WO2011063570A1 - Method and apparatus for estimating frequency offset in lte - Google Patents

Method and apparatus for estimating frequency offset in lte Download PDF

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Publication number
WO2011063570A1
WO2011063570A1 PCT/CN2009/075186 CN2009075186W WO2011063570A1 WO 2011063570 A1 WO2011063570 A1 WO 2011063570A1 CN 2009075186 W CN2009075186 W CN 2009075186W WO 2011063570 A1 WO2011063570 A1 WO 2011063570A1
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Prior art keywords
frequency offset
concerned
sub
carrier
estimated frequency
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PCT/CN2009/075186
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French (fr)
Inventor
Aijun Cao
Johansson Jan
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Zte Wistron Telecom Ab
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Priority to PCT/CN2009/075186 priority Critical patent/WO2011063570A1/en
Publication of WO2011063570A1 publication Critical patent/WO2011063570A1/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2657—Carrier synchronisation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2668—Details of algorithms
    • H04L27/2669—Details of algorithms characterised by the domain of operation
    • H04L27/2672—Frequency domain
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2668—Details of algorithms
    • H04L27/2673—Details of algorithms characterised by synchronisation parameters
    • H04L27/2675—Pilot or known symbols
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
    • H04L27/2691—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation involving interference determination or cancellation

Definitions

  • the present invention relates to communication technology, and especially to estimating of frequency offset in LTE (Long Term Evolution) system.
  • LTE Long Term Evolution
  • LTE is the name given to a project within the Third Generation Partnership Project to improve the UMTS mobile phone standard to cope with future technology evolutions.
  • One of the goals of LTE is improving spectral efficiency. Since the orthogonal frequency provides much more flexibility to allocate the radio resource than the current cellular standards, the orthogonal frequency improves the spectral efficiency.
  • Frequency error can destroy the orthogonality among the sub-carriers, thus can introduce performance degradation into communication systems. It is vital to decrease frequency error. Both fixed frequency offset within a certain time and the frequency spread could contribute to frequency errors.
  • DMRS demodulation reference signal
  • the pilot symbol is located in the middle of one slot and allocated as one pilot symbol per slot.
  • the receiver performs channel estimates on the pilot symbol of several consecutive slots, then measures the phase change in the concerned consecutive slots in time domain, finally estimates the frequency offset according to the measured phase change.
  • Time-domain methods have the following main disadvantages:
  • An object of the present invention is to provide a method for estimating a frequency offset of a concerned sub-carrier of a concerned User Equipment (UE) in a Long Term Evolution (LTE) system, comprising: estimating the frequency offset by using Inter-Channel-Interferences (ICIs) associated with a limited number of adjacent sub-carriers of the concerned sub-carrier of UEs in the LTE system, whereby the frequency offset is estimated in frequency domain.
  • ICIs Inter-Channel-Interferences
  • the ICIs are associated with a left first adjacent sub-carrier and a right first adjacent sub-carrier of the concerned sub-carrier.
  • the method further comprises: using pilot symbols for sub-carriers of the concerned UE and outputs corresponding to the sub- carriers of the concerned UE after a system FFT to estimate the frequency offset.
  • the method further comprises: using one of the following to estimate the frequency offset: Zero-Forcing algorithm and Linear Minimum Mean-squared Error algorithm.
  • the using pilot symbols and outputs to estimate the frequency offset comprises: estimating the frequency offset by an equation below: wherein, is a estimated frequency offset, Re is a real part of ,and
  • P 0 ,... ,P 11 are the pilot symbols for the sub-carriers of the concerned UE
  • the method further comprises: if the plurality of Resource Blocks are two Resource Blocks, revising the estimated frequency offset by to give a final estimated frequency offset.
  • the method further comprises: revising the estimated frequency offset by o give a final estimated frequency offset.
  • the method further comprises: if the frequency offset is not changed for a plurality of consecutive slots, averaging in time domain the estimated frequency offsets from the plurality of consecutive slots, to give a final estimated frequency offset.
  • An object of the present invention is to provide an apparatus for estimating a frequency offset of a concerned sub-carrier of a concerned User Equipment (UE) in a Long Term
  • LTE Long Term Evolution
  • the apparatus comprising: an estimator for estimating the frequency offset by using Inter-Channel-Interferences (ICIs) associated with a limited number of adjacent sub-carriers of the concerned sub-carrier of UEs in the LTE system, whereby the frequency offset is estimated in frequency domain.
  • ICIs Inter-Channel-Interferences
  • the ICIs are associated with a left first adjacent sub-carrier and a right first adjacent sub-carrier of the concerned sub-carrier.
