I. Field
The present invention relates generally to data communication, and more specifically to synchronization in a wireless broadcast system using orthogonal frequency division multiplexing (OFDM).
II. Background
OFDM is a multi-carrier modulation technique that effectively partitions the overall system bandwidth into multiple (N) orthogonal frequency subbands. These subbands are also referred to as tones, sub-carriers, bins, and frequency channels. With OFDM, each subband is associated with a respective sub-carrier that may be modulated with data.
In an OFDM system, a transmitter processes data to obtain modulation symbols, and further performs OFDM modulation on the modulation symbols to generate OFDM symbols, as described below. The transmitter then conditions and transmits the OFDM symbols via a communication channel. The OFDM system may use a transmission structure whereby data is transmitted in frames, with each frame having a particular time duration. Different types of data (e.g., traffic/packet data, overhead/control data, pilot, and so on) may be sent in different parts of each frame. Pilot generically refers to data and/or transmission that are known a priori by both the transmitter and a receiver.
The receiver typically needs to obtain accurate frame and symbol timing in order to properly recover the data sent by the transmitter. For example, the receiver may need to know the start of each frame in order to properly recover the different types of data sent in the frame. The receiver often does not know the time at which each OFDM symbol is sent by the transmitter nor the propagation delay introduced by the communication channel. The receiver 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.
Synchronization refers to a process performed by the receiver to obtain frame and symbol timing. The receiver may also perform other tasks, such as frequency error estimation, as part of synchronization. The transmitter typically expends system resources to support synchronization, and the receiver also consumes resources to perform synchronization. Since synchronization is overhead needed for data transmission, it is desirable to minimize the amount of resources used by both the transmitter and receiver for synchronization.
There is therefore a need in the art for techniques to efficiently achieve synchronization in a broadcast OFDM system.
Techniques for achieving synchronization using time division multiplexed (TDM) pilots in an OFDM system are described herein. In each frame (e.g., at the start of the frame), a transmitter broadcasts or transmits a first TDM pilot on a first set of subbands followed by a second TDM pilot on a second set of subbands. The first set contains L1 subbands and the second set contains L2 subbands, where L1 and L2 are each a fraction of the N total subbands, and L2>L1. The subbands in each set may be uniformly distributed across the N total subbands such that (1) the L1 subbands in the first set are equally spaced apart by S1=N/L1 subbands and (2) the L2 subbands in the second set are equally spaced apart by S2=N/L2 subbands. This pilot structure results in (1) an OFDM symbol for the first TDM pilot containing at least S1 identical “pilot-1” sequences, with each pilot-1 sequence containing L1 time-domain samples, and (2) an OFDM symbol for the second TDM pilot containing at least S2 identical “pilot-2” sequences, with each pilot-2 sequence containing L2 time-domain samples. The transmitter may also transmit a frequency division multiplexed (FDM) pilot along with data in the remaining part of each frame. This pilot structure with the two TDM pilots is well suited for a broadcast system but may also be used for non-broadcast systems.
A receiver can perform synchronization based on the first and second TDM pilots. The receiver can process the first TDM pilot to obtain frame timing and frequency error estimate. The receiver may compute a detection metric based on a delayed correlation between different pilot-1 sequences for the first TDM pilot, compare the detection metric against a threshold, and declare detection of the first TDM pilot (and thus a frame) based on the comparison result. The receiver can also obtain an estimate of the frequency error in the received OFDM symbol based on the pilot-1 sequences. The receiver can process the second TDM pilot to obtain symbol timing and a channel estimate. The receiver may derive a channel impulse response estimate based on a received OFDM symbol for the second TDM pilot, detect the start of the channel impulse response estimate (e.g., based on the energy of the channel taps for the channel impulse response), and derive the symbol timing based on the detected start of the channel impulse response estimate. The receiver may also derive a channel frequency response estimate for the N total subbands based on the channel impulse response estimate. The receiver may use the first and second TDM pilots for initial synchronization and may use the FDM pilot for frequency and time tracking and for more accurate channel estimation.
Various aspects and embodiments of the invention are described in further detail below.
The features and nature of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:
The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.
The synchronization techniques described herein may be used for various multi-carrier systems and for the downlink as well as the uplink. The downlink (or forward link) refers to the communication link from the base stations to the wireless devices, and the uplink (or reverse link) refers to the communication link from the wireless devices to the base stations. For clarity, these techniques are described below for the downlink in an OFDM system.
At base station 110, a 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 received data to generate data symbols. As used herein, a “data symbol” is a modulation symbol for data, a “pilot symbol” is a modulation symbol for pilot, and 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 also processes pilot data to generate pilot symbols and provides the data and pilot symbols to an OFDM modulator 130.
