<font color=Orange>Multiband OFDM moves to higher data rates
By Jaiganesh Balakrishnan, Anuj Batra and Anand Dabak
EE Times </font>
May 12, 2004 (5:28 PM EDT)
The IEEE 802.15.3a task group is developing an alternative wireless personal-area network physical layer to achieve rates of 110 Mbits/second at a distance of 10 meters. One of the options for this layer is the multiband orthogonal frequency-division multiplexing (OFDM) ultrawideband proposal. The multiband OFDM system divides the UWB spectrum (3.1 to 10.6 GHz) into 528-MHz-wide subbands and uses OFDM modulation to transmit the information in each subband. The OFDM symbols are then interleaved over three contiguous subbands across both time and frequency to provide a robust link and maximum range to support multiple access between piconets. Currently, the multiband OFDM proposal defines data rates from 55 up to 480 Mbits/s and restricts the constellation size to quadrature phase-shift keying (QPSK).
To satisfy the growing demand for higher data rates, this technology will need to scale to over 1 Gbit/s. Although enhancements required to achieve that scaling improve the spectral efficiency and overall robustness of the multiband OFDM system, some implementation issues must be considered first.
Today, the multiband OFDM system supports a maximum 480-Mbit/s data rate using a QPSK constellation and a 3/4 convolutional-code rate. By simply increasing the constellation size to 16 quadrature amplitude modulation (Q-AM), the supported data rate doubles, to 960 Mbits/s. To obtain the same performance, a 16-QAM constellation re-quires about 3.9 dB of additional link margin when compared with a QPSK symbol mapping. The increase in system complexity from using a 16-QAM constellation is minimal.
The multiband OFDM system achieves data rates of 480 and 960 Mbits/s over a line-of-sight range of 2.9 and 1.8 meters, respectively. For nonline of sight, the range drops to 2.6 and 1.6 meters. The range for additive white Gaussian noise at the same data rate is 8.9 and 5.6 meters. This range — called the 90 percent outage range — is the distance where the multiband OFDM system has a less than 8 percent packet error rate for 90 percent of the multipath-channel realizations. The range assumes 0-dBi antennas, free-space propagation, a CMOS implementation with a 6.6-dB noise figure referenced at the antenna and a 2.5-dB implementation loss.
The channel model specified in "A Multi-band OFDM System for UWB Communication" (J. Balakrishnan, A. Batra and A. Dabak, Proceedings of the IEEE Conference On Ultra Wideband Systems and Technologies, November 2003) is used to determine the range in the multipath-channel environments. The simulations include the effects of transmit clipping, shadowing, front-end filtering, analog-to-digital quantization, packet acquisition, channel estimation, carrier/time tracking and a 20-ppm crystal mismatch between the transmitter and receiver.
In the late 1990s, multiple antennas at the transmitter and receiver were proposed to enhance the data rates and robustness of narrowband systems in flat-fading channel environments. At the transmitter of a 2 x 2 multiple-input, multiple-output (MIMO) extension of the multiband OFDM system, the encoded bits are interleaved and divided into two parallel bit streams. Each bit stream is mapped onto frequency tones at the inverse fast Fourier transform input. The output streams of the IFFT are transmitted simultaneously on two antennas. At the receiver, the signal received on each antenna is processed separately until the FFT output is obtained.
In a heavy multipath environment, the OFDM system converts a frequency-selective channel into a bank of parallel flat-fading channels. Therefore, the signal vector y from the two streams, corresponding to a given subcarrier k, can be mathematically represented as shown.
In the formula below, si is the transmitted symbol; hij represents the channel coefficient, corresponding to subcarrier k, from transmit antenna i to transmit antenna j; and n is the noise vector. For optimal performance, the symbol vector s would be jointly demodulated for each subcarrier. The two data streams can be demodulated if the channel matrix H is full-rank. Heavy multipath environments like that of UWB usually satisfy this condition. To enhance performance, an optional space-time block code can be introduced between the channel encoder and the IFFT.
A plot of a 2 x 2 multiband OFDM system's capacity shows that at 960 Mbits/s, a range of 7 meters can be achieved. In addition, a 2 x 2 multiband OFDM system effectively allows doubling of the data rate without sacrificing range. The increase in data rate and robustness, however, comes at the expense of more complexity.
For example, a 2 x 2 system would need an additional transmitter and receiver chain. Also, the digital complexity for a 2 x 2 system is effectively double that of a 1 x 1 system. Nevertheless, advances in technology will make 2 x 2 MIMO a realizable approach in the near future.
Channel bonding provides an alternative to the previous two techniques and offers a different trade-off between high-data-rate extensions and complexity. Processing information over a single subband restricts the data rates the multiband OFDM system can support to a maximum of 640 Mbits/s.
By using all three bands simultaneously, the data rate can be increased by a factor of three. This translates to rates of up to 1.44 Gbits/s with QPSK modulation and a 3/4 convolutional code rate.
Given that the duty cycle for the OFDM symbols in each subband increases by a factor of three for the channel-bonded system, the resulting transmitted power per symbol would drop by 4.8 dB. So the 90 percent outage range in multipath channel environments is expected to be 1.5 meters.
The transceiver's analog baseband would need to be modified to handle a larger bandwidth, the analog-to-digital converters would need to operate at a higher speed, and the complexity of the digital baseband would increase by a factor of three. The frequency-synthesis circuitry would be simplified, however, since it would no longer be needed to switch between subbands.
Jaiganesh Balakrishnan (jai@ti.com) and Anuj Batra (batra@ti.com) are members of the technical staff and Anand Dabak (dabak@ti.com) is distinguished member of the technical staff and manager of the mobile wireless branch of Texas Instruments Inc.'s DSPS Research and Development Center (Dallas).
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