WO2005112378A1 - Staggered pilot transmission for channel estimation and time tracking - Google Patents
Staggered pilot transmission for channel estimation and time tracking Download PDFInfo
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- WO2005112378A1 WO2005112378A1 PCT/US2005/014259 US2005014259W WO2005112378A1 WO 2005112378 A1 WO2005112378 A1 WO 2005112378A1 US 2005014259 W US2005014259 W US 2005014259W WO 2005112378 A1 WO2005112378 A1 WO 2005112378A1
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- impulse response
- frequency subbands
- channel
- response estimate
- estimate
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/76—Pilot transmitters or receivers for control of transmission or for equalising
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/0202—Channel estimation
- H04L25/022—Channel estimation of frequency response
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L23/00—Apparatus or local circuits for systems other than those covered by groups H04L15/00 - H04L21/00
- H04L23/02—Apparatus or local circuits for systems other than those covered by groups H04L15/00 - H04L21/00 adapted for orthogonal signalling
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/0202—Channel estimation
- H04L25/0212—Channel estimation of impulse response
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/0202—Channel estimation
- H04L25/0224—Channel estimation using sounding signals
- H04L25/0228—Channel estimation using sounding signals with direct estimation from sounding signals
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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
-
- 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
- H04L27/2662—Symbol synchronisation
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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
- H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
- H04L27/2695—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation with channel estimation, e.g. determination of delay spread, derivative or peak tracking
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L5/00—Arrangements affording multiple use of the transmission path
- H04L5/003—Arrangements for allocating sub-channels of the transmission path
- H04L5/0048—Allocation of pilot signals, i.e. of signals known to the receiver
Definitions
- the present invention relates generally to data communication, and more specifically to pilot transmission, channel estimation, and time tracking in a multi- carrier communication system.
- Orthogonal frequency division multiplexing is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple orthogonal frequency subbands. These subbands are also referred to as tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data.
- a transmitting entity processes data to obtain modulation symbols and further performs OFDM modulation on the modulation symbols to generate OFDM symbols.
- the transmitting entity then conditions and transmits the OFDM symbols via a communication channel.
- a receiving entity typically needs to obtain relatively accurate symbol timing in order to recover the data sent by the transmitting entity.
- the receiving entity often does not know the time at which each OFDM symbol is sent by the transmitting entity nor the propagation delay introduced by the communication channel.
- the receiving entity would then need to ascertain the timing of each OFDM symbol received via the communication channel in order to properly perform the complementary OFDM demodulation on the received OFDM symbol.
- the receiving entity also needs a good estimate of the response of the communication channel in order to perform data detection to obtain good estimates of the modulation symbols sent by the transmitting entity.
- the transmitting entity expends system resources to support channel estimation and time tracking, and the receiving entity also consumes resources to perform these tasks.
- the resources used by the transmitting and receiving entities for channel estimation and time tracking represent overhead. Thus, it is desirable to minimize the amount of resources expended by both the transmitting and receiving entities for these tasks.
- a transmitting entity may transmit a pilot on different groups of subbands in different time intervals (e.g., different symbol periods).
- N subbands in the system may be arranged into M non-overlapping groups.
- the transmitting entity may transmit the pilot on a different subband group in each time interval.
- the transmitting entity may select all M subband groups in M time intervals based on a pilot staggering pattern.
- the transmitting entity may use many or most of the M subband groups in different time intervals, so that a substantial number of all subbands usable for transmission in the system are used for pilot transmission in different time intervals.
- the substantial number of subbands may be, for example, all of the usable subbands, three quarter of the usable subbands, at least half of the usable subbands, or some other significant percentage of the usable subbands.
- the receiving entity may derive an initial impulse response estimate with P channel taps based on the pilot received on one subband group.
- the receiving entity may derive a longer impulse response estimate (with up to N channel taps) by filtering initial impulse response estimates obtained for a sufficient number of different subband groups, as described below.
- Each longer impulse response estimate may be derived based on a different time-domain filter that filters S or more initial impulse response estimates obtained for S or more different subband groups, where S may be Si or S 2 .
- the first P channel taps are for a "main channel”
- the remaining channel taps are for an "excess channel”.
- the coefficients for each time-domain filter may be selected based on various criteria.
- the coefficients for the main channel may be selected to (1) cancel the excess channel, (2) suppress time variation in the main channel, (3) provide an unbiased estimate of the main channel, and so on. Details of the filtering are described below. Various aspects and embodiments of the invention are also described in further detail below.
- FIG. 1 shows a block diagram of a transmitting entity and a receiving entity
- FIG. 2 shows an interlace subband structure
- FIG. 3 shows an impulse response estimate for one interlace
- FIGS. 4A through 4C show three different pilot staggering patterns
- FIG. 5 shows a process for deriving a channel estimate used for data detection
- FIGS. 6A and 6B illustrate ambiguity in a channel impulse response estimate due to timing uncertainty
- FIG. 7 shows a process for performing time tracking
- FIG. 8 shows a channel estimator and a time tracking unit
- FIG. 9 shows a filter for deriving a longer impulse response estimate.
