Verifying a Connected Solutions Design

This chapter explains how to verify your CS design. After creating a CS design and its data display template, you can test it on any ADS system using Connection Manager. Use the following guidelines for running simulations and connecting to instruments to help you verify the design.

DUT Model Simulation to Device Measurement

Each wireless design provides a stimulus-response test environment for an RF DUT. Using a wireless design, you can create and refine stimulus characteristics and measurement parameters on a mathematical model of your device prior to measuring the device itself using connected instrumentation.

The former is referred to as DUT Model measurement, the latter physical device measurement.

Effectively, the design contains a double-pole double-throw switch controlled by the Instrument Connectivity Enabled parameter within the design dialog.
When instrument connectivity is not enabled, the design uses its internal software model for the test instruments (ESG and VSA) and the DUT (cascade of DUT pre-amplifier DUT, and DUT post-amplifier).

When instrument connectivity is enabled, the design uses the external hardware consisting of the hardware test instruments (ESG and VSA) and the hardware DUT (cascade of DUT pre-amplifier, DUT, and DUT post-amplifier).

A model for the Connected Solutions design is shown in Connected Solutions Design Block Diagram.

Connected Solutions Design Block Diagram

For either case the design provides the same signal stimulus generation and signal response measurement.

Use DUT Model simulation (instrument connectivity disabled) to develop your stimulus and measurement schemes and simulate their effect upon a model of your device.

After you are satisfied with your simulated signals and measurements, enable the instrument connectivity and perform the test on your physical device.

Running Simulations without Instrument Connections

Verify the design in Simulation Mode by running simulations for different settings.

Running Simulations with Instruments Connected

When verifying design operation with instrument connections, the instruments to use depend on the design template. See Connecting Instruments for details on these design instrument configurations:

Connecting Instruments

Depending on your requirements, follow the steps listed in the appropriate section to configure systems for testing RF transmitter, RF receiver, or baseband (BB) designs.

RF Transmitter and RF Receiver Design System Connections

The following procedure details system connections for RF transmission designs, including Agilent E4438C ESG, Agilent 896xx VSA, and PC connections.

  1. Connect the E4438C ESG GPIB connector to a USB port on the PC using an Agilent 82357A USB/GPIB gateway.
    Other ways for the connections are available through connection manager installed on the PC; for details, refer to the Connection Manager manual, Getting Started , "Configuring the Server IO on PC".
  2. Connect the E4438C ESG Event 1 BNC output to the Agilent 896xx VSA Ext Trigger BNC input through BNC tee with a 50 ohm termination. Based on the trigger setup guidelines from VSA 89600 menus, the minimum pulse width for the External trigger is 300 ns. For an Agilent 896XX VSA, the maximum bandwidth is 36 MHz. The sampling step can be set to 28 ns and the minimum number of points to set the trigger is about 20.
  3. Connect the Agilent 896xx VSA Ext Clock/Ref BNC output to the E4438C ESG 10 MHz IN BNC input.
  4. Connect the Agilent 896xx VSA IEEE 1394 Port A (or B or C ) to your PC using an IEEE-1394 cable.

    For the DUT, the input connects to ESGc output by an RF cable and the DUT output is connected to Ch1 of VSA by another RF cable.

RF Receiver Design System with Baseband IQ Connections

The following procedure details system connections for RF receiver designs, including Agilent E4438C ESG, Agilent 896xx VSA, and PC connections.

  1. Connect the E4438C ESG GPIB connector to a USB port on the PC using an Agilent 82357A USB/GPIB gateway.
  2. Alternative connection methods are available through Connection Manager installed on the PC. For details, see the ADS documentation Translation & I/O > Connection Manager > Getting Started > Configuring the Server IO on the PC .
  3. Connect the E4438C ESG Event 1 BNC output to the Agilent 896xx VSA Ext Trigger BNC input through BNC Tee plus 50 ohm termination.
  4. Connect the Agilent 896xx VSA Ext Clock/Ref BNC output to the E4438C ESG 10 MHz IN BNC input.
  5. Connect the Agilent 896xx VSA IEEE 1394 Port A (or B or C) to your PC using an IEEE-1394 cable.
  6. Connect the DUT input to ESGc output through an RF cable.
  7. Connect the DUT IQ output to Ch1 and Ch2 of VSA by two ENC cables of the same length and type.