  • the estimator uses pilot symbols for sub-carriers of the concerned UE and outputs corresponding to the sub-carriers of the concerned UE after a system FFT to estimate the frequency offset.
  • the estimator uses one of the following to estimate the frequency offset: Zero-Forcing algorithm and Linear Minimum Mean-squared Error algorithm.
  • the estimator estimates the frequency offset by an equation below:
  • the apparatus further comprises: a first calculator for calculating ⁇ 1 , ⁇ 0 and ⁇ _ 1 for each of a plurality of Resource Blocks; and an averager for the calculated ⁇ 1 , ⁇ 0 and ⁇ _ 1 for the plurality of Resource Blocks to yield the averaged calculated ⁇ 1 , ⁇ 0 and ⁇ _ 1 for computing the estimated frequency offset
  • the apparatus further comprises: a second calculator for revising the estimated frequency offset by to give a final estimated frequency offset.
  • the apparatus further comprises: a calculator for revising the estimated frequency offset by to give a final estimated
  • the apparatus further comprises: an averager for averaging in time domain the estimated frequency offsets from the plurality of consecutive slots, to give a final estimated frequency offset.
  • the apparatus when signal power from one UE in the LTE system is much higher than signal power from the concerned UE, the apparatus further comprises: a first calculator for calculating the estimated frequency offset for the one UE by the equation, an generator for regenerating signal of the one UE by the estimated frequency offset for the one UE, a second calculator for subtracting the regenerated signal of the one UE from a total received signal to yield a revised total received signal, and a third calculator for calculating the estimated frequency offset for the concerned UE by the equation to be a final estimated frequency offset for the concerned UE, based on the revised total received signal.
  • Figure 1 shows ICI caused by frequency offset at different system FFT sizes
  • FIG. 1 shows T Vs normalized frequency offset and its approximation
  • Figure 3 shows simulation results of frequency offset for one UE in the case of noise free.
  • Equation (1) P 1 (k) is the pilot symbol for sub-carrier k of the User Equipment (UE) 1, and P 2 (k) is the pilot symbol for sub-carrier k of the UE 2, since the pilot symbol for each sub- carrier is different.
  • H k (1) is the frequency response of the sub-carrier k of the UE l
  • H k (2) is the frequency response of the sub-carrier k of the UE 2.
  • ⁇ f here is the sub-carrier bandwidth, in LTE, ⁇ f is 15kHz.
  • ⁇ 1 and ⁇ 2 are the frequency offset of the UE 1 and the UE 2 respectively.
  • RBsetl and RBset2 are the sub-carrier set for UE 1 and the sub-carrier set for UE 2 respectively.
  • n ⁇ t) here represents for noise.
  • the initial phases ⁇ 1 and ⁇ 2 can be regarded as an add-on phase on the channel so the initial phases ⁇ 1 and ⁇ 2 in Equations (1) and (2) can be omitted, and and are the
  • n (n) n(n) . Note that if ⁇ 1 is
  • each sub-carrier after system FFT consists of not only the Inter-Channel-Interference (ICI) caused by the frequency offset of the concerned UE (the first item in Equation (6)), but also the ICI caused by the frequency offset of other UEs (the second item in Equation (6)).
  • ICI Inter-Channel-Interference
  • Figure 1 shows ICI caused by frequency offset at different system FFT sizes.
  • the curves for different system FFT sizes are almost completely overlapped. That means the ICI caused by the frequency offset are independent from the system FFT size.
  • the ICI caused by the frequency offset is associated with the adjacent sub-carriers including the left multiple sub-carriers and the right multiple sub- carriers of the concerned sub-carrier. Further more, since the bigger the value of T, the smaller the dB value, ICI caused by the frequency offset can be simplified to be associated with the left first and the right first adjacent sub-carriers.
  • the ICI at the concerned sub-carrier caused by the frequency offset mainly comes from the left first and the right first adjacent sub-carriers.
  • R(k) is the received signal by the concerned UE including the ICI components caused by other UEs.
  • the ICI caused by other UEs interferes with the concerned UE only when (1) RBs are close, (2) the interfering UE has much higher frequency offset than the concerned UE, and (3) the interfering UE has much higher signal power than the interfered UE.
  • R(k) is further simplified as: for UE and fo
  • P 0 , P 1 , ... , P 11 are the pilot symbols in this RB;
  • R(0), R(l ), R(2), ... , R(10), R(11) are the system FFT outputs at the concerned RB ( R(0) and R(11) are not used);
  • Equation (10) The solution to the Equation (10) could be expressed as the following:
  • A is the determinant of the concerned matrix
  • A is the determinant of the concerned matrix and it is equal to :
  • A is apparently a real value, thus it can be ignored in the following frequency estimation procedures.
