Test Solution for UWB System

How UWB signals are generated

Baseband pulse form

The baseband pulse form is the earliest signal form used in UWB communication. It uses baseband pulse sequences with pulse widths in the ns and sub-ns levels for communication, and usually carries information through modulation methods such as pulse position modulation (PPM), pulse polarity modulation or pulse amplitude modulation (PAM). The pulse can use different waveforms, such as Gaussian waveforms, raised cosine waves, etc., and the duty cycle is also very small, so it has strong multipath channel resolution and anti-multipath performance. Because there is no need to modulate the carrier and local oscillator, the transceiver structure is simple and the cost is low. At the same time, the power consumption of the system is much lower than that of traditional radio systems. In addition, this pulse signal has strong penetration ability, and the positioning and ranging accuracy is very high, which can reach the cm level. At the same time, the positioning function can be realized in motion. However, the baseband pulse contains more low-frequency components, so under the FCC's regulations on the power spectrum of UWB communication, the spectrum utilization rate is not high, but it can be improved through pulse waveform optimization design.

Pulse compression form

For the baseband pulse form, due to its narrow pulse width and low duty cycle, the signal energy is relatively small, which is not suitable for long-distance detection and communication. Therefore, in the military field, in order to maximize the detection distance, ultra-wideband signals in the pulse compression mode have been widely used, and its basic forms of expression include linear frequency modulation. For the linear frequency modulation pulse compression system, the corresponding linear frequency modulation can be achieved in a relatively wide time, and its frequency band coverage meets the FCC definition of ultra-wideband signals, so it has the unique advantages of ultra-wideband systems such as strong distance resolution. In addition, since the modulation duration of the linear frequency modulation can be defined according to the needs, its signal energy is much greater than the baseband pulse form, which can meet the needs of long-distance target detection. As shown in Figure 1, the block diagram of the ultra-wideband radar signal generated based on DDS is shown.

Modulation carrier type

By modulating the carrier, the UWB signal can be moved to a suitable frequency band for transmission, which can make more efficient and flexible use of spectrum resources. At the same time, the methods used in existing communication systems can be used, and the technology maturity and process stability are very high, making it easier to implement high-speed systems. When the IEEE 802.15.3a working group solicited proposals in 2003, Intel, TI and Xtreme Spectrum proposed three schemes, namely multi-band, orthogonal frequency division multiplexing (OFDM), and direct sequence code division multiple access (DS-CDMA). Later, the multi-band scheme was merged with the orthogonal frequency division multiplexing scheme, thus forming two major alliances, MB-OFDM led by TI, Intel and other companies, and DS-CDMA led by Xtreme Spectrum, Freescale and other companies.

Technically speaking, MB-OFDM and DS-CDMA cannot compromise with each other. Through the development in recent years, MB-OFDM has gradually replaced DS-CDMA and become a popular technology for future wireless broadband. Figure 2 shows the signal structure of the MB-OFDM system. [page]

● MB-OFDM ultra-wideband system

The core of MB-OFDM is to divide the frequency band into multiple 528MHz sub-bands. Each sub-band adopts the time-frequency interleaving orthogonal frequency division multiplexing (Time-Frequency Interpolation OFDM) method, and data is transmitted on each sub-band. The traditional UWB system uses pulses with a period of less than 1ns, while MB-OFDM uses multiple sub-bands to achieve dynamic bandwidth allocation, increasing the symbol time. The advantage of long symbol time is that it has a strong ability to resist inter-symbol interference (ISI). MB-OFDM is easy to implement technically, has very low power consumption, high frequency band utilization, and multiple frequency sub-bands are parallel, which can avoid certain frequency bands, flexible configuration, and good rate scalability. However, this performance improvement comes at the cost of the complexity of the transceiver equipment, and the influence of inter-channel interference (ICI) must also be considered. MB-OFDM has advantages in performance (initial speed up to 480Mbits/s), and because OFDM technology enables weak signals to have nearly perfect energy capture, its communication distance will also be longer. MB-OFDM technology processes information on sub-bands, simplifies the digital complexity of the receiver, reduces power consumption and cost, improves spectrum flexibility, and helps establish relevant standards worldwide. However, the transmitter structure is relatively complex (additional IFFT, DAC), which can easily cause a high peak-to-average ratio (PAR), which can easily interfere with other systems. If the transmission power is simply reduced, the transmission distance will be reduced.

