Making Noise in RF Receivers Simulate Real-World Signals with Signal Generators

Transcription

1 Making Noise in RF Receivers Simulate Real-World Signals with Signal Generators Noise is an unwanted signal. In communication systems, noise affects both transmitter and receiver performance. It degrades both the modulation quality of a transmitter and the sensitivity of a receiver. Because of this, noise reduction in electronic devices is in the best interest of R&D engineers. However, to simulate a realistic environment you need to inject a noise signal into your design. The noise signal needs to be simple and based on mathematical models. The additive white Gaussian noise (AWGN) is the most common noise for receiver performance test. Another common noise in RF systems is phase noise. Adding phase noise impairment for accurate signal substitution or tolerance testing is helpful in evaluating and troubleshooting your device under test (DUT). In this white paper, you will learn what the AWGN and phase noise are and how to correctly and accurately apply the noise to your desired signal for receiver performance test. Find us at Page 1

2 Adding Real-Time Noise to a Signal What is AWGN and Why It is Important? Noise is part of all communications channels. The Shannon-Hartley theorem below tells you the maximum rate at which information can be transmitted over a communication channel within a specified bandwidth with the presence of noise. C = B * Log2 (1+S/N) where: C is the channel capacity in bits per second (bit/s). B is the signal bandwidth in Hz. S is the average received power over the bandwidth in watts. N is the average power of the noise over the bandwidth in watts. To simulate realistic channel conditions in a repeatable manner, you must add random noise to the desired signal. AWGN is a mathematical model that is used to simulate the channel between the transmitter and the receiver. The model is a linear addition of wideband noise with a constant spectral density and a Gaussian distribution of amplitude. AWGN does not apply to fading, intermodulation, and interference tests. Taking LTE enb receiver tests (3GPP TS ) as an example, apply AWGN to the desired LTE signal for enb dynamic range test in section 7.3 and all non-multipath receiver performance test cases in clause 8. The dynamic range test is specified as a measurement of the capability of the receiver to receive the desired signal in the presence of an interference (AWGN) in the received channel. Figure 1 illustrates a common receiver performance test setup. It requires a signal generator to generate the wanted signal and another one to generate the AWGN. Use a combiner to combine the signals and connect to a DUT. Ensure the isolation between the two signal generators is good enough so that they do not impact the other unit s ALC (automatic leveling control) operation. Figure 1. Measurement system setup for receiver dynamic range test Find us at Page 2

3 The signal generators for receiver tests need AWGN generation capabilities. The following figure depicts the relationship between the carrier signal, AWGN bandwidth, and power. Carrier bandwidth is the occupied bandwidth of the carrier, and the noise bandwidth is the flat noise bandwidth. The actual flat noise bandwidth should be slightly wider than the carrier bandwidth (typically 1.6 times wider). When you combine the carrier and AWGN signal for receiver tests, the carrier now appears larger because of the added noise power. Figure 2. Add AWGN to the wanted signal for receiver tests Table 1 represents the signal level setup following 3GPP TS section 7.3 receiver dynamic range test for 5 and 10 MHz channel bandwidth. The signal levels depend on the channel bandwidths and base station types. The throughput should be less than 95% of the possible maximum throughput of the reference measurement channel. LTE channel bandwidth (MHz) Reference measurement channel 5 FRC A FRC A2-3 Base station type Wanted signal mean power (dbm) Interfering signal mean power (dbm) Wide area Mid-range BS Local area Home BS Wide area Mid-range BS Local area Home BS C/N (db) Table 1. LTE receiver dynamic range test requirement Find us at Page 3

4 It is important to measure the noise power seen within carrier bandwidth as shown in yellow in Figure 2. By knowing your noise power value, you can calculate the carrier to noise ratio (C/N). Additionally, most standards use energy per bit over noise power density at the receiver (Eb/No) to characterize their receiver as opposed to C/N. However, this requires you to know the carrier bit rate. Below is the equation used to convert C/N to Eb/No. (E b/n o) db = C/N db - 10 log 10 (bit rate/carrier bandwidth) It will be very tedious to create the wanted and interfering signals with the specified C/N for different channel bandwidth and base station type. There are ways you can simplify the measurement setups. Simplify Your Measurement Setup Using Real-Time I/Q Baseband AWGN The additional measurements and calculation required to make receiver measurements setup more tedious. Luckily, with evolving digital signal processing (DSP) technology, signal generators add real-time noise (AWGN) to the baseband waveforms digitally. This provides a very accurate amplitude level for both the carrier and noise signal. You don t need to worry about the correction of external accessories. In addition, you can easily select either C/N or Eb/No as the variable controlling the ratio of the carrier power to noise power in the carrier bandwidth. A vector signal generator (VSG) enables you to add AWGN to a carrier in real time. You can easily apply real-time AWGN to the wanted signal using the signal generator s internal DSP. This can be accomplished by using a single vector signal generator. Figure 3 shows the setup of real-time AWGN. You can select different power control modes as a reference Total, Carrier, Total Noise, and Channel Noise. For example, if you select power control mode Total, the mode makes the total power and C/N (or Eb/No) independent variables, while making the carrier power and total noise power dependent variables. Figure 3. The setup of AWGN on Keysight MXG N5182B Find us at Page 4

