HK1120688A - Timing control in orthogonal frequency division multiplex systems based on effective signal-to-noise ratio - Google Patents

Abstract

A spread-spectrum communication system provides an effective signal-to-noise ratio (SNR) of received orthogonal frequency division multiplex (OFDM) slots in the presence of timing errors. Effective SNR can serve as a diagnostic tool for determining whether there was a timing error when a measured packet error rate (PER) remains high, and a predicted PER from the effective SNR remains low. A loop can use the effective SNR to control a time reference used by an OFDM decoder.

Description

Effective snr-based timing control in an orthogonal frequency division multiplexing system

Technical Field

The present invention relates generally to communication systems, and specifically to timing control in orthogonal frequency division multiplexing systems.

Background

In a spread spectrum system, a mobile station may receive transmissions from one or more base stations. Each mobile station and base station may use a particular spreading code to identify its signal transmission.

Disclosure of Invention

Is free of

Drawings

Various embodiments of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. These drawings depict only exemplary embodiments and are not to be considered limiting of its scope.

Fig. 1 illustrates a communication system including a base station and a mobile station.

Fig. 2 illustrates an example of a plurality of Orthogonal Frequency Division Multiplexing (OFDM) tones in the frequency domain that may be transmitted in the system of fig. 1.

Fig. 3 illustrates an example of an OFDM processing path.

Fig. 4A illustrates a total OFDM symbol time period that may carry data on N tones.

Fig. 4B illustrates an OFDM symbol in the frequency domain.

Fig. 4C illustrates an OFDM symbol and its cyclic prefix.

Fig. 5 illustrates a timing acquisition/control device that may be implemented in the system of fig. 1.

Fig. 6A illustrates a timing acquisition/control process that may be performed by the apparatus of fig. 5.

Fig. 6B illustrates an apparatus having means corresponding to the blocks in fig. 6A.

Fig. 7 illustrates a time division multiplexing pattern of code division multiplexed slots and OFDM slots.

Fig. 8 illustrates an example of an actual channel response of a transmitted OFDM signal, an OFDM slot with an incorrect time reference, and an OFDM slot with a correct time reference.

Detailed Description

Any embodiment described herein is not necessarily preferred or advantageous over other embodiments. While various aspects of the invention are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

Fig. 1 illustrates a communication system 100 that includes base stations 102A, 102B and mobile stations 104A, 104B. System 100 may have any number of base stations and mobile stations. Communication system 100 may use one or more communication techniques such as Code Division Multiple Access (CDMA), High Rate Packet Data (HRPD) (also known as High Data Rate (HDR), as indicated in CDMA2000High Rate Packet Data Air Interface Specification, TIA/EIA7 IS-856), CDMA1x Evolution Data Optimized (EV-DO), wideband CDMA (wcdma), Universal Mobile Telecommunications System (UMTS), time division synchronous CDMA (TD-SCDMA), Orthogonal Frequency Division Multiplexing (OFDM), and so on.

Fig. 1 also shows two multipath signals 110A, 110B received by mobile station 104A due to object 106 between base station 102A and mobile station 104A. The repeater 108 or the distance between the repeater 108 and the base station 102A may delay the signal 110B transmitted from the base station 102A to the mobile station 104A.

A "mobile station" as described herein may refer to various types of devices, such as a corded telephone, a wireless telephone, a cellular telephone, a laptop computer, a wireless communication Personal Computer (PC) card, a Personal Digital Assistant (PDA), an external or internal modem, and so forth. A mobile station may be any device that communicates through a wireless channel or through a wired channel, for example using fiber optic or coaxial cables. A mobile station can have a variety of names, such as access terminal, access unit, subscriber unit, mobile device, mobile unit, mobile phone, mobile, remote station, remote terminal, remote unit, user device, user equipment, handheld device, etc. The mobile stations may be mobile or stationary and may be dispersed throughout the communication system 100 of fig. 1. A mobile station may communicate with one or more base station transceiver systems (BTSs), also known as base stations, access networks, access points, node bs, and Modem Pool Transceivers (MPTs).

