Why multi-band OFDM is more suitable for high-speed UWB communication than direct sequence (DS) technology

Why multi-band OFDM is more suitable for high-speed UWB communication than direct sequence (DS) technology

Part 1: Bandwidth and multipath

1 Introduction

About three years ago, the US Federal Communications Commission issued a historic Report & Order report that allocated the 7,500 MHz spectrum from 3.1 GHz to 10.6 GHz for ultra-wideband (UWB) devices. (Figure 1: UWB device emission limits). This report brings many opportunities for innovation and technological progress for industry and academia. The industry has also established various organizations, such as the Multi-band OFDM Alliance (MulTI-band OFDM Alliance), UWB Forum (UWB Forum), and IEEE 802.15.3a The special working group hopes to establish a common standard for UWB technology.

With UWB spectrum, manufacturers can develop new wireless personal area network (PAN) technology and achieve high-speed wireless communication with low power consumption and low cost. UWB promises to provide various data rates in a real-world multipath environment, which extends from 110 Mbps at a distance of 10 meters to 480 Mbps at a distance of 2 meters; in fact, only UWB technology can only be used in the near future Satisfying consumers' endless demands for higher data rates, and can continue to be a low-cost and low-power solution.

From an application point of view, one of the main driving forces of UWB technology is its ability to "cable-free"; with the assistance of UWB technology, it is possible for consumers to purchase a high-definition TV and connect it to power immediately This TV can be compatible with various devices in the home entertainment center (for example: DVD, set-top box, personal video recorder, PlayStaTIon game console, etc.) without connecting any cables. The main reason why UWB can do this is that it can support very high data rates, and the use of network connection quality (QoS) technology to bring this high data rate indoors; in fact, the initial engineering samples of UWB solutions have The advent of many products has also appeared in consumer electronics in 2005.

Figure 1: UWB emission limits for indoor and handheld devices

2. The challenge of designing UWB system

Using the 7,500 MHz spectrum brings some challenges to the design of high-speed wireless UWB communication systems. The most difficult parts are the design of radio frequency circuits (low-noise amplifiers and mixers), analog fundamental frequency circuits (channel selection filters and variable gains) Amplifiers), digital-to-analog converters (DACs) and analog-to-digital converters (ADCs). Because the Federal Communications Commission requires that the bandwidth be at least 500 MHz at any time, these circuits must support a bandwidth of at least 500 MHz, and may even expand all the way up to 7,500 MHz. The greater the bandwidth supported by these circuits, the more difficult the design and the increased power consumption. These factors show that the ideal approach is to use a smaller bandwidth. However, choosing a smaller bandwidth will damage the transmission power (Figure 1: clearly shows that the transmission power is directly related to the bandwidth usage), so when designing UWB systems, it is necessary to transmit power and RF, analog and mixed signals Make an appropriate trade-off between the complexity of the circuit.

Because the length of the communication system's online distance is basically determined by the amount of multipath energy collected, another important challenge in the design of high-speed UWB communication systems is to collect the most energy from multiple paths. Several different technologies, such as equalizers and RAKE receivers in single-carrier system equalizers and cyclic prefixes or zero-padded prefixes in multi-carrier systems, can be used from Collect energy from multiple paths. These technologies are usually operated in the digital field, so that they can use increasingly sophisticated silicon chip technology. With different operating environments, when choosing a single-carrier or multi-carrier system, there must be trade-offs in complexity. The application and multi-path environment will determine which method can provide a solution with lower complexity.

The main disadvantage of allocating 7,500 MHz bandwidth to UWB is that it will cover multiple frequency bands for other uses, including U-NII and WiMax. Designers must consider how to deal with interference caused by other devices in the frequency band, while controlling their own interference to these devices. In addition, the noise emitted by out-of-band devices is also a problem that all UWB designers must solve. Therefore, UWB systems must have strong narrow-band noise resistance and coexist with existing and future devices.

