WlRELESS APPLICATIONS

FIELD OF THE INVENTION

This invention pertains to the field of wireless data transmission and more specifically to a wireless system utilizing multiple-input/multiple-output (MIMO), orthogonal frequency division multiplexing (OFDM).

BACKGROUND OF THE INVENTION As Internet usage has increased- so has the need for fast, efficient transmission of

Internet data. While there exists a plethora of data transmitting equipment, ranging from analog modems that transmit at 56 kilobits per second (kps) to Cable Modems that transmit at 10-30 megabits per second (mps), the present systems have their drawbacks. For example, users with an analog modem sacrifice speed for a product that can be easily installed in a computer and used with an existing phone line. Cable Modem users require special cables from a base station to their homes/offices before they can use the system. Some systems, like Cable Modems, have a further drawback that they are a shared transmission medium so that the rate of transmission decreases with the number of simultaneous users, e.g. neighbors who may also be on-line. More importantly, most conventional systems require a physical transmission line from the customer premise equipment (CPE) to a base station, be it a traditional telephone line or dedicated data line. It is therefore desirable to transmit data without requiring a physical line. However present wireless devices, such as cellular telephones and pagers have a very limited ability to transmit and receive data.

To increase the capacity of wireless data transmission, Orthogonal Frequency Division Multiplexing (OFDM) may be used. OFDM is a modulation method which encodes data onto a radio frequency (RF) signal. While conventional RF transmission schemes

encode data symbols onto one radio frequency, OFDM encodes data symbols onto multiple frequencies or "sub-carriers." The high-speed data signal is divided into tens or hundreds of lower speed signals, dividing the data across the available spectrum into a set of sub-carriers. To prevent interference between the sub-carriers, each sub-carrier is orthogonal (independent or unrelated) to all the other sub-carriers, so that guard intervals are not needed around each sub-carrier but, rather, are needed only around a set of sub-carriers — namely at the edge^of the occupied frequency band- Thus. OFDM systems are spectrally efficient and are much less susceptible to data loss due to multϊpath fading than conventional systems.

An alternative solution is a Multiple Input Multiple Output (MIMO) system which utilizes multiple independent transmitting antennas to communicate with multiple independent receiving antennas. MIMO systems can be used to either increase signal power or increase the data rate transmission. In one configuration, a MIMO system may operate in temporal diversity, i.e. where each transmitting antenna, sends a data signal that is correlated to the data signal sent by another antenna. The result is a stronger signal. In another configuration, the MIMO system operates in spatial diversity, where each transmitting antenna sends a data signal that is independent to the data signal sent by another antenna. For every additional transmitting antenna that is used, tihte data rate increases proportionality, i.e. using two antennas doubles the data rate, using three antennas triples the data rate, etc.

Heretofore, each of these methods has been implemented independently. Therefore, designers of data transmission systems must examine the benefits of each system and determine which type of modulation method to implement. It would therefore be beneficial to provide the benefits of both OFDM and MIMO in a single system to provide for very efficient wireless transmission of data.

However, such a MIMO OFDM system would require a robust time synchronization method in order to synchronize the multiple OFDM signals received at the multiple receiving

stations. Since OFDM has only been utilised in aingle-input/single-output (SISO) systems thus far, current OFDM synchronization methods are of little value. It would therefore be beneficial to provide a robust time synchronization method for a MEMO OFDM system capable of synchronizing the multiple OFDM signals.

SUMMARY OF THE INVENTION

The present invention is directed to time synchronizing the data signals in a multiple- input/multiple-output (MIMO), orthogonal frequency division multiplexing (OFDM) system and to estimating frequency and sampling clock offsets in such a system. According to an aspect of the invention, a multiple-input/multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system is time-synchronized. Multiple data frames are transmitted from at least two transmitting stations. Each frame includes at least one preamble and at least one symbol. The frames from the at least two transmitting stations are received at at least two receiving stations. At each receiving station, a first synchronization algorithm is performed. A simple sliding correlator is utilized. A second synchronization method is also performed at each receiving station, wherein there is correlation of a preamble in a single data frame to a preamble stored in a memory at the receiving station performing the algorithm.

According to another aspect of the invention, the frequency offset in a multiple- input/multiple-output (MIMO), orthogonal frequency division multiplexing (OFDM) system having multiple MIMO channels is estimated. Each channel has multiple sub-carrier channels. Each sub-carrier channel has a pre-determined sub-carder spacing. Multiple data frames are transmitted from at least two transmitting stations. Each frame includes at least one preamble and at least one symbol. The preamble further includes at least one pilot sub- carrier with an index and at least one training symbol. The data frames transmitted from the

at least two transmitting stations are received at at least two receiving stations. The frequency offset for a multiple of the sub-carrier spacing for each of the MIMO channels is calculated. The frequency offset of a fraction of the sub-caπier spacing for each of the MIMO channels is calculated. The total frequency offset for each MlMO channel is calculated by summing the frequency offset of a multiple of the sub-carrier spacing for each MIMO channel with the frequency offset of a fraction of the sub-carrier spacing for each MIMO channel. The average frequency offset for the MlMO channels is calculated by averaging the total frequency offset of each MlMO channel.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flow diagram illustrating the steps of a typical OFDM transmission.

