OFDM-MIMO communication system using smart spatial symbol mapping and associated methods

A transmitter in an OFDM-MIMO wireless communication system uses multiple antennas to transmit each data stream. Before the coded binary bits are mapped into channel symbols, they are divided into two groups. One group is mapped to a channel symbol as in a conventional system. Another group of binary bits is used to generate a spatial mapping index. The spatial mapping index determines which antenna is to be used to transmit the channel symbol for each subcarrier. Effectively, information bits are jointly represented by a combination of a channel symbol and an antenna that transmits the channel symbol. Therefore, to achieve the same data rate, a smaller signal constellation is required. In addition, spatial diversity can be achieved which is similar to traditional switching diversity. The number of non-zero subcarriers is reduced by half on average, which results in a lower peak to average ratio than conventional OFDM systems.

FIELD OF THE INVENTION

The present invention relates to wireless communication systems, and more particularly, to a wireless communication system using orthogonal frequency division multiplex (OFDM) modulation and equipped with multiple transmit and receive antennas.

BACKGROUND OF THE INVENTION

A multiple-input multiple-output (MIMO) wireless communication system includes a plurality of antenna elements at the transmitter and a plurality of antenna elements at the receiver. A respective antenna array is formed at the transmitter and at the receiver based upon the antenna elements associated therewith. The antenna elements are used in a multi-path rich environment such that due to the presence of various scattering objects in the environment, each signal experiences multipath propagation.

MIMO communication systems are advantageous in that they enable the capacity of the wireless link between the transmitter and receiver to be improved. The multipath rich environment enables multiple channels to be generated therebetween. Data for a single user can then be transmitted over the air in parallel over those channels, simultaneously and using the same bandwidth.

Orthogonal frequency division modulation (OFDM) is also effective in multipath environments without involving complicated receiver designs. A combination of OFDM and MIMO techniques has been adapted into various standards, such as 802.11n and Evolved UTRA (E-UTRA), and is promising for next generation wireless data communications.

In an OFDM-MIMO transmitter10, as shown inFIG. 1, a serial bit information stream12is coded by a channel encoder14to improve link reliability. The coded serial bit information stream16is then punctured by a puncturer18to achieve a desirable data rate. The punctured coded serial bit information stream20is then interleaved by an interleaver22to avoid burst errors.

The interleaved bits24are then divided by a multiplexer26into multiple serial bit information substreams28,30to increase total data throughput. A plurality of transmitter chains40is coupled to the multiplexer26, with each transmitter chain receiving a respective serial bit information substream28,30.

Each transmitter chain40includes a serial-to-parallel converter42for converting the respective serial bit information substream to a parallel bit information bit substream. In the illustrated example, 3 channel bits50(1)-50(3),51(1)-51(3) are provided from the serial-to-parallel42to a signal mapper44. The signal mapper44maps the 3 channel bits50(1)-50(3),51(1)-51(3) to a channel symbol52,53.

A block of the channel symbol52is then modulated by an OFDM modulator, such as an inverse fast Fourier transform (IFFT) module46. The length of the channel symbol block to be modulated by the OFDM modulator46is determined by total number of subcarriers. The OFDM modulator converts a frequency domain signal to a time domain signal. The time domain signal is transmitted by a transmit antenna48(1),48(2). Effectively, each channel symbol52,53is transmitted on a subcarrier and the channel symbol block occupies the whole bandwidth.

There are several potential problems associated with the conventional OFDM-MIMO transmitter10. To achieve high data rates in evolving wireless standards, high-order modulation schemes such as 16 QAM and 64 QAM are used. However, these high-order modulation schemes require a higher signal-to-noise (SNR) ratio to achieve certain bit error rates (BER). Modulation schemes that require high SNR to achieve certain bit error rates are adversely affected in multipath fading environments, thus causing the wireless links to be unreliable. Moreover, the peak-to-average ratio (PAR) is high in any OFDM system. High PAR causes problems in RF circuitry design, especially in the power amplifiers.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of the present invention to provide a robust OFDM-MIMO communication system that achieves desired bit error rates with reduced signal-to-noise ratios.

