Digital Microwave Transmission Equipment IDU Measurement Solution

1 Introduction

Microwave communication technology has been around for more than half a century. It is a wireless communication method that transmits information through ground line of sight in the microwave frequency band. The initial microwave communication systems were all analog, and together with the coaxial cable carrier transmission system at the time, they were important transmission methods for long-distance transmission trunk lines of communication networks. Since the 1970s, digital microwave communication systems with small and medium capacities (8Mbit/s, 34Mbit/s) have been developed, which is the inevitable result of the development of communication technology from analog to digital. In the late 1980s, with the promotion and application of synchronous digital hierarchy (SDH) in transmission systems, N×155Mbit/s SDH large-capacity digital microwave communication systems appeared. Now, digital microwave communication, optical fiber and satellite are called the three pillars of modern communication transmission. With the continuous development of technology, digital microwave technology has also attracted more and more attention in the field of fixed broadband access.

Rohde & Schwarz is the largest manufacturer of electronic measuring instruments in Europe and has been active in the fields of test and measurement, information and communication technology. As a leading global manufacturer of test and measurement equipment, R&S is always paying attention to the development of digital microwave communications and currently provides a full set of solutions for IDU systems from signal generation to signal analysis, namely signal source, spectrum analyzer, vector network analyzer and power meter. The signal source can generate arbitrary vector modulation signals with flexible configuration, the spectrum analyzer can perform modulation domain, frequency domain and time domain measurement and analysis on arbitrary vector modulation signals, the vector network analyzer can perform port standing wave tests on IDU and ODU, etc., and the power meter can test the output power of IDU and ODU.

2 Introduction to Digital Microwave Indoor Unit IDU Test Instrument

Rohde & Schwarz offers a range of instruments to meet different IDU test requirements. The test instruments mentioned in this article include signal generation solutions and signal analysis solutions (see Tables 1 and 2).

3 Digital Microwave Transmission Indoor Unit IDU Test Solution

The testing process from product development to production may also be different. During product development, both devices and modules need to be tested, mainly focusing on product performance and extended function testing; during product production, the main focus is on speed and stability. R&S provides corresponding testing solutions for these different testing stages.

(1) R&S provides R&D test solutions of SMU200A+FSQ/FSG+ZVB+NRP

SMU200A can be built with two independent signal sources, one of which is used to generate useful signals, and the other can generate various interference signals defined by the specification. SMU200A can generate fading simulation and AWGN required by the specification, so that one SMU200A can meet the specification requirements for signal sources. The spectrum analyzer FSQ has the best dynamic range among similar products, which is very important for spurious testing and ACLR testing. The vector signal analysis mode of FSQ can analyze the signal in the IQ domain, which is convenient for positioning and fault analysis in the R&D stage. SMU200A+FSQ is the most compact and powerful R&D test solution that the industry can provide.

(2) R&S provides the IDU production solution of SMBV100A+FSV/FSL+ZVL+NRP

The production solution is a solution that comprehensively considers system test items, test speed, reliability, data consistency and cost savings, namely SMBV100A+FSV/FSL+ZVL+NRP, which is suitable for the testing needs of a series of network devices including indoor unit IDU, outdoor unit ODU and antenna, etc. Help customers quickly establish a production line that meets the requirements.

For digital microwave indoor units (IDUs) that mainly use transparent transmission, R&S provides the K70 option for arbitrary vector modulation signal generation and signal analysis as standard functions. This function is briefly introduced below.

●Generation of arbitrary vector modulation signals (see Figure 1)

Figure 1 Custom Digital Modulation Setting
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Any vector modulation signal can be generated inside SMx. The currently supported modulation types are shown in Table 3.

From these configurations, we can see that the modulation mode supported by SMx is as high as 1024QAM, which can basically meet the needs of all users who use transparent transmission. In addition, SMx can also support I and Q data offset settings to simulate IQ impairments. For example, IQ phase offset and IQ orthogonal offset can be set to facilitate users to conduct baseband research and development.

WinIQSIM software

WinIQSIM is a simulation software developed by R&S and runs on a PC. It can flexibly configure the signals generated by the signal source through a GPIB card. It supports various digital standard signals such as GSM, IS-95, cdma2000, WCDMA, TD-SCDMA, WLAN802.11, etc. It also supports the configuration of arbitrary vector modulation signals.