- FIG. 1 shows a block diagram of a transmitting entity 110 and a receiving entity 150 in an OFDM system 100.
- Transmitting entity 110 may be a base station or a wireless device
- receiving entity 150 may also be a base station or a wireless device.
- a base station is generally a fixed station and may also be referred to as a base transceiver system (BTS), an access point, or some other terminology.
- BTS base transceiver system
- a wireless device may be fixed or mobile and may also be referred to as a user terminal, a mobile station, or some other terminology.
- a transmit (TX) data and pilot processor 120 receives different types of data (e.g., traffic/packet data and overhead/control data) and processes (e.g., encodes, interleaves, and symbol maps) the data to generate data symbols.
- a "data symbol” is a modulation symbol for data
- a "pilot symbol” is a modulation symbol for pilot (which is data that is known a priori by both the transmitting and receiving entities)
- a modulation symbol is a complex value for a point in a signal constellation for a modulation scheme (e.g., M-PSK, M-QAM, and so on).
- Processor 120 provides data and pilot symbols to an OFDM modulator 130.
- OFDM modulator 130 multiplexes the data and pilot symbols onto the proper subbands and further performs OFDM modulation on the multiplexed symbols to generate OFDM symbols. For each symbol period, OFDM modulator 130 performs an N-point inverse fast Fourier transform (IFFT) on N multiplexed symbols for N total subbands and obtains a "transformed" symbol that contains N time-domain samples. Each sample is a complex value to be transmitted in one sample period. OFDM modulator 130 then repeats a portion of each transformed symbol to form an OFDM symbol that contains N + C samples, where C is the number of samples being repeated. The repeated portion is often called a cyclic prefix and is used to combat inter-symbol interference (ISI) caused by frequency selective fading.
- ISI inter-symbol interference
- An OFDM symbol period (or simply, a symbol period) is the duration of one OFDM symbol and is equal to N + C sample periods.
- OFDM modulator 130 provides a stream of OFDM symbols to a transmitter unit (TMTR) 132.
- Transmitter unit 132 processes (e.g., converts to analog, amplifies, filters, and frequency upconverts) the OFDM symbol stream to generate a modulated signal, which is then transmitted from an antenna 134.
- the transmitted signal from transmitting entity 110 is received by an antenna 152 and provided to a receiver unit (RCNR) 154.
- Receiver unit 154 processes (e.g., filters, amplifies, frequency downconverts, and digitizes) the received signal and provides a stream of input samples.
- An OFDM demodulator (Demod) 160 performs OFDM demodulation on the input samples and provides received data and pilot symbols.
- a detector 170 performs data detection (e.g., equalization or matched filtering) on the received data symbols with a channel estimate from a channel estimator 172 and provides detected data symbols, which are estimates of the data symbols sent by transmitting entity 110.
- a receive (RX) data processor 180 processes (e.g., symbol demaps, deinterleaves, and decodes) the detected data symbols and provides decoded data.
- the processing by OFDM demodulator 160 and RX data processor 180 is complementary to the processing by OFDM modulator 130 and TX data and pilot processor 120, respectively, at transmitting entity 110.
- Channel estimator 172 derives impulse response estimates based on the received pilot symbols from OFDM demodulator 160 and further derives frequency response estimates used by detector 170.
- a synchronization unit 162 performs time tracking and determines symbol timing based on the impulse response estimates from channel estimator 172.
- OFDM demodulator 160 performs OFDM demodulation based on the symbol timing from unit 162.
- Controllers 140 and 190 direct operation at transmitting entity 110 and receiving entity 150, respectively.
- Memory units 142 and 192 provide storage for program codes and data used by controllers 140 and 190, respectively.
- Data and pilot may be transmitted in various manners in system 100.
- data and pilot may be transmitted (1) simultaneously in the same symbol period using frequency division multiplexing (FDM), (2) sequentially in different symbol periods using time division multiplexing (TDM), or (3) using a combination of FDM and TDM.
- FDM frequency division multiplexing
- TDM time division multiplexing
- the N total subbands may also be used for data and pilot transmission in various manners.
- An exemplary data/pilot transmission scheme is described below.
- FIG. 2 shows an interlace subband structure 200 that may be used for data and pilot transmission in system 100.
- the N total subbands may be arranged into M "interlaces" or disjoint subband groups.
- the M interlaces are disjoint or non-overlapping in that each of the N total subbands belongs in only one interlace.
- the P subbands for each interlace may be uniformly distributed across the N total subbands such that consecutive subbands in the interlace are spaced apart by M subbands.
- the P subbands in each interlace are thus interlaced with the P subbands in each of the other M - 1 interlaces.
