Frequency synchronization in wireless communication systems

The present invention provides a method of wireless communication over a communication link including at least one carrier that comprises a plurality of sub-carriers. The method includes modifying at least one frequency of at least one uplink sub-carrier in response to a signal indicating a modification of the frequency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to communication systems, and, more particularly, to wireless communication systems.

2. Description of the Related Art

Wireless communication systems typically include one or more base stations or access points for providing wireless connectivity to mobile units in a geographic area associated with each base station or access point. Mobile units and base stations communicate by transmitting modulated radiofrequency signals over a wireless communication link, or air interface. The air interface includes downlink (or forward link) channels for transmitting information from the base station to the mobile unit and uplink (or reverse link) channels for transmitting information from the mobile unit to the base station. The uplink and downlink channels are typically divided into data channels, random access channels, broadcast channels, paging channels, control channels, and the like.

The channels in a wireless communication system are defined by one or more transmission protocols. For example, in wireless communication systems that operate according to the Frequency Division Multiple Access (FDMA) protocol, the transmission bandwidth allocated to the air interface is divided into a number of frequencies and these frequencies are allocated to the various channels. For another example, Code Division Multiple Access (CDMA) protocols implement coding sequences that may be used to modulate transmitted information so that multiple users may transmit on the same frequency or set of frequencies. Other transmission protocols include Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier-FDMA (SC-FDMA). In an OFDMA system, the available bandwidth may be divided into a plurality of orthogonal subcarrier frequencies (commonly referred to as subcarriers), which may be allocated to one or more subchannels so that multiple users may transmit data concurrently using separate groups of subchannels. In SC-FDMA, the available bandwidth is also divided into a plurality of orthogonal subcarriers similar to OFDMA, but discrete Fourier transform (DFT) pre-coding is used to provide low Peak-to-Average-Power Ratio (PAPR) compared with OFDMA transmission.

Conventional wireless communication systems typically implement techniques for ensuring timing synchronization between the base stations and the mobile units. However, these wireless communication systems typically do not implement techniques for frequency synchronization between the base stations and the mobile units. Consequently, a frequency offset may be formed between the expected frequency of each sub-carrier and the actual frequency that is transmitted and/or received by a base station or mobile unit. For example, wireless communication protocols such as the Evolved UMTS Terrestrial Radio Access (E-UTRA) system require network operation at user equipment (UE) speeds of up to 350 kmph or even at higher speed of 500 kmph. The maximum frequency offset foffsetseen at an eNode B receiver may be expressed as:
foffset=ΔfBS+ΔfUE+2×fDoppler—max(Eq. 1)
where ΔfBS, ΔfUE, and fDoppler—maxdenote the base station frequency drift, the UE frequency error, and the maximum Doppler frequency, respectively. In the UMTS W-CDMA system, the frequency error at the base station and the UE is required to be less that 0.05 ppm and 0.1 ppm of the carrier frequency, respectively. For a carrier frequency of 2.1 GHz, the maximum frequency offset is therefore 781 Hz for a UE moving at the velocity 120 kmph, and can be as large as 2260 Hz, when the velocity is 500 kmph.

FIG. 1shows the frequency offset of one mobile unit as a function of the velocity of the mobile unit. The vertical axis indicates the frequency offset in Hertz and the horizontal axis indicates the velocity in kilometers per hour. The frequency offset at 0 kph corresponds to the base station frequency drift and the user equipment frequency error. The frequency offset increases approximately linearly with increasing velocity as the Doppler shift of the moving mobile unit increases. InFIG. 1, the frequency offset increases from approximately 350 Hertz to approximately 2260 Hertz.

FIG. 2shows the normalized frequency offset of one mobile unit as a function of the velocity of the mobile unit. In theFIG. 2, the frequency offset is normalized to the sub carrier spacing. For example, the normalized frequency offset (denoted as ε) is obtained as

ɛ=foffsetΔ⁢⁢fsub⁢-⁢carrier(Eq.⁢2)
The vertical axis indicates the normalized frequency offset as a percentage of the subcarrier spacing and the horizontal axis indicates the velocity in kilometers per hour. The normalized frequency offset at 0 kmph corresponds to the base station frequency drift and the user equipment frequency error. InFIG. 2, the normalized frequency offset at 0 kmph is approximately 0.03 of the subcarrier spacing. However, the Doppler shift causes the normalized frequency offset to increase to approximately 23% of the subcarrier spacing at velocities of 500 kph. Furthermore, when there are two UEs travelling in opposite directions, the amount of residual frequency offset between the two UEs can be twice that of the frequency offset between a single UE and the eNodeB.

