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Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
Radio Conformance Test
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Radio Conformance Test

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RF testing has remained hype for most of us. But seriously it is not so. It can be very interesting and one can develop a lot of interest in this if given an opportunity. …

RF testing has remained hype for most of us. But seriously it is not so. It can be very interesting and one can develop a lot of interest in this if given an opportunity.
In this paper, authors have started with the some basic concepts of radio engineering which we studied in engineering and built upon these concepts to use in practical applications.
We have also described the basic principles of Signal Analyzer and Signal Generator which are the most common test tools used for any radio testing.

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  • 1. January 1, 2014 || Wayne Turner, Nisha Malik, Preet Rekhi, Rahul Sharma, Sukhvinder Malik Radio Conformance 1. Introduction RF testing has remained hype for most of us. But seriously it is not so. It can be very interesting and one can develop a lot of interest in this if given an opportunity. In this paper, authors have started with the some basic concepts of radio engineering which we studied in engineering and built upon these concepts to use in practical applications. We have also described the basic principles of Signal Analyzer and Signal Generator which are the most common test tools used for any radio testing. 2. Basic Concepts . Contents 1. Introduction 2. Basic concepts 3. RF components of a Transceiver 4. RF test equipment 5. LTE Radio conformance Tests 6. Conclusion A SIGNAL is a function that conveys information about the behavior or attributes of some phenomenon. It can be an electric current or electromagnetic field used to convey data from one place to another. In terms of signal processing these can be analog or Digital. A SPECTRUM is a collection of sine waves that, when combined properly; produce the time-domain signal under examination. 7. References
  • 2. MODULATION is the addition of information (or the signal) to an electronic or optical signal carrier. NOISE is a random process unwanted byproduct or unwanted data without meaning. Noise can be intentional too called dither which can reduce overall noise and allows retrieval of signals below the nominal detection threshold of an instrument. PHASE NOISE is the noise in the oscillator which increases at frequencies close to the oscillation frequency or its harmonics. With the noise being close to the oscillation frequency, it cannot be removed by filtering without also removing the oscillation signal. The noise close in to the center frequency is due to FLICKER NOISE. It is usually referred to as 1/f noise or pink noise. The noise signal adds to the modulated signal which adds together to end up in the wrong place on the constellation. DISTORTION is any sort of undesired change in waveform as it passes from Input to Output of some device. The total number of bit errors divided by the total number of transferred bits is BIT ERROR RATE and is therefore unit less. BLOCK ERROR RATE will similarly be total number of erroneous blocks divided by the total number of received blocks. An OSCILLATOR is used to convert a DC source to an AC source. The output will swing from a maximum to minimum voltage with a fixed period. An analogy can be a pendulum. In RF we mostly use sine wave oscillators however we can have square, pulse, triangle or sawtooth. The Error Vector Magnitude is a measure of the difference between the ideal symbols and the measured symbols after the equalization. This difference is called the EVM.
  • 3. The FREQUENCY ERROR is the difference in frequency, after adjustment for the effect of the modulation and phase error, between the RF transmission from the mobile station and the test set. In digital communications, modulation is often expressed in terms of I/Q FORMATS. I/Q diagrams are particularly useful because they mirror the way I /Q modulator creates most digital communications signals. Independent dc voltages (I and Q components) provided to the input of an I/Q modulator correlate to a composite signal with a specific amplitude and phase at the modulator output. Conversely, a modulated signal’s amplitude and phase sent to an I/Q demodulator correspond to discrete dc values at the demodulator’s output. The digital modulation maps the data to a number of discrete points on the I/Q plane. These are known as CONSTELLATION POINTS. For e.g. QPSK has 2 bits per symbol so 4 points on the constellation diagram, 16 QAM has 4 bits per symbol so 16 points and likewise 64 QAM has 6 bits per symbol so 64 points. THERMAL NOISE is a random fluctuation in voltage caused by the random motion of charge carriers in any conducting medium at a temperature above absolute zero. The formula to find the RMS thermal noise voltage of a resistor is: Vn = 4kTRB Where: k = Boltzmann constant (1.38*10-23 Joules/Kelvin) T = Temperature in degrees Kelvin (K= +273 Celsius) R = Resistance in ohms B = Bandwidth in Hz in which the noise is observed (RMS voltage measured across the resistor is also function of the bandwidth in which the measurement is made).
