Patent Application Titled "Method and Wireless Communication Device for Estimating Frequency Offset of Received Signal" Published Online (USPTO...

Telecommunications Weekly |

Patent Application Titled "Method and Wireless Communication Device for Estimating Frequency Offset of Received Signal" Published Online (USPTO 20170279652)

By a News Reporter-Staff News Editor at Telecommunications Weekly -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventors Guo, Zhiheng (Beijing, CN); Wang, Hai (Beijing, CN), filed on , was made available online on .

No assignee for this patent application has been made.

Reporters obtained the following quote from the background information supplied by the inventors: "This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.

"In Long Term Evolution (LTE) systems where Single Carrier Frequency Division Multiple Access (SC-FDMA) is used for uplink (i.e., from a User Equipment (UE) to an evolved NodeB (eNB)), the uplink reception performance may be seriously degraded by a frequency offset between a carrier frequency of a signal received at the eNB and a frequency of the eNB's local oscillator. Likewise, the downlink reception performance suffers from a frequency offset between a carrier frequency of a signal received at the UE and the eNB's local oscillator.

"To guarantee an acceptable reception performance, it is thus necessary to estimate and compensate the frequency offset which mainly results from a Doppler frequency shift due to UE mobility.

"In prior art (referring to US 2013/0070874 A1 for example), the estimation of the frequency offset is done by making use of the fact that the frequency offset causes a phase change between two OFDM reference symbols.

"For illustration, the downlink subframe structure and the uplink subframe structure for LTE systems will be described with respect to FIGS. 1 and 2. As shown in the Figures, each subframe consists of a slot #0 and a slot #1, and each slot consists of seven OFDM symbols in case the subframe has a normal Cylic Prefix. Although not shown here, those skilled in the art will appreciate that each slot consists of six OFDM symbols in case the subframe has an extended CP.

"More specifically, as illustrated in FIGS. 1 and 2 for the normal CP case, symbols #0, #4, #7 and #11 in a downlink subframe are used as reference symbols for channel estimation while the other symbols in the subframe are used as data symbols, and symbols #3 and #10 in an uplink subframe are used as reference symbols while the other symbols are used as data symbols. Likewise, for the extended CP case, both downlink and uplink subframes contain reference and data symbols.

"According to the prior art frequency offset estimation approach US 2013/0070874 proposes, a phase change O.sub.diff.epsilon.[-.pi.pi.) between two reference symbols are measured at first. Then, a preliminary frequency offset f.sub.m.sub._.sub.est is calculated as

"f m_est = .0. diff 2 .pi. t d , ##EQU00001##

"where t.sub.d denotes a time distance between the two reference symbols. By way of example, for a downlink subframe as illustrated in FIG. 1, the two reference symbols may be selected as symbols #4 and #7. In this case, t.sub.d is equal to 0.215 ms. For an uplink subframe as illustrated in FIG. 2, the two reference symbols may be selected as symbols #3 and #10. Accordingly, t.sub.d is equal to 0.5 ms.

"Based on the preliminary frequency offset f.sub.m.sub._.sub.est, a plurality of frequency offset candidates can be determined as f.sub.n,offset=f.sub.m.sub._.sub.est+n.times.f.sub.es, where n.epsilon.{0, .+-.1, .+-.2 . . . } and f.sub.es denotes an observation frequency which may take a value of 1/t.sub.d. Then, the received OFDM signal is decoded multiple times, with one of the plurality of frequency offset candidates applied to the received OFDM signal each time. In case the received OFDM signal is successfully decoded when a specific one of the plurality of frequency offset candidates is applied thereto, the frequency offset is determined as the specific frequency offset candidate.

"One of the drawback of the prior art frequency offset estimation approach is that it cannot be applied to estimate the frequency offset for uplink reception in case frequency hopping is employed in the uplink. This is because, in the case of frequency hopping where different subcarriers are allocated to one UE in different slots, the phase change between the reference symbols #3 and #10 shown in FIG. 2 is no longer equal to 2.pi.t.sub.df.sub.m.sub._.sub.est.

"As another drawback of the prior art approach, the multiple attempts of decoding the received OFDM signal, to which the plurality of frequency offset candidates are respectively applied, consume large amounts of computation and power resources. The requirement for large amounts of computation resources may adversely incur high costs for the eNB and the UE. The consumption of large amounts of power resources may significantly reduce the UE's battery life."

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "In view of the foregoing, an object of the present disclosure is to eliminate or at least alleviate one of the drawbacks of the prior art frequency offset estimation approach.