  • beta values For One RB, one set of beta values can be obtained. If there are more than one RBs, then beta values are averaged at first before computing rvalues below.
  • Figure 2 shows that the proposed algorithm has higher sensitivity at the low frequency offset than at half of the sub-carrier bandwidth 7.5 kHz, which means that large variation of the observed value would not cause a large variation of the estimated value. This is a very good property for an estimator because the concerned frequency offset is located at the low end.
  • ⁇ is the estimated value of ⁇
  • A is the estimated value of ⁇
  • Figure 3 shows simulation results of frequency offset for one UE in the case of noise free.
  • the frequency offset is estimated with 50 RBs, the real frequency offset almost equals to the estimated frequency offset.
  • This frequency offset of the concerned sub-carrier is only associated with an arbitrary limited number of adjacent sub-carriers.
  • Working range can be up to one sub-carrier bandwidth 15kHz. This is not possible for any time domain method.
  • frequency offset estimators can be designed in accordance with the above embodiments. These frequency offset estimators can implement the above frequency offset estimation method and can achieve the same advantages describe above.

Abstract

The present invention provides a method and apparatus for estimating a frequency offset of a concerned sub-carrier of a concerned UE in a Long Term Evolution (LTE) system. The method comprises: estimating the frequency offset by using Inter-Channel-Interferences (ICIs) associated with a limited number of adjacent sub-carriers of the concerned sub-carrier of UEs in the LTE system, whereby the frequency offset is estimated in frequency domain.

Description

Method and Apparatus for Estimating Frequency Offset in
LTE
Technical Field
The present invention relates to communication technology, and especially to estimating of frequency offset in LTE (Long Term Evolution) system.
Background
With the increased demand for higher data rate provided by mobile communication systems, multi-carriers with orthogonal frequencies are introduced into the next generation industry standard, such as OFDM for downlink or SC-FDMA for uplink in LTE system. LTE is the name given to a project within the Third Generation Partnership Project to improve the UMTS mobile phone standard to cope with future technology evolutions. One of the goals of LTE is improving spectral efficiency. Since the orthogonal frequency provides much more flexibility to allocate the radio resource than the current cellular standards, the orthogonal frequency improves the spectral efficiency.
But one inherent disadvantage with orthogonal frequency is sensitivity to frequency error. Frequency error can destroy the orthogonality among the sub-carriers, thus can introduce performance degradation into communication systems. It is vital to decrease frequency error. Both fixed frequency offset within a certain time and the frequency spread could contribute to frequency errors.
There are many different ways to obtain frequency offset estimation. In which estimating of frequency offset in OFDM systems is generally performed by looking at the known segments in time domain for example, the so-called DMRS (demodulation reference signal) which are specially designed OFDM pilot symbols known by the receiver. In LTE uplink, the pilot symbol is located in the middle of one slot and allocated as one pilot symbol per slot. At first the receiver performs channel estimates on the pilot symbol of several consecutive slots, then measures the phase change in the concerned consecutive slots in time domain, finally estimates the frequency offset according to the measured phase change. Time-domain methods have the following main disadvantages:
• Since there is one pilot symbol per slot (0.5ms), thus the maximum frequency offset should be less than 1000Hz.
• The time-domain methods won't work at all in the case of slot-hopping mode, because the two consecutive slots could have different sub-carriers, thus the channel phase change in the interval is not measurable.
Disclosure of Invention
An object of the present invention is to provide a method for estimating a frequency offset of a concerned sub-carrier of a concerned User Equipment (UE) in a Long Term Evolution (LTE) system, comprising: estimating the frequency offset by using Inter-Channel-Interferences (ICIs) associated with a limited number of adjacent sub-carriers of the concerned sub-carrier of UEs in the LTE system, whereby the frequency offset is estimated in frequency domain.
In accordance with an embodiment of the invention, wherein the ICIs are associated with a left first adjacent sub-carrier and a right first adjacent sub-carrier of the concerned sub-carrier.
In accordance with a further embodiment of the invention, the method further comprises: using pilot symbols for sub-carriers of the concerned UE and outputs corresponding to the sub- carriers of the concerned UE after a system FFT to estimate the frequency offset.
In accordance with a further embodiment of the invention, the method further comprises: using one of the following to estimate the frequency offset: Zero-Forcing algorithm and Linear Minimum Mean-squared Error algorithm.