At present, only the United States has completed the configuration and specification of UWB spectrum, which is another challenge of UWB system design. Europe, Japan, South Korea and other parts of the world are configuring spectrum for UWB products, but the relevant specifications have not yet been finalized. The final spectrum configuration and emission limit values ​​determined by these regions may be different from those set by the United States, so the system design must be Provide sufficient spectrum flexibility. It is best to complete the spectrum adjustment as long as a simple software modification, so that a set of solutions can be applied around the world without modifying any hardware.

Finally, from a market perspective, the UWB solution must meet low cost, and its power consumption should also meet the requirements of the target application. These are the keys to the UWB system's success in the market.

In summary, to design a successful high-speed UWB communication system, these challenges include:
1. Bandwidth optimization: trade-off between radio frequency, analog and mixed signal design and transmission power;
2. Multi-path energy collection;
3. Strong ability to resist narrow-band interference and coexist with existing and future devices;
4. Spectrum flexibility and compatibility with global telecommunications regulations; and
5. Complexity and power consumption limitations.
3. Overview of pulse radio and multi-band OFDM UWB systems

The traditional UWB communication system design method is to use impulse radio (impulse radio), it will put the information into a very narrow time domain pulse, the use of a very narrow time domain pulse is to produce a wide enough spectrum (usually equal to or greater than 2,000 MHz ). Before encoding the information bits into a very narrow time-domain pulse, the information is usually scattered through a long quasi-random sequence or mapped to a multi-dimensional multi-signal group (mulTI-dimensional constellaTIon), for example, using M- ary two-bit orthogonal keying signal group (M-ary binary orthogonal keying constellation). The main advantage of using spread spectrum technology to develop UWB communication systems is that these techniques are widely understood and proven in other commercial technologies such as IEEE 802.11b. The UWB Forum is a supporter of pulsed radio UWB technology, and the UWB technology developed by it is also commonly referred to as single-carrier direct-sequence UWB technology.

The latest UWB communication system design method is based on multi-band orthogonal frequency modulation (multi-band OFDM) technology, which divides the frequency spectrum into several sub-bands, while making each sub-band width slightly larger than 500 MHz To meet UWB signal requirements. Then it will use a time-interleaved method to transmit information in a very narrow time-domain OFDM symbol in each sub-band (please refer to Figure 2), so that at any time, the transmission signal is limited to only one Sub-band. The main advantage of using multiple sub-bands to transmit symbols in a time-interleaved manner is that UWB systems can transmit the same average power-as if they were using the entire bandwidth, that is, the frequency obtained by multiplying the bandwidth of the sub-band by the number of previous band Wide value. The advantage of multiple frequency bands is that the instantaneous processing bandwidth is very small (about 500 MHz), which not only increases the flexibility of spectrum use and compatibility with global telecommunications regulations, but also lowers power consumption and costs. Therefore, the reason for adopting OFDM is that the efficiency of its receiver to collect multipath energy is higher than that of a single carrier system using the same bandwidth. Multi-band OFDM Alliance (Multi-band OFDM Alliance) is the promoter of multi-band OFDM (MB-OFDM) UWB technology.

Both technologies have been submitted to the IEEE 802.15.3a working group for review, and will be compared based on some of the challenges listed in Section 2, such as their bandwidth, multipath energy collection capabilities, and receiver complexity. These two methods. In the follow-up article, we will further compare the two methods based on the other challenges listed in Section 2.

Figure 2: An example of using multiple sub-bands to transmit OFDM symbols in a time-interleaved manner

4. Comparison of MB-OFDM and DS-UWB systems

In this chapter, we will compare the multi-band OFDM (MB-OFDM) system with the direct sequence UWB (DS-UWB) system in terms of bandwidth optimization, multipath energy harvesting capabilities, and receiver complexity. In particular, the single-carrier direct sequence UWB system uses a 16 finger rake receiver at a chip rate of 1,368 MHz; the multi-band OFDM system uses a 128-point fast Fourier transform (FFT), a 60.6 ns zero-padded prefix, and 507 MHz instantaneous operating bandwidth and 1,521 MHz average operating bandwidth.