"According to a first aspect of the present disclosure, there is provided a method implemented in a communication device for estimating a frequency offset between a carrier frequency of a received signal and a frequency of a local oscillator. The method comprises determining a plurality of phase change candidates for a phase change between a data symbol and a first reference symbol in the signal. The method further comprises generating a collection of constellation symbols from the data symbol and rotating the collection of constellation symbols by the plurality of phase change candidates. Then, one of the phase change candidates corresponding to one of the rotated collections of constellation symbols is selected in such a manner that said one of the rotated collections of constellation symbols matches a set of constellation points best. Next, the frequency offset is determined based on the selected phase change candidate.

"Instead of basing the frequency offset estimation on a phase change between two reference symbols, the frequency offset estimation solution according to the present disclosure is based on a phase change between a data symbol and a reference symbol. Accordingly, it may work well even if frequency hopping is employed for a subframe structure where only one reference symbol exists in a single slot.

"Additionally, unlike the prior approach which relies on decoding processes that are computation-intensive, the frequency offset estimation solution according to the present disclosure involves less complex operations, such as selecting one of a plurality of phase change candidates corresponding to one of rotated collections of constellation symbols which matches a set of constellation points best and determining the frequency offset based on the selected phase change candidate. Accordingly, the frequency offset estimation solution according to the present disclosure consumes less computation and power resources than the prior art approach.

"In an embodiment, the generating the collection of constellation symbols from the data symbol may comprise demodulating the data symbol to obtain multiple constellation symbols. Then, a subset of the multiple constellation symbols may be selected as the collection of constellation symbols.

"In an embodiment, the selecting one of the phase change candidates corresponding to one of the rotated collections of constellation symbols may comprise determining SNRs for the rotated collections of constellation symbols. Then, one of the phase change candidates corresponding to one of the rotated collections of constellation symbols may be selected, in such a manner that the SNR for said one of the rotated collections of constellation symbols is the highest one of said SNRs.



"In an embodiment, the selecting one of the phase change candidates corresponding to one of the rotated collections of constellation symbols may comprise performing hard decision on the rotated collections of constellation symbols to obtain corresponding collections of constellation points and determining differences between the rotated collections of constellation symbols and their corresponding collections of constellation points. Then, one of the phase change candidates corresponding to one of the rotated collections of constellation symbols may be selected, in such a manner that the difference between said one of the rotated collections of constellation symbols and its corresponding collection of constellation points is the minimum one of the differences.

"In an embodiment, the differences between the rotated collections of constellation symbols and their corresponding collections of constellation points may be determined as ((v.sub.n-v.sub.n)w.sub.n).times.((v.sub.n-v.sub.n)w.sub.n).sup.H, wherein v.sub.n denotes a vector formed by one of the rotated collections of constellation symbols, v.sub.n denotes a vector formed by one of the collections of constellation points corresponding to said one of the rotated collections of constellation symbols, w.sub.n denotes a vector formed by amplitudes of said one of the rotated collections of constellation symbols, and (.).sup.H denotes a conjugate transpose operation.

"In an embodiment, the determining the phase change candidates may comprise determining the phase change candidates as a sequence of values in a range of

"[ - .pi. 4 , .pi. 4 ) . ##EQU00002##

"Optionally, the sequence of values may be equally spaced in the range of

"[ - .pi. 4 , .pi. 4 ) . ##EQU00003##

"In an embodiment, the determining the phase change candidates may comprise estimating a preliminary frequency offset based on a phase change between a second and a third reference symbols in the signal and determining a plurality of frequency offset candidates as a sequence of values centered around the preliminary frequency offset. Then, the plurality of phase change candidates may be determined based on the frequency offset candidates and a time offset between the data symbol and the first reference symbol. Optionally, the sequence of values are equally spaced by an observation frequency, wherein the observation frequency is equal to a reciprocal of a time distance between the second and the third reference symbols.

"In an embodiment, the first reference symbol may be one of the second and the third reference symbols.

"In an embodiment, the determining the frequency offset may comprise determining the frequency offset as the selected phase change candidate divided by 2.pi.t.sub.i-iref, wherein t.sub.i-iref denotes a time offset between the data symbol and the first reference symbol.