OFDM modulator 130 multiplexes the data and pilot symbols onto the proper subbands and symbol periods and further performs OFDM modulation on the multiplexed symbols to generate OFDM symbols, as described below. A transmitter unit (TMTR) 132 converts the OFDM symbols into one or more analog signals and further conditions (e.g., amplifies, filters, and frequency upconverts) the analog signal(s) to generate a modulated signal. Base station 110 then transmits the modulated signal from an antenna 134 to wireless devices in the system.
At wireless device 150, the transmitted signal from base station 110 is received by an antenna 152 and provided to a receiver unit (RCVR) 154. Receiver unit 154 conditions (e.g., filters, amplifies, and frequency downconverts) the received signal and digitizes the conditioned signal to obtain a stream of input samples. An OFDM demodulator 160 performs OFDM demodulation on the input samples to obtain received data and pilot symbols. OFDM demodulator 160 also performs detection (e.g., matched filtering) on the received data symbols with a channel estimate (e.g., a frequency response estimate) to obtain detected data symbols, which are estimates of the data symbols sent by base station 110. OFDM demodulator 160 provides the detected data symbols to a receive (RX) data processor 170.
A synchronization/channel estimation unit 180 receives the input samples from receiver unit 154 and performs synchronization to determine frame and symbol timing, as described below. Unit 180 also derives the channel estimate using received pilot symbols from OFDM demodulator 160. Unit 180 provides the symbol timing and channel estimate to OFDM demodulator 160 and may provide the frame timing to RX data processor 170 and/or a controller 190. OFDM demodulator 160 uses the symbol timing to perform OFDM demodulation and uses the channel estimate to perform detection on the received data symbols.
RX data processor 170 processes (e.g., symbol demaps, deinterleaves, and decodes) the detected data symbols from OFDM demodulator 160 and provides decoded data. RX data processor 170 and/or controller 190 may use the frame timing to recover different types of data sent by base station 110. In general, the processing by OFDM demodulator 160 and RX data processor 170 is complementary to the processing by OFDM modulator 130 and TX data and pilot processor 120, respectively, at base station 110.
Controllers 140 and 190 direct operation at base station 110 and wireless device 150, respectively. Memory units 142 and 192 provide storage for program codes and data used by controllers 140 and 190, respectively.
Base station 110 may send a point-to-point transmission to a single wireless device, a multi-cast transmission to a group of wireless devices, a broadcast transmission to all wireless devices under its coverage area, or any combination thereof. For example, base station 110 may broadcast pilot and overhead/control data to all wireless devices under its coverage area. Base station 110 may further transmit user-specific data to specific wireless devices, multi-cast data to a group of wireless devices, and/or broadcast data to all wireless devices.
The four fields 212 through 218 are time division multiplexed in each super-frame such that only one field is transmitted at any given moment. The four fields are also arranged in the order shown in
In an embodiment, field 212 carries one OFDM symbol for TDM pilot 1, and field 214 also carries one OFDM symbol for TDM pilot 2. In general, each field may be of any duration, and the fields may be arranged in any order. TDM pilots 1 and 2 are broadcast periodically in each frame to facilitate synchronization by the wireless devices. Overhead field 216 and/or data field 218 may also contain pilot symbols that are frequency division multiplexed with data symbols, as described below.
The OFDM system has an overall system bandwidth of BW MHz, which is partitioned into N orthogonal subbands using OFDM. The spacing between adjacent subbands is BW/N MHz. Of the N total subbands, M subbands may be used for pilot and data transmission, where M<N, and the remaining N−M subbands may be unused and serve as guard subbands. In an embodiment, the OFDM system uses an OFDM structure with N=4096 total subbands, M=4000 usable subbands, and N−M=96 guard subbands. In general, any OFDM structure with any number of total, usable, and guard subbands may be used for the OFDM system.
TDM pilots 1 and 2 may be designed to facilitate synchronization by the wireless devices in the system. A wireless device may use TDM pilot 1 to detect the start of each frame, obtain a coarse estimate of symbol timing, and estimate frequency error. The wireless device may use TDM pilot 2 to obtain more accurate symbol timing.
A smaller value is used for L1 so that a larger frequency error can be corrected with TDM pilot 1. A larger value is used for L2 so that the pilot-2 sequence is longer, which allows a wireless device to obtain a longer channel impulse response estimate from the pilot-2 sequence. The L1 subbands for TDM pilot 1 are selected such S1 identical pilot-1 sequences are generated for TDM pilot 1. Similarly, the L2 subbands for TDM pilot 2 are selected such S2 identical pilot-2 sequences are generated for TDM pilot 2.