RF Receiver Design System with Logic Analyzer Connections

This procedure details system connections for WiMAX BER DIGITAL-IF workbenches. The following instruments will require some sort of IO connection cable or, if appropriate, converter box (LAN, GPIB, USB-GPIB, LAN-GPIB gateway, etc.) linked to your PC which has the control software installed, ADS 2006 Update 2 or higher + 89601 VSA software):

  1. Using a BNC cable, connect the E4438C ESG 10 MHz REF Out BNC output to the Encode Clock ESG 10 MHz REF IN BNC input.
  2. Using a BNC cable, connect the E4438C ESG Event 1 Marker output to the 16900 Logic Analyzer TRIG IN (rear) BNC input.
  3. Using two similar-length BNC cables, connect the E4438C ESG RF Out to the DUT A IN , and the Encode Clock ESG RF Out to the DUT BNC input.
  4. Using the E5383A Flying Lead Adaptor or similar, connect the 14 data bits and clock to the 16900-series mainframe via Pod 1 of the 16910A LA input plug-in

Configuring the Agilent Logic Analyzer Interface

The Logic Analyzer software will control the logic analyzer as a remote instrument via the LAN. A file included with ADS Connected Solution release will be used to configure the LA.
The following instructions are a concise version of those found in the Agilent 89600 VSA Help, Chapter 9, "Linking Logic Analyzers to the Agilent 89600 VSA."

  1. With the LA (which should have the Agilent Connection Manager software installed) connected to the LAN and powered on, start the Logic Analyzer application on the PC.
  2. Click Start > Programs > Agilent 89600 VSA > Logic Analyzer > IO Connections. When the "Agilent 89600 I/O Connections" dialog box appears, click Add Instrument and type localhost in the "Hostname" text box. Click Test Connection to make sure the connection works. Click OK.
  3. Select Go Online, Select Add Remote System, select it and Connect Remote:

    The window should show remote operation as shown below:
  4. Load .ala file from the ADS installation folder C:\ADS2006U2\adsptolemy\templates\ 16900_CSW_Setup_File.ala:
  5. The Logic Analyzer window should automatically configure as shown
  6. In ADS, load the example C:\ADS2005A\examples\System-TestWorkbench\STW_WMAN_16e_OFDMA_Downlink_prj\STW_WMAN_802_16e_DL_RX_Measurement.dsn. The Uplink BER example may also be used.
  7. Enable the VSA Digital IF Option and other parameters as needed in the main VAR block, and set the Oversampling Ratio Option =2 on the VAR_SourceParameters item as shown. This sets the Oversampling Ratio to 4 and is required for correct BER measurement with the Digital IF configuration.
  8. The other parameters for this measurement are set appropriately for the WiMAX example and DUT used here. If a different DUT is used, it may be necessary to adjust the RF and VSA frequencies as well as the Encode Clock frequency. After powering on the DUT, enabling Instrument Connectivity and setting the desired power sweep parameters, a measurement may be run at this time.

    The above BER/PER example used 4 sweep points, from -20 to -14 dBm, with AWGN enabled. In the above display, the default number of 3 decimal places was increased to 4 decimal places. A more typical test would have set Eb/No to a larger value, e.g. 20, and would have resulted in 0 PER.

Configuring the IO

As described above, at a minimum you need to link your PC's Agilent IO Library interface to your ESG-C signal generator. Although there are multiple supported ADS/Agilent IO Library IO Interface combinations (LAN, GPIB, USB-GPIB, LAN-GPIB gateway etc.), one approach using LAN cable connections to the PSG and Infiniium scope is described below.