In accordance with a further embodiment of the invention, wherein the using pilot symbols and outputs to estimate the frequency offset comprises: estimating the frequency offset by an equation below:
Figure imgf000003_0001
wherein, is a estimated frequency offset, Re is a real part of ,and
Figure imgf000003_0002
Figure imgf000003_0003
Figure imgf000004_0001
wherein P0,... ,P11 are the pilot symbols for the sub-carriers of the concerned UE, R(n) is the output corresponding to the nth sub-carrier of the concerned UE after the system FFT with n=1 ,..,10, and is Conjugate Pn .
In accordance with a further embodiment of the invention, wherein before computing the estimated frequency offset
Figure imgf000004_0002
by the equation, calculating β1 , β0 and β_1 for each of a plurality of Resource Blocks; and averaging the calculated β1 , β0 and β_1 for the plurality of Resource Blocks to yield the averaged calculated β1 , β0 and β_1 for computing the estimated frequency offset . In accordance with a further embodiment of the invention, the method further comprises: if the plurality of Resource Blocks are two Resource Blocks, revising the estimated frequency offset by to give a final estimated frequency offset.
Figure imgf000005_0001
In accordance with a further embodiment of the invention, f the method further comprises: revising the estimated frequency offset by o give a final estimated frequency offset.
Figure imgf000005_0002
In accordance with a further embodiment of the invention, the method further comprises: if the frequency offset is not changed for a plurality of consecutive slots, averaging in time domain the estimated frequency offsets from the plurality of consecutive slots, to give a final estimated frequency offset.
In accordance with a further embodiment of the invention, wherein when signal power from one UE in the LTE system is much higher than signal power from the concerned UE, calculating the estimated frequency offset for the one UE by the equation, regenerating signal of the one UE by the estimated frequency offset for the one UE, subtracting the regenerated signal of the one UE from a total received signal to yield a revised total received signal, and based on the revised total received signal, calculating the estimated frequency offset for the concerned UE by the equation to be a final estimated frequency offset for the concerned UE.
An object of the present invention is to provide an apparatus for estimating a frequency offset of a concerned sub-carrier of a concerned User Equipment (UE) in a Long Term
Evolution (LTE) system, the apparatus comprising: an estimator for estimating the frequency offset by using Inter-Channel-Interferences (ICIs) associated with a limited number of adjacent sub-carriers of the concerned sub-carrier of UEs in the LTE system, whereby the frequency offset is estimated in frequency domain.
In accordance with an embodiment of the invention, wherein the ICIs are associated with a left first adjacent sub-carrier and a right first adjacent sub-carrier of the concerned sub-carrier.
In accordance with a further embodiment of the invention, wherein the estimator uses pilot symbols for sub-carriers of the concerned UE and outputs corresponding to the sub-carriers of the concerned UE after a system FFT to estimate the frequency offset. In accordance with a further embodiment of the invention, wherein the estimator uses one of the following to estimate the frequency offset: Zero-Forcing algorithm and Linear Minimum Mean-squared Error algorithm.
In accordance with a further embodiment of the invention, wherein the estimator estimates the frequency offset by an equation below:
Figure imgf000006_0001
wherein, is a estimated frequency offset, Re is a real part of ,and
Figure imgf000006_0002
Figure imgf000006_0003
Figure imgf000006_0004
with
Figure imgf000006_0005
and
Figure imgf000006_0006
wherein P0,...,P11 are the pilot symbols for the sub-carriers of the concerned UE, R{n) is the output corresponding to the nth sub-carrier of the concerned UE after the system FFT with n=1 ,..,10, and is Conjugate Pn .
In accordance with a further embodiment of the invention, the apparatus further comprises: a first calculator for calculating β1 , β0 and β_1 for each of a plurality of Resource Blocks; and an averager for the calculated β1 , β0 and β_1 for the plurality of Resource Blocks to yield the averaged calculated β1 , β0 and β_1 for computing the estimated frequency offset
Figure imgf000007_0003
.
In accordance with a further embodiment of the invention, wherein if the plurality of Resource Blocks are two Resource Blocks, the apparatus further comprises: a second calculator for revising the estimated frequency offset by to give a final estimated frequency offset.
Figure imgf000007_0001
In accordance with a further embodiment of the invention, the apparatus further comprises: a calculator for revising the estimated frequency offset by to give a final estimated
Figure imgf000007_0002
frequency offset.
In accordance with a further embodiment of the invention, wherein if the frequency offset is not changed for a plurality of consecutive slots, the apparatus further comprises: an averager for averaging in time domain the estimated frequency offsets from the plurality of consecutive slots, to give a final estimated frequency offset.
