WLAN Layer 1 Testing

Transcription

1 Application Note #43 WLAN Layer 1 Testing December 2002 P/N REV A

2 Spirent Communications, Inc Agoura Road Calabasas Hills, CA USA Support Contacts Web: < Toll Free: SPIRENT ( ) Phone: Fax: Copyright 2002 Spirent Communications, Inc. All Rights Reserved. All of the company names and/or brand names and/or product names referred to in this document, in particular, the name Spirent and its logo device, are either registered trademarks or trademarks of Spirent plc and its subsidiaries, pending registration in accordance with relevant national laws. All other registered trademarks or trademarks are the property of their respective owners. The information contained in this document is subject to change without notice and does not represent a commitment on the part of Spirent Communications. The information in this document is believed to be accurate and reliable, however, Spirent Communications assumes no responsibility or liability for any errors or inaccuracies that may appear in the document. Warranty Spirent Communications, Inc.( Spirent ) warrants that its Products will conform to the description on the face of order, that it will convey good title thereto, and that the Product will be delivered free from any lawful security interest or other lien or encumbrance. Spirent further warrants to Customer that hardware which it supplies and the tangible media on which it supplies software will be free from significant defects in materials and workmanship for a period of twelve (12) months, except as otherwise noted, from the date of delivery (the Warranty Period ), under normal use and conditions. To the extent the Product is or contains software ( Software ), Spirent also warrants that during the Warranty Period, the Software which it supplies will operate substantially in accordance with the specifications supplied by Spirent for such Software. Spirent does not warrant that the functions contained in the Software will meet a specific requirement or that the operation will be uninterrupted or error free. Spirent shall have no warranty obligations whatsoever with respect to any Software which has been modified in any manner by Customer or any third party. The Warranty Period for Wireless software products shall be defined as ninety (90) days from the date of delivery. Defective Products and Software under warranty shall be, at Spirent s discretion, repaired or replaced or a credit issued to Customer s account for an amount equal to the price paid for such Product provided that: (a) such Product is returned to Spirent after first obtaining a return authorization number and shipping instructions, freight prepaid, to Spirent s location in the United States; (b) Customer provides a written explanation of the defect or Software failure claimed by Customer; and (c) the claimed defect actually exists and was not caused by neglect, accident, misuse, improper installation, improper repair, fire, flood, lightning, power surges, earthquake, or alteration. Spirent will ship repaired Products to Customer, freight prepaid, within ten (10) working days after receipt of defective Products. Except as otherwise stated, any claim on account of defective materials or for any other cause whatsoever will conclusively be deemed waived by Customer unless written notice thereof is given to Spirent within the Warranty Period. Spirent reserves the right to change the warranty and service policy set forth above at any time, after reasonable notice and without liability to Customer. TO THE EXTENT PERMITTED BY APPLICABLE LAW, ALL IMPLIED WARRANTIES, INCLUDING BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY, NONINFRINGEMENT AND FITNESS FOR A PARTICULAR PURPOSE, ARE HEREBY EXCLUDED, AND THE LIABILITY OF SPIRENT, IF ANY, FOR DAMAGE RELATING TO ANY ALLEGEDLY DEFECTIVE PRODUCT SHALL BE LIMITED TO THE ACTUAL PRICE PAID BY THE CUSTOMER FOR SUCH PRODUCT. THE PROVISIONS SET FORTH ABOVE STATE SPIRENT S ENTIRE RESPONSIBILITY AND CUSTOMER S SOLE AND EXCLUSIVE REMEDY WITH RESPECT TO ANY BREACH OF ANY WARRANTY.

3 This application note will describe some of the capabilities of the Spirent Wireless WLAN Layer 1 test offering. This application note contains the following topics Introduction to TAS4500 FLEX5 RF Channel Emulator Test Configuration Example and Equipment Setup Testing Overview Channel Impairments Examples Conclusion Spirent Communications Application Note #43 1

4 Introduction to TAS4500 FLEX5 RF Channel Emulator Introduction to TAS4500 FLEX5 RF Channel Emulator The Spirent Communications FLEX5 RF Channel Emulator possesses all the advanced capabilities necessary to provide accurate and repeatable multipath fading for current and developing WLAN applications. The FLEX5 operates between 25 and 3000 MHz with an RF bandwidth of 26 MHz. The FLEX5 is a 2-channel, 12-path device supporting both JTC 94 and Exponentially Decaying (Naftali) channel models. It also supports customized channel models with the powerful Dynamic Environment Emulation (DEE) feature. Support for a is also available utilizing the FLEX5 6 GHz Option. Test Configuration Example and Equipment Setup This application note concentrates on Access Point-to-Access Point (AP-to-AP) testing, although any two RF isolated wireless devices (such as an AP and a Network Interface Card [NIC]) with the capability of mating to the N-terminated connectors on the FLEX5 could be tested. (The FLEX5 provides N-Type connectors for both the input and output of each channel.) In order to use the FLEX5 directly in your experimental configuration, each AP must have at least two antenna connectors and support a simplex transmission mode. If the Device Under Test (DUT) has only one antenna connector, or does not support a simplex transmission mode, then circulators that cover the desired frequency range must be used to allow for a unidirectional connection between the AP and the FLEX5. Refer to Figure 1 on page 3 for the dual antenna simplex transmission configuration, and refer to Figure 2 on page 4 for the duplex antenna configuration. The following circulators may be used in testing: For B testing Alcatel 20A11-41 ( GHz) circulator. For A testing Alcatel 40E89-41 (5 6 GHz) or the Narda 4924 (4 8 GHz) circulators. 2 Spirent Communications Application Note #43