R&S Test Solutions

Compared with the original communication system, the UWB system has the advantages of large channel capacity, high transmission rate, strong anti-interference ability, and high distance resolution, but it also poses corresponding challenges to system testing. For the reception test of the UWB system, a signal generator that can generate ultra-wideband signals is required, while for the transmission test of UWB, ultra-wideband signal analysis equipment is required. In order to meet the corresponding testing needs, R&S has launched the corresponding UWB system test solution, which can meet the needs of customers for test equipment in different R&D and production stages, and combine different options to meet different application needs.

Ultra-wideband baseband signal test solution AFQ100B

In the early stage of UWB system development, in order to carry out corresponding waveform design and corresponding algorithm analysis, it is necessary to simulate and calculate the baseband signal. The newly launched IQ baseband signal source AFQ100B of R&S has better performance and more functions. The broadband mode of AFQ100B can reach 600MHz, RF bandwidth can reach 528MHz, and signal storage depth can reach 1G sampling, which is suitable for BER testing that requires ultra-long signals.

For MB-OFDM systems that meet the WiMedia Alliance (ECMA-368) standard, the corresponding AFQ-K264 option can be added to configure ultra-wideband signals that meet the standard, so it is very suitable for ultra-wideband communications. Figure 3 shows the menu interface of the K264 option. For baseband pulse system UWB systems used in the military field, the AFQ-K6 option can be used to generate narrow pulse signals and complex pulse sequence signals for radar applications to meet military applications. As shown in Figure 4, the AFQ-K6 option is used to design a complex pulse sequence to achieve the UWB signal waveform. For UWB systems of other systems, combined with the ARB mode of AFQ, most of the UWB waveform designs and corresponding signal processing tasks we need can be completed. In addition, AFQ100B can provide a variety of data interfaces including digital IQ and analog IQ for customers to choose freely.

Ultra-wideband RF receiving system test solution AFQ100B+SMBV100A

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In the middle of the development of UWB system, in order to verify the performance of the whole receiving system, we need an ultra-wideband RF signal generator. The external modulation bandwidth of the newly launched vector signal source SMBV100A of R&S reaches 528MHz. Combined with the R&S AFQ100B wideband I/Q source, SMBV can generate RF signals with a bandwidth of up to 528 MHz.

Ultra-wideband RF transmission system test solution ZVT

In the design of UWB system, the analysis of UWB transmission signal is also an important part. Since the signal bandwidth of ultra-wideband system is very large, if digital sampling is performed directly, an extremely high sampling rate is required, which will bring great difficulties to the subsequent data transmission and data processing. Therefore, the common method is to perform corresponding signal processing through channelized receiver, that is, divide the ultra-wideband signal into several frequency bands, and each frequency band corresponds to a corresponding channelized receiver. Then, different frequency bands are mixed to a lower frequency through different local oscillators for corresponding sampling and signal processing. Figure 5 shows the implementation block diagram of the channelized receiver.

For channelized receivers, if the accuracy of subsequent processing is to be guaranteed, the performance of each channel receiver needs to be as consistent as possible, so the consistency test of channelized receivers becomes the main content of the test. The 8-port vector network analyzer ZVT launched by R&S contains four independent sources, and the power, frequency and phase of each source can be freely set. Therefore, different sources can be used as the input RF signal and LO signal of the channelized receiver, and then the corresponding channel consistency test can be performed, thereby completing the transmission test of the UWB system.

For UWB systems implemented with narrow pulse signals, how to measure the quality of the intra-pulse signal is also an issue we need to pay attention to. The ZVA-K7 option based on ZVA/ZVT of R&S can conveniently test the S parameters of narrow pulses. Its function is equivalent to an oscilloscope with a time resolution of 12.5ns and a test bandwidth of 30MHz. The block diagram of the pulse S parameter test is shown in Figure 6.