4.1 Bandwidth selection

Basically, the average operating bandwidth used by DS-UWB and MB-OFDM systems is the same, so if you ignore the slight changes in the spectrum, the transmission power of the two technologies is actually the same. The main advantage of the MB-OFDM solution is that its instantaneous bandwidth will not exceed 528 MHz at any time, which means that the bandwidth that the fundamental channel selection filter and variable gain amplifier must support will be much lower than the DS-OFDM bandwidth. UWB solution (about three times the difference); In addition, the mixed signal circuit of MB-OFDM, especially the digital-to-analog converter and the analog-to-digital converter, can also work at a lower rate than the DS-UWB solution. Although the MB-OFDM solution requires one more number of bits than the DS-UWB solution, the power consumption of the analog-to-digital converter can be reduced by at least 1.5 times.

This shows that the bandwidth performance of the MB-OFDM solution is clearly superior to the DS-UWB solution.

4.2 Multi-path energy collection

The multi-path channel environment is quite challenging for the design of wireless communication systems. Because the efficiency and robustness of wireless communication systems are usually determined by the amount of multipath energy that the receiver can collect; and the UWB channel model may be highly divergent in the range of 4 to 10 meters. The channel environment of the general non-visible range usually has an RMS delay spread of 14 ns, and the worst-case channel environment has a rms delay spread of 25 ns.

With different types of systems, receivers usually use two methods to collect multipath energy: either use a rake receiver in a single carrier system, or add a cyclic prefix or a zero-fill prefix at the front end of the transmitted code. Multi-carrier receivers such as OFDM usually take the latter approach. In most UWB communication systems, when the reciprocal of the sampling rate is always much smaller than the total delay spread time, the OFDM system is more attractive than the single-carrier solution, especially when considering the digital complexity.

The performance of a single carrier system is limited by two effects in a highly divergent channel. First, it requires a considerable number of RAKE fingers to collect enough multipath energy. Secondly, the time divergence characteristics of the channel will cause interference between symbols (Inter-Symbol Interference, ISI for short), which in turn leads to a decrease in system performance. Although the use of an equalizer can reduce the impact of inter-symbol interference, it needs to be at the cost of higher computational complexity.

Let r (n) be equivalent to the discrete sampling receive sequence at the fundamental frequency, then it can be written as:

Where s (n) is the transmission sequence, h (k) is the channel impulse response of length L, and w (n) is the noise sequence. Let y (n) represent the output of the rake receiver, which covers a total of L coefficients, of which only M fingers are not equal to zero, and let δ (k) represent M delay times, respectively corresponding to the L-tap rake receiver The non-zero coefficient of the response function f (n), then:

If the receiver response matches the channel impulse response exactly, the receiver can collect all the multipath energy, but it also means that M must be equal to L, which will make the design of the rake receiver more complicated.

Figure 3 shows the multi-path energy loss and signal-to-ISI ratio of the DS-UWB system as a function of the total number of RAKE fingers. The figure shows the 90th percentile in the channel This is equivalent to a channel environment (CM3) with a non-visible range of 4 to 10 meters, and a data rate of 114 Mbps. The figure also assumes that the selected RAKE finger can obtain the maximum multipath coefficient within a distance of approximately 40 μs. It can be seen from Figure 3 that even with the ideal 16 finger RAKE, the DS-UWB system can only collect 56% of the multipath energy; when using the 16 finger rake receiver, the inter-symbol interference is only 9 dB less than the signal energy , Which will cause a decrease in system performance. It is worth noting that when the data rate is doubled, the processing gain is reduced by half, which increases the inter-symbol interference by 3 dB. For example, for a 16 finger rake receiver, the inter-symbol interference at a data rate of 200 Mbps is about the signal energy It is 5 dB smaller, so without the aid of an equalizer, the signal-to-noise ratio (SINR) will not be enough to allow the information bits to be successfully decoded.

