Comprehensive UWB product testing: Part 2

Due to the wide spectrum of UWB, frequency-dependent tilt (Figure 2) severely compromises the dynamic range of the link. Since RF attenuation increases with frequency, the wider the frequency band the more tilted the receive spectrum and the more dynamic range is lost to receive equalization or transmit pre-distortion.

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Figure 2: Channel tilt— the wider the channel the greater the attenuation tilt between high and low frequencies in the channel. To correct the tilt distortion, equalization can be performed in the receiver or reverse tilt pre-distortion can be done in the transmitter.

WiMedia
The WiMedia specification broke up the available UWB spectrum into 5 Band Groups that are further subdivided into 528 MHz sub-bands (Figure 3). Data transmissions can be frequency hopped among the three sub-bands to reduce the average transmit power while maximizing the instantaneous power of symbol transmissions.

For example, the OFDM signal can be pulsed in the time domain over any of the 3 frequency sub-bands with one third duty cycle, thereby reducing the average transmit power by a factor of 3 or 4.77 dB. The WiMedia techniques for spreading the power include what WiMedia calls Time-Frequency Interleaving (TFI) and Fixed Frequency Interleaving (FFI).

TFI is essentially a technique of frequency hopping the 528 MHz wide OFDM pulses over three bands. The FCC relaxed the -41.3 dBm/MHz limit to -36.5 dBm/MHz for peak power in the 528 MHz sub-bands since the 1/3 duty cycle averages to -41.3 dBm/MHz.

To avoid the UNII band 5.8 GHz interference from Wi-Fi, the current generation of WiMedia products operate in Band Group 1.

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Figure 3: WiMedia MB-OFDM channel assignment in the 3.1 to 10.6 GHz band. Most existing products support Band Group 1. The 528 MHz OFDM sub-bands in each Band Group can be used to interleave the signal and spread its power.

WiMedia uses MB-OFDM with data rates of 53.3, 80, 106.7, 160, 200, 320, 400 and 480 Mbps. QPSK modulation is used for data rates up to 200 Mbps and DCM (dual-carrier modulation) is used for data rates of 320 Mbps and higher.

On the TX side a single 4 to 6 bit DAC running at 1 GHz is typically used to generate the 528 MHz TX spectrum and on the RX side two 4-bit, 1 GHz A/D converters (one for "I", the other for "Q" component) are typically required to detect and recover the MB-OFDM sub-carriers. One only has to look at the power consumption for these components alone to see this is not a low power technology and that it has substantial complexity in both the TX and RX sections.

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Figure 4: Example of the WiMedia Band Group 1 spectrum showing 3 sub-bands (adapted from an FCC report). For UWB spectrum measurement Agilent has provided E4440A PSA Series Spectrum Analyzer and ETS Lindgren has provided the Model 3117 Double-Ridged Waveguide Horn antenna. Both the analyzer and the antenna cover the entire 3.1 to 10.6 GHz range.