Sounding reference signal processing for LTE
A wireless communication receiver including a serial to parallel converter receiving an radio frequency signal, a fast Fourier transform device connected to said serial to parallel converter converting N FFT corresponding serial signals into a frequency domain; an EZC root sequence unit generating a set of root sequence signals; an element-by-element multiply unit forming a set of products including a product of each of said frequency domain signals from said fast Fourier transform device and a corresponding root sequence signal, an N SRS -length IDFT unit performing a group cyclic-shift de-multiplexing of the products and a discrete Fourier transform unit converting connected cyclic shift de-multiplexing signals back to frequency-domain.
1. A wireless communication receiver comprising;
a serial-to-parallel converter receiving a time sample sequence and generating parallel signals;
a fast Fourier transform (FFT device connected to said serial-to-parallel converter and converting said signals from a time domain into a frequency domain;
an Extended Zadoff-Chu (EZC) root sequence unit generating a set of root sequence signals;
an element-by-element multiply unit connected to said FFT device and said EZC root sequence unit, said element-by-element multiply unit forming a set of products Y including a product of each of said frequency domain signals from said FFT device and a complex conjugate of one of said corresponding root sequence signals X;
an Inverse Discrete Fourier Transform (IDFT) unit connected to said element-by-element multiply unit performing a group cyclic-shift de-multiplexing of said products to compute an impulse response for each multiplexed channel; and
a discrete Fourier transform unit connected to said IDFT unit to convert said impulse responses back to frequency domain.
2. A wireless communication receiver comprising;
a serial-to-parallel converter receiving a time sample sequence and generating parallel signals;
a fast Fourier transform (FFT) device connected to said serial-to-parallel converter and converting said signals from a time domain into a frequency domain;
an Extended Zadoff-Chu (EZC) root sequence unit generating a set of root sequence signals;
an element-by-element multiply unit connected to said FFT device and said EZC root sequence unit, said element-by-element multiply unit forming a set of products Y including a product of each of said frequency domain signals from said FFT device and a complex conjugate of one of said corresponding root sequence signals X;
an Inverse Discrete Fourier Transform (IDFT) unit connected to said element-by-element multiply unit performing a group cyclic-shift de-multiplexing of said products to compute an impulse response for each multiplexed channel, wherein said group cyclic shift de-multiplexing performed by said discrete Fourier transform unit includes:
Y=F N SRS N FFT( r )
y=F N SRS −1 diag( X*Y T )
where: Y is the frequency domain product of an element-wise multiplication, r is the received time sample sequence, y is the time domain product of an element-wise multiplication, F N SRS N FFT is a N SRS by N FFT matrix corresponding to N FFT -point FFT and N SRS sub-carriers de-mapping, X is the expected root sequence; and F N SRS is a N N SRS by N SRS matrix, thereby producing for each Sounding Reference Signal (SRS) comb a concatenated Channel Impulse Response (CIR) sequence y of all received wireless users multiplexed on a same root sequence; and
a discrete Fourier transform unit connected to said IDFT unit to convert said impulse responses back to frequency domain.
3. A wireless communication receiver comprising:
a serial-to-parallel converter receiving a time sample sequence and generating parallel signals;
a fast Fourier transform (FFT) device connected to said serial-to-parallel converter and converting said signals from a time domain into a frequency domain;
an Extended Zadoff-Chi (EZC) root sequence unit generating a set of root sequence signals;
an element-by-element multiply unit connected to said FFT device and said EZC root sequence unit, said element-by-element multiply unit forming a set of products Y including a product of each of said frequency domain signals from said FFT device and a complex conjugate of one said corresponding root sequence signals X;
an Inverse Discrete Fourier Transform (IDFT) unit connected to said element-by-element multiply unit performing a group cyclic-shift de-multiplexing of said products to compute an impulse response for each multiplexed channel, wherein said group cyclic shift de-multiplexing performed by said discrete Fourier transform unit includes:
y u =(0, . . . , 0, y n 1 (u) , y n 2 (u) , . . . , y n L (u) , 0, . . . , 0) T
Ĥ=F N SRS y N
where: y is the time domain product of an element-wise multiplication, F N SRS is a N SRS by N SRS matrix corresponding to N SRS -point DFT; and n 1 (U), . . . , n L (U) are the samples defining the cyclic shift window of user u, involving zeroing-out the remaining samples outside the cyclic shift window of user u and Ĥ N is the channel estimate across frequency chunk c produced by the last stage N SRS -length DFT-based frequency interpolation; and
a discrete Fourier transform unit connected to said IDFT unit to convert said impulse responses back to frequency domain.