This and other objects, features, and advantages in accordance with the present invention are provided by an OFDM-MIMO wireless communication system comprising a transmitter comprising a multiplexer for dividing a serial bit information stream into a plurality of serial bit information substreams, and a plurality of transmitter chains coupled to the multiplexer.

Each transmitter chain receives a respective serial bit information substream and may comprise a serial-to-parallel converter coupled to the multiplexer for converting the respective serial bit information substream to a parallel bit information substream, and a signal mapper is coupled to the serial-to-parallel converter for receiving as input a first group of bits from the parallel bit information substream. Each signal mapper corresponds to a specific subcarrier. An antenna selector having a first input is coupled to the serial-to-parallel converter for receiving a second group of bits from the parallel bit information substream, and a second input is coupled to the signal mapper for receiving a channel symbol therefrom.

A plurality of OFDM modulators may be coupled to a plurality of outputs from the antenna selector. A transmit antenna is coupled to each OFDM modulator. The antenna selector selects one of the transmit antennas for transmitting the channel symbol for each subcarrier based upon the second group of bits from the serial-to-parallel converter.

The antenna selector in each transmitter chain provides the channel symbol to the OFDM modulator associated with the selected transmit antenna, while also providing placeholders to the OFDM modulators associated with the non-selected transmit antennas. The antenna selector in each transmitter chain alternates selecting each one of the transmit antennas associated therewith for transmitting the channel symbols based upon the second group of bits from the serial-to-parallel converter.

The OFDM-MIMO communication system in accordance with the present invention improves robustness in the data link between the transmitter and a receiver. The transmitter uses a smaller signal constellation for the same data rate, therefore requires less SNR to achieve the same packet error rate (PER) under certain circumstances. This is based upon the transmitted information bits being jointly represented by channel symbols and antenna selection bits.

Another advantage is that spatial diversity is achieved since coded bits are effectively distributed among multiple transmit antennas. In addition, for each OFDM modulator receiving the placeholders, the average non-zero inputs is reduced on average to half as compared to conventional OFDM-MIMO communication systems, which results in a lower peak to average ratio of the resulting OFDM waveform.

Each transmit antenna may comprise a directional antenna. Alternatively, each transmit antenna may comprise an omni-directional antenna.

The transmitter may also periodically transmit reference signals from each transmit antenna for each transmitter chain. The wireless communication system further comprises a receiver comprising a plurality of receive antennas, a plurality of OFDM demodulators coupled to the plurality of receive antennas, and a plurality of channel estimators coupled to the plurality of OFDM demodulators.

Each channel estimator may receive the periodically transmitted reference signals from each transmit antenna, and may estimate radio frequency (RF) characteristics between each respective transmit antenna and a respective receive antenna associated with the channel estimator.

The receiver may further comprise a plurality of signal demappers coupled to each OFDM demodulator, with each signal demapper corresponding to a respective subcarrier. Each demapper may determine which channel symbol was transmitted and which transmit antenna was used to transmit the channel symbol. This may be determined by each demapper comparing the channel symbol to the estimated channel symbols transmitted from each transmit antenna. If necessary, the demapper may also calculate soft bit output to facilitate channel decoding.

Another aspect of the present invention is directed to a method for communicating between a transmitter and a receiver in a wireless communication system as defined above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An OFDM-MIMO wireless data communication system in accordance with the present invention combines antenna pattern modulation with traditional OFDM modulation techniques. Information bits are jointly represented by channel symbols and antenna selection bits. This combination effectively uses a smaller signal constellation as compared to using only traditional modulation techniques to achieve the same data transfer rate. Moreover, this combination requires less transmission power to achieve the same bit error rate (BER).

Another advantage of the OFDM-MIMO communication system is that spatial diversity is achieved since coded bits are effectively distributed among multiple transmit antennas. In addition, for each OFDM modulator receiving placeholders, the average number of non-zero subcarriers is reduced on average to half as compared to conventional OFDM-MIMO communication systems, which results in a lower peak to average ratio of the resulting OFDM waveform.

Referring now to the transmitter100illustrated inFIG. 2, a serial bit information stream112is coded by a channel encoder114to improve link reliability. The coded serial bit information stream116is then punctured by a puncturer118to achieve a desirable data rate. The punctured coded serial bit information stream120is then interleaved by an interleaver122to avoid burst errors.