  • 4. Thermal noise can be given as - 174 dBm/Hz. In context of LTE we might need to calculate the thermal noise over a Resource Block or a subcarrier or over the channel bandwidth. Thermal noise is the main contributor for the Receiver sensitivity calculation.    Thermal Noise over 1 Resource Block (180 KHz) = - 121 dBm Thermal Noise over 1Sub-carrier (15 KHz) = - 132 dBm Thermal Nosie over 20 MHz Channel BW (18MHz) = - 101dBm Thermal Noise for a Bandwidth of Interest can be calculated as = -174 + 10*log10(bandwidth in Hz) . The NOISE FIGURE of a circuit element represents the amount the S/N ratio degraded from the input to the output of that circuit element, when and only when the noise input power = KTB. The higher the noise figure, the less sensitive the system. FRIIS formula for noise factor states that NF Total= NF1 + (NF2-1)/G1 + (NF3-1) / (G1*G2) + . . . Where NF1 = noise figure of amp1 in linear ratio NF2 = noise figure of amp2 in linear ratio NF 3 = noise figure of amp3 in linear ratio G1 = gain of amp1 in linear ratio G2= gain of amp2 in linear ratio etc. It is always advised to put a lot of gain in the first stage in a multistage amplifier system, which should have a low noise figure. The Base station have a NF in order of 2-3 dB while mobile handsets have NF in order of 7-8 dB. Reason for this high NF in mobile handsets is the limitation of the size and weight which leads us to use an inefficient power amplifier while in Base station we do not have limitation of size and weight so we can use a better linear power amplifier stage which reduce the NF with high gain capability.
  • 5. 3. RF Component of a RF Transceiver The basic components of a radio transceiver are local oscillators (LO), RF mixer, phase local loop (PLL) and synthesizer. Under this section we will discuss about all these components. HOMODYNE RECEIVER or Zero IF or Direct Conversion Reception is the most natural solution to detect information transmitted by a carrier in just one conversion stage. It has at least one mixer stage less than in a heterodyne system. But from the other hand the flexibility to change receiving frequency is reduced significantly, because the narrow band filtering is not possible before down conversion and the only narrow band filter is the output low pass. Though, in case of the BPM signal processing there is no need to change the input frequency. That means a narrow band filter can be used already at the front-end of the electronics. The principle is that the signal is first amplified at a low noise stage and then directly converted to the baseband or even to a direct current signal. When the frequencies of the RF and the LO signals are equal, this scheme works as a phase detector. Suppose that the IF in a heterodyne is reduced to zero. The LO will then translate the center of the desired channel to 0 Hz, and the portion of the channel translated to the negative frequency half-axis becomes the “image” to the other half of the same channel at the positive frequency half-axis. To achieve maximum information, we should take both parts of signal. It’s done by a method, which is called quadrature down conversion. The principle of this method is that the signal is at first divided into two channels and then down converted by an LO signal, which has a phase shift of 90 in one channel with respect to another. The vector of the resulting signal is described as: Signal  I 2  Q 2 arg( Signal )    arctg Q I The main problem in homodyne technique is an offset caused by the LO signal leakage to the RF port of the mixer. The propagated signal reflects from the components in the front-end of the receiver and goes back to the mixer, where it is mixed down to DC. The offset can be considerable with respect to the signals to be measured. This leads to a narrower dynamic range of the electronics, because the active components get saturated easier than it would be in case of a zero offset. Another problem of the homodyne receiver, or, more concretely, of the I/Q (in-phase/quadrature) mixer, is mismatches in its branches.
  • 6. In HETERODYNE RECEIVER the Radio frequency (RF) signal is first amplified in a frequency selective (but usually broadband) low noise stage, then translated to a lower intermediate frequency (IF) : fIF = fRF – fLO. After a significant amplification and an additional filtering on the intermediate frequency it is finally down converted to the baseband (i.e. to the original or desired signal frequency). This technique is flexible in changing the receiving frequency, because the only change to be done is the frequency of the first local oscillator (LO) in the way that at the frequency of the signal at the output of the mixer would not change. The rest of electronics is free from additional readjustments, what is very important in the applications of the heterodyne receiver in the radio, TV, satellite and other communications. The main problem in heterodyne technique is the half IF and image frequencies, received signals on these frequencies act as a blocker e.g. reducing the sensitivity of the receiver. If the LO frequency is a 1100 MHz and the received signal is at 900 MHz, this is a high side injected system, the Image frequency will be at: High side injection Image LO + IF IF = 200 MHz Image = 1100MHz + 200MHz = 1300MHz PHASE NOISE is short term random fluctuation of the signal which is expressed in dBm/Hz i.e. the power measured in a 1 Hz resolution bandwidth, the noise profile follows the 1/frequency profile. To measure phase noise of an oscillator the measuring equipment has to have a phase noise performance exceeding the phase noise performance of the oscillator you are testing. The phase noise is proportional to the Q (quality factor) of the resonant circuit used in the oscillator circuit. A PLL or PHASE LOCKED LOOP is a control system that generated an output signal whose phase is related to the phase of an input signal. When using a phase locked loop as a frequency synthesizer the phase noise may be amplified by 20 logs (N-divider number) and is directly to the ratio of the output frequency of the VCO and the comparison frequency of the phase detector used in the PLL.
