mapped to subcarriers spaced apart by p, where p is a prime number that does not divide NFFT, and NFFT is an FFT size for the generated OFDM symbols.

### Claim 2 (depends on 1)

The method of claim 1, further comprising:
generating a third pilot for a third transmit antenna based on a third cyclic delay larger than the second cyclic delay by at least the cyclic prefix length.

### Claim 3 (depends on 1)

The method of claim 1, wherein the cyclic delay for each transmit antenna is
t
m
=
∑
i
=
0
m
⁢
⁢
N
C
,
i
,
for m=0,1, . . . ,M−1,
where NC,0≧0, NC,i≧NCP ∀i≧1, NCP is the cyclic prefix length, m is a transmit antenna index, and tm is the cyclic delay for transmit antenna m, for m=0,1, . . . ,M−1.

### Claim 4 (depends on 1)

The method of claim 1, wherein the first cyclic delay is zero and the second cyclic delay is equal to or greater than the cyclic prefix length.

### Claim 5 (depends on 1)

The method of claim 1, wherein the first and second cyclic delays are not sent by signaling.

### Claim 6 (depends on 1)

The method of claim 1, wherein:
generating the first pilot further comprises generating a first sample sequence comprising the first pilot and cyclically delaying the first sample sequence by the first cyclic delay; and
generating the second pilot further comprises generating a second sample sequence comprising the second pilot and cyclically delaying the second sample sequence by the second cyclic delay.

### Claim 7 (depends on 1)

The method of claim 1, wherein the generated OFDM symbols comprise a first OFDM symbol comprising the first pilot and having the first cyclic delay and a second OFDM symbol comprising the second pilot and having the second cyclic delay.

### Claim 8 (depends on 7)

The method of claim 7, wherein the pilot symbols are mapped to the same set of subcarriers for both the first and second OFDM symbols.

### Claim 9 (depends on 7)

The method of claim 7, wherein:
S
≥
∑
i
=
0
M
-
1
⁢
⁢
N
C
,
i
,
where S is number of subcarriers with pilot symbols, M is number of transmit antennas, and
∑
i
=
0
m
⁢
⁢
N
C
,
i
is the length of cyclic delay for transmit antenna m, for m=0, . . . ,M−1.

### Claim 10 (depends on 7)

The method of claim 7, wherein
p
≤
N
FFT
∑
i
=
0
M
-
1
⁢
N
C
,
i
,
where M is a number of transit antennas and
∑
i
=
0
m
⁢
⁢
N
C
,
i
is the length of cyclic delay for transmit antenna m, for m=0, . . . , M−1.

### Claim 11 (depends on 1)

The method of claim 1, further comprising:
delaying OFDM pilot symbols transmitted from each antenna by a different amount to create frequency diversity.

### Claim 12 (depends on 11)

The method of claim 11, further comprising:
performing channel estimation within a Multiple Input Multiple Output (MIMO) system based on the delayed OFDM pilot symbols.

### Claim 13

A method of performing channel estimation in a wireless communication system, comprising:
obtaining first input samples comprising first and second pilots, the first pilot being generated based on a first cyclic delay and sent from a first transmit antenna, the second pilot being generated based on a second cyclic delay and sent from a second transmit antenna, the second cyclic delay being larger than the first cyclic delay by at least a cyclic prefix length, and the first input samples being from a first receive antenna; and
processing the first input samples based on pilot subcarriers spaced apart by p to obtain a first channel estimate for the first transmit antenna and a second channel estimate for the second transmit antenna, where p is a prime number that does not divide NFFT, and NFFT is an FFT size for an OFDM symbol.

### Claim 14 (depends on 13)

The method of claim 13, further comprising:
obtaining second input samples comprising the first and second pilots, the second input samples being from a second receive antenna; and
processing the second input samples to obtain a third channel estimate for the first transmit antenna and a fourth channel estimate for the second transmit antenna.

### Claim 15 (depends on 14)

The method of claim 14, wherein the processing the second input samples comprises:
processing the second input samples to obtain observations for the pilot subcarriers; and
processing the observations to obtain the third and fourth channel estimates.

### Claim 16 (depends on 15)

The method of claim 15, wherein the processing the second input samples to obtain observations comprises:
performing OFDM demodulation on the second input samples to obtain received pilot symbols for the pilot subcarriers; and
removing pilot modulation from the received pilot symbols to obtain the observations for the pilot subcarriers.

### Claim 17 (depends on 15)

The method of claim 15, wherein the processing the observations comprises processing the observations based on a minimum mean square error (MMSE) technique to obtain the third and fourth channel estimates.

