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Application Note | 802.11 a/b/g WLANReceiver and PER Testing With SeaSolve’s WiLANTA LVSG11bg

Over the years WLAN devices have evolved into complex products that offer extensive features; such as multi-band capability and higher data rates, and their chipsets (RF ICs) are also multifac-eted, since they integrate radio-frequency (RF), analog, and digital technology into single devices. The transmitted data is also mapped onto the WLAN frequencies using complex modulation schemes.

As such, it is necessary to test/verify many of these features to meet the functionality, standards-compliance and interoperability specifications before it can be shipped.

Therefore a test solution should provide the functionality and accu-racy required of the specification, the flexibility to be updated and improved in-line with the development of the standard and the execution speed needed to perform these tests as quickly as pos-sible.

A number of different methods are employed to test WLAN devices and chipsets, including sinusoidal generation testing, spec-trum analysis, and the ‘golden radio’ approach.

Older test and measurement tools were designed for narrowband systems where the response of a DUT to a sinusoidal input was checked, yielding certain S-parameters that related information about the DUT’s performance.

This approach is impractical for WLAN devices since they rely on wideband signals that consist of complex modulation. In order to effectively test WLAN devices that operate in the 2.4 GHz and 5 GHz bands with bandwidths of 20 MHz, it is necessary to use signal generators and analyzers that are capable of complex modulation and demodulation, respectively.

Spectral analysis can verify some of the PHY (physical layer) char-acteristics of the DUT output signal, but it falls short when it comes to data demodulation and actual testing of the MAC layer.

Overview

Traditional Test Methods

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Application Note

802.11 a/b/g WLAN Receiver

& PER TestingWith SeaSolve’s

WiLANTA LVSG11bg

A versatile test-bench should be capable of both signal and data analysis.

The ‘golden radio’ approach is where one WLAN device transmits common data to DUT(s) in the production line; a common method for verifying the receiver characteristics of WLAN devices. This, though sometimes accurate, limits the testability of the DUTs by subjecting them to only one constant signal/data pattern that will have to be changed manually. ‘Golden radios’ also need regular calibration to keep test results accurate (unless they use transmit algorithms that take this factor into consideration). Another disad-vantage of ‘golden radios’ is that they cannot be used to measure Error Vector Magnitude (EVM), a measure of modulation accuracy which is required to check the quality of complex modulation schemes (such as 16- or 64-QAM) used in 802.11 systems.

The main problem with these methods is that they all require further equipment to carry out a complete analysis of 802.11 devices; a combination of both spectral analysis (spectrum analyzer) and modulation/demodulation tests (calibrated golden radio). There-fore, they are not only expensive but also limit the testability of 802.11 devices.

This application note will show how SeaSolve’s WiLANTA LVSG11bg signal generator software, in combination with the National Instruments PXI-5670 RFSG vector signal generator, can be used to provide extensive WLAN receiver testing, with transmit-ted parameters defined by the user.

The combination of SeaSolve’s WiLANTA LVSG11bg, WLAN Vector Signal Generation software, and National Instruments’ PXI-5670/71, RF Signal Generator hardware, provides a flexible platform for testing the receiver characteristics of a WLAN DUT at both the PHY and MAC layers.

Designed to generate, and test a DUT’s response to real-world 802.11 b/g signals at the RF and baseband levels.

WiLANTA LVSG11bg transmits 802.11 standard-specific signals at user-defined power levels and specific channels; which eliminates the concern of fixed reference signal transmission mentioned earlier.

With WiLANTA LVSG11bg, the ‘golden radio’ method is replaced by the generator itself, since the user is able to configure the exact signal and MAC data that is to be transmitted to the DUT. The data payloads of the signal can also be changed in order to test the DUT’s response over a broad & varied range of signal and data inputs.

WiLANTA LVSG11bg – WLAN Signal Generation

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Application Note 802.11 a/b/g

WLAN Receiver & PER Testing

With SeaSolve’s WiLANTA LVSG11bg

And lastly, calibration of the VSG instrument is a much less frequent process.

This document highlights the use of WiLANTA LVSG11bg in verify-ing a receiver and PER (Packet-Error-Rate) characteristics. Receiver characteristics, in this context, shall apply to the DUT’s response to WiLANTA LVSG’s signal and data generation.

WiLANTA LVSG runs either on a standard PC/Laptop with a MXI interface to the PXI hardware or an embedded PXI controller. There are a number of hardware options available, depending upon the specific application. Please call to speak to a SeaSolve appli-cation engineer for advice.

When testing the receiver characteristics of a wireless DUT it is critical to generate standard-compliant signals. WiLANTA LVSG11bg offers a host of options for generating signals according to the user-selected IEEE 802.11 standard specifications.

Generating the Right PHY with WiLANTA

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Synchronizing with the DUT Receiver

Before a receiver can be tested it is necessary to synchronize the RF Signal Generator with the DUT, WiLANTA offers the following options for configuring the NI RFSG

1. Power Level

Before a receiver can be tested it is necessary to synchronize the RF Signal Generator with the DUT, WiLANTA offers the following options for configuring the NI RFSG

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Standards, modulations, and data rates

The user has the option of transmitting data in accordance with the IEEE 802.11b/g standards along with their respective modulations and data rates. This enables testing of a DUT’s response to various modulation schemes and data throughput.