- Each interlace is further associated with a staggering phase m, which is equal to the index k of the first subband in the interlace.
- system 100 may utilize any OFDM structure with any number of total, usable, and guard subbands. Any number of interlaces may also be formed. Each interlace may contain any number of subbands and any one of the N total subbands. The interlaces may contain the same or different numbers of subbands. For clarity, the following description is for the interlace subband structure shown in FIG. 2 with M interlaces and each interlace containing P uniformly distributed subbands. This interlace subband structure provides several advantages. First, frequency diversity is achieved since each interlace contains subbands taken from across the entire system bandwidth. Second, a receiving entity may recover data/pilot symbols sent on a given interlace by performing a partial P-point FFT instead of a full N-point FFT, which can simplify the processing at the receiving entity.
- a communication channel between transmitting entity 110 and receiving entity 150 in OFDM system 100 may be characterized by either a time-domain channel impulse response or a corresponding frequency-domain channel frequency response.
- a “channel impulse response” or “impulse response” is a time-domain response of the channel
- a “channel frequency response” or “frequency response” is a frequency-domain response of the channel.
- the channel frequency response is the discrete Fourier transform (DFT) of the channel impulse response. This relationship may be expressed in matrix form, as follows:
- the Fourier matrix W NxN is defined such that the (l, ⁇ ) -th entry, W ⁇ n , is given as:
- the channel impulse response h Nxl is composed of N channel taps, with each channel tap h t being defined by a zero or non-zero complex gain value at a specific tap delay £ .
- the channel frequency response H Nxl is composed of N channel gains for the N total subbands, with each channel gain H k being a complex gain value for a specific subband k.
- pilot symbols are transmitted on the P subbands in interlace m
- the received pilot symbols for this interlace may be expressed as: where X m is a P x 1 vector with P pilot symbols sent on the P subbands in interlace m; Y m is a P x l vector with P received pilot symbols obtained by the receiving entity for the P subbands in interlace m; H m is a P l vector for the actual channel frequency response for interlace m; N is a P x 1 noise vector for the P subbands in interlace m; and " o " denotes the Hadamard product, which is an element-wise product, where the z ' -th element of Y m is the product of the i-th elements of X m and H m .
- the vector H m contains only P entries of the vector H Nxl for the P subbands in interlace m.
- the noise N m is assumed to be additive white Gaussian noise (AWGN) with zero mean and a variance of ⁇ 2 .
- AWGN additive white Gaussian noise
- Y m IX m [y m i I p m ⁇ ... y m ⁇ ? lp m ⁇ , and y m i and p m i are respectively the received and transmitted pilot symbols for the i-th subband in interlace m
- H m is a P x l vector for the initial frequency response estimate for interlace m.
- H m contains P channel gain estimates for the P subbands in interlace m, which may be obtained based on P element- wise ratios of the received pilot symbols to the transmitted pilot symbols, as shown in equation (5). If interlace m contains unused subbands with no received pilot symbols, then extrapolation, interpolation, and/or some other technique may be used to estimate the channel gains for these unused subbands.
- the slope of the phase ramp is determined by the staggering phase m of interlace m.
- the phase ramp may be removed by multiplying each element of Wp xP • H m with W ⁇ m ' p to obtain a corresponding element of h m .
- h m contains P channel taps and is obtained based on H m , which contains P channel gain estimates for the P subbands in interlace m. Since the actual channel impulse response h Nxl is composed of N channel taps, the initial impulse response estimate h m is undersampled in the frequency domain by the P subbands in interlace m.
- the full-length actual channel impulse response h Nxl is composed of M segments. Each segment s contains P consecutive channel taps in h Nxl and is represented by a vector h ⁇ . Equation (7) indicates that the M segments alias and combine when undersampled in the frequency domain, and the combining coefficients are given by the aliasing pattern.
- FIG. 3 shows an impulse response estimate 300 obtained based on pilot symbols received on P subbands in one interlace.
- the full-length channel impulse response h Nxl is composed of N channel taps with indices of 1 through N.
- the first P channel taps in h Nxl are contained in h, and may be referred to as the main channel.
- the remaining N - P channel taps in h Nxl are contained in h 2 through h M and may be referred to as the excess channel.
- the P channel taps in h m thus contain P main channel taps as well as N - P excess channel taps. Each aliased excess channel tap causes error in the estimation of the corresponding main channel tap.
- a longer impulse response estimate with more than P channel taps may be obtained by transmitting pilot symbols on multiple interlaces.
- One interlace may be used for pilot transmission in each symbol period, and different interlaces may be used for pilot transmission in different symbol periods.
- the use of multiple interlaces for pilot transmission allows the receiving entity to obtain a longer channel estimate, which may improve performance.
- By using all M interlaces for pilot transmission it is possible to estimate the entire full-length channel impulse response with N channel taps.
- the specific interlace to use for pilot transmission in each OFDM symbol period may be determined by a pilot staggering pattern.