  • 7. The idea is to keep the N-Divider ratio to a minimum value as possible; this can be achieved by using a fractional-N SYNTHESISER. A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed time base or oscillator. There are discrete spur like signals which appear on the output of the local oscillator spectrum (integer N), these are the reference spurs i.e. they appear on both the upper and lower side of the spectrum at frequency offset from the center equal to that of the caparison frequency. These occur as the charge pump is constantly pushing current in or pulling from the loop filter ( due to capacitor current leakage and constants loop correction), a tight loop filter can reduce these spurs but would increase the lock time of the phase locked loop. If the local oscillator used in a transceiver has a bad phase noise performance then the following performance can be degraded. 1. 2. Transmitter and Receiver EVM ACS (Adjacent Channel Selectivity) A RF MIXER is a 3 port device which is used to produce a sum a difference signal of 2 signals. For example is a mixer is fed with 900MHz and 1100 the sum will be at 2 GHZ and difference will be at 200MHz, in a down converter the difference will be used and in a up convertor the sum is used. When a transmission line (cable) is terminated by impedance that does not match the characteristic impedance (Z0) of the transmission line, not all of the power is absorbed by the termination. Part of the power is reflected back down the transmission line. The forward (or incident) signal mixes with the reverse (or reflected) signal to cause a voltage standing wave pattern on the transmission line. The ratio of the maximum to minimum voltage is known as VSWR, or VOLTAGE STANDING WAVE RATIO. A VSWR of 1:1 is ideal and means that there is no power being reflected back to the source. A VSWR of 1.2 could be excellent.
  • 8. 4. RF Tests Equipment There are a lot of radio test equipment like power meter, VSWR meter, Signal Analyzer and Signal Generators. The Signal Analyzer and Signal generator are the most power test equipment for radio engineers which are mostly used for lab testing. In this section we will discuss about the principle of operation and basic building blocks of signal analyzer and signal generator which help us to understand the functioning of these test equipment. A SPECTRUM ANALYSER measures the magnitude of an input signal versus frequency. Its primary use is to measure the power of the spectrum of known and unknown signals. The swept-tuned analyzer “sweeps” across the frequency range of interest, displaying all the frequency components present. This enables measurements to be made over a large dynamic range and wide frequency range. Inside the analyzer, the mixer converts the input signal from one frequency to another. The input signal is applied to one port, and the local oscillator’s (LO) output signal is applied to the other. The mixer is a nonlinear device, so frequencies will be present at the output that weren’t present at the inputs. These frequencies are the original input signals, plus the sum and difference frequencies of the two signals. The difference frequency is called the IF signal. Block Diagram of a Spectrum Analyzer
  • 9. The analyzer’s IF filter is a band pass filter used as a “window” for detecting signals. Its bandwidth, the analyzer’s resolution bandwidth (RBW), can be changed by means of the instrument’s front panel. A broad range of variable RBW settings allows the analyzer to be optimized for different sweep and signal conditions and enables the user to trade off frequency selectivity, signal-to-noise ratio (SNR), and measurement speed. Narrowing RBW, for example, improves selectivity and SNR. However, sweep speed and trace update rate degrade. The optimum RBW setting depends heavily on the characteristics of the signals of interest. Agilent’s and R&S Spectrum Analyzer The detector allows the analyzers IF signal to be converted to a baseband or video signal so it can be digitized and viewed on the LCD. This is accomplished with an envelope detector whose video output is digitized with an analog-to-digital converter (ADC) and represented as the signal’s amplitude on the Y-axis of the analyzer display. For e.g. Agilent’s N9020A MXA Signal Analyzer which supports a frequency range from 10 Hz to 26.5 GHz. The CW SIGNAL GENERATOR, the RF CW source splits into three sections: reference, synthesizer, and output. The reference section supplies a sine wave with a known frequency to the phase-locked loop (PLL) in the synthesizer section. Its reference oscillator determines the accuracy of the source’s output frequency. The synthesizer section produces a sine wave at the desired frequency and supplies a stable frequency to the output section. Block Diagram of a CW Signal Generator
  • 10. Agilent’s Signal Generator Creating a VECTOR SIGNAL GENERATOR simply involves adding an IQ modulator to the basic CW generator. To generate baseband IQ signals, a baseband generator takes binary data containing the desired “information” to be transmitted, maps it to digital symbols and then to digital I and Q signals, converts the digital IQ signals to analog IQ signals, and sends them to the IQ modulator to be coded onto the carrier signal. The same thing we can see in the block diagram of a vector signal generator. After the data undergoes symbol mapping, the digital signals are digitally filtered using two sets of filters in the baseband generator. The filters are designed to limit the bandwidth of the I and Q symbols and slow down the transitions between symbols. Many types of baseband filters exist, with each having different attributes that must be set in the signal generator. Common filter types are Root Raised Cosine, Gaussian, and Rectangular. Block Diagram of a Vector Signal Analyzer The above picture shows some of the Agilent’s signal analyzer which is capable of generation a wide range of signals for different technology like LTE, WiMAX and Microware Radar signals.