FIGS. 3 and 4show the impact of frequency offset on the modulation symbol in a SC-FDMA system. The vertical axes in these figures indicate the imaginary part of the modulation symbol and the horizontal axes indicate the real part of the modulation symbol.FIG. 3shows a received signal constellation in the presence of frequency offset for QPSK modulation and a normalized frequency offset of 0.1.FIG. 4shows a received signal constellation in the presence of frequency offset for 16-QAM modulation and a normalized frequency offset of 0.1. As in any single-carrier system, frequency offset introduces rotation of the signal constellation. The amount of rotation depends on the sampling rate and the symbol duration. Conventional wireless communication systems implement a frequency offset estimation and compensation algorithm on the receiver-side. The frequency offset estimation and compensation algorithms can rotate the signal approximately back to the original constellation. However, in a multi-user FDM system, frequency offsets also exacerbate inter-carrier interference (ICI) and these effects cannot be corrected by simply applying a frequency offset estimation and compensation algorithm, at least in part because of the difficulty in separating contributions from symbols transmitted using the different carriers with different frequency offset. In some cases, the ICI can be significant and limit the performance of multi-user SC-FDMA system. The signal degradation caused by ICI may be particularly acute when users with different received SNR requirements are scheduled in adjacent subcarriers.

FIG. 5shows the degradation in received signal power due to frequency offset. The vertical axis indicates the received signal power degradation in decibels and the horizontal axis indicates the normalized frequency offset. In the illustrated embodiment, ICI is assumed to be produced by a single interfering user. The symbol SNR of the user of interest and the symbol SNR of the interfering user are assumed to be equal. Increasing the normalized frequency offset causes the received signal-to-interference-plus-noise ratio (SINR) to be increasingly degraded.FIG. 5shows that the SINR can degrade up to 4 dB when the frequency offset is large.

FIG. 6shows the degradation in the signal-to-interference-plus-noise ratio (SINR) caused by ICI between users having a normalized frequency offset. The vertical axis indicates the approximate SINR and the horizontal axis indicates the normalized frequency offset.FIG. 6shows that even in the presence of small residual frequency offset, the amount of SINR degradation can be significant. The degradation of the SINR is larger when the SNR is large. Although the symbol constellation of the user of interest may be rotated to approximately the original constellation, conventional frequency offset estimation and compensation algorithms are not typically able to compensate for frequency offset of the interfering user. Consequently, ICI from the frequency offset signal from the interfering user may result in significant degradation in the SINR for the user of interest. This can be a limiting factor in achieving a high target data rate on the uplink in high date rate systems such as the Long Term Evolution (LTE) system.

SUMMARY OF THE INVENTION

The present invention is directed to addressing the effects of one or more of the problems set forth above. The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.

In one embodiment of the present invention, methods are provided for wireless communication over a communication link including at least one carrier that comprises a plurality of sub-carriers. One embodiment of the method includes modifying at least one frequency of at least one uplink sub-carrier in response to a signal indicating a modification of the frequency. Another embodiment of the method includes providing a signal indicating a modification of at least one frequency of at least one uplink sub-carrier.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring now toFIG. 7, one exemplary embodiment of a wireless communication system700is illustrated. In the illustrated embodiment, the wireless communication system700includes a base station705. Although a single base station705is depicted in the illustrated embodiment, the present invention is not limited to one base station705for providing wireless connectivity. In alternative embodiments, the wireless communication system700may include any number of the base stations705and/or other devices for providing wireless connectivity, such as access points, base station routers, nodes-B, e-nodes-B, and the like. The wireless communication system700operates according to Single Channel-FDMA (SC-FDMA). However, the present invention is not limited to systems that operate exclusively according to SC-FDMA. For example, alternative embodiments of the wireless communication system700may operate according to Orthogonal Frequency Division Multiple Access (OFDMA) and/or other wireless indication protocols or standards.

The wireless communication system700also includes two mobile units710(1-2). The distinguishing indices (1-2) may be used to indicate individual mobile units710(1-2) four subsets thereof. However, the indices may be dropped when the mobile units710are referred to collectively. This convention may also be applied to other elements shown in the drawings and indicated by reference numerals and one or more distinguishing indices. The mobile units710(1-2) communicate with the base station705over wireless communication links (also known as air interfaces) that include an uplink (or reverse link)715and a downlink (or forward link)720. A band of carrier frequencies is allocated to the wireless communication link and the available frequency spectrum is then divided into a plurality of sub-carriers within the carrier frequency bandwidth. For example, in SC-FDMA, the available bandwidth is divided into a plurality of orthogonal subcarriers similar to OFDMA, and discrete Fourier transform (DFT) pre-coding is used to provide low Peak-to-Average-Power Ratio (PAPR) compared with OFDMA transmission. Each sub-carrier has a selected frequency and the frequencies associated with different sub-carriers are selected to be orthogonal to each other. Information may then be transmitted between a base station705and the mobile unit710concurrently over one or more channels of the uplink715and downlink720that are formed using the sub-carriers.