- Various staggering patterns may be used for pilot transmission.
- the -1 and +1 in equation (8) account for an interlace index numbering scheme that starts with ' 1 ' instead of '0'.
- Different "complete" staggering patterns may be formed with different values of Am .
- a complete staggering pattern is one that selects all M interlaces for pilot transmission, e.g., in M symbol periods.
- FIG. 4A shows a complete staggering pattern 400 that may be used for pilot transmission.
- the vertical axis represents interlace indices, and the horizontal axis represents time.
- M 8 and one interlace is used for pilot transmission in each symbol period.
- the pilot is thus transmitted on interlace 1 in symbol period 1, then interlace 2 in symbol period 2, and so on, then interlace 8 in symbol period 8, then back to interlace 1 in symbol period 9, and so on. All eight interlaces are used for pilot transmission in each 8-symbol period duration.
- FIG. 4B shows a complete staggering pattern 410 that may also be used for pilot transmission.
- M 8 and one interlace is used for pilot transmission in each symbol period.
- the pilot is thus sent on interlace 1 in symbol period 1, then interlace 4 in symbol period 2, then interlace 7 in symbol period 3, and so on.
- the pilot is transmitted on all eight interlaces in each 8-symbol period duration.
- staggering pattern 410 selects interlaces with relative offsets of ⁇ 1, 4, 7 ⁇ while staggering pattern 400 selects interlaces with relative offsets of ⁇ 1, 2, 3 ⁇ .
- Staggering pattern 410 is thus more "spread out" than staggering pattern 400 and may provide better performance.
- FIG. 4C shows a complete staggering pattern 420 that does not satisfy equation (8) but may also be used for pilot transmission.
- This complete staggering pattern may be expressed as ⁇ 1, 5, 2, 6, 3, 7, 4, 8 ⁇ .
- the pilot is transmitted on all eight interlaces in each 8-symbol period duration.
- the pilot may be transmitted on any number of interlaces and on any one of the M interlaces in each symbol period.
- the particular interlace to use for pilot transmission in each symbol period may be selected based on any staggering pattern, three of which are shown in FIGS. 4A through 4C.
- the pilot may be transmitted on all M interlaces using a complete staggering pattern or on a subset of the M interlaces using a "partial" staggering pattern.
- a longer impulse response estimate h Lxl (t) with L channel taps, where P ⁇ L ⁇ N may be obtained by filtering multiple P-tap initial impulse response estimates obtained for multiple interlaces. This time-domain filtering may be performed, e.g., with a finite impulse response (FLR) filter, as follows:
- ]-U( IA, ⁇ ( ⁇ , 2 ( — ⁇ S , P ( ] ⁇ is a P ⁇ l vector that is an estimate of the channel impulse response h s (t) for segment 5 in symbol period t; s t (i) is a coefficient for the i-th filter tap used to derive the ⁇ -th channel tap in segment 5; N f is the number of non-causal taps for the time-domain filter; and N b is the number of causal taps for the time-domain filter.
- Equation (9) indicates that the P channel taps for each segment 5 may be obtained by filtering N f + N b initial impulse response estimates h(t + N f ) through h(t - N b + 1) , which may be obtained over N f + N b symbol periods for N f + N b different interlaces.
- Equation (9) also indicates that each channel tap h s ( (t) in h Lxl (t) may be obtained by multiplying N f + N b channel taps ⁇ ,(t - N b + 1) through ⁇ ,(t + N f ) with N f + N b coefficients ⁇ (N b - l) through ⁇ s ,(-N f ) , respectively, and combining the N f + N b resultant products.
- the coefficients for each channel tap h s ( (t) of each segment s may be selected separately.
- N f and N b may be selected for each channel tap of each segment s.
- N f + N b coefficients may be used for all P channel taps in each segment, and S sets of coefficients may be defined for the S segments of h Lxl (t) .
- the coefficients ⁇ s (i) ⁇ for each segment s are not a function of channel tap index £ .
- the time-domain filtering may also be performed using other types of filter, such as an infinite impulse response (IIR) filter.
- IIR infinite impulse response
- a non-causal filter with N f > 1
- Equation (10) is derived based on equation (7) and assumes that segments 3 through M contain channel taps with zero magnitude.
- the vector h,(t) contains the first P channel taps in h Nxl (t) for the main channel.
- the vector h 2 (t) contains the next P channel taps in h Nxl (t) for the excess channel.
- Equation (l ib) cancels the linear component of the channel variation over the N f + N b symbol periods, which would be the dominant component at low speeds and/or small N f + N b .
- the first constraint in equation (11a) cancels the contribution from the excess channel h 2 (t) , so that h,(t) contains mostly components from the main channel h,(t) .
- the second constraint in equation (1 lb) suppresses time variation in the main channel h x (t) across the N f + N b symbol periods.