  1. Make sure that the following icons are active in your Windows toolbar:
    • Agilent IO Libraries blue "IO" icon (Agilent IO Library version "M.01.01.04" recommended)
    • Agilent 2005A Add-on Connection Manager Server (instrument display screen icon)
  2. With your control PC (has ADS installed) on the LAN, also connect your ESG-C to your LAN hub. Power "ON" the ESG.
  3. Right-click on the blue Agilent IO Libraries "IO" icon, and select Run IO Config .
  4. Remove all Configured Interfaces and select * Auto Config .
  5. For the ESG connected on LAN: Left-click to select the TCPIP0 Configured Interface. Then click Edit > Edit VISA Config > Add device . Enter the requested IP address of your ESG source and click OK to close this configured window.

Troubleshooting

If you need instruction on using the Connection Manager's server diagnostic tool, or you experience problems connecting to instruments, see the ADS documentation Translation & I/O > Connection Manager > Troubleshooting .

Wireless Measurement Definitions

The following sections briefly describe the most commonly used measurements in transmitter and receiver testing for WLAN. For additional information on the Connection Manager theory and client-server architecture, see the ADS documentation Translation & I/O > Connection Manager > Operational and Functional Concepts .

Transmission Test Measurements

RF Envelope

The RF envelope measurement represents the magnitude of the RF signal's envelope versus time. By observing a signal's RF envelope versus time, you can see the signal's frame/burst structure. Some wireless standards specify a mask to which the signal's RF envelope must conform. The mask is typically specified during the ramp up and ramp down transient of the signal.

Constellation

The constellation measurement displays the general signal quality. For a more accurate measurement of a signal's modulation quality, perform an EVM analysis.

A constellation measurement is most appropriate when a QAM modulation scheme is used, directly or indirectly. For example WLAN 802.11a uses OFDM modulation, where a set of subcarriers is modulated using some QAM scheme. An ideal QAM signal will have a constellation that consists of a set of distinct points on the IQ plane. Constellation for Ideal 16-QAM Signal shows the constellation for an ideal 16-QAM signal. This constellation has 16 points and is symmetric around the X and Y axes. All points are equally spaced.

Constellation for Ideal 16-QAM Signal

Different signal distortions modify the ideal constellation in different ways. Observing the constellation may help in determining the signal distortion type. Constellation for 16-QAM Signal with Gain Imbalance through Constellation for 16-QAM Signal with Multiple Types of Distortion show examples of signal distortions appearing in the constellation.

Constellation for 16-QAM Signal with Gain Imbalance shows the constellation for a 16-QAM signal with gain imbalance. This can be deduced by the fact that in the ideal constellation the span of the points across both axes is equal (approximately 0.425), whereas in the constellation with the gain imbalance the Y axis has a greater span (approximately 0.475).

Constellation for 16-QAM Signal with Gain Imbalance

Constellation for 16-QAM Signal with Phase Imbalance shows the constellation for a 16-QAM signal with phase imbalance. This can be deduced by the fact that in the ideal constellation the points are lined up parallel to the X and Y axes, whereas in the constellation with the phase imbalance the points are lined up parallel to the X axis but not to the Y axis.

Constellation for 16-QAM Signal with Phase Imbalance

Constellation for 16-QAM Signal with ISI and AWGN shows the constellation for a 16-QAM signal with ISI (intersymbol interference) and AWGN (additive white gaussian noise). Notice that instead of 16 distinct points there are 16 clusters of points centered around the ideal 16-QAM points.

Constellation for 16-QAM Signal with ISI and AWGN

Constellation for 16-QAM Signal with Gain Compression shows the constellation for a 16-QAM signal with gain compression (from a nonlinear amplifier). Notice how the outer points (especially the four corner points) that have the highest power have been compressed (moved closer to the center of the IQ plane).