One or more base stations 102 may transmit signals, such as broadcast/multicast content, to multiple mobile stations 104, i.e., multiple mobile stations 104 receive the same broadcast content. The broadcast transmission may use OFDM communication techniques. As shown in fig. 2, OFDM distributes data over a large number of equally spaced frequency subcarriers (also referred to as "carriers," frequency "tones," or frequency "bins").

Fig. 2 illustrates an example of a plurality of OFDM tones 200A-200E in the frequency domain (horizontal axis), with amplitude represented on the vertical axis. Each tone 200 is "orthogonal" to every other tone due to the spacing of the tones at precise frequencies. The peak of each tone 200 corresponds to the zero level or null (null) of each other tone. Thus, there is no interference between tones 200A-200E. When the receiver samples at the center frequency of each tone 200, the only energy present is that of the desired signal plus any noise that happens to be in the channel. The detector of a given tone 200 is not affected by the energy in other tones 200. OFDM allows the spectrum of each tone 200 to overlap and, because they are orthogonal, they do not interfere with each other.

The sinusoidal waveform that makes up tone 200 in OFDM has a special characteristic that is the only eigenfunction of the linear channel. This particular characteristic prevents adjacent tones in an OFDM system from interfering with each other, and as such, the human ear can clearly distinguish each of the tones produced by adjacent keys of a piano. This property, and the incorporation of a small amount of guard time into each OFDM symbol 400 (fig. 4A), enables the orthogonality between tones 200 to be preserved with multipath signal propagation.

A portion of the user's data is modulated onto each tone 200 by adjusting the amplitude, phase, or both of the tones. In one configuration, the tone 200 may be present or disabled to indicate a1 or 0 bit of information. In other configurations, Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) may be used to modulate the data onto each tone.

Fig. 3 illustrates an example of an OFDM processing path 300, which includes a channel coding unit 302, a modulation unit 304, a frequency mapping unit 306, an Inverse Fast Fourier Transform (IFFT) processing unit 308, a cyclic prefix insertion unit 310, a carrier modulation unit 312, and transmit circuitry 314 (e.g., antennas). The channel coding unit 302 takes a data stream, codes the data stream with a turbo code or a convolutional code, interleaves or permutes the stream, and divides it into N parallel data streams, each at a rate of 1/N of the original rate. The input to channel coding unit 302 is a packet (not shown in fig. 3), and the output is transmitted as one or more OFDM symbols 400 in fig. 4.

The modulation unit 304 and frequency mapping unit 306 map each data stream to tones having unique frequencies, and these tones are referred to as "data tones". Meanwhile, it is known that "pilot symbols" are transmitted on a different set of tones called "pilot tones". The receiver may use these pilot tones to estimate the frequency response of the composite channel and perform demodulation on the received OFDM signal. The modulation unit 304 may use QAM. OFDM modulation may be implemented using Digital Signal Processing (DSP) software. OFDM transmission may be considered as Discrete Multitone (DMT) modulation with a common spreading code, e.g. a spreading code comprising all zeros.

The pilot tones and data tones are combined together using an IFFT 308 to produce a time domain waveform. Cyclic prefix insertion unit 310 inserts cyclic prefix 402 (fig. 4A). The output of cyclic prefix insertion unit 310 is provided to a carrier modulation unit 312 and transmit circuitry 314, which synthesize a Radio Frequency (RF) signal.

FIG. 4A illustrates a total OFDM symbol time period T_symDuring this time, data may be carried on the N tones. In the total symbol time periodPeriod T_symDuring which each tone may carry an OFDM symbol 400 and a cyclic prefix 402. Fig. 4B illustrates a plurality of OFDM symbols in the frequency domain. Fig. 4C illustrates an OFDM symbol 400 and its cyclic prefix 402.