5 Test Case Manager for LTE & LTE-Advanced FDD/TDD enb Receiver Tests The Keysight N7649B Test Case Manager software tool offers a simple and easy-to-use user interface and works with the Keysight Signal Studio software to perform standard-required conformance tests. Test Case Manager (TCM) reduces the time you spend on configuration by creating signals that are compliant with the conformance test requirements and automatically setting up the signal generators. Figure 4 shows the user interface for setting the 3GPP TS section 7.3 Receiver Dynamic Range and the base station type Wide area. You just need to configure channel frequency and bandwidth. Figure 4. Test Case Manager setup for 3GPP TS section 7.3 Find us at Page 5

6 Optimize Signal Generator s Phase Noise Profile Signal generators provide several ways to optimize phase noise for your applications. Let s start with signal generators phase noise profile and when the profile impacts your measurements. Then you will learn how to optimize a signal generator s phase noise profile for your applications. What is Phase Noise? Phase noise is a frequency-domain view of the noise spectrum around the oscillator signal. It describes the frequency stability of an oscillator. Frequency stability can be broken into two components: long-term stability and short-term stability. Long-term stability (e.g. accuracy, drift, and aging) is characterized in terms of hours, days, months, or years. Short-term stability (e.g. phase noise) occurs in a few seconds or less. The short-term variations have a greater effect on systems, especially for phase noise. Let s take a closer look at phase noise measurement. Unit of Measure The most commonly used phase noise unit of measure is the single-sideband (SSB) power within onehertz bandwidth at a specific frequency away from the carrier frequency power. (f) = Noise power in a 1-Hz Bandwidth / total signal power where (f) has units of dbc/hz. Figure 5 represents SSB phase noise measurement results. Both frequency and amplitude are in log scale. The log plot shows phase noise measurements over the range of frequencies specified by the minimum and maximum offset frequencies. The yellow trace is a raw measurement result, and the blue trace is a smoothened result. The table below lists the decade frequency offsets and the corresponding noise power (normalized to a 1 Hz bandwidth). Figure 5. SSB phase noise measurement with a log plot and decade table Find us at Page 6

7 Signal Generators Architecture and Phase Noise Most signal generator architecture includes reference oscillator, synthesizer, voltage-controlled/yig oscillator, and output section. Each component has different effects on the phase noise characteristics as shown in Figure 6. For offsets below 1 khz, the noise is dominated by the performance of the reference oscillator, which is multiplied up to the carrier frequency. From offsets 1 khz to roughly 100 khz, synthesizer influences the most. The VCO/YIG oscillator is from 100 khz to 2 MHz, and the output amplifier is at offsets above 2 MHz. Next, we will discuss measurement applications and how to optimize phase noise to meet the test needs. Figure 6. Contributions to the phase noise performance When Phase Noise Matters Signal source phase noise performance is a key factor in obtaining accurate measurements. It can be a limiting factor for specific applications in aerospace and defense, as well as in digital communications. It is important to understand the impact of phase noise on your tests. Radar Applications Radar systems require excellent phase noise performance. A radar transmits pulses at a specific frequency and measures the change of each returning pulse s frequency. Each returning pulse s change in frequency is related to the velocity of the moving object based on the Doppler Effect. If the object moves very slowly, the frequency shift of the returning pulse is small. In Figure 7 below, the returning pulse of a moving object is the signal of interest, and the returning pulse of a fixed object (e.g. ground) is the interfering signal. The radar receiver cannot identify the moving object if the downconverted signal of interest is masked by the phase noise. Find us at Page 7