Figure 2 illustrates a coarse synchronization apparatus of the invention.

Figure 3illustrates a fine synchronization method apparatus of the invention

Figure 4 illustrates multiple sub-carrier spacing offset estimation apparatus of the invention.

Figure 5 illustrates sampling clock /fractional sub-carrier spacing offset apparatus of the invention.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates a typical OFDM transmission in a known single-input/single- output (SISO) system.

Referring to Figure 1, a number of bits are to be transmitted via a radio-frequency

(RF) transmission to a receiving location. The bits are divided into segments of A bits at step

100 and input into an encoder every T _p + T ₅ seconds at step 200, where T ₅ is an OFDM symbol interval, and T _p is a cyclic prefix or guard interval. The encoder then sub-divides the

A bits into N sub-segments of m bits. For example, a stream of bits that is to be transmitted may be input into the encoder in segments of 1024 bits. The encoder then divides the 1024 bits into 256 sub-segments of 4 bits each. Thus, in this example, A = 1024, m = 4 , N =128. However, m may be an integer other than 4. In a typical OFDM transmission, there are N +1 sub-carriers, including a zero frequency DC sub-carrier that is not used to transmit data. The encoder may further perform a modulation, such as a quadrature amplitude modulation (QAM), so that each N sub- segment of m bits has a corresponding complex valued sub-symbol Ck=ak + jb _k , where -N/2 ≤ k ≤ N/2. There now exists a sequence of N frequency domain sub-symbols that encodes all the A data bits.

After encoding, an inverse fast Fourier transform (IFFT) is carried out on the sequence of N sub-symbols at step 300. The IFFT uses each of sub-symbols, Ck , to modulate the phase and amplitude of a corresponding one of N+1 sub-carrier frequencies over the symbol interval T ₅ . The sub-carriers may be given b and therefore have baseband frequencies of f _k = k / T _s where k is the frequency number and an integer within —N/2 < k ≤ N/2. The IFFT may produce a digital time-domain OFDM symbol of duration T _s given by the relation:

The modulation of the OFDM sub-carriers by frequency-domain sub-symbols over

symbol intervals of T ₅ seconds results in the OFDM sub-carriers each having a spectrum in the frequency domain. By spacing the N+1 sub-carriers 1/ T ₅ in the frequency

domain, the peak of each sub-carrier's spectrum coincides with a minimum of the spectrum of every other sub-carrier. Thus, although the sub-carriers may overlap, they are

orthogonal to each other.

The digital time domain OFDM symbols produced by IFFT are then passed to a digital signal processor (DSP) which may add a cyclic prefix or guard interval of length T _p to each OFDM symbol at step 400. The cyclic prefix may simply be a repetition of part of the symbol. As an example, a "48 prefix" may be the last 48 sub-symbols of the OFDM symbol. The cyclic prefix prevents interference between consecutively transmitted OFDM symbols.

After adding the prefix, the information is transmitted, as shown at 500, via a channel having a minimum channel bandwidth of (1/ T _p )*(N + 1) Hz, so as to accommodate all the OFDM sub-carriers. The OFDM data is typically transmitted in data frames composed of plural symbols at a transmission rate determined by a sampling clock. For example, a data frame may have 100 symbols, each symbol being 9000 samples, as a function of the sampling rate of the sampling clock.

The OFDM data frames are transmitted serially by an RF transmitter, which typically includes an IF bandpass filter, an RF mixer, a local oscillator, an RF bandpass filter, an RF power amplifier, and an antenna.

The OFDM data frames are then received by an OFDM receiver comprised of an RF receiver, which may include an antenna, a low noise amplifier, an RF bandpass filter, an automatic gain control circuit, an RF mixer, an RF carrier frequency local oscillator, and an IF bandpass filter. The received data frames are then delivered to an OFDM receiving system which: a) synchronizes the receiver to the timing of the symbols and data frames; b) estimates and corrects for the carrier frequency offset; c) removes the cyclic prefixes from the received OFDM signal; and d) computes a fast Fourier transform (FFT) in order to recover the sequences of frequency-domain sub-symbols. The system may also estimate and correct the sampling clock offset.

The MlMO-OFDM system of the present invention uses two or more transmitting base stations, each including an RF transmitter in place of the single transmitter described above and two or more receiving base stations, each including an RF receiver. The MIMO- OFDM system is not limited as such and may include more than two transmitting and receiving antennas.

Because the receiving base stations may begin to operate alter the transmitting stations have been transmitting for some lime, the first signal received at the receiving base station may not be the first signal transmitted by the transmitting base station. Thus, in such circumstances synchronization is carried out to determine the appropriate start and end points of the transmitted data stream.

Preambles are periodically inserted into the OFDM data frames prior to serial transmission. For example, a preamble may be inserted every 2.5 ms or every 5 ms. Each data frame transmitted from an OFDM transmitter is defined by a sequence, such as "preamble, symbol, preamble, symbol, etc..." or "preamble, symbol, symbol, preamble, symbol, symbol, etc."