To compensate for multipath delays, the cyclic prefix 402 is designed to be larger than the delay spread (time delay between the longest and earliest channel multipath). The cyclic prefix 402 provides a guard time to the OFDM symbol 400 to ensure orthogonality between subcarriers in the frequency domain, i.e., to prevent subcarriers from interfering with each other. If the delay spread is too large, the subcarriers may overlap in the frequency domain and orthogonality may be lost.

Cyclic prefix 402 may have a fixed length and may be appended at the beginning of each OFDM symbol 400 such that the linear convolution of the channel becomes a "cyclic convolution". It is desirable that the OFDM symbol length be large relative to the cyclic prefix length to minimize overhead. A fundamental tradeoff arises because the cyclic prefix 402 must be long enough to account for the expected multipath delay spread experienced by the system 100. In other words, the cyclic prefix length should be "longer" than the length of the effective impulse response experienced at the receiver.

Timing acquisition

The performance of an OFDM system may rely on accurate timing acquisition. Larger timing acquisition errors may result in inter-symbol interference (ISI) from adjacent channels, erroneously estimated channels, and larger performance degradation of the modem.

The following description analyzes the channel estimation algorithm and shows that the effective signal-to-noise ratio (SNR) metric, when properly calculated, is a robust measure of actual channel performance, even with timing acquisition errors. Based on this analysis, a timing acquisition (or timing control) outer loop may be implemented to detect and correct timing acquisition errors.

The following description is generally applicable to any receiver that receives multipath OFDM signals. In particular, the following description may apply to a mobile station 104 receiving multipath signals 110A, 110B, 110C broadcast from one or more base stations 102A, 102B.

Channel estimation with timing error

The composite channel response in the time domain (e.g., at mobile station 104A receiving L multipath signals 110A-110C transmitted from one or more base stations 102A, 102B) may be expressed as:

where L represents the total number of multipath components, α_lAnd τ_lRespectively, the complex-valued amplitude and delay of the l-th multipath ray, p (T) the complex time-domain filter response of the transmit and receive impulse filters, and T the chip duration. For the sake of simplicity, a static channel is assumed. With correct timing, the frequency response of the channel on the pilot tone can be expressed as:

where h (f) is the discrete fourier transform of h (nt), and k denotes the index of the kth pilot tone at frequency k/PT, where P is the number of pilot tones.

Assume that due to timing errors, a start time is erroneously assigned to a sample delayed by Δ chips after the first arrival path. Then it can be shown that the estimated channel response at the pilot tone can be expressed as:

equation (1) above leads to two important observations. First, at the pilot tone, the amplitude response of the channel is estimated regardless of the error in the timing referenceEqual to the amplitude response H of the actual channel_k. Second, on the pilot channel, errors in the timing reference result in a linear phase shift across the pilot tones. Due to FFT-based channel estimation in OFDM systems, the time domain response of the estimated channel derived from these phase shifted pilot tones is a cyclic shift of the time domain response of the actual channel.

Unfortunately, because the frequency response on the data tones is derived by interpolating the frequency response on the pilot tones, the interpolated amplitude and phase on the data tones can differ significantly from the values of the actual channel when the timing is incorrect. In the case of a timing error with Δ chips, the frequency response on the data tone (the least squares interpolated from the pilot tones using the FFT) can be approximated by the following equation:

where N is the number of tones (both pilot and data tones) in an OFDM symbol and k represents the data tone index. In this equation, the first summation includes a subset of multipath components that occur before the timing reference Δ, and the second summation includes all paths that arrive after Δ. It is clear that the estimated channel response at the data tones can be significantly different from the response of the actual channel, and this can lead to significant errors during data demodulation.

Effective SNR with timing error

When there is no timing error, a metric called effective SNR can be used to accurately predict OFDM performance. For a packet encoded with a certain rate code (e.g., 3/4 rate code) and modulated using a certain transmission cluster (e.g., 16-QAM), if the effective SNR metric is less than a certain threshold (e.g., 11.4dB), then the packet is most likely not decodable. Conversely, if the effective SNR metric exceeds the threshold, the packet is most likely to be decoded correctly. For a typical OFDM system, the shannon/nyquist sampling theorem ensures that an effective SNR metric can be calculated from the channel frequency response at the pilot tones only, as long as the number of pilot tones exceeds the maximum delay path in the channel.