In accordance with a further embodiment of the invention, when signal power from one UE in the LTE system is much higher than signal power from the concerned UE, the apparatus further comprises: a first calculator for calculating the estimated frequency offset for the one UE by the equation, an generator for regenerating signal of the one UE by the estimated frequency offset for the one UE, a second calculator for subtracting the regenerated signal of the one UE from a total received signal to yield a revised total received signal, anda third calculator for calculating the estimated frequency offset for the concerned UE by the equation to be a final estimated frequency offset for the concerned UE, based on the revised total received signal.
The following is the steps for estimating the frequency offset, wherein
• P0,...,P11 : 12 pilot symbols in one Resource Block (RB) R(n) : the output corresponding to the nth sub-carrier after the system FFT in one RB, n = 0 1 1
(1) calculating R01 as
Figure imgf000008_0002
(2) calculating R02 as
(3) calculating R12 as
Figure imgf000008_0001
(4) calculating Q0 as
(5) calculating Ql as
(6) calculating Q2 as
Figure imgf000008_0003
(7) calculating β1 as
(8) calculating β0 as
(9) calculating β-1 as
Figure imgf000008_0004
(10) averaging the above three β values from each RB;
(1 1) calculating Tl as
Figure imgf000008_0005
(12) calculating T2 as
Figure imgf000008_0006
(13) calculating T as T = Re {T1 - T2 } ;
(14) computing the normalized estimate over the frequency offset as
Figure imgf000009_0002
(15) computing the final frequency offset estimate as 15000;
Figure imgf000009_0003
(16) correcting the frequency offset estimate by a factor of 0.8071 if the number of RBs is 1 :
- 15000/0.8071 18585 ;
Figure imgf000009_0005
Figure imgf000009_0004
(17) correcting the frequency offset estimate by a factor of 0.7757 if the number of RBs is 2: 15000/0.7757 19338.
Figure imgf000009_0006
Figure imgf000009_0007
Brief Description of the Drawings
Figure 1 shows ICI caused by frequency offset at different system FFT sizes;
Figure 2 shows T Vs normalized frequency offset and its approximation; and
Figure 3 shows simulation results of frequency offset for one UE in the case of noise free.
Detailed Description of the Invention
In our embodiments, we estimate frequency offset in frequency domain. Assume that there are two user equipments (UEs) connected with one eNodeB, then the received samples of one pilot SC-FDMA symbol can be written as:
Equation (1)
Figure imgf000009_0001
In Equation (1), P1(k) is the pilot symbol for sub-carrier k of the User Equipment (UE) 1, and P2(k) is the pilot symbol for sub-carrier k of the UE 2, since the pilot symbol for each sub- carrier is different. Hk (1) is the frequency response of the sub-carrier k of the UE l, Hk (2) is the frequency response of the sub-carrier k of the UE 2. k is the sequence number of a sub-carrier, it ranges according to the total bandwidth and the sub-carrier bandwidth, for example, for 15kHz sub-carrier bandwidth, 20MHz total bandwidth, k=0~2047. Δf here is the sub-carrier bandwidth, in LTE, Δf is 15kHz. ε1 and ε2 are the frequency offset of the UE 1 and the UE 2 respectively. RBsetl and RBset2 are the sub-carrier set for UE 1 and the sub-carrier set for UE 2 respectively. n{t) here represents for noise.
After sampling at the system for NTs, in which Ts is sample interval:
Figure imgf000010_0001
Where the initial phases φ1 and φ2 can be regarded as an add-on phase on the channel so the initial phases φ1 and φ2 in Equations (1) and (2) can be omitted, and and are the
Figure imgf000010_0002
Figure imgf000010_0003
normalized frequency offset to the sub-carrier bandwidth of the two UEs and we assume that both of their absolute values are less than 1.
System FFT is performed on the received signal:
R
-
=
Figure imgf000010_0004
Wherein, / will refer to all sub-carriers in general. We have n (n) = n(n) . Note that if γ1 is
Figure imgf000010_0005
not zero, then
Figure imgf000010_0006
Otherwise, if γ1 is zero, then
Figure imgf000011_0001
Thus R{k) can be rewritten as the followin for the non-zero frequency offset:
Figure imgf000011_0002
From the above equation, it can be seen that the output of each sub-carrier after system FFT consists of not only the Inter-Channel-Interference (ICI) caused by the frequency offset of the concerned UE ( the first item in Equation (6)), but also the ICI caused by the frequency offset of other UEs (the second item in Equation (6)).