- the third constraint in equation (l ie) provides an unbiased estimate of h,(t) , so that the expected magnitude of h (t) is equal to h l (t) .
- the fourth constraint in equation (l id) minimizes the noise variance in the main channel estimate h, (t) .
- the number of taps ( N f + N b ) for the time-domain filter determines (1) the number of degrees of freedom for selecting the coefficients and (2) the number of constraints that may be applied in selecting the coefficients. [0051]
- the coefficients for the time-domain filter for the excess channel h 2 (t) may be selected based on the various constraints such as:
- the first constraint in equation (12a) cancels the contribution from the main channel h,(t), so that h 2 (t) contains mostly components from the excess channel h 2 (t).
- the second constraint in equation (12b) suppresses time variation in the main channel h,( .
- the third constraint in equation (12c) provides an unbiased estimate of h 2 (t) .
- a 3-tap time-domain filter may be used to derive the 2P channel taps in h 2 p ⁇ t ( based on h(t-l), h(t), and h(t + l) for three symbol periods.
- the 3-tap time-filter may be designed as follows. Using equation (10), the £ -th channel tap in symbol periods t - 1 , t, and t + 1 , prior to the time-domain filtering, may be expressed as:
- h t (t- ⁇ ) h (t- ⁇ ) +h (t- ⁇ )-W ⁇ +n e (t-l) ,
- h e (t + ⁇ ) h (t + ⁇ ) +h u (t + V)-WTM- +n e (t + l) , Eq(13) where h ( (t), h ( (t), h 2( (t), and n t (t) are the ⁇ -th element of h(t), 11 1 (t) , h 2 (t) , and n(t) , respectively; and r _,, m t , and m, +1 are the interlaces used for pilot transmission in symbol periods t - 1 , t, and t + 1 , respectively.
- the constraints used to select the coefficients for the main channel estimate h,(t) may be expressed as:
- the solution to the above set of equations for the main channel is given as:
- Equation (14) indicates that the coefficients for the main channel estimate h,(t) are independent of symbol period t. This set of coefficients suppresses time variation in the main channel h,(t) but does not suppress time-variation in the excess channel h 2 (t) .
- Time-variation error is proportional to the energy of the channel taps, which is typically small for the excess channel and significant only when the transmitting and/or receiving entity is moving at high speeds. Thus, not suppressing time variation in the excess channel h 2 (t) may only marginally degrade performance, if at all.
- the constraints used to select the coefficients for the excess channel estimate h 2 (t) may be expressed as:
- Equation (15) indicates that the coefficients for the excess channel are dependent on the staggering phase m t of interlace m t used for pilot transmission in symbol period t.
- Equation (16) is derived based on equation (7) and assumes that segments 4 through M contain channel taps with zero magnitude.
- the vectors h ⁇ t) , h 2 (t) , and h 3 (t) contain P channel taps for the first, second, and third segments, respectively, of h Nxl (t) .
- a 3-tap time-domain filter may also be used to derive the 3P elements of h 3Px , (t) based on h(t - 1) , h(t) , and h(t + 1) obtained in three symbol periods.
- the £ -th channel tap in symbol periods t - 1 , t, and t + 1 prior to the time-domain filtering, may be expressed in matrix form, as follows:
- the 3-tap time-domain filter does not have enough degrees of freedom to suppress time variation in h,(t) , h 2 (t) , or h 3 (t) .
- equation (17) further assumes that h,(t) , h 2 (t) , and h 3 (t) are constant over the three symbol periods t - 1 , t, and t + 1 .
- a least-squares estimate of h,(t) , h 2 (t) , and h 3 (t) may be obtained as follows:
- the 3-tap time-domain filter for h x (t) , h 2 (t) , and h 3 (t) may be expressed in matrix form, as follows:
- the coefficients for the 3-tap time-domain filter may be derived based on equation (18) and given as:
- the main channel estimate h,(t) may be obtained by applying the coefficients ⁇ ,(l) , ⁇ ,(0) , and a x (-X) to h(t -l) , h(t) , and h(t + l) , respectively.
- the excess channel estimate h 2 (t) may be obtained by applying the coefficients 2 ( ⁇ ) , cc 2 (0) , and ⁇ 2 (-l) to h(f - 1) , h(t) , and h(t + 1) , respectively.
- the excess channel estimate h 3 (t) may be obtained by applying the coefficients « 3 (1), ⁇ 3 (0) , and « 3 (-l) to h(t - l) , h(t) , and h(t + 1) , respectively.
- the 3-tap time-domain filter does not have sufficient degrees of freedom to apply many of the constraints shown in equation sets (11) and (12).
- the coefficients for this time-domain filter do not suppress time variation in the main channel h,(t) or the excess channel h 2 (t) and h 3 (t) .
- the various constraints described above may be applied by using a time-domain filter with more than three taps.
- a different set of coefficients ⁇ a s (i) ⁇ may be derived for the time- domain filter for the impulse response estimate s (t) for each segment s.