Based on the channel response H at the pilot tones as follows_kAn effective SNR metric is calculated. First, the noise variance on the pilot tones may be estimated using an Estimation procedure similar to commonly assigned U.S. patent application No. 11/047,347 entitled "noise variance Estimation in Wireless Communications for Diversity Combining and log-likelid Scaling," filed on 28.1.2005. Suppose σ²Representing the estimated noise variation. Then, the effective SNR metric may be calculated by the following formula

Where c (x) is the capacity of the gaussian channel whose SNR x and input are constrained to a selected modulation type (e.g., 64QAM or 16 QAM). For example, if the transmitted cluster is limited to a set of points { c ] in the complex plane_i: 1.. J), wherein the points are normalized according to the following formula:

then, the limiting capacity function c (x) is given by the following equation:

this set of equations is in "Principles and Practice of Information Theory" of r.e. blahut, AddisonWesley, 1991, section 7.8, pages 272 and 279.

Because the effective SNR metric calculated from the pilot tones depends only on the amplitude of the channel frequency response, and because the channel amplitude at the pilot tones is independent of timing errors (see equation (1) above), the effective SNR metric is also independent of timing errors. However, in the presence of timing errors, the noise variations may include inter-symbol interference and inter-tone interference, and thus the noise variations with timing errors will be no less than the noise variations without timing errors. Thus, the effective SNR metric computed with timing errors will typically be slightly less than the effective SNR metric without timing errors. Thus, the effective SNR metric provides a conservative prediction of whether a packet should be decodable.

Since the effective SNR based on pilot tones is still a good predictor of supportable or achievable Packet Error Rate (PER) even in the presence of timing errors, the effective SNR can be used with PER measured in the outer loop for timing acquisition/control, as described below.

Timing control outer loop based on effective SNR

Fig. 5 illustrates a timing acquisition/control apparatus 520, which may be implemented in the mobile station 104 or the base station 102 of fig. 1. The apparatus 520 may be implemented in software, hardware, or a combination of software and hardware. The apparatus 520 includes a receiver 506, a signal searcher 504, an OFDM demodulator and decoder 500, and a timing control outer loop 502, which may also be referred to as a timing control module. OFDM demodulator and decoder 500 may be separate from or integrated with a CDM demodulator and decoder. Apparatus 520 may include other software and hardware components, such as a deinterleaver, channel estimator, and the like, in addition to or in place of the components shown in fig. 5.

Fig. 6A illustrates a timing acquisition/control process, which may be performed by the apparatus 520 of fig. 5. Fig. 6B illustrates an apparatus having means 611-614 corresponding to block 601-604 in fig. 6A.

Fig. 7 illustrates a time division multiplexing pattern of transmitted code division multiplexed slots 702 (e.g., unicast data) and OFDM slots 700 (e.g., broadcast data). Fig. 7 also shows an example of a particular OFDM slot structure.

Receiver 506 in fig. 5 receives signals transmitted (e.g., from one or more base stations) in CDM and OFDM time slots 702, 700A, 700B (fig. 7). A signal searcher 504, such as a CDMA signal searcher, determines coarse timing values. An OFDM demodulator and decoder 500 demodulates and decodes OFDM symbols 400 from OFDM slots 700A, 700B, the OFDM symbols 400 containing data tones and pilot tones.