Let τ=(l-k), when τ=-1/+1 , it means the ICI caused by the frequency offset of the left first sub-carrier and the right first sub-carrier; when τ=-2/+2, it means the ICI caused by the frequency offset of the left second sub-carrier and the right second sub-carrier; when τ=-3/+3, it means the ICI caused by the frequency offset of the left third sub-carrier and the right third sub- carrier; and so on.
Figure 1 shows ICI caused by frequency offset at different system FFT sizes. The curves for different system FFT sizes are almost completely overlapped. That means the ICI caused by the frequency offset are independent from the system FFT size. Note that when τ= +1/-1 , it reaches to -l0dB for 1200FIz. When τ= +2/-2, +3/-3,... , the dB value decreases gradually.
From Figure 1, in one embodiment, the ICI caused by the frequency offset is associated with the adjacent sub-carriers including the left multiple sub-carriers and the right multiple sub- carriers of the concerned sub-carrier. Further more, since the bigger the value of T, the smaller the dB value, ICI caused by the frequency offset can be simplified to be associated with the left first and the right first adjacent sub-carriers.
Thus as another embodiment, we can conclude:
• The ICI at the concerned sub-carrier caused by the frequency offset mainly comes from the left first and the right first adjacent sub-carriers.
• Within the concerned three sub-carriers, the channel can be regarded as the same. From Equation (6), R(k) is the received signal by the concerned UE including the ICI components caused by other UEs. The ICI caused by other UEs interferes with the concerned UE only when (1) RBs are close, (2) the interfering UE has much higher frequency offset than the concerned UE, and (3) the interfering UE has much higher signal power than the interfered UE.
In one embodiment, if the signal power from different UEs is similar, and ignore the boundary sub-carriers between two UEs, then ICI caused by the frequency offset from other UEs can be neglected since the distance is much more than one sub-carrier. This user self closure is very attractive and good for the estimation. Then R(k) is further simplified as: for UE
Figure imgf000012_0001
and fo
Figure imgf000012_0002
We define:
Figure imgf000012_0003
where τ=(l-k) which equals to {-1, 0, 1 } .
Because of user self-closure, the weights defined above are only dependent on the concerned user's frequency offset.
For one RB, if we ignore two edge sub-carriers, we could obtain the following equation:
Where • P0, P1, ... , P11 are the pilot symbols in this RB;
• R(0), R(l ), R(2), ... , R(10), R(11) are the system FFT outputs at the concerned RB ( R(0) and R(11) are not used); and
Figure imgf000013_0001
We could use Zero-Forcing (ZF) algorithm and Linear Minimum Mean-squared Error (LMMSE) algorithm to solve the Equation (10), so as to estimate the frequency offset.
The solution to the Equation (10) could be expressed as the following:
Figure imgf000013_0002
If we introduce three variables as:
Figure imgf000013_0003
Then we have
Figure imgf000014_0001
wherein, A is the determinant of the concerned matrix
Figure imgf000014_0002
Let
Figure imgf000014_0003
wherein is Conjugate Pn
Then
Figure imgf000014_0004
Where A is the determinant of the concerned matrix and it is equal to :
Figure imgf000014_0005
A = 1000 - 10(
Figure imgf000015_0003
A is apparently a real value, thus it can be ignored in the following frequency estimation procedures.
Note that
Figure imgf000015_0001
Let
Figure imgf000015_0004
For One RB, one set of beta values can be obtained. If there are more than one RBs, then beta values are averaged at first before computing rvalues below.
And let
then
Figure imgf000015_0002
In order to reduce the amount of the computation, the above calculation can further be simplified as the following since the imaginary part of T is equal to sin{PilN), which is very small:
Figure imgf000015_0005
Figure 2 shows that the proposed algorithm has higher sensitivity at the low frequency offset than at half of the sub-carrier bandwidth 7.5 kHz, which means that large variation of the observed value would not cause a large variation of the estimated value. This is a very good property for an estimator because the concerned frequency offset is located at the low end.
Please be noted that if we assume that γ is less than 1 which means the frequency
Figure imgf000016_0001
offset ε is less than 15kHz, then
Figure imgf000016_0002
r r
which corresponds to the red line in the above figure. So we can obtain the normalized frequency offset estimation based on this RB:
Then the frequency offset estimate is:
Figure imgf000016_0003
Wherein γ is the estimated value of γ, and A is the estimated value of ε.