8 Figure 7. Poor LO phase noise affects radar receiver sensitivity Digital Modulation Let's look at digital modulation. Figure 8 represents a simplified QPSK digital receiver block diagram. The phase noise of the LO signal is translated into the output of the mixers. The direct effect of the phase noise on the constellation diagram is the radial smearing of the symbols (as shown in green). For a high order modulation scheme (e.g. 256 QAM), the symbols are closer, and the symbols smearing results in a bad receiver sensitivity and higher bit error rate (BER). Figure 8. A simplified digital receiver block diagram with a poor phase noise LO Find us at Page 8

9 Orthogonal Frequency-Division Multiplexing (OFDM) OFDM is a popular modulation scheme for wideband digital communication. OFDM uses many closely spaced orthogonal sub-carrier signals to transmit data in parallel as shown in Figure 9 below. During frequency conversion with a poor phase noise LO, the sub-carrier with phase noise spreads into other sub-carriers as interference. The phase noise degrades the modulation quality of the OFDM signal. Figure 9. OFDM signal upconvert with a poor phase noise LO Table 1 below illustrates sub-carrier spacing of modern wireless standards using OFDM modulation scheme. Sub-carrier spacing IEEE ac IEEE ax khz khz LTE/LTE-A 7.5, 15 khz 5G NR 15, 30, 60, 120, 240, 480 khz Table 1. Sub-carrier spacing of OFDM signals. From the above table, the sub-carrier spacings are located in a signal generator s synthesizer or oscillator session. In order to get the best performance of modulation quality, you need to reduce the carrier s phase noise of specific frequency offset as low as possible. Find us at Page 9

10 Optimize Signal Generator s Phase Noise Close to Carrier Phase Noise Reference Phase-Lock Loop (PLL) Bandwidth At frequency offsets below approximately 1 khz, the stability and phase noise are determined by the internal or external frequency reference. It is straightforward to have a stable and extremely low phase noise reference oscillator that improves the carrier s phase noise in the offset frequency range below 1 khz. Keysight PSG signal generator offers options to improve close-in phase noise. The reference oscillator bandwidth (sometimes referred to as loop bandwidth) in the signal generator is adjustable in fixed steps for either an internal or external 10 MHz frequency reference. You can optimize the phase noise performance of the signal generator for your applications. Figure 10 shows phase noise curves with the different setting of oscillator bandwidth. The PSG with Option UNR/UNX/ UNY is adjustable in fixed steps for either an internal or external 10 MHz frequency reference. Figure 10. Reference oscillator s PLL bandwidth adjustments Find us at Page 10

11 Synthesizer Session PLL Bandwidth In the synthesizer session, you can set the phase-lock loop (PLL) bandwidth to optimize phase noise above or below 150 khz on Keysight PSG signal generators, as shown in the figure below. The light blue curve is optimized for < 150 khz frequency offset and the yellow curve is optimized for > 150 khz. Evaluate your application to choose the appropriate phase noise setting for wider offset frequencies. This capability is supported on all PSG models with Option UNY. Figure 11. Optimizing pedestal phase noise Learn about phase noise signal generator fundamentals and ways to optimize phase noise for your application. Download the application note Understanding Phase Noise Needs and Choices in Signal Generation. Find us at Page 11

12 Phase Noise Impairment Optimizing phase noise performance is not always necessary or even desirable. Some applications and tests require a specific amount of phase noise for accurate signal substitution or tolerance testing of phase noise. Keysight RF signal generator N5182B/N5172B allows users to adjust phase noise impairment of the synthesizer section. This feature allows you to degrade the phase noise performance of the signal generator by controlling two frequency points and amplitude values as shown in Figure 12. The signal generator bases the resultant phase noise shape on three settings Lmid (amplitude), f1 (start frequency), and f2 (stop frequency). This customized phase noise is produced by internal algorithms of the signal generator operating on a real-time baseband ASIC and processor accelerator. This allows you to simulate a more realistic signal and is helpful in evaluating and troubleshooting your device under test. This feature is available only in Keysight X-Series vector signal generators. Figure 12. Phase noise impairment setting and measurement Find us at Page 12

13 Conclusion Signal generators provide precise, highly stable test signals for a variety of component and system test applications. Performance requirements vary for different applications. Keysight s signal generators offer flexibility and capabilities to optimize performance and simplify measurement setups. The best solutions will come from your experience, insight, and creativity, combined with signal generators and measurement software that allow you to generate the signals required to effectively test your DUT. For more tips on making better measurements, visit the RF Test blog. For more information about Keysight signal generators, visit Learn more at: For more information on Keysight Technologies products, applications or services, please contact your local Keysight office. The complete list is available at: Find us at Page 13 This information is subject to change without notice. Keysight Technologies, 2018, Published in USA, October 30, 2018, EN