In block 601 in fig. 6A, the OFDM demodulator and decoder 500 (fig. 5) measures PER of the received decoded OFDM packets using the coarse timing value and calculates an effective SNR metric from pilot tones of the OFDM symbols. The demodulator processes one or more OFDM symbols and provides this to the decoder, which then attempts to reproduce the packets input to the channel encoding unit 302 in fig. 3. The demodulator and decoder 500 (or, alternatively, the outer loop 502) may use the effective SNR metric to predict supportable PERs (predicted PERs'). For example, the effective SNR metric is compared to an SNR threshold that depends on the data rate of the packet; if the effective SNR metric exceeds the threshold, the packet is predicted to have no errors, otherwise the packet is predicted to be erroneous. The predicted PERs may be suitable time averages of these predictions. Demodulator and decoder 500 includes a timing control inner loop 501 that provides the current time reference to a timing acquisition/control outer loop 502. The timing control inner loop 501 also refines the coarse timing value from the signal searcher 504 or updates the previous timing value by using the received OFDM symbol.

In block 602, the timing control outer loop 502 receives the current time reference,Effective SNR metric, measured PER and predicted PER as inputs, and at N_SA timing error is declared when all three of the following conditions are met during a consecutive OFDM slot 700. As shown in fig. 7, "consecutive" OFDM slots 700 may be separated in time by unicast CDM slots 702. N is a radical of_SMay be equal to 64, 100, 256, or some other value.

Parameters (also referred to as values, variables, thresholds, etc.) N as described herein_S、N_C、P₁、P₂、P₃、T₁、T₂、T₃Can be selected, programmed and/or optimized according to various system parameters such as inner timing loop update rate, pilot tone SNR measurement accuracy and network topology. These parameters may be set or determined by the device manufacturer or wireless operator, etc., and the default values may be programmed at the time the device is manufactured or during immediate operation in the field.

i. The measured PER is close to 100% (or PER > P)₁In which P is₁May be 30% (for example));

predicted PER based on the effective SNR metric is close to 0% (or PER' < P₂In which P is₂May be 2% (for example)); and

the current timing reference remains the same (or for T from a previous reference)₁With the last chip, the current timing reference does not change, where T₁May be equal to 10 (for example)).

If these three conditions have not been met, the outer loop 502 may continuously check for these three conditions.

Thus, if several consecutive OFDM slots decode incorrectly (i.e., a higher measured PER), then either (a) the channel is not "good" or (b) the current time reference is incorrect. If the channel is "good" (i.e., if the effective SNR metric is high and the predicted PER is low), then the time reference is likely to be incorrect and adjustments should be made.

In block 603, if assertedReferring to timing error, the outer loop 502 sends an "advance" or shift signal to the timing control inner loop 501 such that the reference timing (also referred to as the current time reference) is advanced by N for the next OFDM slot_COne chip as shown in fig. 8. For example, N_CMay be equal to 5 to 8 chips. After a timing error is declared, the timing estimate from the timing control inner loop 501 will not affect the reference timing. The last timing reference before the timing error is declared continues to be updated only by the timing control outer loop 502.

Fig. 8 illustrates an example of an actual channel response of a transmitted OFDM signal 800, an OFDM slot with an erroneous time reference 802, and an OFDM slot with a correct time reference 804.

In block 604, the process may repeat block 603 until any of the following conditions are met:

i. the measured PER is close to the predicted PER ' (or IPER-PER ' I/PER ' < P)₃In which P is₃May be equal to 5);

the total number of advanced chips exceeds a threshold T of chips determined a priori based on the maximum possible delay in the channel₂Wherein T is₂May be equal to 80 (for example); or

Changing T relative timing estimate of first arriving path detected by inner loop timing algorithm₃More than one chip, where T₃May be equal to 30 (for example).

If condition iii is satisfied, the timing reference for the next OFDM slot will be reset to the timing estimate from the timing control inner loop 501.

The above description provides an analysis of the estimated channel and effective SNR of received OFDM symbols in the presence of timing errors. The analysis explains why effective SNR can serve as a diagnostic tool to determine whether there is a timing error when measured PER remains high but predicted PER based on the effective SNR metric remains low. The outer loop 502 may control timing based on the effective SNR metric.