### Claim 18 (depends on 13)

The method of claim 13, wherein the processing the first input samples comprises:
processing the first input samples to obtain observations for the pilot subcarriers; and
processing the observations to obtain the first and second channel estimates.

### Claim 19 (depends on 18)

The method of claim 18, wherein the processing the second input samples to obtain observations comprises:
performing OFDM demodulation on the first input samples to obtain received pilot symbols for the pilot subcarriers; and
removing pilot modulation from the received pilot symbols to obtain the observations for the pilot subcarriers.

### Claim 20 (depends on 18)

The method of claim 18, wherein the processing the observations comprises processing the observations based on a minimum mean square error (MMSE) technique to obtain the first and second channel estimates.

### Claim 21

An apparatus for transmitting pilots in a wireless communication system, comprising:
logic for generating a first pilot for a first transmit antenna based on a first cyclic delay; and
logic for generating a second pilot for a second transmit antenna based on a second cyclic delay larger than the first cyclic delay by at least a cyclic prefix length,
wherein the logic for generating the first pilot and the logic for generating the second pilot comprise logic for generating OFDM symbols that comprise pilot symbols mapped to subcarriers spaced apart by p, where p is a prime number that does not divide NFFT, and NFFT is an FFT size for the generated OFDM symbols.

### Claim 22 (depends on 21)

The apparatus of claim 21, further comprising:
logic for generating a third pilot for a third transmit antenna based on a third cyclic delay larger than the second cyclic delay by at least the cyclic prefix length.

### Claim 23 (depends on 21)

The apparatus of claim 21, wherein the cyclic delay for each transmit antenna is
t
m
=
∑
i
=
0
m
⁢
⁢
N
C
,
i
,
for m=0,1, . . . ,M−1,
where NC,0≧0, NC,i≧NCP ∀i≧1, NCP is the cyclic prefix length, m is a transmit antenna index, and tm is the cyclic delay for transmit antenna m, for m=0,1, . . . ,M−1.

### Claim 24 (depends on 21)

The apparatus of claim 21, wherein the first cyclic delay is zero and the second cyclic delay is equal to or greater than the cyclic prefix length.

### Claim 25 (depends on 21)

The apparatus of claim 21, wherein the first and second cyclic delays are not sent by signaling.

### Claim 26 (depends on 21)

The apparatus of claim 21, wherein:
the logic for generating the first pilot further comprises logic for generating a first sample sequence comprising the first pilot and cyclically delaying the first sample sequence by the first cyclic delay; and
the logic for generating the second pilot further comprises logic for generating a second sample sequence comprising the second pilot and cyclically delaying the second sample sequence by the second cyclic delay.

### Claim 27 (depends on 21)

The apparatus of claim 21, wherein the generated OFDM symbols comprise a first OFDM symbol comprising the first pilot and having the first cyclic delay and a second OFDM symbol comprising the second pilot and having the second cyclic delay.

### Claim 28 (depends on 27)

The apparatus of claim 27, wherein the pilot symbols are mapped to the same set of subcarriers for both the first and second OFDM symbols.

### Claim 29 (depends on 27)

The apparatus of claim 27, wherein:
S
≥
∑
i
=
0
M
-
1
⁢
⁢
N
C
,
i
,
where S is number of subcarriers with pilot symbols, M is number of transmit antennas, and
∑
i
=
0
m
⁢
⁢
N
C
,
i
is the length of cyclic delay for transmit antenna m, for m=0, . . . ,M−1.

### Claim 30 (depends on 27)

The apparatus of claim 27, wherein:
p
≤
N
FFT
∑
i
=
0
M
-
1
⁢
⁢
N
C
,
i
,
where M is number of transmit antennas and
∑
i
=
0
m
⁢
⁢
N
C
,
i
is the length of cyclic delay for transmit antenna m, for m=0, . . . ,M−1.

### Claim 31

An apparatus for performing channel estimation in a wireless communication system, comprising:
logic for obtaining first input samples comprising first and second pilots, the first pilot being generated based on a first cyclic delay and sent from a first transmit antenna, the second pilot being generated based on a second cyclic delay and sent from a second transmit antenna, the second cyclic delay being larger than the first cyclic delay by at least a cyclic prefix length, and the first input samples being from a first receive antenna; and
logic for processing the first input samples based on pilot subcarriers spaced apart by p to obtain a first channel estimate for the first transmit antenna and a second channel estimate for the second transmit antenna, where p is a prime number that does not divide NFFT, and NFFT is an FFT size for an OFDM symbol.

