Introduction With the increasing popularity of wireless technology, people are increasingly demanding consumer electronics. According to demand, these devices can be divided into two major camps: (1) Indoor wireless video playback (compressed or uncompressed) (2) High-speed connection of low-power handheld devices. In terms of video playback applications, it provides different users with relatively high data transmission rates, strong performance, and low power consumption requirements (because video sources and displays are generally connected to external power supplies). On the contrary, handheld devices have high requirements for low cost and low power consumption. At the same time, high-speed data conversion requirements can be extended to extremely high data transfer rates (1 Gbps and higher).
In this article, we will introduce two wireless technologies that can meet the needs of these applications, direct sequence ultra-wideband technology (DS-UWB, which is the first UWB standardization proposal proposed by the IEEE organization) and 802.11 wireless LAN technology (802.11 a / g and 802.11n on this basis is still in the research and development stage). In the comparison of these technologies, we can find the obvious difference between DS-UWB and WLAN technologies, that is to say, ultra-wideband technology will induce different solutions and different levels of power consumption efficiency, because these two technologies have been improved. To meet people's demand for high data rate handheld devices.
Introduction to the two technologies DS-UWB was developed for wireless personal area network (WPAN) and draws on the strengths of ultra-wideband (UWB) communication technology. Currently, the DS-UWB solution being considered by the IEEE organization will enable devices based on the 802.15.3a standard to provide both high performance and low-power and low-cost expansion capabilities for high-speed multimedia and handheld devices.
The application of DS-UWB equipment will be based on the FCC's UWB regulations for the US market and will be basically consistent with the regulations for the rest of the world. The current FCC regulations allow the use of 7.5 GHz spectrum (
DS-UWB equipment will occupy 1.5 or 3 GHz spectrum at any time), but its transmission power is very low, please refer to the chart. In fact, the transmission level of DS-UWB basically reaches the limit of -41.3 dBm per MHz spectrum.
Compared with DS-UWB, 802.11 wireless local area network technology is developed for operation under different FCC rules, and can be operated on channels that specifically target unauthorized wireless devices. The 802.11a / g / n operating bandwidth is narrower than DS-UWB, and the occupied spectrum is about 17 MHz (or 80 times lower than the bandwidth of DS-UWB, or 40 times lower than the 11n system using 34 MHz), but Transmission power consumption is higher. The nominal transmission power consumption of an 802.11 device is about 50 mW. Please note that this level of average transmission power consumption is 500 times higher than the average power consumption of a DS-UWB device, or the difference is 27 dB.
The above two different characteristics as well as signal bandwidth and transmission power consumption have led to very different aspects of communication system design.
We note that there is a 500-fold difference in transmission power consumption between these two technologies (but not in terms of total power consumption, namely: the power emitted by the antenna and the power consumed by all circuits). What caused such a big difference? A basic rule of the communication system is that the received signal power is attenuated in a relatively close range-does this mean that 802.11 technology has a range of _ (500) or about 22 times the same data transmission rate? In fact, DS-UWB technology provides a range of 110 m or more suitable for 110 Mbps in a multi-channel environment-this is roughly equal to the range provided by 802.11a / g technology for its maximum rate of 54 Mbps. 802.11n extension technology can also provide 100 Mbps or higher in these ranges, but the specific range depends on the antenna and multi-channel hypothesis. So what other than the factor of transmission range has caused such a large difference in transmission power? By understanding the internal causes of this difference, we can further understand the fundamental differences between DS-UWB and 802.11 technologies, and require us to consider many aspects of wireless system design and performance.
Signal bandwidth and transmission power Compared with DS-UWB, there are two basic reasons why the difference in signal bandwidth causes the 802.11a / g / n system to have higher requirements for transmission power. One is due to the relatively high data transmission rate obtained through modulation in a relatively narrow radio channel; the second is due to the basic physical characteristics of radio frequency propagation in multiple channels. The modulation format describes how to encode data into a radio frequency signal for transmission in a wireless medium.
