Method of channel estimation by phase rotation in an orthogonal frequency division multiplexing (OFDM) system

The method includes receiving communication signals in a time domain and to aa frequency domain, providing resource blocks in the frequency domain including a first and second resource block, selecting first pilot signals from first resource block and second pilot signals from second resource block, calculating a first average value based on the first pilot signals, calculating a second average value, determining a phase difference between the first and second pilot signals using the first and second average values, adjusting a first phase of first resource block using the phase difference, providing a first waveform using the first resource block with adjusted the first phase and the second resource block, applying a smoothing filter against the first waveform to generate a second waveform, generating a third waveform using at least the first and third set of phase and amplitude differences, and converting third waveform from frequency domain to time domain.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to application Ser. No. 14/233,165, entitled ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM) CHANNEL ESTIMATION TO IMPROVE THE SMOOTHING PROCESS, filed concurrently herewith and, application Ser. No. 14/131,926, entitled CHANNEL ESTIMATION METHOD FOR OVERCOMING CHANNEL DISCONTINUITY IN SUBBANDS OF AN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM) SYSTEM, filed concurrently herewith, which are incorporated by reference in their entirety for any and all purposed.

BACKGROUND

In general communication systems, signal transmission in wireless channel presents channel characteristics such as multipath information, doppler frequency shift, and phase rotation of received data. Therefore, when a receiver receives a signal, it estimates and uses current channel information to equalize the received data and restore the sent data.

In an orthogonal frequency division multiplexing (OFDM) system, some pilot sequences known to the receiver are transmitted on a time-frequency domain at intervals. For example, as shown inFIG. 1, the Long Term Evolution (LTE) system is provided with spaced pilot sequences, by which the receiver can restore channels on subcarriers and perform smoothing on the time-frequency domain so as to obtain channel estimation results on the whole time-frequency domain by common methods such as the Wiener filtering method, the simple linear interpolation method or FFT method with time-frequency domain transformation.

However, in the LTE system, beam forming of smart antennas may cause discontinuous phases among subbands of the OFDM in transmission modes7or8. Therefore, in the situation of smoothing among subbands, the channel estimation error can be significant and the subbands may become narrow. For example, in the LTE channel, a resource block generally presents a discontinuous channel (in LTE, one resource block includes 12 subcarriers, and there are 3 or 4 UE-RS pilot frequencies thereon); in the situation of smoothing by a traditional filter in the resource block, the error may also be significant due to the edge effect of the filter.

In an OFDM receiver, channel smoothing is performed on the estimated channel in order to reduce the effects of noise on the estimated channel, thereby improving the system packet error performance.FIG. 2illustrates a single stream OFDM transmitter202accepting an input stream s1204to a baseband encoder206which encoded stream is provided to an inverse fast Fourier transform (IFFT)208to produce a plurality of baseband subcarriers such as 2 through 2024 or 2 through 512and the subcarriers are modulated to a carrier frequency for coupling to an antenna212as transmitted signal X. The transmitted signal X is coupled through a channel with a frequency dependent characteristic H to receive antenna232of receiver230to form received signal Y=HX. The receiver230receives signal Y, which is baseband converted using RF Front End233and applied to FFT234to channel compensator238and to decoder240which generates the received stream S1′. Channel estimator236estimates the channel characteristic H during a long preamble interval, and the channel characteristic H is applied to channel compensator238.

FIG. 3Aillustrates a Multiple Input Multiple Output (MIMO) receiver340operative on two transmit streams s1and s2304encoded306and provided to first stream IFFT308which generates baseband subcarriers, which are provided to RF modulator and amplifier310and coupled as X1to antenna316. Second stream IFFT312and RF modulator and amplifier314similarly generate subcarriers which are upconverted and coupled to antenna318as X2. Receiver340has three antennas342,344,346, which couple to receivers348,350,352and to output decoder354which forms decoded streams s1′ and s2′. Each receiver348,350,352performs the receive functions as described forFIG. 2, however the channel estimation function349,351,353for each receiver uses the long preamble part of the packet to characterize the channel from each transmit antenna316,318to each receive antenna342,344,346. For example, receiver348must characterize and compensate the channel h11from316to342as well as channel h21from318to342. Each channel characteristic h11and h21is a linear array containing real and imaginary components for each subcarrier, typically1through1024. The channel estimator349therefore contains h11and h21, estimator351contains h21and h22, and channel estimator353contains h31and h32. The 2.times.3 MIMO case ofFIG. 3Ashows the case where the number of remote transmitters Nt=2 and the number of local antennas and receivers Nr=3. For a MIMO receiver where the number of remote transmitters is Nt and the number of local antennas and receivers is Nr, the Nt*Nr channels have a frequency response which may be smoothed over a range of subcarrier frequencies using a finite impulse response (FIR) filter for I and Q channels. Such a channel smoothing filter would require a total of 2*Nt*Nr filters. For a 13 tap FIR filter, each tap would have an associated multiplier, so such an implementation would require 13 complex multipliers for each filter, or 26*Nt*Nr multipliers total at each receiver station.

It is to be appreciated that communicaiton interfaces can have other MIMO configurations.FIG. 3Bis a simplified diagram illustrating various types of MIMO configuration.

Accordingly, due to channel discontinuity, the smoothing processing will result in significant errors in the channel estimation results. These errors affecting the performance, and signal quality. Thus, a method for obtaining more accurate channel estimation results is needed.