### Claim 32 (depends on 31)

The apparatus of claim 31, further comprising:
logic for obtaining second input samples comprising the first and second pilots, the second input samples being from a second receive antenna; and
logic for processing the second input samples to obtain a third channel estimate for the first transmit antenna and a fourth channel estimate for the second transmit antenna.

### Claim 33 (depends on 32)

The apparatus of claim 32, wherein the logic for processing the second input samples comprises:
logic for processing the second input samples to obtain observations for the pilot subcarriers; and
logic for processing the observations to obtain the third and fourth channel estimates.

### Claim 34 (depends on 33)

The apparatus of claim 33, wherein the logic for processing the second input samples to obtain observations comprises:
logic for performing OFDM demodulation on the second input samples to obtain received pilot symbols for the pilot subcarriers; and
logic for removing pilot modulation from the received pilot symbols to obtain the observations for the pilot subcarriers.

### Claim 35 (depends on 33)

The apparatus of claim 33, wherein the logic for processing the observations comprises logic for processing the observations based on a minimum mean square error (MMSE) technique to obtain the third and fourth channel estimates.

### Claim 36 (depends on 31)

The apparatus of claim 31, wherein the logic for processing the first input samples comprises:
logic for processing the first input samples to obtain observations for the pilot subcarriers; and
logic for processing the observations to obtain the first and second channel estimates.

### Claim 37 (depends on 36)

The apparatus of claim 36, wherein the logic for processing the first input samples to obtain observations comprises:
logic for performing OFDM demodulation on the first input samples to obtain received pilot symbols for the pilot subcarriers; and
logic for removing pilot modulation from the received pilot symbols to obtain the observations for the pilot subcarriers.

### Claim 38 (depends on 36)

The apparatus of claim 36, wherein the logic for processing the observations comprises logic for processing the observations based on a minimum mean square error (MMSE) technique to obtain the first and second channel estimates.

### Claim 39

An apparatus for transmitting pilots in a wireless communication system, comprising:
means for generating a first pilot for a first transmit antenna based on a first cyclic delay; and
means for generating a second pilot for a second transmit antenna based on a second cyclic delay larger than the first cyclic delay by at least a cyclic prefix length,
wherein the means for generating the first pilot and the means for generating the second pilot comprise means for generating OFDM symbols that comprise pilot symbols mapped to subcarriers spaced apart by p, where p is a prime number that does not divide NFFT, and NFFT is an FFT size for the generated OFDM symbols.

### Claim 40 (depends on 39)

The apparatus of claim 39, further comprising:
means for generating a third pilot for a third transmit antenna based on a third cyclic delay larger than the second cyclic delay by at least the cyclic prefix length.

### Claim 41 (depends on 39)

The apparatus of claim 39, wherein the cyclic delay for each transmit antenna is
t
m
=
∑
i
=
0
m
⁢
⁢
N
C
,
i
,
for m=0,1, . . . ,M−1,
where NC,0≧0, NC,i≧NCP ∀i≧1, NCP is the cyclic prefix length, m is a transmit antenna index, and tm is the cyclic delay for transmit antenna m, for m=0,1, . . . ,M−1.

### Claim 42 (depends on 39)

The apparatus of claim 39, wherein the first cyclic delay is zero and the second cyclic delay is equal to or greater than the cyclic prefix length.

### Claim 43 (depends on 39)

The apparatus of claim 39, wherein the first and second cyclic delays are not sent by signaling.

### Claim 44 (depends on 39)

The apparatus of claim 39, wherein:
the means for generating the first pilot further comprises means for generating a first sample sequence comprising the first pilot and cyclically delaying the first sample sequence by the first cyclic delay; and
the means for generating the second pilot further comprises means for generating a second sample sequence comprising the second pilot and cyclically delaying the second sample sequence by the second cyclic delay.

### Claim 45 (depends on 39)

The apparatus of claim 39, wherein the generated OFDM symbols comprise a first OFDM symbol comprising the first pilot and having the first cyclic delay and a second OFDM symbol comprising the second pilot and having the second cyclic delay.

### Claim 46 (depends on 45)

The apparatus of claim 45, wherein the pilot symbols are mapped to the same set of subcarriers for both the first and second OFDM symbols.

### Claim 47 (depends on 45)

The apparatus of claim 45, wherein:
S
≥
∑
i
=
0
M
-
1
⁢
⁢
N
C
,
i
,
where S is number of subcarriers with pilot symbols, M is number of transmit antennas, and
∑
i
=
0
m
⁢
⁢
N
C
,
i
is the length of cyclic delay for transmit antenna m, for m=0, . . . ,M−1.