"In an embodiment, the determining the frequency offset may comprise determining a first group of phase change candidates as a sequence of values centered around the selected phase change candidate and spaced by

".pi. 2 , ##EQU00004##

"estimating a phase change between the second and the third reference symbols in the signal and determining a second group of phase change candidates as a sequence of values centered around the estimated phase change and spaced by 2.pi.. The determining the frequency offset may further comprise calculating absolute differences between individual phase change candidates among the first group of phase change candidates scaled by t.sub.d/t.sub.i-iref and individual phase change candidates among the second group of phase change candidates, wherein t.sub.i-iref denotes a time offset between the data symbol and the first reference symbol and t.sub.d denotes a time distance between the second and the third reference symbols. Then, one of the first group of phase change candidates and one of the second group of phase change candidates may be selected, in such a manner that the absolute difference between said one of the first group of phase change candidates scaled by t.sub.d/t.sub.i-iref and said one of the second group of phase change candidates is the minimum one of the calculated absolute differences. Next, the frequency offset may be determined as said one of the first group of phase change candidate divided by 2.pi.t.sub.i-iref or said one of the second group of phase change candidates divided by 2.pi.t.sub.d.

"In an embodiment, the data symbol may be the closest data symbol to the first reference symbol. Additionally, the signal may be an OFDM signal and the data and reference symbols may be OFDM data and reference symbols.

"By determining the phase change candidates as the sequence of values in the range of

"[ - .pi. 4 , .pi. 4 ) ##EQU00005##

"and determining the frequency offset as the selected phase change candidate divided by 2.pi.t.sub.i-iref, a maximum frequency offset of about 1.75 kHz may be estimated for uplink reception. At the 2.6 GHz frequency band which is the operation band for LTE systems, even a moving speed up to 201 km/h cannot cause a Doppler frequency shift higher than 1.75 kHz. Accordingly, an accurate frequency offset can be estimated for a majority of LTE terminal devices in the real world which move at a speed lower than 201 km/h.

"By determining the phase change candidates based on the plurality of frequency offset candidates derived from the preliminary frequency offset f.sub.m.sub._.sub.est instead of as the sequence of values in the range of

"[ - .pi. 4 , .pi. 4 ) ##EQU00006##

"or by determining the frequency offset based on the first and second groups of phase change candidates instead of as the selected phase change candidate divided by 2.pi.t.sub.i-iref, the limitation on the maximum estimable frequency offset may be eliminated. Accordingly, an accurate frequency offset can be estimated for LTE terminal devices moving at a speed higher than 201 km/h.

"According to a second aspect of the present disclosure, there is provided a communication device for estimating a frequency offset between a carrier frequency of a received signal and a frequency of a local oscillator. The communication device comprises a phase change candidate determination section, a constellation symbol collection generation section, a rotation section, a phase change selection section and a frequency offset determination section. The phase change candidate determination section is configured to determine a plurality of phase change candidates for a phase change between a data symbol and a first reference symbol in the signal. The constellation symbol collection generation section is configured to generate a collection of constellation symbols from the data symbol. The rotation section is configured to rotate the collection of constellation symbols by the plurality of phase change candidates. The phase change selection section is configured to select one of the phase change candidates corresponding to one of the rotated collections of constellation symbols, in such a manner that said one of the rotated collections of constellation symbols matches a set of constellation points best. The frequency offset determination section is configured to determine the frequency offset based on the selected phase change candidate.

"According to a third aspect of the present disclosure, there is provided a communication device for estimating a frequency offset between a carrier frequency of a received signal and a frequency of a local oscillator. The communication device comprises a processor and a memory. The memory has machine-readable program code stored therein. When executed by the processor, the program code causes the communication device to perform the method according to the first aspect of the present disclosure.

"According to a fourth aspect of the present disclosure, there is provided a communication device for estimating a frequency offset between a carrier frequency of a received signal and a frequency of a local oscillator. The communication device comprises means adapted to determine a plurality of phase change candidates for a phase change between a data symbol and a first reference symbol in the signal, to generate a collection of constellation symbols from the data symbol and to rotate the collection of constellation symbols by the plurality of phase change candidates. The means is further adapted to select one of the phase change candidates corresponding to one of the rotated collections of constellation symbols, in such a manner that said one of the rotated collections of constellation symbols matches a set of constellation points best, and to determine the frequency offset based on the selected phase change candidate."

For more information, see this patent application: Guo, Zhiheng; Wang, Hai. Method and Wireless Communication Device for Estimating Frequency Offset of Received Signal. Filed and posted . Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PG01&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.html&r=1&f=G&l=50&s1=%2220170279652%22.PGNR.&OS=DN/20170279652&RS=DN/20170279652

Keywords for this news article include: Patents, Electronics, Wireless Technology, Wireless Communication.

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