For an 802.11a / g system, to obtain a data rate of 54 Mbps in a 17 MHz bandwidth radio frequency channel requires the use of "high-order modulation" to achieve higher spectral efficiency. In particular, 802.11a / g (and 11n) uses 64-QAM to draw 6 digits into each transmission symbol (802.11a / g combines this 64-QAM with OFDM, and the intention is roughly the same). When introducing bandwidth consumption for forward-error-correcTIon (FEC) and OFDM guidance signals and prefixes, 802.11a / g obtains approximately 3.3 digits per second for each occupied spectrum Hertz. By using 64-QAM to achieve this higher spectral efficiency, the cost is that the receiver requires a stronger SNR in order to demodulate the signal at the same level of error rate performance (relative to the BPSK or QPSK as the bottom limit System). The newly introduced 802.11n technology also uses 64-QAM at its highest data rate, but adds more mature technology to achieve better spectrum efficiency through multi-antenna technology.
DS-UWB operating environment is different from 802.11a / g or 801.11n technology. Because it can obtain a wider bandwidth, DS-UWB uses BPSK to provide demodulation of power coefficients. A simple comparison between the two technologies is the difference in efficacy between BPSK and 64-QAM. For these two modulation formats, the Eb / N0 required by BPSK at the receiver is 9.6 dB and the rate is 10-5 bit-error-rate (BER), while 64-QAM can obtain 10 dB higher on the same BER s level. It should be noted that these numbers are used to describe the operating state of the non-coding technology in the pure AWGN channel, but the basic result is that the high order modulation method requires higher transmission power to provide the same BER on the receiver. In the actual operating system, there are many other factors that affect the receiver SNR requirements, including the use of mature FEC. A key environmental factor that has an impact on actual operating system requirements is multi-channel propagation efficiency.
The effect of signal bandwidth on complexity and power consumption We know that narrow-band systems need to have higher transmission power to support the higher SNR requirements of the receiver because different modulation methods require higher modulation and multi-channel attenuation. For OFDM, the effect of higher transmission power is mixed with the ratio of the high peak to the average value of the OFDM signal, because the latter requires a low-power power amplifier. For example, an output of 50 mW transmission power may require a total power consumption of several hundred to 500 mW to achieve the linearity required for better system performance. On the contrary, no DS-UWB system needs a PA, because the smaller transmission power (-10 dBm) can be directly driven by the RF ASIC.
Different signal bandwidths also have other effects on system complexity and power consumption because of differences in signal processing requirements.
_Analog-to-digital converter: DS-UWB receivers can use low-resolution (eg, 3-bit) ADCs at high rates (1.35 GHz) to simulate broadband signals. 802.11 OFDM systems use high-resolution ADCs at lower rates (9 bits at 80 MHz) to support 64-QAM demodulation.
_ Forward error correction: Both methods use convolu code (convoluTIonal code) to correct the transmission of digital errors. 802.11a / g / n uses higher complexity FEC to compensate for multi-channel attenuation. DS-UWB encoding can reduce the complexity of decoding (2-8 times lower), because the performance of the encoding is less affected by multi-channel attenuation under ultra-wideband operation. This difference is more pronounced when devices reach 500Mbps or higher rates in DS-UWB or implement 802.11n.
When we are considering DS-UWB or 802.11n to a higher rate to meet the other effects of future applications, we need to understand how to increase the DS-UWB to increase the symbol rate (shorten the symbol length) Higher speeds, such as 1 Gbps. The digital processing complexity of most receivers (slope combining, equal sign, FEC decoding, etc.) increases linearly with the data rate. The requirements for the equalizer length can increase as the symbol length decreases, but this effect will be resolved when the delay propagation is increased by a smaller range in the highest data rate mode.
The current recommendation to upgrade 802.11 systems to higher rates in 802.11n (500 Mbps or higher) is based on continued use of 64-QAM. Through MIMO technology (multiple input and output) we can increase to a higher rate, because it uses multiple antennas to send multiple data streams in parallel in the wireless channel. In this regard, the complexity of processing also increases ((FEC decoding, FFT / iFFT, equalization, etc.). Due to the requirement of up to 4 transmission / reception processing chains (multiple ADC / DAC, filters, amplifiers, etc.), the complexity And power consumption will also increase.
When we evaluate these two technologies in terms of high-speed, low-power applications, we find that the bandwidth of the system has a greater impact in many areas. As narrow-band designs are expanded to higher rates, the use of high order modulation and multi-antenna technology can provide extended and stronger performance, but may also cause greater complexity and power consumption. Those systems that use broadband, such as DS-UWB, can use completely different design methods to provide wireless connection solutions to achieve higher rates, more scalability and low complexity.
Chart: Power spectral density and bandwidth on different wireless technologies
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