### Claim 48 (depends on 45)

The apparatus of claim 45, wherein:
p
≤
N
FFT
∑
i
=
0
M
-
1
⁢
⁢
N
C
,
i
,
where M is number of transmit antennas and
∑
i
=
0
m
⁢
⁢
N
C
,
i
is the length of cyclic delay for transmit antenna m, for m=0, . . . ,M−1.

### Claim 49

An apparatus for performing channel estimation in a wireless communication system, comprising:
means for obtaining first input samples comprising first and second pilots, the first pilot being generated based on a first cyclic delay and sent from a first transmit antenna, the second pilot being generated based on a second cyclic delay and sent from a second transmit antenna, the second cyclic delay being larger than the first cyclic delay by at least a cyclic prefix length, and the first input samples being from a first receive antenna; and
means for processing the first input samples based on pilot subcarriers spaced apart by p to obtain a first channel estimate for the first transmit antenna and a second channel estimate for the second transmit antenna, where p is a prime number that does not divide NFFT, and NFFT is an FFT size for an OFDM symbol.

### Claim 50 (depends on 49)

The apparatus of claim 49, further comprising:
means for obtaining second input samples comprising the first and second pilots, the second input samples being from a second receive antenna; and
means for processing the second input samples to obtain a third channel estimate for the first transmit antenna and a fourth channel estimate for the second transmit antenna.

### Claim 51 (depends on 50)

The apparatus of claim 50, wherein the means for processing the second input samples comprises:
means for processing the second input samples to obtain observations for the pilot subcarriers; and
means for processing the observations to obtain the third and fourth channel estimates.

### Claim 52 (depends on 51)

The apparatus of claim 51, wherein the means for processing the second input samples to obtain observations comprises:
means for performing OFDM demodulation on the second input samples to obtain received pilot symbols for the pilot subcarriers; and
means for removing pilot modulation from the received pilot symbols to obtain the observations for the pilot subcarriers.

### Claim 53 (depends on 51)

The apparatus of claim 51, wherein the means for processing the observations comprises means for processing the observations based on a minimum mean square error (MMSE) technique to obtain the third and fourth channel estimates.

### Claim 54 (depends on 49)

The apparatus of claim 49, wherein the means for processing the first input samples comprises:
means for processing the first input samples to obtain observations for the pilot subcarriers; and
means for processing the observations to obtain the first and second channel estimates.

### Claim 55 (depends on 54)

The apparatus of claim 54, wherein the means for processing the first input samples to obtain observations comprises:
means for performing OFDM demodulation on the first input samples to obtain received pilot symbols for the pilot subcarriers; and
means for removing pilot modulation from the received pilot symbols to obtain the observations for the pilot subcarriers.

### Claim 56 (depends on 54)

The apparatus of claim 54, wherein the means for processing the observations comprises means for processing the observations based on a minimum mean square error (MMSE) technique to obtain the first and second channel estimates.

### Claim 57

A computer-program product for transmitting pilots in a wireless communication system, comprising a non-transitory computer readable medium having instructions stored thereon, the instructions being executable by one or more processors and the instructions comprising:
instructions for generating a first pilot for a first transmit antenna based on a first cyclic delay; and
instructions for generating a second pilot for a second transmit antenna based on a second cyclic delay larger than the first cyclic delay by at least a cyclic prefix length,
wherein the instructions for generating the first pilot and the instructions for generating the second pilot comprise instructions for generating OFDM symbols that comprise pilot symbols mapped to subcarriers spaced apart by p, where p is a prime number that does not divide NFFT, and NFFT is an FFT size for the generated OFDM symbols.

### Claim 58 (depends on 57)

The computer-program product of claim 57, wherein the instructions further comprise:
instructions for generating a third pilot for a third transmit antenna based on a third cyclic delay larger than the second cyclic delay by at least the cyclic prefix length.

### Claim 59 (depends on 57)

The computer-program product of claim 57, wherein the cyclic delay for each transmit antenna is
t
m
=
∑
i
=
0
m
⁢
⁢
N
C
,
i
,
for m=0,1, . . . ,M−1,
where NC,0≧0, NC,i≧NCP ∀i≧1, NCP is the cyclic prefix length, m is a transmit antenna index, and tm is the cyclic delay for transmit antenna m, for m=0,1, . . . ,M−1.

### Claim 60 (depends on 57)

The computer-program product of claim 57, wherein the first cyclic delay is zero and the second cyclic delay is equal to or greater than the cyclic prefix length.