WinIQSIM software currently allows users to customize test modes and modify any configuration, including symbol rate, encoding method, filter selection and forming coefficient.

●Analysis of arbitrary vector modulation signal FS-K70

R&S vector analyzers FSQ/FSG/FSV/FSU all have the function of arbitrary vector signal analysis. Taking FSQ as an example, FSQ has a standard 28MHz demodulation bandwidth, which can be extended to 120MHz, and the IQ sampling point storage depth is up to 705M. The analysis process is directly completed by digital signal processor hardware. The functional block diagram of FS-K70 and extended demodulation bandwidth is shown in Figure 2.

Figure 2 FSQ-K70 option block diagram

Option FSQ-K70 can analyze various digital modulation signals, such as MSK, PSK, QAM and other modulation modes, and obtain analysis results. For example, EVM, frequency error, phase error, IQ non-ideality, eye diagram, etc., see Figure 3 for details. See Figure 4 for test results.

Figure 3 FS-K70 measurement display window

Figure 4 FS-K70 measurement results display

4 System Performance Test

In actual environments, digital microwave transmission transceivers are greatly affected by wireless propagation channels. For a long time, the performance tests of digital microwave transmission equipment in actual environments were mostly conducted on-site. The disadvantage of this approach is that the cost is too high. For example, on-site testing at several sites far apart requires the construction of equipment, microwave antennas and other equipment and facilities, as well as personnel, which is time-consuming and labor-intensive. In addition, there are many unknown and uncontrollable error factors during the test (see Figure 5).

Figure 5 Digital microwave transmission wireless channel model

Since the dual-channel signal source SMU200A of R&S can simulate the influence of multipath fading and Gaussian white noise, it can solve the factors that may appear in the actual application of digital microwave IDU, such as fading, Doppler effect, delay, loss, etc. In other words, a wireless channel simulator is needed to realize the above effects for testing. See Figure 6 for the test device and block diagram.

Figure 6 Scheme formed by SMU200A and FSQ

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For this test, the test solution recommended by R&S is the RF fading simulator implemented using SMU200A+FSQ:

●SMU200A: Vector signal source (SMU-B14: Fading simulator; SMU-B17: Baseband input).

●FSQ: Signal analyzer (FSQ-B17: digital baseband interface).

In this solution, the R&S test platform can not only complete the fading simulation test, but also simulate Gaussian white noise AWGN, simplifying the C/N and BER test of the digital microwave transmission equipment IDU, as shown in Figure 7.

Figure 7 SMU200A BER~C/N test block diagram

5 Digital baseband test

In the R&D stage, hierarchical testing is necessary to ensure that the functions and performance of each level of circuits meet the design requirements. Digital circuits are one of the core parts of digital microwave indoor unit signal processing. The configuration, programming, networking and communication capabilities of digital microwave transmission equipment depend largely on the digital part. Therefore, testing the digital circuit part is essential.

As the largest manufacturer of radio test and measurement instruments in Europe, Rohde & Schwarz has rich practical experience in RF testing of base stations of various communication standards. The full range of high-performance instruments of Rohde & Schwarz has been widely used in the development and production of base stations and terminals. At present, the EX-IQ-Box device launched by R&S can provide flexible digital interface input and output functions, which can be easily connected to R&S signal sources and signal analyzers, and can perform receiver measurement, transmitter measurement, signal source simulation and RF front-end simulation.

Usually, when testing the digital circuit of the transmitting part, the digital IQ signal output by the circuit is directly tested to analyze its modulation quality, such as EVM; when testing the digital circuit of the receiving part, the receiving performance of the input digital IQ signal is directly tested, and noise and fading effects can also be added to the source part for further testing (see Figure 8).

Figure 8 Digital baseband test platform

According to the above requirements, the test solution recommended by R&S is:

●SMU200A: vector signal source.

●FSQ: Vector signal analyzer.

●EX-IQ-Box: Digital IQ signal format converter.

The vector signal source SMU200A and the vector signal analyzer FSQ have powerful signal generation and analysis capabilities and are appropriate tools for testing transmission and analysis performance. In addition, for the test of the digital circuit part, both SMU200A and FSQ provide digital IQ input/output interface, which is TRV290 (R&S internal digital interface format). In order to match the digital interface of the external device under test, R&S provides EX-IQ-Box, which can convert the TVR290 interface format to a universal digital signal interface format, and vice versa.