Frequency offsets may exist between the mobile unit710and a base station705. For example, the sub-carrier frequency transmitted by the mobile unit710, e.g., over the uplink715, may differ from a sub-carrier frequency received at the base station705. Similarly, the sub carrier frequency transmitted by the base station705, e.g., over the downlink720, may differ from a sub carrier frequency received at the mobile unit710. Frequency drifts and/or errors within the transmitting or receiving elements (not shown) of the mobile unit710and/or the base stations705may also contribute to the frequency offsets. For example, the frequency offset foffsetfor a subcarrier transmitted between the mobile unit710and the base station705may be expressed as:
foffset=ΔfBS+ΔfUE+2×fDoppler—max(Eq. 3)
where ΔfBS, ΔfUE, and fDoppler—maxdenote the base station frequency drift, the mobile unit (or user equipment, UE) frequency error, and the maximum Doppler frequency, respectively.

In the illustrated embodiment, the mobile unit710(1) is moving approximately towards the base station705, as indicated by the velocity vector725(1). Thus, the Doppler shift of the sub-carrier increases the frequency of the sub-carrier relative to the transmitted frequency of the sub-carrier. In the illustrated embodiment, the mobile unit710(2) is moving approximately away from the base station705, as indicated by the velocity vector725(2). Thus, the Doppler shift of the sub-carrier decreases the frequency of the sub-carrier relative to the transmitted frequency of the sub-carrier.

FIG. 8Aconceptually illustrates the transmitted frequency800of the sub-carrier and the received frequency805. The vertical axis indicates the amplitude of the received signal (in arbitrary units) and the horizontal axis indicates the frequency of the sub-carrier. In the embodiment shown inFIG. 8A, the transmitting and receiving elements are moving towards each other. The Doppler shift and any other frequency offsets result in the received frequency805being larger than the transmitted frequency800by a frequency error810.FIG. 8Bconceptually illustrates the transmitted frequency815of the sub-carrier and the received frequency820. The vertical axis indicates the amplitude of the received signal (in arbitrary units) and the horizontal axis indicates the frequency of the sub-carrier. In the embodiment shown inFIG. 8A, the transmitting and receiving elements are moving away from each other. The Doppler shift and any other frequency offsets result in the received frequency820being smaller than the transmitted frequency815by a frequency error825.

Referring back toFIG. 7, the frequency of one or more sub-carriers may be modified to compensate for associated frequency offsets. In one embodiment, the base station705may monitor a channel, such as a random access channel, for signals such as pilot signals transmitted by the mobile units710. When the base station705receives a pilot signal over the random access channel, the base station705may determine a frequency offset or error by comparing the received frequency to an expected frequency of the pilot signal. The base station705may then determine an appropriate correction that approximately compensates for the measured frequency offset or error and may transmit information indicating this correction to the mobile unit710that transmitted the received pilot signal over the random access channel. The mobile unit710may then use this correction signal to modify the frequency of one or more sub-carriers used to transmit information over the uplink715. In one embodiment, the station705may determine modifications to the sub-carrier frequency periodically or in response to the measured frequency offset or error exceeding a threshold value.

FIG. 9conceptually illustrates one exemplary embodiment of a method900of frequency synchronization. In the illustrated embodiment, a base station receives (at905) a pilot signal from a mobile unit. For example, the base station may be monitoring a random access channel and may receive (at905) a pilot signal transmitted by the mobile unit over the random access channel. In one embodiment, the received pilot signal may include one or more reference symbols, such as the reference symbols that may be used for channel estimation. For example, the received signal may be a channel sounding reference signal, which may be used for timing and/or frequency estimation, at least in part because the channel sounding reference signal has a large bandwidth and has better timing resolution relative to other reference signals, such as a data demodulation reference signal. The channel sounding reference signal and the data demodulation reference signal are both defined by LTE. Alternatively, the received signal may include a cyclic prefix, such as the cyclic prefix that may be attached to orthogonal frequency division multiplexed (OFDM) signals to reduce or avoid inter-symbol interference. However, persons of ordinary skill in the art having benefit of the present disclosure should appreciate that the present invention is not limited to signals that include these particular symbols and/or prefixes.