In an embodiment, a pseudo-random number (PN) generator 420 is used to generate data for both TDM pilots 1 and 2. PN generator 420 may be implemented, for example, with a 15-tap linear feedback shift register (LFSR) that implements a generator polynomial g(x)=x15+x14+1. In this case, PN generator 420 includes (1) 15 delay elements 422a through 422o coupled in series and (2) a summer 424 coupled between delay elements 422n and 422o. Delay element 422o provides pilot data, which is also fed back to the input of delay element 422a and to one input of summer 424. PN generator 420 may be initialized with different initial states for TDM pilots 1 and 2, e.g., to ‘011010101001110’ for TDM pilot 1 and to ‘010110100011100’ for TDM pilot 2. In general, any data may be used for TDM pilots 1 and 2. The pilot data may be selected to reduce the difference between the peak amplitude and the average amplitude of a pilot OFDM symbol (i.e., to minimize the peak-to-average variation in the time-domain waveform for the TDM pilot). The pilot data for TDM pilot 2 may also be generated with the same PN generator used for scrambling data. The wireless devices have knowledge of the data used for TDM pilot 2 but do not need to know the data used for TDM pilot 1.
A bit-to-symbol mapping unit 430 receives the pilot data from PN generator 420 and maps the bits of the pilot data to pilot symbols based on a modulation scheme. The same or different modulation schemes may be used for TDM pilots 1 and 2. In an embodiment, QPSK is used for both TDM pilots 1 and 2. In this case, mapping unit 430 groups the pilot data into 2-bit binary values and further maps each 2-bit value to a specific pilot modulation symbol. Each pilot symbol is a complex value in a signal constellation for QPSK. If QPSK is used for the TDM pilots, then mapping unit 430 maps 2L, pilot data bits for TDM pilot 1 to L1 pilot symbols and further maps 2L2 pilot data bits for TDM pilot 2 to L2 pilot symbols. A multiplexer (Mux) 440 receives the data symbols from TX data processor 410, the pilot symbols from mapping unit 430, and a TDM_Ctrl signal from controller 140. Multiplexer 440 provides to OFDM modulator 130 the pilot symbols for the TDM pilot 1 and 2 fields and the data symbols for the overhead and data fields of each frame, as shown in
rn=xn+wn Eq (1)
where
For the embodiment shown in
where
Within frame detector 710, a shift register 812 (of length L1) receives, stores, and shifts the input samples {rn} and provides input samples {rn-L
A post-processor 834 detects for the presence of the pilot-1 OFDM symbol, and hence the start of the super-frame, based on the detection metric Sn and a threshold Sth, which may be a fixed or programmable value. The frame detection may be based on various criteria. For example, post-processor 834 may declare the presence of a pilot-1 OFDM symbol if the detection metric Sn (1) exceeds the threshold Sth, (2) remains above the threshold Sth for at least a predetermined percentage of the pilot-1 OFDM symbol duration, and (3) falls below the threshold Sth for a predetermined time period (one pilot-1 sequence) thereafter. Post-processor 834 may indicate the end of the pilot-1 OFDM symbol (denoted as TC) as a predetermined number of sample periods prior to the trailing edge of the waveform for the detection metric Sn. Post-processor 834 may also set a Frame Timing signal (e.g., to logic high) at the end of the pilot-1 OFDM symbol. The time TC may be used as a coarse symbol timing for the processing of the pilot-2 OFDM symbol.
Frequency error estimator 712 estimates the frequency error in the received pilot-1 OFDM symbol. This frequency error may be due to various sources such as, for example, a difference in the frequencies of the oscillators at the base station and wireless device, Doppler shift, and so on. Frequency error estimator 712 may generate a frequency error estimate for each pilot-1 sequence (except for the last pilot-1 sequence), as follows:
where
The frequency error estimates may be used in various manners. For example, the frequency error estimate for each pilot-1 sequence may be used to update a frequency tracking loop that attempts to correct for any detected frequency error at the wireless device. The frequency tracking loop may be a phase-locked loop (PLL) that can adjust the frequency of a carrier signal used for frequency downconversion at the wireless device. The frequency error estimates may also be averaged to obtain a single frequency error estimate Δf for the pilot-1 OFDM symbol. This Δf may then be used for frequency error correction either prior to or after the N-point DFT within OFDM demodulator 160. For post-DFT frequency error correction, which may be used to correct a frequency offset Δf that is an integer multiple of the subband spacing, the received symbols from the N-point DFT may be translated by Δf subbands, and a frequency-corrected symbol {tilde over (R)}k for each applicable subband k may be obtained as {tilde over (R)}k={tilde over (R)}k+Δf. For pre-DFT frequency error correction, the input samples may be phase rotated by the frequency error estimate Δf, and the N-point DFT may then be performed on the phase-rotated samples.