DETAILED DESCRIPTION

Aspects of the present invention relate to the use of the pilot signals within each resource blocks to determine the phase, and adjust the phase of each resource block in order to get a continuous wave. Then, smoothing is performed on the continuous wave, and then the original waveform is restored.

A further aspect of the present invention includes obtaining an average of channel information in a subband by calculating the average value of the pilot frequency descrambled in the subband. Then, obtaining channel estimation results according to phase difference among the subcarriers in the subband, average value of the channel and phase rotation. The method can be used to obtain better channel estimation result in a small subband and also overcomes the traditionally significant channel estimation error due to discontinuous phases.

Further, phase differences of a subcarrier interval can be obtained directly by the pilot frequency. The phase differences among pilot frequencies are normalized to one subcarrier by distance and averaged. Channel estimation results obtained by the phase rotation can also be obtained by pilot frequency rotation.

FIGS. 4A and 4Billustrate a method400for performing processing of communication signals, according to one embodiment of the invention. At process block402a communication signal in a time domain is received. The communication signal to a frequency domain is converted (process block404). At process block406, a plurality of resource blocks based on the communication signal in the frequency domain are provided. In one embodiment, the plurality of resource blocks includes a first resource block and a second resource block.

At process block408, a first plurality of pilot signals from the first resource block and a second plurality of pilot signals from the second resource block are selected. A first average value based on the first plurality of pilot signals is then calculated (process block410). A second average value based on the second plurality of pilot signals is also calculated (process block412).

At process block414, a phase difference between the first plurality of pilot signals and the second plurality of pilot signals using the first average value and the second average value is determined. At this point the method400continues toFIG. 4Bat point A. Then, at process block416, a first phase of first resource block using the phase difference is adjusted.

At process block418, a first waveform using the first resource block with adjusted first phase and the second resource block is provided. Then, a smoothing filter against the first waveform to generate a second waveform is applied (process block420). In one embodiment, a Weiner filter, a discrete Fourier Transform or an inverse discrete Fourier Transform may be used. At process block422, a third waveform using at least the first and third set of phase and amplitude differences is generated. Then, the third waveform from the frequency domain to the time domain is converted (process block424).

Furthermore, a number of pilot signals to use may be selected as well as a fourth waveform on the edge of the second resource block may also be selected. Method400also calculates a phase average of the first and second resource blocks, adjusts the phase of the first resource block, and adjusts the phase of the second resource block.

Turning now toFIG. 5which illustrates a method500for performing processing of communication signals, according to a further embodiment of the invention. According toFIG. 1, pilot frequencies from a subband (such as one RB) in an OFDM symbol are selected, and set it as Ploti, i=0, 1, 2, . . . N−1 (process block502). Then, average value of pilot frequencies as

Ave=1N⁢∑i=0N-1⁢⁢Ploti
is set, and a phase of Ave as Phase_Ave is also set (process block504).

The phase of each pilot frequency is calculated (process block506) and set as Phase_Ploti. A distance between the subcarrier corresponding to each pilot frequency and averaging intermediate subcarrier as Distiis set, and the phase difference between each pilot frequency and Ave and normalize the difference to one subcarrier is calculated, and set as: PhasePerSubcarri=(Ave−Phase_Ploti)/Disti. The PhasePerSubcarriis not calculated if Distiis0.

The average value in the subband and set it as AvePhasePreSubCar is solved, the channel estimation results of all subcarriers in the subband is calculated, and set as Rmby calculating the distance between the current subcarrier and the subcarrier corresponding to average value Ave; m=0, 1, M−1, where M is number of subcarriers in the subband; calculate phase difference as AvePhasePreSubCar×Rm. Then, the channel estimation result by Ave phase rotation AvePhasePreSubCar×Rmis obtained (process block508).

Turning now toFIG. 6which illustrates multiple waveforms in accordance with embodiments of the present invention. Graph605shows the original waveform with resource block RB_0, RB_1, RB_2. Then, graph610shows smoothing within the RBs by smoothing the amplitudes. Graph615shows smoothing the phase within the RBs, and graph620shows obtaining the avg. Finally, graph625shows restoring the waveform back to the original waveform after smoothing occurs.

FIG. 7illustrates a block diagram of an exemplary computer system700that may be used to implement various embodiments. Some embodiments may employ a computer system (such as the computer system700) to perform methods in accordance with various embodiments of the invention. The computer system may be implemented using various circuits, microchips, and connections within a mobile device. According to a set of embodiments, some or all of the procedures of such methods are performed by the computer system700in response to processor710executing one or more sequences of one or more instructions (which might be incorporated into the operating system740and/or other code, such as an application program745) contained in the working memory735. Such instructions may be read into the working memory735from another computer-readable medium, such as one or more of the storage device(s)725. Merely by way of example, execution of the sequences of instructions contained in the working memory735might cause the processor(s)710to perform one or more procedures of the methods described herein.

The communications subsystem730(and/or components thereof) generally will receive signals, and the bus705then might carry the signals (and/or the data, instructions, etc. carried by the signals) to the working memory735, from which the processor(s)710retrieves and executes the instructions. The instructions received by the working memory735may optionally be stored on a non-transitory storage device725either before or after execution by the processor(s)710.