Scrambler

Scrambling a long sequence of data makes it less prone to burst errors. The scrambler applies to the entire data sequence with one exception – OFDM modulated MAC headers are never scrambled. Note that whenever a transmission is scrambled, it must be descrambled at the receiver/analyzer end, as can be seen later in WiLANTA LVSA11bg, SeaSolve’s complimentary WLAN Analysis software application.

Standard-Compliant signals

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There are 14 channels within the WLAN protocol; any channel can be selected within WiLANTA LVSG

2. Channel

3. Transmission Mode

Unmodulated Carrier – Transmits only the center frequency. Used in checking for frequency offsets or frequency-synchronizing with the DUT.

Modulated Spectrum – Transmits continuous modulated basebanddata, where received power spectral density may be checked at the receiver end

Packet Transmission – WiLANTA can transmit packets of user-defined data in bursts (with a delay between packets) to the DUT. As this application note will discuss later, this mode can be used to send custom MAC data or a sequence of frames to the DUT

Figure 1 : NI RFSG Settings


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The following demonstrates how a user can generate a fully cus-tomized MAC layer using WiLANTA LVSG11bg.

With WiLANTA’s ability to generate custom frames or sequences of frames, a DUT can be tested for its response to different MAC frames and network scenarios.

Generating a MAC Layer

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Custom MAC Data Payload

On the front panel of WiLANTA LVSG11bg, the user can define the exact number of octets to be transmitted in a MAC data frame.

In addition, the user can define the bits that will be carried in the MAC data payload by selecting from the adjacent tabbed list. Note that the data payload patterns and number of octets only apply to a data frame

If necessary, data payloads may be used to check the DUT’s response to a defined pattern of data.

Figure 2 : MAC Data Payload Settings

However, it is possible to fully customize the data contained in the transmitted frame body as discussed below.


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Custom Frame Data

One of the main features of WiLANTA LVSG11bg is its ability to generate custom MAC frames with user-defined fields that are completely network-specific to 802.11 standards.

The user is able to send certain types of network-specific frames with custom settings by altering values in the ‘Frame Control Field’. Source and destination addresses can be set so that the MAC data will be sent to the DUT within the Basic Service Set (BSS). Duration/ID may also be set to test the timing of the frames at the receiver. The frame body containing the data to be transmitted can be altered down to the bit level in the MAC frame window.

With these custom MAC settings, it is possible to test the DUT’s response to varying types and number of frames. Source and destination addresses, BSSID, and SSID help keep data trans-mission within the BSS under test.

Figure 3 : Advanced MAC frame configuration panel


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With WiLANTA LVSG11bg, the user also has the option of trans-mitting a sequence of frames, as opposed to sending multiple packets of the same frame, as discussed above.

With the ‘Advanced Settings’ dialog of the ‘Build Sequence of Frames’ option, the user can generate a custom sequence of frames, complete with the number of sequences and the number of frames transmitted in each sequence.

This feature comes in handy when testing the DUT’s response to different network scenarios, such as the exchange of control frames.

For example, if WiLANTA transmits an RTS (Request-to-Send), the DUT should respond with a corresponding CTS (Clear-to-Send) frame. A more complex scenario - WiLANTA would send a CTS after which it receives and acknowledges a data frame from the DUT, and then not respond for a set time.

Here, the user can check whether the DUT will drop the 2nd data frame and move on to the next one.

Below is an example scenario where two network stations are communicating by exchanging RTS, CTS, ACK, and data frames in a CSMA/CA methodology.

Checking the DUT’s response to the transmitted frames would require either a 3rd-party controller/analyzer software or WiLANTA LVSA11bg Online signal analyzer (acting as a ‘sniffer’ to view the DUT’s response).

Testing Network, CSMA/CA Scenarios with Frame Sequences


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We have seen how data frames can be generated with WiLANTA LVSG11bg and sent to the DUT .

As mentioned above, a ‘sniffer’ or analyzer application such as WiLANTA LVSA11bg can be used in conjunction with the NI RFSA 5660/1 vector-signal-analyzer, to demodulate the data received from the DUT in response to frames sent by WiLANTA LVSG11bg.

In this section, we shall explore how the PER (Packet-Error-Rate) characteristics of a DUT can be found after the DUT has received frames from WiLANTA LVSG11bg.

The PER value (as a percentage of received frames out of the total number of frames transmitted) gives a good indication of how well the DUT receives and demodulates the data received from a transmitter.

Testing Receiver PER Characteristics

DUT Controller Software and PER Calculation

In order to measure PER at the DUT receiver, it becomes neces-sary to have the DUT ‘controller’ software that is capable of counting the number of frames received successfully by the device.

Some controller software for WLAN devices will even do a PER calculation for the user based on the number of frames expected versus the number of frames received while the DUT is in ‘Re-ceive’ mode.