Broadcast content

The broadcast transmissions from multiple base stations 102 may be time synchronized with each other such that the base stations 102 transmit the same broadcast content using the same waveform or modulation (e.g., the same spreading code) at the same time. In this way, multiple broadcast transmissions may be viewed as multipath transmissions at the receiver. In other words, the synchronized broadcast transmission creates virtual multipath, thereby providing improved reception quality at the receiver with appropriate signal processing. An advantage of generating signals that exhibit multipath is that the receiver is able to maximize macro diversity gain, where an attenuated signal from one base station is cancelled by the same strongly received signal with a differential propagation delay from another base station. The synchronized broadcast may provide the same spreading code for multiple transmitters.

Time synchronization between base stations may be beneficial when the synchronized broadcast transmission uses OFDM for the broadcast portion 700 (fig. 7) of the transmission. If the base station transmissions are not time synchronized, the difference in timing can effectively become a multipath delay, which can increase the delay spread. Thus, time-synchronized transmissions from multiple base stations 102 are used to align OFDM transmissions and avoid introducing additional delay spread.

As shown in fig. 7, a base station may broadcast data in interleaved broadcast slots 700, the broadcast slots 700 being interleaved between slots 702 for user-specific (unicast) data transmission. One embodiment uses OFDM waveforms for synchronous broadcasting. Each broadcast slot may have three or four OFDM symbols, where one symbol may have more pilot tones than the other two OFDM symbols. Each mobile station may estimate the channel response using pilot tones of one or more OFDM symbols and derive a timing reference for demodulating the OFDM symbols.

Information and signals described herein may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure.

The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with the following means: a general purpose processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable optical disk, a CD-ROM, or any other form of storage medium known in the art. A storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

Headings are included herein for reference and to aid in locating certain sections. These headings are not intended to limit the scope of the concepts described therein under, and these concepts may have applicability in other sections throughout the entire specification.

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 disclosure. 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.

Claims (27)

1. A method, comprising:

decoding a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols in a plurality of slots using a current time reference;

measuring a packet error rate of the decoded OFDM symbol;

determining an effective signal-to-noise ratio (SNR) metric from pilot tones of the demodulated OFDM symbols;

predicting a supportable packet error rate based on the determined effective SNR metric;

comparing the measured packet error rate with the predicted packet error rate; and

based on the comparison, the current time reference is adjusted prior to decoding a next OFDM slot.

2. The method of claim 1, wherein comparing the measured packet error rate to the predicted packet error rate comprises:

determining whether the measured packet error rate is greater than a first threshold; and

determining whether the predicted packet error rate is less than a second threshold.

3. The method of claim 1, wherein adjusting the current time reference based on the comparison comprises: if the measured packet error rate is greater than the first threshold and the predicted packet error rate is less than the second threshold, then the current time reference is adjusted before decoding the next OFDM slot.

4. The method of claim 1, wherein determining an effective signal-to-noise ratio (SNR) metric comprises using:

where C (x) is the capacity of the Gaussian channel whose SNR x and input are constrained to a selected modulation type, where σ²Represents the estimated noise variation, where P is the number of pilot tones, and where H_kRepresenting the amplitude response of the channel.

5. The method of claim 4, further comprising implementing function C (x) using a look-up table, and estimating the noise variance.

6. The method of claim 1, further comprising:

determining whether the current time reference is unchanged for a number of consecutive OFDM slots; and

based on the determination, advancing the current time reference by a configured number of chips for the next OFDM slot to be decoded.

7. The method of claim 1, further comprising:

determining whether the current time reference has not changed from a previous time reference by a configured number of chips; and based on the determination, advancing the current time reference by a configured number of chips for the next OFDM slot to be decoded.

8. The method of claim 1, further comprising:

wirelessly receiving a multipath transmission containing the OFDM symbol in the slot; and

searching the multipath transmission to determine the current time reference.

9. The method of claim 1, wherein adjusting the current time reference comprises advancing the current time reference by a configured number of chips.