Constellation for 16-QAM Signal with Gain Compression

Constellation for 16-QAM Signal with Multiple Types of Distortion shows the constellation for a 16-QAM signal that includes all of the distortions discussed above.

Constellation for 16-QAM Signal with Multiple Types of Distortion

Power

The power measurement is a set of power-related measurements that provide information about a signal's statistical properties. The power measurement includes:

In Power CCDF Curve for a WLAN 802.11a 54 Mbps Signal, the instantaneous signal power exceeds the average signal power (0 dB) for approximately 20% of the time. Also, the instantaneous signal power exceeds the average signal power by 5 dB for only 0.7% of the time.

For signals that are transmitted in bursts or in slots, this measurement should be performed only when the signal is active (on) and not during the idle intervals or inactive slots.

Power CCDF Curve for a WLAN 802.11a 54 Mbps Signal

CDP Measurement for 3GPP FDD Test Model 3 Signal with 16 DPCHs

CDP Measurement for 3GPP FDD Test Model 3 Signal with 16 DPCHs, clearly displays the 16 active DPCHs (occupying codes in the 64 to 128 range), as well as the primary CPICH (code 0), the P-CCPCH+SCH (code 1), and the PICH (code 16).

Spectrum

The spectrum measurement shows the spectrum of a signal that is the distribution of the signal's power in the frequency domain. Most wireless standards specify a mask to which the signal's spectrum must conform. The transmitted signal spectrum must conform to a spectral mask to minimize the interference to adjacent channels. Spectrum Measurement and Spectral Mask for WLAN 802.11a Signal is an example of a spectrum measurement and a spectral mask for a WLAN 802.11a signal.

Spectrum Measurement and Spectral Mask for WLAN 802.11a Signal

Some wireless standards specify additional spectrum related measurements. For example, the 3GPP FDD standard defines the following spectrum measurements:

Occupied Bandwidth Definition

EVM

The EVM (error vector magnitude) measurement provides a metric for the modulation quality/accuracy of a signal. EVM is a measure of the difference between the measured signal and an ideal reference signal. While EVM may be defined differently in each wireless standard, the basic concept is described here.

Let S(k) , k = 1, ..., N, be the ideal transmitted signal sampled at one sample-per-symbol (or chip) at the optimal (zero-ISI) instance. The actual transmitted signal can be modeled as

where:
W = e Dr+jDa , accounts for both a frequency offset ( Da radians/symbol phase rotation) and an amplitude change rate ( Dr nepers/symbol)
C 0 is a complex constant representing origin offset
C 1 is a complex constant representing the transmitter's arbitrary phase and output power
E(k) is the residual vector error on sample S(k)
The sum square error vector is

where C 0 , C 1 , W are chosen such as to minimize the above expression.
EVM (rms) is defined to be the rms value of | E(k) | normalized by the rms value of | S(k) |. Therefore,

In addition to the EVM rms value, the EVM analysis provides other useful results, such as:

For signals that use code division multiple access (CDMA) techniques, peak code domain error (PCDE) is another useful measurement related to EVM. To get the PCDE value, the error vector E(k) is first projected on the code domain. This is done by calculating the inner-product between E(k) and all the orthogonal vectors in the channelization code set (all codes belonging to the one spreading factor). Then the maximum projection is selected and normalized by the rms value of the reference signal.

Receiver Test Measurements

BER Measurements

A receiver's performance is determined by its ability to receive and demodulate a wanted signal in the presence of noise and/or other interfering signals. Although there are several measurements used to test a receiver's performance, all of them measure the same quantity under different conditions. The measured quantity is the bit error rate (BER). BER is the probability that a transmitted bit will be received and detected in error. Of course, better receivers have a lower BER.