PER is therefore, the ratio of lost frames to the total number of transmitted frames that the DUT is set to receive, calculated as follows:

PER% = (1 - (Number of frames correctly received/Number of frames sent)) x 100%

Ideally, this value should be 0%, but this is almost never encoun-tered in real-life WLAN scenarios. However, PER should be kept to a minimum in order to ensure a device’s accuracy of recep-tion, necessary for WLAN manufacturers to fulfill the promises of high data rates and QoS.

In the absence of DUT controller software, it is possible to calcu-late PER with the following method: Using an analyzer/sniffer that can capture DUT transmissions, and then count the number of ‘Acknowledge’ frames sent back by the DUT as it receives frames from the NI RFSG generator. This number of ACK frames is the number of frames successfully received by the DUT, and


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can be used in the equation given above.

A DUT can be tested under different conditions to see whether it’s PER values can stay within acceptable limits. For instance, PER can be seen at varying power levels, varying distances (from the transmitter/NI RFSG), and various impairments imposed on the RF channel.

The next section will deal with checking the DUT’s response to signals that are impaired using the ‘Signal Impairments’ feature of WiLANTA LVSG11bg.

Impairments – Simulating Non-ideal Signals and Environments

WiLANTA LVSG11bg enables performance-testing of a DUT receiver under intentionally non-ideal scenarios with a wide range of signal impairments.

Impairments can be set at user-defined levels so that the user has con-trol over the exact ‘non-idealities’ that are imposed on the RF channel.

The simulation of impairments on a signal can be used to test the DUT’s response in field environments, wherein signals may be impaired by noise, multipath, frequency offset, etc. WiLANTA LVSG11bg provides for the configuration of the following signal impairments:

Memoryless Non-linearity

Frequency offset

I/Q Gain Imbalance

Phase noise

AWGN

DC Offset

Quadrature skew

Figure 7 : Signal Impairments Settings


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As discussed in the previous section, it is possible to check the PER value of the DUT’s receiver after the transmitted signal has been impaired with the above settings. Designers can employ this method of testing PER under varying signal conditions to check the performance bounds of a WLAN DUT.

The WiLANTA AdvantageIn conclusion, we highlight the advantages of the WiLANTA LVSG11bg signal generation platform over similar approaches to WLAN testing.

Replacement of the WLAN ‘Golden Radio’With WiLANTA LVSG11bg and the NI 5670 RFSG, a complete signal generation test-bench is available that replaces the inefficient ‘golden radio’ test for production-line WLAN devices. The NI hardware gener-ates accurate test signals through WiLANTA’s direction and requires very infrequent calibration.

Configurable Signal GenerationSinusoidal input test -methods are outdated and inaccurate for testing current 802.11 devices. WiLANTA LVSG11bg can be configured to transmit user-defined data at varying power levels and channels, with or without additive signal impairments. Thus, it is a flexible signal genera-tion utility for extensively testing the receiver performance of a DUT, down to the frame- and bit-level.

Simulation of Network ScenariosWith the ‘Frame Sequences’ feature of WiLANTA LVSG11bg, it is pos-sible to fully emulate different network scenarios in order to test the DUT’s response to them.

Integration with the Versatile NI RF PXI PlatformWiLANTA LVSA11bg’s seamless integration with the NI PXI 5660 RF module requires minimal hardware knowledge and interaction on the user’s part. The NI platform also enables testing of a DUT at any stage of development, rather than having the designer wait to test a finished product. Therefore, the SeaSolve/NI test-bench is useful for production testing of 802.11 b/g devices.

PER Measurement after Impairments

SeaSolve Software Inc (SSI) is a leading supplier of real-time test and measurement solutions for testing and analyzing the performance of IEEE 802.15.4/ZigBee (WPAN), IEEE 802.11a/b/g (WLAN), IEEE 802.16-2004 (WiMAX) and IEEE 802.16e (Mobile WiMAX) transmitters and receivers.SeaSolve is headquartered in San Jose, CA and has regional offices in Europe and Asia

SeaSolve’s signal generation and signal analysis products imple-ment complete PHY and MAC layer functionality. They are designed to help investigate and isolate functional issues that arise during design and validation testing - quickly and efficiently.

Our manufacturing test tools can be provided as stand-alone APIs, a .dll library, a set of TestStand Custom Step Types (CSTs) and/or a fully integrated automated application, designed to work seam-lessly with NI’s LabVIEW & TestStand applications as well as many other industry standard ADEs, such as C/C++, VB etc.

SeaSolve also has a suite of IP cores that can be incorporated into 3rd-party device designs. They include entire PHY and MAC imple-mentations of various wireless standards as well as components and algorithms that can be utilized as functional blocks of a wireless and/or embedded device. These are available as fixed point imple-mentations or as synthesizable VHDL source codes.

SeaSolve's products seamlessly integrate with RF PXI modules to provide efficient transmitter and receiver testing of WLAN/WPAN and WiMAX devices.

About SeaSolve

www.seasolve.comWireless Test & Measurement Solutions

© 2006 SeaSolve Software Inc

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