The interleaved bits124are then divided by a multiplexer126into multiple serial bit information substreams128,130to increase total data throughput. A plurality of transmitter chains140is coupled to the multiplexer146, with each transmitter chain receiving a respective serial bit information substream128,130.

Each transmitter chain140includes a serial-to-parallel converter142for converting the respective serial bit information substream to a parallel bit information bit substream. In the illustrated example, 3 channel bits150(1)-150(3),151(1)-151(3) are provided from the serial-to-parallel142.

Also in each transmitter chain140, a signal mapper144is coupled to the serial-to-parallel converter142for receiving as input a first group of bits150(1)-150(2),151(1)-151(2) from the parallel bit information substream128,130. An antenna selector145has a first input coupled to the serial-to-parallel converter142for receiving a second group of bits150(3),150(3) from the parallel bit stream information substream, and a second input is coupled to the signal mapper144for receiving a channel symbol152,153therefrom.

In the illustrated example, a pair of OFDM modulators, such as inverse fast Fourier transform (IFFT) modules146, is coupled to a respective pair of outputs from the antenna selector145. Each IFFT module146(1),146(2) buffers the received channel symbol152,153to form a block of channel symbols. The length of the channel symbol block is determined by the total number of data subcarriers. Each channel symbol within the channel symbol block represents the data to be transmitted over a specific subcarrier. Each IFFT module146(1),146(2) then modulates a block of received channel symbols152,153and converts a frequency domain signal to a time domain signal to be transmitted by a transmit antenna148(1),148(2).

On a subcarrier by subcarrier basis, the antenna selector145selects one of the transmit antennas148(1),148(2) for transmitting the channel symbol152,153based upon the second group of bits150(3),151(3) (i.e., an antenna select bit) from the serial-to-parallel converter142. More particularly, the antenna select bit150(3),151(3) in each transmitter chain140(1),140(2) is used to select a particular transmit antenna148(1) or148(2).

In the illustrated example, the antenna select bit150(3),151(3) is a single bit, and consequently, can be used to select 1 of 2 different antennas148(1) or148(2). If more than two transmit antennas148(1),148(2) are coupled to the antenna selector145, the antenna select bit150(3),151(3) will be more than one bit in order to provide more than two different transmit antenna selections, as readily appreciated by those skilled in the art.

If antenna select bit150(3),151(3) of a specific subcarrier takes a value of 0, then the channel symbol152,153for this subcarrier is sent to OFDM modulator146(1), while a placeholder of 0 would be sent to OFDM modulator146(2) for this specific subcarrier. Similarly, if antenna select bit150(3),151(3) of a specific subcarrier takes a value of 1, then the channel symbol152,153for this subcarrier is sent to OFDM modulator146(2), while a placeholder of 0 would be sent to OFDM modulator146(1). This is repeated in each transmitter chain140(1),140(2).

For each OFDM modulator146(1),146(2) receiving the placeholders, the number of non-zero inputs (subcarriers) is reduced on average to half as compared to conventional OFDM-MIMO transmitters, which results in a lower peak to average ratio of the resulting OFDM waveform. Even though the illustrated OFDM-MIMO transmitter100has only two transmitter chains140(1) and140(2), the present invention may be applied to an OFDM-MIMO transmitter with more than two transmitter chains, as readily appreciated by those skilled in the art.

As noted above, information bits transmitted by the transmit antennas148(1),148(2) in each transmitter chain140(1),140(2) are jointly represented by channel symbols152,153and the antenna selection bit150(3),151(3). As a result of the antenna selection bit150(3),151(3) selecting between different OFDM modulators146(1),146(2) and their corresponding transmit antennas148(1),148(2), a smaller signal constellation is required. This combination also requires less transmission power to achieve the same bit error rate (BER).

For comparison purposes, the signal mapper44inFIG. 1modulates the 3 channel bits50(1)-50(3),51(1)-51(3) for generating a channel symbol52,53. Since there are 3 information bits, the modulation may be 8 PSK. In contrast, the signal mapper144inFIG. 2is QPSK since each symbol is made up of 2 information bits150(1) and150(2),151(1) and151(2).