- the coefficients for each segment s may be selected based on various constraints such as: canceling the other segments, suppressing estimation error due to time variation in the channel, providing an unbiased estimate of h s (t) , minimizing the noise variance in h s (t) , and so on.
- the number of taps for the time-domain filter determines the number of constraints that may be applied to the coefficients.
- a longer impulse response estimate with L channel taps may be obtained based on pilot symbols received on L different subbands in one or more symbol periods.
- the pilot may be transmitted on one interlace in each symbol period to limit the amount of overhead for the pilot.
- the pilot may be transmitted on different interlaces with staggered subbands in different symbol periods. This allows the receiving entity to obtain a longer impulse response estimate with more than P channel taps.
- a full-length impulse response estimate with all N channel taps may be obtained if the pilot is transmitted on all M interlaces using a complete staggering pattern.
- the receiving entity may derive a longer impulse response estimate h Lxl (t) of length L by filtering initial impulse response estimates h of length P for a sufficient number of (S or more) different interlaces. If the pilot is transmitted on a different interlace in each symbol period, then the time-domain filtering may be performed over a sufficient number of (S or more) symbol periods to obtain h Lxl (t) . A progressively longer impulse response estimate may be obtained by filtering over more symbol periods. Time-domain filtering over fewer symbol periods provides better tracking of changes in the channel, is thus more robust to Doppler effects, and can provide an impulse response estimate with a shorter length.
- a longer impulse response estimate contains excess channel taps. Since each channel tap contains the complex channel gain at that tap position as well as noise, a progressively longer impulse response estimate contains more information regarding the channel but also contains more noise. The noise from the excess channel taps may be viewed as noise enhancement resulting from extending the length of the channel estimate beyond P. If the excess channel energy is relatively small, or if the excess channel taps are not needed, then better performance may be achieved with a shorter impulse response estimate (e.g., h 2Pxl (t)).
- a longer impulse response estimate (e.g., h 3Pxl (t)) may provide better performance even with the noise enhancement.
- Channel estimates with different lengths may be derived and used for different purposes at the receiving entity.
- FIG. 5 shows a flow diagram of a process 500 for deriving a channel estimate used for data detection and decoding.
- Received pilot symbols are obtained for the subbands in interlace m t used for pilot transmission in the current symbol period t (block 512).
- An initial frequency response estimate H(t) is derived based on the received pilot symbols, as shown in equation (5) (block 514).
- An initial impulse response estimate h(t) is derived based on the initial frequency response estimate H(t) , as shown in equation (6) (block 516).
- Post-processing may be performed on the ⁇ channel taps in h L ⁇ Xl (t) to further improve channel estimation performance (block 520).
- the post-processing may include truncation, e.g., setting channel taps P + 1 through L, ! for the excess channel estimate to zeros.
- the post-processing may alternatively or additionally include thresholding, e.g., setting channel taps in the main and/or excess channel estimates having energy below a given threshold to zeros.
- the unprocessed or post-processed longer impulse response estimate h L xl (t) may then be extended to length N by zero-padding to obtain a vector ]1 NXI (0 of length N (also block 520).
- An N-point FFT may then be performed on h Nxl (t) to obtain a frequency response estimate H Nxl (t) for all N subbands (block 522), as follows:
- Process 500 may be performed for each symbol period with pilot transmission.
- the M frequency response estimates H m (t) for the M interlaces may have different noise variances depending on the particular staggering pattern used for pilot transmission. In general, a staggering pattern that is more spread out (e.g., staggering pattern 410) may result in less noise variation across H m (t) for the M interlaces than a staggering pattern that is more closely spaced (e.g., staggering pattern 400). 4. Time Tracking
- the receiving entity performs time tracking to estimate and track symbol timing across different OFDM symbols.
- the symbol timing is used to capture a window of N input samples (often called an FFT window) from among the N + C input samples for each received OFDM symbol.
- FFT window a window of N input samples
- Accurate symbol timing is pertinent since performance of both channel estimation and data detection is affected by the placement of the FFT window.
- the timing of the received OFDM symbol for each symbol period may be estimated by deriving a longer impulse response estimate for that symbol period and detecting for the timing based on an appropriate criterion, e.g. maximizing the energy that falls within the cyclic prefix.
- a longer impulse response estimate with L channel taps may be derived but only L/2 channel taps may be resolved without any ambiguity. This is because a negative timing error results in earlier channel taps aliasing and appearing at the end of the impulse response estimate. Thus, it is not possible to determine whether the channel taps at the end of the impulse response estimate are later channel taps (if the symbol timing is co ⁇ ect) or earlier channel taps that have aliased (if there is a negative timing e ⁇ or).
- a longer channel impulse response estimate with up to N channel taps may be obtained by filtering the initial impulse response estimates for M different interlaces. The resolvable length of the communication channel is increased by the use of the longer impulse response estimate.