10. The method of claim 1, further comprising repeating the adjusting the current time reference until the measured packet error rate approaches the predicted packet error rate.

11. The method of claim 1, further comprising repeating the adjusting the current time reference until adjusting the current time reference exceeds a threshold number of chips determined a priori based on a maximum possible delay in a channel.

12. The method of claim 1, further comprising repeating the adjusting the current time reference until a relative timing of a first arrival path changes by more than a threshold number of chips.

13. The method of claim 1, wherein the OFDM symbol comprises broadcast content intended for a plurality of mobile stations.

14. The method of claim 1, wherein the time slots containing OFDM symbols are time division multiplexed with time slots containing user specific data.

15. An apparatus configured to receive a signal from a base station, the apparatus comprising:

means for decoding a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols in a plurality of slots using a current time reference;

means for measuring a packet error rate of the decoded OFDM symbol;

means for determining an effective signal-to-noise ratio (SNR) metric from pilot tones of the decoded OFDN symbols;

means for predicting a supportable packet error rate based on the determined effective SNR metric;

means for comparing the measured packet error rate with the predicted packet error rate; and

means for adjusting the current time reference prior to decoding a next OFDM slot based on the means for comparing.

16. The apparatus of claim 15, wherein the means for comparing comprises:

means for determining whether the measured packet error rate is greater than a first threshold;

means for determining whether the predicted packet error rate is less than a second threshold; and

means for adjusting the current time reference before decoding a next OFDM slot if the measured packet error rate is greater than the first threshold and the predicted packet error rate is less than the second threshold.

17. An apparatus configured to receive Orthogonal Frequency Division Multiplexing (OFDM) symbols from a base station, the apparatus comprising:

a timing control module configured to:

determining whether a measured packet error rate of the received OFDM symbol is greater than a first threshold;

determining whether the predicted packet error rate is less than a second threshold; and

if the measured packet error rate is greater than the first threshold and the predicted packet error rate is less than

The second threshold, then the current time reference is adjusted before decoding the next OFDM slot.

18. The apparatus of claim 17, further comprising:

a receiver configured to wirelessly receive a multipath transmission containing OFDM symbols in a plurality of slots; and

a searcher to search the multipath transmissions to determine the current time reference.

19. The apparatus of claim 18, wherein the time slots containing OFDM symbols are time division multiplexed with time slots containing user specific data.

20. The apparatus of claim 17, further comprising:

a decoder operable to:

decoding a plurality of OFDM symbols in a plurality of slots using a current time reference;

measuring a packet error rate of the decoded OFDM symbol;

determining an effective signal-to-noise ratio (SNR) metric from pilot tones of the decoded OFDM symbol; and

based on the determined effective SNR metric, a supportable packet error rate is predicted.

21. The apparatus of claim 17, wherein the timing control module is further configured to:

determining whether the current time reference is unchanged for a number of consecutive OFDM slots; and

based on the determination, advancing the current time reference by a configured number of chips for the next OFDM slot to be decoded.

22. The apparatus of claim 17, wherein the timing control module is further configured to:

determining whether the current time reference has not changed from a previous time reference by a configured number of chips; and

based on the determination, advancing the current time reference by a configured number of chips for the next OFDM slot to be decoded.

23. The apparatus of claim 17, wherein adjusting the current time reference comprises advancing the current time reference by a configured number of chips.

24. The apparatus of claim 17, wherein the timing control module is configured to repeat the adjusting the current time reference until the measured packet error rate approaches the predicted packet error rate.

25. The apparatus of claim 17, wherein the timing control module is configured to repeat the adjusting the current time reference until adjusting the current time reference exceeds a threshold number of chips determined a priori based on a maximum possible delay in a channel.

26. The apparatus of claim 17, wherein the timing control module is configured to repeat the adjusting the current time reference until a relative timing of a first arrival path changes by more than a threshold number of chips.

27. The apparatus of claim 17, wherein the OFDM symbol comprises broadcast content intended for a plurality of mobile stations.