The signal mapper44inFIG. 1generates a signal constellation with 8 possible symbol points on an x-y plane, whereas the signal mapper144inFIG. 2only generates a signal constellation with 4 possible symbol points on the x-y plane.

For the signal mapper144to generate an equivalent 8 symbol points in the x-y plane, antenna pattern modulation is used. The antenna selector145selects one of 2 transmit antennas148(1),148(2) to transmit the channel symbol152,153. The 2 transmit antennas148(1),148(2) in each transmitter chain140have unique RF characteristics so that 2 different antenna patterns will be seen by the receiver. Since each of the 4 possible symbol points can be transmitted using one of two different antenna patterns, 8 possible symbols can be detected by an OFDM-MIMO receiver.

The signal constellation for the QPSK modulation used by the signal mapper62is significantly less than the signal constellation for the 8 PSK modulation used by the signal mapper20. Therefore, for the same transmission power, the minimum distance between two distinct signal points of QPSK constellation is much greater than the minimum distance between two distinct signal points of 8 PSK constellation. Consequently, to achieve the same BER, it will require less transmission power when QPSK modulation is used as opposed to when 8 PSk modulation is used.

In this specific example, the signal constellation size for the signal mapper144is one-half the size of the signal constellation for the signal mapper44. Nonetheless, the data rate for the data being transmitted from each transmitter10,100is the same. Under certain circumstances, the transmitter100in accordance with the present invention requires less transmission power to achieve the same quality of service or cover a wider range with the same quality of service.

The antenna patterns for the transmit antennas148(1),148(2) may be directional or omni-directional. When there is more than one omni-directional antenna, each omni-directional antenna will still be seen differently by the receiver due to multipath fading.

As will be now explained in greater detail, the transmitter100is required to periodically transmit know reference bits so that the receiver is able to differentiate between the different values of the antenna selection bit150(3) and151(3). These reference bits are referred to as pilot bits, for example. A block diagram of the receiver200for receiving the antenna modulated channel symbols is provided inFIG. 3.

For a receiver200to measure the RF characteristics of each transmit antenna148(1),148(2) associated with each OFDM modulator146(1),146(2) for each transmitter chain140(1),140(2), known reference bits are periodically transmitted by the transmitter100. These reference bits are referred to as pilot bits, for example.

A block diagram of the receiver200for receiving the antenna modulated channel symbols is provided inFIG. 3. In the illustrated embodiment, the receive antennas248(1),248(2) are coupled to respective OFDM demodulators246(1),246(2). The OFDM demodulators246(1),246(2) are fast Fourier transform (FFT) modules, for example, for converting the received signals from time domain to frequency domain. The OFDM-demodulated signals on each subcarrier include an antenna modulated channel symbol.

As noted above, the transmitter100periodically transmits known reference bits so that the receiver200is able to differentiate between the different values of the antenna selection bit150(3),151(3). These reference bits are referred to as pilot bits, for example.

For a signal received by each receive antenna248(1),248(2), it is applied as input to an OFDM demodulator246(1),246(2). The OFDM demodulator246(1),246(2) buffers the received signal to form a block of receive signals. The length of the receive signal block is determined by the total number of data subcarriers. The received signal block is a time domain signal and is converted into a frequency domain signal by the OFDM demodulator246(1),246(2). The output of the OFDM demodulator246(1),246(2) is passed to a plurality of demappers244(1)-244(N).

The output of the OFDM demodulator246(1),246(2) is also sent to a channel estimator250, where the channel estimator250extracts the known reference bits from the received signal. These reference bits, which may be pilot bits, for example, are used to estimate the radio frequency (RF) characteristics of the received signal associated with each transmit and receive antenna pair, as readily understood by those skilled in the art. The channel estimator250estimates the RF characteristics, and passes the estimated RF characteristics to the demappers244(1)-244(N).

The RF characteristics of the received signal associated with each transmit and receive antenna pair may include attributes such as amplitude, phase, delay spread and frequency response. The demappers244(1)-244(n) compute the Euclidean distance (ED) between the received signal and the estimated signals associated with each transmit and receive antenna pair, on a subcarrier basis. The demappers244(1)-244(N) choose the bits associated with the smallest Euclidean distance as the final output for each subcarrier.