- FIGS. 6A and 6B illustrate ambiguity in a channel impulse response estimate due to timing uncertainty.
- FIG. 6 A shows a channel impulse response estimate 610 of length L for an actual channel with an impulse response of length greater than L/2.
- the symbol timing is correct and channel impulse response estimate 610 properly includes responses 612 and 614 of the actual channel at the proper locations.
- FIG. 6B shows an impulse response 620 of length greater than L/2 for another actual channel. If there is no timing e ⁇ or, then a channel impulse response estimate for this channel would include responses 622 and 624 at the locations as shown in FIG. 6B. However, if there is a timing error of x, then response 622 would alias and appear as response 632. Thus, a channel impulse response estimate for this channel, with timing e ⁇ or of x, would be similar to channel impulse response estimate 610 in FIG. 6 A [0076] FIGS. 6A and 6B illustrate that channel impulse response estimate 610 may be obtained for (1) a channel having the impulse response shown in FIG. 6 A, with no timing e ⁇ or, or (2) a channel having the impulse response shown in FIG.
- the longer channel impulse response estimate has additional noise due to the excess channel taps, and greater error due to channel time-variations.
- time tracking is likely to be less sensitive to the additional noise since the goal of time tracking is to determine less detailed information such as the general location of the channel energy rather than the complex channel gains of each tap.
- the tradeoff between channel quality and length is consistent with the requirements for data detection and time tracking.
- the communication channel may be assumed to be 3P/2 taps long for data detection purpose.
- a 3P/2 -tap channel may be estimated by obtaining a longer impulse response estimate with 2P channel taps and truncating the last 256 channel taps.
- FIG. 7 shows a flow diagram of a process 700 for performing time tracking.
- Blocks 712, 714, 716, and 718 in FIG. 7 are as described above for block 512, 514, 516, and 518, respectively, in FIG. 5.
- the channel estimate h L xl (t) is then processed to determine the timing of the received OFDM symbol for the cu ⁇ ent symbol period t (block 720).
- One method to determine the timing is as follows.
- a window of length L 2 / 2 is placed such that the left edge of the window is initially at tap index 1.
- the energy of all channel taps falling within the window is computed.
- the window is then moved to the right, one tap position at a time until tap index L 2 / 2 is reached.
- the channel tap energy is computed for each tap position.
- the peak energy among all of the L 2 II window starting positions is then determined. If multiple window starting positions have the same peak energy, then the leftmost window starting position with the peak energy is identified.
- the leftmost window starting position with the peak energy uniquely determines the FFT window for the received OFDM symbol. Timing detection may also be performed using other techniques. In any case, symbol timing estimate is updated with the timing information obtained for the current received OFDM symbol (block 722).
- the same or different impulse response estimates may be used for data detection decoding and time tracking.
- the use of the same impulse response estimate can reduce the amount of computation at the receiving entity.
- the channel length L and the time-domain filter for this impulse response estimate may be selected to provide good performance for both data detection and time tracking.
- Different impulse response estimates may also be used for data detection/decoding and time tracking in order to achieve better performance for both, and may be derived with two time-domain filters.
- the channel length and the time-domain filter coefficients for each impulse response estimate may be selected to provide good performance for data detection or time tracking.
- FIG. 8 shows an embodiment of OFDM demodulator 160, channel estimator 172, and time tracking unit 162 at receiving entity 150.
- a cyclic prefix removal unit 812 captures N input samples for each received OFDM symbol based on the symbol timing provided by time tracking unit 162.
- An FFT unit 814 performs an N-point FFT on each window of N input samples and obtains N received symbols for the N subbands.
- FFT unit 814 provides received data symbols to detector 170 and received pilot symbols to channel estimator 172.
- Detector 170 also receives the frequency response estimate H Nxl (t) from channel estimator 172, performs data detection on the received data symbols, and provides detected data symbols.
- a pilot detector 822 removes the modulation on the received pilot symbols and may perform extrapolation and/or interpolation to obtain the initial frequency response estimate H(t) composed of P channel gains for the P subbands in the interlace used for pilot transmission in the cu ⁇ ent symbol period t.
- An IFFT unit 824 performs a P-point IFFT on H(t) to obtain the modulated impulse response estimate h m (t) with P channel taps.
- a rotator 826 removes the phase ramp in the P elements of h m (t) and provides the initial impulse response estimate h(t) .
- a time-domain filter 830 filters the initial impulse response estimates h(t) obtained for Sj or more interlaces obtained in Si or more symbol periods and provides the longer impulse response estimate h L[Xl (t) with ⁇ channel taps.
- a post-processor 832 performs post-processing (e.g., truncation, thresholding, and so on) and zero-padding on h L]Xl (t) and provides a vector h Nxl (t) with N channel taps.
- An FFT unit 834 performs an N-point FFT on h Nxl (t) to obtain the frequency response estimate H Nx] (t) for the N total subbands.