Different wireless standards give different names to various BER measurements such as: Minimum Input Power Sensitivity, Minimum Input Level Sensitivity, Adjacent Channel Rejection, Adjacent Channel Selectivity, Reference Sensitivity Level, Dynamic Range, Blocking, Intermod. All of these measurements are BER measurements under different conditions. These different conditions include additive white gaussian noise (AWGN), modulated interference signals, and CW interference signals. The interference signals can be in band and/or out of band. Typically, the standards specify that the BER should not exceed a certain value for certain power levels of the wanted and interfering signals, and a certain frequency offset (between the wanted signal's channel frequency and the frequency of the interfering signals).

E b /N o Definition

Bit error rate (BER) and frame and packet error rate (FER/PER) are typically reported with respect to E b /N 0 . This section defines E b /N 0 and relates it to signal to noise ratio (SNR). Distinction is made of local and system E b /N 0 . The following discussion is based on similar discussions published by Bernard Sklar [1\], [2\].

For this discussion Typical RF Communication System Receiver Block Diagram illustrates a typical RF communication system receiver block diagram.

Typical RF Communication System Receiver Block Diagram

In Typical RF Communication System Receiver Block Diagram:

Local values of S / N o and E b / N 0 are directly measurable at each point in the system and are typically the preferred S / N o and E b / N 0 values used by RF/analog designers.
At points A, B, and C, the local S / N o values are:
S A /N OA = S A /(k T a )
S B /N OB = (S A G 1 )/(k (T a +T 1 ) G 1 ) = S A /(k (T a +T 1 ))
S C /N OC = (S A G 1 G 2 )/(G 1 G 2 k (T a +T 1 )+G 2 k T 2 ) = S A /(k (T a +T 1 +T 2 /G 1 )
System values of E b / N 0 and S / N o are directly measurable only at the pre-detection system point (point C in Typical RF Communication System Receiver Block Diagram). These are the system values because they characterize the overall system performance. The system values are typically the preferred S / N o and E b / N 0 values used by System/DSP designers.

In all cases,
E b /N 0 = S/N o /R
At point C, the local E b / N 0 and S / N o values are the same as the system E b / N 0 and S / N o values.

Use of Local and System E b /N 0 in Designs

In designs, the E b / N o value used is the local E b / N o value at the Pre-Amplifier input (equivalent to point A in Simplified Amplifier Line-up Used with ESG and VSA), or the local E b / N o value at the DUT input (equivalent to point B in Simplified Amplifier Line-up Used with ESG and VSA), or the system E b / N o value.

Points A and B in Typical RF Communication System Receiver Block Diagram are related to the design's block diagram in Simplified Amplifier Line-up Used with ESG and VSA where the DUT 1 and DUT 2 in Typical RF Communication System Receiver Block Diagram represent the Pre-Amp and DUT in Simplified Amplifier Line-up Used with ESG and VSA respectively.

Simplified Amplifier Line-up Used with ESG and VSA

For the WLAN designs, the E b / N o value used is always the local E b / N o value at the Pre-Amplifier input.

The design E b / N o is set in WLAN Transmission designs using the parameters AWGN Specification Type = E b / N o in dB and AWGN Specification Value , and set in WLAN BER designs using parameter AWGN defined by E b / N o at ESG output .

Be aware that Local E b / N o ( dB ) at ESG output references the generated noise to the ESG output, whereas the signal power specified by the user is always referenced to the DUT input. These two different reference points for E b / N o and signal power require adjustment by the user if interest is in the equivalent signal-to-noise ratio.

Summary

E b /N o References

  1. Sklar, Bernard, Digital Communications: Fundamentals and Applications , 2nd Edition, Prentice-Hall, N.J., 2001.
  2. Sklar, Bernard, "RF Design: Will the Real E b /N o Please Stand Up," Communication Systems Design , April, 2003.

References

  1. Agilent Application Note "Characterizing Digitally Modulated Signals with CCDF Curves."
    http://literature.agilent.com/litweb/pdf/5968-6875E.pdf]
  2. Agilent PN (Product Note) "Using Error Vector Magnitude Measurements to Analyze and Troubleshoot Vector-Modulated Signals."
    http://literature.agilent.com/litweb/pdf/5965-2898E.pdf]
 

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