Further numerical results show that if the RB number is no more than 2, then the above estimator is under-estimated with the estimate of 80.71% of the real value when RB number is 1, and 77.57% of the real value when RB number is 2. Thus in other embodiments, we can make extra revisions:
Figure imgf000016_0004
Multiuser Interference Cancellation
In another embodiment, if there are multiple UEs, and the power from one UE is much higher than other UEs, for example, 20dB stronger, then the assumption is not valid for the weak users. So one possible way to apply this algorithm is similar as interference cancellation:
(1) First perform frequency offset estimation on the UE with the strongest power, and after performing demodulating and decoding on UE with the strongest power, the UE's signal is re-generated and subtracted from the total received signal.
(2) The frequency offset estimation for the UE with weaker power is performed on the
residue signal after cancelling the strongest UE's signal. Time Domain Averaging
In another embodiment, it is very natural to perform an average in the time domain by averaging the estimates from several consecutive slots, because for each slot, one independent estimate can be obtained via this frequency offset estimation algorithm. And this will possibly lead to a higher accuracy of the estimate as long as the frequency offset doesn't change during the several consecutive slots.
Multiple RBs Averaging
Figure 3 shows simulation results of frequency offset for one UE in the case of noise free. In one embodiment, the frequency offset is estimated with 50 RBs, the real frequency offset almost equals to the estimated frequency offset. We can see that the more the number of RBs is, the more accuracy the estimated frequency offset will be.
The advantages of this frequency offset estimating method are:
• This frequency offset estimation is valid for both slot-hopping and none slot-hopping cases, because it is performed purely in frequency domain.
• This frequency offset estimation does not required channel estimates because it is only dependent on the output of system FFT and reference signals, thus the frequency offset estimation and channel estimation can be done in parallel.
• This frequency offset of the concerned sub-carrier is only associated with an arbitrary limited number of adjacent sub-carriers.
• Working range can be up to one sub-carrier bandwidth 15kHz. This is not possible for any time domain method.
• The solution has a certain resistance to the noise by nature because of the implicit "LSE (Least Squared Estimation)" principle.
Accordingly, frequency offset estimators can be designed in accordance with the above embodiments. These frequency offset estimators can implement the above frequency offset estimation method and can achieve the same advantages describe above.
While several embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes and modifications may be made without departing from the spirit and scope of the invention.

Claims

Claims What is claimed is:
1. A method for estimating a frequency offset of a concerned sub-carrier of a concerned User Equipment (UE) in a Long Term Evolution (LTE) system, comprising:
estimating the frequency offset by using Inter-Channel-Interferences (ICIs) associated with a limited number of adjacent sub-carriers of the concerned sub-carrier of UEs in the LTE system, whereby the frequency offset is estimated in frequency domain.
2. A method according to claim 1, wherein the ICIs are associated with a left first adjacent sub-carrier and a right first adjacent sub-carrier of the concerned sub-carrier.
3. A method according to claim 2, further comprising:
using pilot symbols for sub-carriers of the concerned UE and outputs corresponding to the sub-carriers of the concerned UE after a system FFT to estimate the frequency offset.
4. A method according to claim 3, further comprising:
using one of the following to estimate the frequency offset: Zero-Forcing algorithm and Linear Minimum Mean-squared Error algorithm.
5. A method according to claim 3, wherein the using pilot symbols and outputs to estimate the frequency offset comprises:
estimating the frequency offset by an equation below:
Figure imgf000018_0001
wherein, is a estimated frequency offset, is a real part of ,and
Figure imgf000018_0003
Figure imgf000018_0004
Figure imgf000019_0001
with
Figure imgf000019_0002
,and
Figure imgf000019_0003
wherein P0,...,P11 are the pilot symbols for the sub-carriers of the concerned UE, R(n) is the output corresponding to the nth sub-carrier of the concerned UE after the system FFT with n=1 ,..,10, and P is Conjugate Pn .
6. A method according to claim 5, wherein before computing the estimated frequency offset by the equation, calculating β1 , β0 and β_1 for each of a plurality of Resource Blocks; and averaging the calculated β1 , β0 and β_1 for the plurality of Resource Blocks to yield the averaged calculated β1 , β0 and β_1 for computing the estimated frequency offset
Figure imgf000019_0005
.
7. A method according to claim 6, further comprising:
if the plurality of Resource Blocks are two Resource Blocks, revising the estimated frequency offset by to give a final estimated frequency offset.
Figure imgf000019_0004
8. A method according to claim 5, further comprising: revising the estimated frequency offset by to give a final estimated frequency
Figure imgf000020_0001
offset.
9. A method according to claim 5, further comprising:
if the frequency offset is not changed for a plurality of consecutive slots, averaging in time domain the estimated frequency offsets from the plurality of consecutive slots, to give a final estimated frequency offset.