Frame detection and frequency error estimation may also be performed in other manners based on the pilot-1 OFDM symbol, and this is within the scope of the invention. For example, frame detection may be achieved by performing a direct correlation between the input samples for pilot-1 OFDM symbol with the actual pilot-1 sequence generated at the base station. The direct correlation provides a high correlation result for each strong signal instance (or multipath). Since more than one multipath or peak may be obtained for a given base station, a wireless device would perform post-processing on the detected peaks to obtain timing information. Frame detection may also be achieved with a combination of delayed correlation and direct correlation.
Referring back to
Referring back to
The detection window size LW may be selected based on the expected delay spread of the system. The delay spread at a wireless device is the time difference between the earliest and latest arriving signal components at the wireless device. The delay spread of the system is the largest delay spread among all wireless devices in the system. If the detection window size is equal to or larger than the delay spread of the system, then the detection window, when properly aligned, would capture all of the energy of the channel impulse response. The detection window size LW may also be selected to be no more than half of L2 (or LW<L2/2) to avoid ambiguity in the detection of the beginning of the channel impulse response. The beginning of the channel impulse response may be detected by (1) determining the peak energy among all of the L2 window starting positions and (2) identifying the rightmost window starting position with the peak energy, if multiple window starting positions have the same peak energy. The energies for different window starting positions may also be averaged or filtered to obtain a more accurate estimate of the beginning of the channel impulse response in a noisy channel. In any case, the beginning of the channel impulse response is denoted as TB, and the offset between the start of the sample window and the beginning of the channel impulse response is TOS=TB−TW. Fine symbol timing may be uniquely computed once the beginning of the channel impulse response TB is determined.
Referring to
The pilot-2 OFDM symbol may also be used to obtain a more accurate frequency error estimate. For example, the frequency error may be estimated using the pilot-2 sequences and based on equation (3). In this case, the summation is performed over L2 samples (instead of L1 samples) for the pilot-2 sequence.
The channel impulse response from IDFT unit 918 may also be used to derive a frequency response estimate for the communication channel between base station 110 and wireless device 150. A unit 922 receives the L2-tap channel impulse response, circularly shifts the channel impulse response so that the beginning of the channel impulse response is at index 1, inserts an appropriate number of zeros after the circularly-shifted channel impulse response, and provides an N-tap channel impulse response. A DFT unit 924 then performs an N-point DFT on the N-tap channel impulse response and provides the frequency response estimate, which is composed of N complex channel gains for the N total subbands. OFDM demodulator 160 may use the frequency response estimate for detection of received data symbols in subsequent OFDM symbols. The channel estimate may also be derived in some other manner.
A wireless device may use TDM pilots 1 and 2 for initial synchronization, e.g., frame synchronization, frequency offset estimation, and fine symbol timing acquisition (for proper placement of the DFT window for subsequent OFDM symbols). The wireless device may perform initial synchronization, for example, when accessing a base station for the first time, when receiving or requesting data for the first time or after a long period of inactivity, when first powered on, and so on.
The wireless device may perform delayed correlation of the pilot-1 sequences to detect for the presence of a pilot-1 OFDM symbol and thus the start of a super-frame, as described above. Thereafter, the wireless device may use the pilot-1 sequences to estimate the frequency error in the pilot-1 OFDM symbol and to correct for this frequency error prior to receiving the pilot-2 OFDM symbol. The pilot-1 OFDM symbol allows for estimation of a larger frequency error and for more reliable placement of the DFT window for the next (pilot-2) OFDM symbol than conventional methods that use the cyclic prefix structure of the data OFDM symbols. The pilot-1 OFDM symbol can thus provide improved performance for a terrestrial radio channel with a large multi-path delay spread.
The wireless device may use the pilot-2 OFDM symbol to obtain fine symbol timing to more accurately place the DFT window for subsequent received OFDM symbols. The wireless device may also use the pilot-2 OFDM symbol for channel estimation and frequency error estimation. The pilot-2 OFDM symbol allows for fast and accurate determination of the fine symbol timing and proper placement of the DFT window.
The wireless device may use the FDM pilot for channel estimation and time tracking and possibly for frequency tracking. The wireless device may obtain an initial channel estimate based on the pilot-2 OFDM symbol, as described above. The wireless device may use the FDM pilot to obtain a more accurate channel estimate, particularly if the FDM pilot is transmitted across the super-frame, as shown in
The synchronization techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing units at a base station used to support synchronization (e.g., TX data and pilot processor 120) 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 arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof. The processing units at a wireless device used to perform synchronization (e.g., synchronization and channel estimation unit 180) may also be implemented within one or more ASICs, DSPs, and so on.
For a software implementation, the synchronization 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 192 in
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
This application claims the benefit of provisional U.S. Application Ser. No. 60/499,951, entitled “Method for Initial Synchronization in a Multicast Wireless System Using Time-Division Multiplexed Pilot Symbols,” filed Sep. 2, 2003.
Number | Date | Country | |
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60499951 | Sep 2003 | US |