4. The wireless communication receiver of claim 1 , wherein:
said group cyclic shift de-multiplexing performed by said discrete Fourier transform unit includes performing Channel Impulse Response extraction and cyclic shift de-multiplexing simultaneously by selecting the appropriate user's cyclic shift window.
5. The wireless communication receiver of claim 4 , wherein:
selecting a cyclic shift window includes shrinking a SRS bandwidth by 10% to minimize the interpolation errors.
6. A wireless communication receiver comprising;
a serial-to-parallel converter receiving a time sample sequence and generating parallel signals;
a fast Fourier transform (FFT) device connected to said serial-to-parallel converter and converting said signals from a time domain into a frequency domain;
an Extended Zadoff-Chu (EZC) root sequence unit generating a set of root sequence signals;
an element-by-element multiply unit connected to said FFT device and said EZC root sequence unit, said element-by-element multiply unit forming a set of products Y including a product of each of said frequency domain signals from said FFT device and a complex conjugate of one of said corresponding root sequence signals X:
an Inverse Discrete Fourier Transform (IDFT) unit connected to said element-by-element multiply unit performing a grog cyclic-shift de-multiplexing of said products to compute an impulse response for each multiplexed channel,
a non-biased per-antenna per-sub-carrier channel gain estimator performing
Ĝ 0 ( a )=| Ĥ ( a )| 2−{circumflex over (σ)} N 2
where: Ĝ 0 ( a )is the channel gain estimate per sub-carrier per antenna, Ĥ( a ) is the channel estimate {circumflex over (σ)} N 2 an estimate of the noise variance σ N 2 =a 2 σ H 2 , involving estimating and removing the noise variance; and
a discrete Fourier transform unit connected to said IDFT unit to convert said impulse responses back to frequency domain.
7. A wireless communication receiver comprising:
a serial-to-parallel converter receiving a time sample sequence and generating parallel signals;
a fast Fourier transform (FFT) device connected to said serial-to-parallel converter and converting said signals from a time domain into a frequency domain;
an Extended Zadoff-Chu (EZC) root sequence unit generating a set of root sequence signals;
an element multiply unit connected to said FFT device and said EZC root sequence unit, said element-by-element multiply unit forming a set of products Y including a product of each of said frequency domain signals from said FFT device and a complex conjugate of one of said corresponding root sequence signals X;
an Inverse Discrete Fourier Transform (IDFT) unit connected to said element-by-element multiply unit performing a group cyclic-shift de-multiplexing of said products to compute an impulse response for each multiplexed channel,
a non-biased per-antenna per-sub-carrier channel gain estimator performing
Ĝ 0 ( a )=| Ĥ ( a )| 2−{circumflex over (σ)} N 2
where: Ĝ 0 ( a ) is the channel gain estimate per sub-carrier per antenna, Ĥ( a )is the channel estimate, {circumflex over (σ)} N 2 is an estimate of the noise variance σ N 2 =a 2 σ H 2 , involving estimating and removing the noise variance, and
wherein said gain estimator includes a negative gain avoidance by applying a simple clipping threshold of 0.01 according to
Ĝ Clip ( a )=max{| Ĥ ( a )| 2 −{circumflex over (σ)} N 2 ; G floor }
where: Ĝ Clip ( a ) is the channel gain estimate per sub-carrier per antenna, Ĥ( a ) is the channel estimate, {circumflex over (σ)} N 2 is an estimate of the noise variance σ N 2 =a 2 σ H 2 and G floor is the clipping threshold; and
a discrete Fourier transform unit connected to said IDFT unit to convert said impulse responses back to frequency domain.