- Channel estimator 172 may also derive a frequency response estimate H m (t) for just one or more selected interlaces.
- a time-domain filter 840 filters the initial impulse response estimates h(t) for S 2 or more interlaces obtained in S 2 or more symbol periods and provides the longer impulse response estimate h L x] (t) with L 2 channel taps.
- a timing detector 842 determines the timing for the current received OFDM symbol, e.g., based on the energy of the channel taps in h L2Xl (t) .
- a time tracking loop 844 (which may be a loop filter) adjusts the symbol timing from the timing used for the current received OFDM symbol. [0083] FIG.
- FIG. 9 shows a block diagram of a time-domain filter 830x, which may be used for filters 830 and 840 in FIG. 8.
- the £ - ⁇ channel tap in h(t) is provided to N f + N b -1 series-coupled delay elements 912.
- Each delay element 912 delays its input channel tap by one symbol period.
- N f + N b -1 multipliers 914 couple to the input of the N f + N b -1 delay elements, and one multiplier 914 couples to the output of the last delay element.
- the N f + N b multipliers receive and multiply their channel taps ⁇ (t + N f ) through h t (t - ⁇ N b + Y) with coefficients ⁇ -N,.) through c s l N b -1) , respectively.
- the same coefficients may be used for all P channel taps in each segment, in which case the coefficients may be denoted as ⁇ -N f ) through « s (N b -1) , without subscript £ for tap index.
- a summer 916 receives and sums the outputs of all N f + N b multipliers and provides the £ -th channel tap in segment 5 of h Lxl (t) .
- L may be equal to Lj for data detection and to L 2 for time tracking.
- the filtering for only one channel tap in h Lxl (t) is shown in FIG. 9.
- the filtering for each of the remaining channel taps in h Lxl (t) may be performed in similar manner.
- the pilot transmission, channel estimation, and time tracking techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof.
- the processing units used for pilot transmission at the transmitting entity may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate a ⁇ ays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.
- ASICs application specific integrated circuits
- DSPs digital signal processors
- DSPDs digital signal processing devices
- PLDs programmable logic devices
- FPGAs field programmable gate a ⁇ ays
- processors controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.
- these techniques may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein.
- the software codes may be stored in a memory unit (e.g., memory unit 142 or 192 in FIG. 1) and executed by a processor (e.g., controller 140 or 190).
- the memory unit may be implemented within the processor or external to the processor.
Abstract
Description
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CN200580022669XA CN1981498B (en) | 2004-05-04 | 2005-04-25 | Staggered pilot transmission for channel estimation and time tracking |
PL05739529T PL1745618T3 (en) | 2004-05-04 | 2005-04-25 | Staggered pilot transmission for channel estimation and time tracking |
EP05739529A EP1745618B1 (en) | 2004-05-04 | 2005-04-25 | Staggered pilot transmission for channel estimation and time tracking |
CA2565527A CA2565527C (en) | 2004-05-04 | 2005-04-25 | Staggered pilot transmission for channel estimation and time tracking |
JP2007511412A JP5048481B2 (en) | 2004-05-04 | 2005-04-25 | Staggered pilot transmission for channel estimation and time tracking |
DE602005013765T DE602005013765D1 (en) | 2004-05-04 | 2005-04-25 | Tiered pilot transmission for channel estimation and time tracking |
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US7457231B2 (en) | 2008-11-25 |
JP5048481B2 (en) | 2012-10-17 |
KR20070007956A (en) | 2007-01-16 |
US7907593B2 (en) | 2011-03-15 |
CA2699640A1 (en) | 2005-11-24 |
JP5474728B2 (en) | 2014-04-16 |
JP2007536803A (en) | 2007-12-13 |
KR20080098084A (en) | 2008-11-06 |
KR100925909B1 (en) | 2009-11-09 |
JP2011066902A (en) | 2011-03-31 |
TWI361600B (en) | 2012-04-01 |
US20090129511A1 (en) | 2009-05-21 |
JP5080546B2 (en) | 2012-11-21 |
EP2066088A1 (en) | 2009-06-03 |
DE602005013765D1 (en) | 2009-05-20 |
CA2565527A1 (en) | 2005-11-24 |
EP1745618B1 (en) | 2009-04-08 |
PL1745618T3 (en) | 2009-09-30 |
CA2565527C (en) | 2014-04-22 |
CN1981498B (en) | 2010-09-29 |
EP1745618A1 (en) | 2007-01-24 |
CA2699640C (en) | 2011-07-12 |
ATE428247T1 (en) | 2009-04-15 |
KR100878430B1 (en) | 2009-01-13 |
ES2322077T3 (en) | 2009-06-16 |
US20050249181A1 (en) | 2005-11-10 |
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TW200625881A (en) | 2006-07-16 |
JP2010093827A (en) | 2010-04-22 |
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