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DS-UWB vs. 802.11n: What's the best connectivity option?

Posted: 12 Sep 2005 ?? ?Print Version ?Bookmark and Share

Keywords:Freescale?

System complexity and power consumption

For OFDM, the impact of higher transmit power is compounded by the fact that the high peak-to-average ratios of the OFDM signals require a power amplifier (PA) that operates at relatively low power efficiency. For example, a 50mW transmit power output might require several hundred to 500mW of total power consumption to achieve the linearity required for good system performance. In contrast, there is no need for a PA in a DS-UWB system, as the small amount of transmit power (-10dBm) can be driven directly by the RF ASIC.

The different signal bandwidths have other effects on system complexity and power consumption due to differences in signal processing requirements:

Analog-to-digital converters: DS-UWB receivers can use low resolution (e.g. 3 bits) ADCs at high rates (1.35GHz) to sample the wideband signals. 802.11 OFDM systems typically use high resolution ADCs at lower rates (e.g. 9 bits at 80MHz) to support demodulation of 64-QAM.

Forward Error Correction: Both approaches use convolutional codes to correct bit errors caused during transmission. 802.11a/g/n uses higher complexity FEC to compensate for multipath fading. DS-UWB codes enable lower decode complexity (2-8x lower) since the code performance is less impacted by multipath fading due to the ultra-wideband operation. This difference can become more significant as devices are scaled to 500 Mbits/s or higher for high-rate DS-UWB or 802.11n implementations.

When considering other effects of scaling DS-UWB or 802.11n to higher rates for future applications, it is helpful to understand that DS-UWB scales to rates of 1Gbps or higher by increasing the symbol rate (shortening symbol length). Most of the receiver digital processing complexity (rake combining, symbol equalization, FEC decode, etc.) increases linearly with the data rate. Requirements for equalizer length can increase with decreasing symbol length, however this effect may be mitigated by natural scaling to lower delay spreads at the shorter ranges supported by the highest data rate modes.

Current proposals for scaling 802.11 systems to higher rates (500Mbps or more) in 802.11n are based on the continued use of 64-QAM. Scaling to higher rates will be enabled through the use of multiple-input-multiple-output (MIMO) techniques that use multiple antennas to send multiple data streams in parallel through the wireless channel. For this approach, the processing complexity also increases with data rate (FEC decode, FFT/iFFT, equalization, etc). There will also be increased complexity and power consumption due to the requirement for up to 4 transmit/receive processing chains (multiple ADC/DAC pairs, filters, amplifiers, etc).

As digital process technology scales, the digital portions of each system will scale much faster to lower cost and power. The significant analog potions of the system will scale more slowly and will thus have a proportionally bigger impact when these functions represent a larger portion of the implementation. The power consumption and area required for large ADCs and linear PAs becomes a bigger factor as digital technology scales in the future.

As we evaluate the two technologies for very high rate, low power applications, we see that the impact of system bandwidth is significant in many areas. As the narrowband designs are extended to higher rates, the use of high order modulation and multiple-antenna technologies can provide scalable and robust performance, but will also likely lead to increased complexity and power consumption. Systems that use wider bandwidths, such as DS-UWB, can use fundamentally different design approaches to provide wireless connectivity solutions that scale to even higher data rates with more scalable and lower complexity implementations.

References:

Coffey, Sean et al. "WWiSE IEEE 802.11n Proposal," January 6, 2005, IEEE document number 802.11-05-1591r3.

Kohno, R.; McLaughlin, M.; and Welborn, M. "DS-UWB Physcal Layer Submission to 802.15 Task Group 3a," IEEE Document number 802.15-04-0137r4.

Meng, Theresa et al. "Digitally Assisted Analog Circuit Design for Communication SoCs," SiPS 2004 Keynote Presentation, www.spsworkshops.com/SiPS2004/Keynote_Slides_Meng.pdf.

Mujtaba, Syaed et al. "TGn Synch Complete Proposal," January 18, 2005, IEEE document number 802.11-04-888r8.

About the author

Matt Welborn is a wireless architect in Freescale Semiconductor's UWB group. Prior to Freescale, he was the wireless systems architect and standards technical lead at XtremeSpectrum. Matt can be reached at matt.welborn@freescale.com.

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