The base station may then determine (at910) the frequency (υSC) of the subcarrier used to transmit the pilot signal. As discussed herein, the received frequency (υSC) may not correspond to the frequency (υ0) used or expected by the receiver in the base station. The base station may therefore determine (at915) whether the received frequency (υSC) is equal to the frequency (υ0) expected at the base station. In one embodiment, the received signal may be down-modulated at the expected subcarrier frequency (υSC) and then the autocorrelation properties may be used to estimate the frequency offset in the equivalent complex baseband representation of the received and down-modulated signal. For example, the autocorrelation properties of reference symbols corresponding to the received pilot signal may be used to estimate the frequency offset. For another example, the autocorrelation properties of the cyclic prefix included in the received signal may be used to estimate the frequency offset. In one embodiment, the base station may determine (at915) whether the received frequency (υSC) is within a selected tolerance of the frequency (υ0). For example, if the received frequency (υSC) is within the selected tolerance of the frequency (υ0), then the receiver may determine that the received frequency (υSC) is substantially equal to the frequency (υ0). However, if the received frequency (υSC) is not within the selected tolerance of the frequency (υ0), then the receiver may determine that the received frequency (υSC) is not substantially equal to the frequency (υ0).

If the base station determines (at915) that the received frequency (υSC) is substantially equal to the frequency (υ0), then the base station may wait for additional signals from the mobile unit. For example, the base station may monitor a random access channel or other channels or sub-channels for signals that may be used to determine (at910) sub-carrier frequencies associated with mobile units in communication with the base station. If the base station determines (at915) that the received frequency (υSC) is not substantially equal to the frequency (υ0), then the base station may determine (at920) a sub-carrier frequency error. For example, the sub-carrier frequency error may be defined as the difference between the received frequency (υSC) and the expected frequency (υ0).

A change in the sub-carrier frequency may then be determined (at925) using the sub-carrier frequency error. In one embodiment, the received frequency (υSC) and the expected frequency (υ0) may also be used to determine (at925) the change in the sub-carrier frequency. For example, the change in the sub-carrier frequency may be determined (at925) to be approximately equal to the difference between the received frequency (υSC) and the expected frequency (υ0). In one embodiment, the estimated frequency offset can be quantized, e.g., into quantized steps that are a selected fraction of the received frequency (υSC). The quantized value may be used to signal a quantized frequency offset to the mobile station, which then compensates for the corresponding quantized frequency offset. In this embodiment, the residual frequency offset (residual error) after frequency compensation may be limited to a value that is less than the quantization stepsize.

The base station may then provide (at930) a signal to the mobile unit indicating a modified sub-carrier frequency. In one embodiment, the signal may be a Frequency Advance signal that includes information indicating the value of the sub-carrier frequency that should be used by the mobile unit. Alternatively, the Frequency Advance signal may include information indicating a change in the sub-carrier frequency that should be used by the mobile unit to modify the subcarriers. The change may be indicated as a frequency shift, a percentage change in the frequency, a frequency step (in the case when the possible subcarrier frequencies are predetermined and known to the mobile unit), and the like. The mobile unit may utilize the signal from the base station to modify one or more sub-carrier frequencies to improve frequency synchronization between the mobile unit and the base station. For example, the Frequency Advance signal may be transmitted to the mobile station on a random access channel with a Timing Advance signal prior to initialization of dedicated communication channels between the mobile unit and the base station.

In the embodiment of the method900described above, the pilot signal transmitted on a random access channel is used to estimate frequency errors and/or offsets that are used for frequency synchronization. However, the present invention is not limited to use of the pilot signal. In one embodiment, dedicated communication channels or sub-channels may already be formed between the mobile unit and the base station. Signals transmitted on these communication channels or sub-channels may therefore be used to determine (at910) the frequency (υSC) of the subcarrier used to transmit the signals on these communication channels or sub-channels. A closed-loop frequency synchronization mechanism may therefore be implemented to maintain frequency synchronization during communications between the mobile unit and the base station. For example, the base station may periodically determine (at910) the frequency (υSC) of the subcarrier used to transmit the signals on the established communication channels or sub-channels. The determined frequency (υSC) may then be used to synchronize frequencies, as discussed above. For another example, the base station may monitor the frequency (υSC) of the subcarrier and may initiate the frequency synchronization procedure described herein in response to the frequency (υSC) of the subcarrier differing from the expected frequency (υ0) by a selected value or fractional value.