  • 11. 5. LTE Radio Conformances Tests: 3 GPP standard 36.141 and 36.104 talks about the Radio conference test case. In this standard these testcases are broadally classified in three categories.  Transmitter conformance Test  Receiver conformance Test  Performance Test For these test case standard defines some test model called EUTRA-Test Models (ETM) and Fixed Reference channel (FRC). The ETM models are basically used for the transmitter conformance testing and FRC are used for receiver and performance testing. The standard also defines the minimum criteria for passing for a particular test In this section we will discuss about some transmitter and receiver test case. SPECTRUM FLATNESS is how much amplitude variation you have in the wanted bandwidth i.e. all subcarrier have the same average power. CCDF (Cumulative Distributive Function) is a way to express statistically how often a static peak to mean ration happens. For e.g. a peak to mean ratio of 6 dBs occurs 0.001% of the time.
  • 12. The amount of energy in the adjacent channel compared to wanted will be the ACLR (ADJACENT CHANNEL LEAKAGE RATIO). Reasons could be improper filtering, frequency control or improper tuning. For e.g. the ratio of power in my channel and the adjacent channel must be greater than 44dB in LTE. However the ACLR is more of an integrated measurement. The SEM (SPECTRUM EMISSION MASK) is kind of absolute measurement so a spike of RF energy could pass the ACLR but would surely fail the SEM measurement.
  • 13. TRANSMIT OFF POWER measurement is used to verify whether the RRC filtered mean power versus time meets the specified mask. For e.g. in LTE Transmit OFF power should be less than -85 dBm/MHz One can use a Limiter which brings down the On Power only and there is no effect on the Transmit off Power. RECEIVER SENSITIVITY is the lowest power level at which the receiver can detect an RF signal and demodulate data. As the signal propagates away from the transmitter, the power density of the signal decreases, making it more difficult for a receiver to detect the signal as the distance increases. Improving the sensitivity on the receiver (making it more negative) will allow the radio to detect weaker signals, and can dramatically increase the transmission range. Sensitivity is vitally important in the decision making process since even slight differences in sensitivity can account for large variations in the range. For e.g. for Base station planning if the sensitivity is more negative, we need less Base Stations and vice versa. The basic formula to calculate the Receiver sensitivity RX Sensitivity = -174 + 10*log10 (Bandwidth) + SINR + Noise figure The range over which the Receiver is sensitive enough to operate will be its DYNAMIC RANGE. The low end of the range is governed by its sensitivity whilst at the high end it is governed by its overload or strong signal handling performance. The overall dynamic range of the receiver is very important because it is just as important for a set to be able to handle strong signals well as it is to be able to pick up weak ones. This becomes very important when trying to pick up weak signals in the presence of nearby strong ones. Under these circumstances a set with a poor dynamic range may not be able to hear the weak stations picked up by a less sensitive set with a better dynamic range.