10. A method according to claim 5, wherein when signal power from one UE in the LTE system is much higher than signal power from the concerned UE,
calculating the estimated frequency offset for the one UE by the equation,
regenerating signal of the one UE by the estimated frequency offset for the one UE, subtracting the regenerated signal of the one UE from a total received signal to yield a revised total received signal, and
based on the revised total received signal, calculating the estimated frequency offset for the concerned UE by the equation to be a final estimated frequency offset for the concerned UE.
11. An apparatus for estimating a frequency offset of a concerned sub-carrier of a concerned User Equipment (UE) in a Long Term Evolution (LTE) system, the apparatus comprising:
an estimator for estimating the frequency offset by using Inter-Channel-Interferences (ICIs) associated with a limited number of adjacent sub-carriers of the concerned sub-carrier of UEs in the LTE system,
whereby the frequency offset is estimated in frequency domain.
12. An apparatus according to claim 11, wherein the ICIs are associated with a left first adjacent sub-carrier and a right first adjacent sub-carrier of the concerned sub-carrier.
13. An apparatus according to claim 12, wherein the estimator uses pilot symbols for sub- carriers of the concerned UE and outputs corresponding to the sub-carriers of the concerned UE after a system FFT to estimate the frequency offset.
14. An apparatus according to claim 13, wherein the estimator uses one of the following to estimate the frequency offset: Zero-Forcing algorithm and Linear Minimum Mean-squared Error algorithm.
15. An apparatus according to claim 13, wherein the estimator estimates the frequency offset by an equation below:
Figure imgf000021_0001
wherein, is a estimated frequency offset, Re is a real part of and
Figure imgf000021_0002
Figure imgf000021_0003
Figure imgf000021_0004
with
Figure imgf000021_0005
and
Figure imgf000021_0006
wherein P0,...,P11 are the pilot symbols for the sub-carriers of the concerned UE, R{n) is the output corresponding to the nth sub-carrier of the concerned UE after the system FFT with n=1 ,..,10, and P* is Conjugate Pn .
16. An apparatus according to claim 15, further comprising: a first calculator for calculating β1 , β0 and β_1 for each of a plurality of Resource Blocks; and an averager for the calculated β1 , β0 and β_1 for the plurality of Resource Blocks to yield the averaged calculated β1 , β0 and β_1 for computing the estimated frequency offset
Figure imgf000022_0001
17. An apparatus according to claim 16, wherein if the plurality of Resource Blocks are two Resource Blocks, the apparatus further comprises: a second calculator for revising the estimated frequency offset by to give a final
Figure imgf000022_0002
estimated frequency offset.
18. An apparatus according to claim 15, further comprising: a calculator for revising the estimated frequency offset by to give a final estimated
Figure imgf000022_0003
frequency offset.
19. An apparatus according to claim 15, wherein if the frequency offset is not changed for a plurality of consecutive slots, the apparatus further comprises:
an averager for averaging in time domain the estimated frequency offsets from the plurality of consecutive slots, to give a final estimated frequency offset.
20. An apparatus according to claim 15, when signal power from one UE in the LTE system is much higher than signal power from the concerned UE, the apparatus further comprises:
a first calculator for calculating the estimated frequency offset for the one UE by the equation, an generator for regenerating signal of the one UE by the estimated frequency offset for the one UE,
a second calculator for subtracting the regenerated signal of the one UE from a total received signal to yield a revised total received signal, and
a third calculator for calculating the estimated frequency offset for the concerned UE by the equation to be a final estimated frequency offset for the concerned UE, based on the revised total received signal.
PCT/CN2009/075186 2009-11-27 2009-11-27 Method and apparatus for estimating frequency offset in lte WO2011063570A1 (en)

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CN1722718A (en) * 2004-07-17 2006-01-18 中国科学技术大学 A kind of frequency synchronization device of orthogonal frequency-division multiplex singal and method thereof
CN101272369A (en) * 2007-03-20 2008-09-24 富士通株式会社 Lead code detection and integer-time carrier wave frequency deviation estimation device and method
US20080273641A1 (en) * 2007-05-04 2008-11-06 Jingnong Yang Ofdm-based device and method for performing synchronization
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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN1722718A (en) * 2004-07-17 2006-01-18 中国科学技术大学 A kind of frequency synchronization device of orthogonal frequency-division multiplex singal and method thereof
CN101272369A (en) * 2007-03-20 2008-09-24 富士通株式会社 Lead code detection and integer-time carrier wave frequency deviation estimation device and method
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