### Claim 61 (depends on 57)

The computer-program product of claim 57, wherein the first and second cyclic delays are not sent by signaling.

### Claim 62 (depends on 57)

The computer-program product of claim 57, wherein:
the instructions for generating the first pilot further comprise instructions for generating a first sample sequence comprising the first pilot and cyclically delaying the first sample sequence by the first cyclic delay; and
the instructions for generating the second pilot further comprise instructions for generating a second sample sequence comprising the second pilot and cyclically delaying the second sample sequence by the second cyclic delay.

### Claim 63 (depends on 57)

The computer-program product of claim 57, wherein the generated OFDM symbols comprise a first OFDM symbol comprising the first pilot and having the first cyclic delay and a second OFDM symbol comprising the second pilot and having the second cyclic delay.

### Claim 64 (depends on 63)

The computer-program product of claim 63, wherein the pilot symbols are mapped to the same set of subcarriers for both the first and second OFDM symbols.

### Claim 65 (depends on 63)

The computer-program product of claim 63, wherein:
S
≥
∑
i
=
0
M
-
1
⁢
⁢
N
C
,
i
,
where S is number of subcarriers with pilot symbols, M is number of transmit antennas, and
∑
i
=
0
m
⁢
⁢
N
C
,
i
and is the length of cyclic delay for transmit antenna m, for m=0, . . . ,M−1.

### Claim 66 (depends on 63)

The computer-program product of claim 63, wherein:
p
≤
N
FFT
∑
i
=
0
M
-
1
⁢
⁢
N
C
,
i
,
where M is number of transmit antennas and
∑
i
=
0
m
⁢
⁢
N
C
,
i
is the length of cyclic delay for transmit antenna m, for m=0, . . . ,M−1.

### Claim 67

A computer-program product for performing channel estimation in a wireless communication system, comprising a non-transitory computer readable medium having instructions stored thereon, the instructions being executable by one or more processors and the instructions comprising:
instructions for obtaining first input samples comprising first and second pilots, the first pilot being generated based on a first cyclic delay and sent from a first transmit antenna, the second pilot being generated based on a second cyclic delay and sent from a second transmit antenna, the second cyclic delay being larger than the first cyclic delay by at least a cyclic prefix length, and the first input samples being from a first receive antenna; and
instructions for processing the first input samples based on pilot subcarriers spaced apart by p to obtain a first channel estimate for the first transmit antenna and a second channel estimate for the second transmit antenna, where p is a prime number that does not divide NFFT, and NFFT is an FFT size for an OFDM symbol.

### Claim 68 (depends on 67)

The computer-program product of claim 67, wherein the instructions further comprise:
instructions for obtaining second input samples comprising the first and second pilots, the second input samples being from a second receive antenna; and
instructions for processing the second input samples to obtain a third channel estimate for the first transmit antenna and a fourth channel estimate for the second transmit antenna.

### Claim 69 (depends on 68)

The computer-program product of claim 68, wherein the instructions for processing the second input samples comprise:
instructions for processing the second input samples to obtain observations for the pilot subcarriers; and
instructions for processing the observations to obtain the third and fourth channel estimates.

### Claim 70 (depends on 69)

The computer-program product of claim 69, wherein the instructions for processing the second input samples to obtain observations comprise:
instructions for performing OFDM demodulation on the second input samples to obtain received pilot symbols for the pilot subcarriers; and
instructions for removing pilot modulation from the received pilot symbols to obtain the observations for the pilot subcarriers.

### Claim 71 (depends on 69)

The computer-program product of claim 69, wherein the instructions for processing the observations comprise instructions for processing the observations based on a minimum mean square error (MMSE) technique to obtain the third and fourth channel estimates.

### Claim 72 (depends on 67)

The computer-program product of claim 67, wherein the instructions for processing the second input samples comprise:
instructions for processing the first input samples to obtain observations for the pilot subcarriers; and
instructions for processing the observations to obtain the first and second channel estimates.

### Claim 73 (depends on 72)

The computer-program product of claim 72, wherein the instructions for processing the first input samples to obtain observations comprise:
instructions for performing OFDM demodulation on the first input samples to obtain received pilot symbols for the pilot subcarriers; and
instructions for removing pilot modulation from the received pilot symbols to obtain the observations for the pilot subcarriers.

### Claim 74 (depends on 72)

The computer-program product of claim 72, wherein the instructions for processing the observations comprise instructions for processing the observations based on a minimum mean square error (MMSE) technique to obtain the first and second channel estimates.