  • 14. Problems like blocking, inter-modulation distortion and the like within the receiver may mask out the weak signals, despite the set having a very good level of sensitivity. These parameters are obviously important when determining what equipment should be used in a radio communications system. The BLOCKING characteristic is a measure of the receiver’s ability to receive a wanted signal at its assigned channel frequency in the presence of an unwanted interferer on frequencies other than those of the spurious response or the adjacent channels, without this unwanted input signal causing a degradation of the performance of the receiver beyond a specified limit. Digital receiver measurement are usually performed by varying a signal (unwanted, or wanted) and observing the bit error rate. To measure the effect of a potential inferring on a receiver you follow the steps bellow:      Find baseline sensitivity with a standard receiver data pattern i.e. 0.1 % BER Increase the received signal level to a specified level maybe 3 dB, specification radio access technology test specification will state this level. Introduce the blocker at the frequency you wish to test and start below the expected limit Measure the BER Keep increasing the blockers level and measuring the BER until the BER level reaches 0.1% and this is the level the system fails at. There are three types of blocking.  In band: - In Band the wanted and the Interferer are both inside the band and the interferer and the wanted are usually mirror images of each other.  Outer band: - The Interferer is placed outside the band at a particular required offset. For e.g., it can be a 5 MHz E-UTRA can act as an interferer.  Narrow band: - A single RB of the 5 MHz will cause the interference in this case. SPURIOUS RESPONSE is a measure of the receiver’s ability to receive a wanted signal on its assigned channel frequency without exceeding a given degradation due to the presence of an unwanted CW interfering signal at any other frequency at which a response is obtained i.e. for which the blocking limit is not met.
  • 15. TAYLOR SERIES is a representation of a function as an infinite sum of terms that are calculated from the values of the function's derivatives at a single point. A one-dimensional Taylor series is an expansion of a real function about a point is given by When Harmonics of in band signals mix together THIRD PRODUCTS are formed. ORDER If the input frequencies are f1 and f2, then the new frequencies produced will be at 2f1 - f2, 3f1 - 2f2, 4f1 - 3f2 and so forth. On the other side of the two main or original signals products are produced at 2f2 - f1, 3f2 - 2f2, 4f2 3f1 and so forth. These are known as odd order inter-modulation products. Two times one signal plus one times another makes a third order product, three times one plus two times another is a fifth order product and so forth. The main signals are first the third order product, then fifth, seventh and so forth. One of the intermodulation products will fall in to the wanted channel, an example is shown below: F wanted = 2140 MHz Fu interferer_cw = 2150 MHz; Fu interferer _mod = 2160 MHz Higher Side intermodulation test frequencies close in third order products will be at 2*Fu interferer cw – Fu interferer _mod and 2*Fu interferer_mod - Fu interferer_cw 2 * 2150MHz – 2160MHz = 2140 MHz !!!! Will fall on your wanted channel, produce and interfere 2 * 2160MHz -2150MHz = 2170MHz Lower Side intermodulation test frequencies Close in third order products will be at 2*Fl interferer _cw – Fl interferer _mod and 2*Fl interferer _mod - Fl interferer _cw
  • 16. Authors 2 * 2130MHz – 2120MHz = 2140 MHz wanted channel, produce and interfere 2 * 2120MHz -2130MHz = 2110MHz Nisha Malik Student M.Tech !!!! Will fall on your To overcome these 3rd order distortion which produce an on channel interferer a more linear RX chain is required i.e. output intermodulation intercept point is high (OIP3). 6. Conclusion The authors have explained the basic fundamentals of RF and Radio conformance. They explained about test tools, test thresholds and some basic calculations .We believe this paper will give the basic understanding and confidence for anybody who has interest in radio testing. Wayne Turner System Design Engineer 7. References    Preet Rekhi System Test Engineer Rahul Sharma System Development Engineer    3 GPP 36 141 LTE: Evolved Universal Terrestrial Radio Access (E- UTRA); Base Station (BS) conformance testing 3GPP 36104 LTE: Evolved Universal Terrestrial Radio Access (E-UTRA); Base Station (BS) radio transmission and reception R& S LTE Base Station Tests according to TS 36.141 application notes Agilent X-Series Signal Analyzer application notes Agilent X-Series Signal Generator application notes LTE, The UMTS long Terms Evolution: From Theory to Practise Disclaimer: Authors state that this whitepaper has been compiled meticulously and to the best of their knowledge as of the date of publication. The information contained herein the white paper is for information purposes only and is intended only to transfer knowledge about the respective topic and not to earn any kind of profit. Every effort has been made to ensure the information in this paper is accurate. Authors does not accept any responsibility or liability whatsoever for any error of fact, omission, interpretation or opinion that may be present, however it may have occurred Sukhvinder Malik System Test Engineer

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