A description of the preambles and how they are designed is set out in U.S. Application No. 09/751,879 entitled "Preamble Design for MIMO and OFDM System for Multi-User Fixed Wireless Access Application", filed concurrently herewith, which is incorporated herein by reference. As noted in Application No. 09/751 ,879, the preamble may contain training symbols and pilots.

The synchronization, namely the process whereby the starting and ending points of the transmitted data are determined, is earned out in two stages. A processor may perform mathematical calculations in each stage. The first stage is typically a coarse synchronization, whereby a rough estimate of the starting point of the data is determined. The coarse synchronization is conducted quickly and easily, such as using a simple sliding correlator.

For example, a simple sliding correlator for the coarse synchronization stage may be defined by the relation:

In the equations listed above, τ _t is the received signal. r\is the conjugate of the received signal, SNR is signal-to-noise ratio, Nsampi _e is the number of samples in one OFDM symbol, and Ie is the threshold. Because the SNR may not be known at the receiving end, a fixed SNR (for example SNR=4 dB) may be assumed on the initial synchronization. The coarse synchronization method does not determine the starting point but provides a starting position from which to commence fine synchronization.

The fine synchronization uses a preamble stored in a memory that is matched to the preamble corresponding to the first received data frame. Starting from the coarse synchronization starting point and traversing along a forward or backward direction along the data stream, the encountered preambles are compared to the stored preamble until a match is found.

More specifically, prime number (PN) pilots in the training symbols of the preamble are typically used to conduct fine synchronization, in either the time domain or in the frequency domain. For example, assuming that there are two transmitting antennas and two receiving antennas so that there are 2 x 2 MIMO possible channels, the correlation functions for fine synchronization in the time domain are defined by the relations, :

j=l _j 2 represents Rx antenna 1 and Rx antenna 2,

In the above equations, PN ₁ and PN ₂ are the known PN codes used for respectively modulating odd indexed pilots, i.e. for Tx antenna 1 and the even indexed pilots i.e. for Tx antenna 2 in the training symbols of the preamble. N _fine is the number of samples to be searched backwards and forwards around the starting position obtained from the coarse synchronization.

Timing may be tracked by repeating the second timing synchronization step several frames. Then _* the frequency and sampling clock offsets may now be estimated and corrected. Alternatively, the frequency and the sampling clock offsets are estimated and corrected prior to conducting the time synchronization as the two processes are independent It is currently impossible to manufacture oscillators in exactly the same way.

Therefore, the RF carrier frequency oscillator in the RJF transmitter is not identical to the RJF carrier frequency oscillator in the RF receiver. The mismatch between oscillators results in a frequency offset which causes a loss of orthogonality between the OFDM sub-carriers and results in inter-carrier interference between sub-carriers as well as a large increase in the bit error rate (BBR) of the recovered data at the receiver. For the same reason, the clock sampling rate at the OFDM transmitter is not identical to the clock sampling rate at the OFDM receiver which creates a sampling clock offset, that adds to the frequency offset. Both the frequency offset and the sampling clock offset are corrected prior to extracting data

from the samples received.

The frequency offset is divided into two parts. The first part is defined as a multiple of the sub-carrier spacing {δf _mt ). The second part is defined as a fraction of the sub-carrier spacing (δf _frac ). The total frequency offset (δf) is thus represented by the following equations:

Here, Q is an integer, ε is a fractional number and Δf is the sub-carrier spacing. Since δf _int may cause a shift of all the sub-carriers, it may be found by finding the shift of the pilot sub-carrier indices (Z). The following equation is provided for one system having two transmitting antennas and two receiving antennas:

/) , where

In the above equations, R _train is the received training symbol in the frequency domain, k _min is the index of the first useful sub-carrier, k _max is the last useful sub-carrier, and PN _i are the known PN codes used by Tx antenna 1 (i=1) and Tx antenna 2 (i=2) respectively. K _max is a value obtained from the expected maximum frequency offset and is based on the precision of the reference frequency.

The fractional frequency offset and the sampling clock offset can be concurrently estimated by using two identical training symbols, R _train1 and R _train2 in the frequency domain, that are received successively. The following correlation may exist:

The sampling clock offset may be obtained from the slope of the curve . Furthermore, the ftactional frequency offset may be obtained from the values and the slope of the curve.

The respective offsets may be estimated for each MIMO channels. The final frequency offset and sampling clock offset estimations may then be determined by averaging the results obtained from the different MIMO channels.

The tracking of the frequency offset and sampling clock offset is achieved by using continual pilots. After the offsets have been estimated and corrected, a Fast Fourier Transform may be performed on the OFDM sub-symbols, again converting the value into complex-valued sub-symbols in the frequency domain, These complex valued sub-symbols may be decoded so that the original A bits of data may be recovered.

The following claims are intended to cover all of the generic and specific features of the invention described herein, and all statements of the scope of the invention which, as a matter of language, might be said to fall there between.