The demappers244(1)-244(N) will now be described in greater detail while referring to the superimposed constellations as shown inFIGS. 4aand4b. Each of the demappers244(1)-244(N) see multiple superimposed constellations, each one being associated with a receive antenna248(1),248(2).

Assuming BPSK modulation is used for the channel symbols, and two transmitter chains140(1),140(2) are employed at the transmitter100, then each superimposed constellation at each receiver demapper244(1)-244(N) would see 16 signal points. Each of 16 signal points is uniquely associated with a bit sequence a1c1a2b2, where c1and c2are the respective channel symbols152,153from transmitter chains140(1) and140(2). Since BPSK is assumed, one channel symbol only consists of 1 bit. In the bit sequence, a1and a2are the respective antenna selection bits150(3),151(3) for the transmitter chains140(1) and140(2).

When the receiver200receives channel estimations from the channel estimators250, it reconstructs the signal constellation for each subcarrier of the receive antenna248(1),248(2).FIGS. 4a,4billustrate an example of reconstructed superimposed constellations for the receive antennas248(1) and248(2), on a specific subcarrier. If the RF channel characteristics are frequency selective, the reconstructed superimposed constellation would vary subcarrier to subcarrier.

Specifically,FIG. 4ais a reconstructed constellation for receive antenna248(1) andFIG. 4bis a reconstructed constellation for receive antenna248(2). The received signal324,326is also marked inFIGS. 4a,4b. For each receive antenna248(1),248(2), the demappers244(1)-244(N) calculate the distance between the received signal324,326and each of the sixteen candidates. In other words, for each subcarrier received by the receive antenna248(1),248(2), there would be sixteen distances, with each distance associated with a unique bit sequence a1c1a2b2.

The distance associated with the same bit sequence is then squared and summed across all receive antennas248(1),248(2). The bit sequence associated with the minimum total distance is then selected as final output of the demappers244(1)-244(N). In the illustrated example, the bit sequence0010330is selected since the total distance d=d1+d2is minimum among all possible bit sequences.

The distance between the received signal and the reconstructed signal point0000330is minimum for receive antenna248(1), as shown inFIG. 4a. However, the total distance between signal point0010332and the received signal330is minimum. Therefore, bit sequence0010332is selected instead of0000330.

Bits from the same spatial stream (e.g., same transmitter chain140(1) or140(2)) are then fed to a parallel-to-serial converter242, where parallel bits in multiple subcarriers are converted into a serial bit sequence. Serial data of all spatial data streams are then demultiplexed by a demultiplexer226into a single stream. The single stream is then fed to a deinterleaver222, a depuncturer218and a decoder214.

The calculation of the Euclidean distance may depend on the RF characteristics. In the example ofFIGS. 4aand4b, RF characteristics between each transmit and receive antenna pair is represented by a complex channel gain.

In the aforementioned example, a hard bit value is output from the demappers244(1)-244(N). To facilitate channel decoding, it is necessary for the demappers244(1)-244(N) to output a soft bit value under certain circumstances. Still referring to the OFDM-MIMO receiver200illustrated inFIG. 3, signals from the different receive antennas248(1),248(2) but same subcarrier are fed into demapping blocks244(1)-244(N), where soft bits, including both channel bits (e.g.,152and153) and antenna selection bits (150(3) and151(3)) are calculated. One example of a soft bit is a log-likelihood ratio (LLR) of coded bits. If the bit of interest is b0, then the LLR of b0is defined as:

Before calculating LLR, the receiver200first makes channel estimations via the channel estimators250using known reference signals that are regularly transmitted from the transmitter100. Soft bits corresponding to the same spatial stream (e.g., data streams128,130) are then fed to a respective parallel-to-serial converter242, which converts the parallel data of all the subcarriers into serial data. Serial data of all spatial data streams are then demultiplexed by a demultiplexer226into a single stream. As discussed above, the single stream is then fed to a deinterleaver222, a depuncturer218and a decoder214.