8. The wireless communication receiver of claim 1 , further comprising:
a per-antenna time domain noise variance estimator reserving a cyclic shift per SRS comb and averaging the squared noise samples across a noise window selected from the samples outside the cyclic shift windows of one or more users.
9. The wireless communication receiver of claim 8 , wherein:
said per-antenna time domain noise variance estimator maximizes the number of noise samples while not including samples carrying adjacent users' energy in spill-over regions.
10. The wireless communication receiver of claim 8 , wherein:
said per-antenna time domain noise variance estimator maximizes a UE-geometry-based selective cyclic shift window reduction as:
NOISE REDUCTION
LMS
CYCLIC SHIFT
TECHNIQUE
FILTERING
WINDOW SHRINK
Channel Model
TU
PA
TU
PA
Channel estimation MSE
3
dB
3
dB
6
dB
6
dB
Channel gain mean error
2
dB
1.2
dB
2.1
dB
2.5
dB
Channel gain standard
1
dB
0.5
dB
1
dB
1.2
dB.
deviation
11. The wireless communication receiver of claim 1 , further comprising:
a per-chunk signal to noise ratio (SNR) estimator from the achieved per-antenna per-subcarrier channel gain estimates based on selectively using a selected one of low-complexity arithmetic averaging or harmonic averaging depending on both a channel type and a user's received SNR.
12. A wireless communication Sounding Reference Signal (SRS) receiver comprising:
a serial to parallel converter receiving a radio frequency signal and generating N FFT corresponding parallel signals;
a fast Fourier transform device connected to said serial to parallel converter receiving said N FFT corresponding parallel signals and converting said N FFT corresponding serial signals from a time domain into a frequency domain;
a EZC root sequence unit generating a set of root sequence signals;
an element-by-element multiply unit connected to said fast Fourier transform device and said EZC root sequence unit, said element-by-element multiply unit forming a set of products including a product of each of said frequency domain signals from said fast Fourier transform device and a corresponding root sequence signal;
an N SRS -length IDFF unit connected to said element-by-element multiply unit performing a group cyclic-shift de-multiplexing of said products employing a sounding reference symbol Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiple Access symbol structure and a Constant Amplitude Zero Auto-Correlation sequence to compute an impulse response for each multiplexed channel through a frequency-domain computed periodic correlation; and
a discrete Fourier transform unit connected to said IDFT unit and receiving cyclic shift de-multiplexing signals to convert them back to frequency-domain
a timing offset estimator combining the amplitude delay profiles across antennas from the concatenated delay profiles sequence y and searching for the highest peak in the user's timing offset window according to:
{
i
^
u
=
arg
max
i
{
p
i
}
;
i
∈
I
τ
,
u
;
p
i
=
∑
a
=
1
A
y
i
,
a
2
τ
^
u
=
(
i
^
u
-
C
u
)
T
S
where: A is the number of antenna; C u is the cyclic shift of user u; T S is the sampling period of sequence y; and I τ,u is the timing offset window of user u, defined as:
{
I
τ
,
u
=
{
-
N
early
,
…
,
-
1
,
0
,
1
,
…
,
N
late
}
N
early
=
⌈
max
(
0.5
µs
,
τ
max
)
/
T
S
⌉
N
late
=
⌈
[
W
M
+
max
(
0.5
µs
,
τ
max
)
]
/
T
S
⌉
W
M
=
min
(
1
µs
,
τ
)
where: I τ,u (N early +1)=0 coincides with the first sample of the cyclic shift window of user U; ±τ max is the maximum expected timing error; W M is the main energy region within the user delay spread; and τis the delay spread of the user; and
a discrete Fourier transform unit connected to said IDFT unit to convert said products back to frequency-domain.