Figure 3: The 90th percentile multipath energy collected by the DS-UWB system in a channel environment of 4 to 10 meters in the non-visible range

An OFDM system only needs to use a low-complexity receiver to provide strong resistance to multipath divergence. The inherent characteristic of this system is due to the addition of a cyclic prefix or its equivalent zero-fill prefix. In order to support zero-filled prefixes, the only change the receiver has to do is to collect additional sample values ​​of the same length as the prefix, and then use overlap-and-add to obtain the characteristics of circular convolution. We can prove that when a receiver using a cyclic prefix performs a linear convolution operation with the channel impulse response, the result is equivalent to performing a circular convolution. Since the circular convolution operation in the time domain is equivalent to the multiplication operation in the discrete Fourier domain (DFT domain), a single-tap frequency domain equalizer is sufficient to eliminate the effect of the multipath channel on the transmitted signal.

The length of the zero-fill prefix determines the multipath energy it can collect. Multi-path energy outside the ZP window will cause inter-carrier interference (ICI), so choose the zero-fill prefix For the length, the impact of inter-carrier interference should be minimized, the multipath energy collected should be maximized, and the additional processing burden caused by the cyclic prefix should be reduced. (Figure 4: How the length of the zero-filling prefix affects the energy collected by the receiver in the channel environment of 4 to 10 meters in the non-visible range, and the inter-carrier interference caused by the multipath energy outside the zero-filling prefix interval). Comparing with the zero-filling prefix length of 60.6 ns in the MB-OFDM solution, the 90th percentile channel realized in this channel environment can collect about 95% of the multipath channel energy.

Figure 4: Representing the multipath energy collected in a 4 to 10 meter, non-visible range channel environment as a function of cyclic prefix length

It can be seen that the multi-path energy collection efficiency of the MB-OFDM solution clearly outperforms the DS-UWB solution.

4.3 The complexity of the receiver

Receiver complexity is an important parameter when selecting the physical layer. The complexity of a single carrier system increases linearly with the number of RAKE fingers and receiver sampling rate. For DS-UWB systems, each chip of an M-finger rake receiver must perform M complex multiplications, so 16 finger rake receivers that sample at the chip rate must perform on average every nanosecond The complex multiplication of 21.9 times is only for the complexity analysis of the DS-UWB system, and does not include the complexity required to introduce a high-speed adaptive equalizer. Single-carrier systems with high information data rates (> 200 Mbps) usually require this type of equalizer.

The complexity of the OFDM system changes logarithmically with the size of the Fast Fourier Transform (FFT) log2 (N) ï‚´. If N-point Fast Fourier Transform is used, each OFDM symbol needs to be executed (N / 2) times complex Operation. Due to the influence of the zero-filling prefix, the OFDM symbol usually exceeds N sample values. For multi-band OFDM systems, the Fast Fourier Transform only needs to perform 1.48 complex multiplications per nanosecond, while the single-tap frequency domain equalizer requires an additional 0.42 complex multiplications per nanosecond, so the entire receiver only needs to perform per nanosecond 1.9 multiplications, which is ten times less complex than the DS-UWB system. In addition, multi-carrier systems do not have to increase their complexity in order to increase the data rate of information.

In terms of highly divergent channels, MB-OFDM receivers are much more efficient than DS-UWB solutions in terms of multipath energy collection.

5 Conclusion

The historic Report & Order report of the Federal Communications Commission has opened many new research areas for the technological development of ultra-high-speed wireless personal area networks. This article discusses several important challenges that must be overcome to successfully build UWB systems. We also compared the two main candidate technologies for high-speed UWB communication systems based on the requirements introduced in the second section. The results prove that the MB-OFDM solution is more suitable for high-speed UWB communication systems than the DS-UWB solution.

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