5 Test Configuration Example and Equipment Setup AP 10/100 Ethernet AX/ /100 Ethernet AP Receive Transmit Ch 1 In Ch 2 Out Ch 1 Ch 2 TAS4500 Ch 1 Out Ch 2 In Receive Transmit Figure 1. Example of a Simplex Transmission Access Point Connection Spirent Communications Application Note #43 3

6 Test Configuration Example and Equipment Setup Figure 2 displays the connection configuration for duplex antenna only access points. AP AX/ /100 Ethernet 10/100 Ethernet AP Port 1 Port 2 Receive/Transmit Port 3 Circulator Receive Ch 2 Out Receive/Transmit Transmit Ch 1 In Port 3 Port 1 Ch 1 Ch 2 TAS4500 Port 3 Ch 2 In Transmit Port 3 Circulator Port 2 Port 1 Ch 1 Out Receive Port 1 Figure 2. Example of a Duplex Antenna Access Point Connection In order to optimize the test configuration for adding the channel impairment characteristics, the input signal level and the Input Reference Level (IRL) of the FLEX5 must be set appropriately. Although there is an auto range feature available in the FLEX5 it assumes a constant waveform. Therefore, due to the nature of a WLAN modulated signal, it is recommended that the IRL of the FLEX5 be set manually in conjunction with the input signal level and its expected peak to average ratio (crest factor). Therefore the input signal level, based on a fixed IRL, should be set to a value based on the following equation: Transmit Signal Power (in dbm) applied at the RFIN of the FLEX5 = IRL - (Crest Factor of the Transmitted Signal [in db] -5 db), or the IRL of the FLEX5, based on a fixed input signal level, should be: IRL = (Transmit Signal Power [in dbm] applied at the RFIN of the FLEX5) + (Crest Factor of the Transmit Signal [in db] -5 db). 4 Spirent Communications Application Note #43

7 Test Configuration Example and Equipment Setup For example, if you have a transmit signal with a crest factor of 10 db, and the IRL on the FLEX5 fixed to -5 dbm, the transmit signal power applied to the RFIN of the FLEX5 should be set to approximately -10 dbm (-5 dbm - [10 db -5 db] = -10 dbm). Note: For optimum performance it is recommended to set the IRL of the FLEX5 between the range of -10 to +5 dbm. If the red overload light comes on, the transmit signal level is too high. Either adjust the transmit power level on the AP or use an inline coaxial attenuator to adjust the signal power level. In addition to the input signal level, the FLEX5 must also be configured to use the same channel as the APs. Refer to Table 1 or Table 2 on page 6 for a list of channel numbers and their frequencies Table 1. Channel Numbers and Frequencies for B Testing Channel Number Frequency (MHz) Spirent Communications Application Note #43 5

8 Test Configuration Example and Equipment Setup Table 2. Channel Numbers and Frequencies for A Testing Band (GHz) Channel Number Frequency (MHz) UNII Lower Band ( ) UNII Middle Band ( ) UNII Upper Band ( ) In order to ensure a proper test configuration, the APs should be unable to communicate with each other if all of the paths in the FLEX5 are turned off, or if the APs are disconnected from the FLEX5. If the APs are still able to communicate under either of these two conditions the following tips will be helpful: Verify that all external antennas have been removed, and if configurable verify the correct antenna (the one connected to the test port) has been properly selected. Increase the physical spacing between the APs (i.e., use longer cables). If the AP has adjustable transmitter power, set the output power to the lowest setting. Use an RF shielded enclosure for one or both of the AP units. Note: Before testing, make sure that the APs are properly connected to the TAS4500, the RF receiver has adequate isolation from the RF transmit signal, and the levels and carrier frequencies are properly set. 6 Spirent Communications Application Note #43

9 Testing Overview Testing Overview Due to the nature of wireless communications, significant care must be taken to establish a repeatable and accurate laboratory test setup. Therefore, the goal of the testing discussed in this document is to provide a controlled base (or experimental set up) that can be utilized to conduct performance testing in a repeatable fashion. Note that the following items should be kept in mind before making any AP comparisons: The IRL on the FLEX5 should remain the same for all APs under consideration. Either adjust the output level of the AP or use inline attenuators to avoid overloading the signal. Radio performance comparisons should be made only between setups involving an identical pair of APs to avoid any transparent interoperability issues that could exist. Note: If interoperability testing is being performed, the FLEX5 will not be able to determine which vendor (if any) is at fault. As previously indicated, this application note concentrates on AP-to-AP testing, therefore the test configuration for the baseline and impairment examples utilized two APs configured for bridging connected through a FLEX5 RF Channel Emulator. The APs used were programmed for channel 11 (2.462 GHz) with a transmit power of 1 mw. (All of the other AP programmable values were left to their default settings.) The FLEX5 was also programmed for channel 11, and set to use the internal local oscillator through the TASKIT software. In addition, a Spirent Communications Adtech AX/4000 Broadband Emulator was used to generate and analyze the data as illustrated by the following screen captures. In all test cases the AX/4000 produced a 4 Mbps stream of test traffic and graphed the streams packet rate, latency, and the test interfaces percent utilization in real time. Figure 3 on page 8 displays the results of non-impaired traffic. The chart is displaying packet rate, bandwidth utilization (for a 10 Mbps link), and latency (also known as average transfer delay). The generated traffic has a constant packet size and rate, which yields a constant magnitude for each of the three metrics. Spirent Communications Application Note #43 7

10 Testing Overview Note: In all graphs, time progresses from right to left, i.e., 0 seconds is now and 100 seconds is in the past. Transfer Delay Packet Rate Bandwidth Figure 3. Non-impaired Traffic Chart Display 8 Spirent Communications Application Note #43

11 Channel Impairments Examples Channel Impairments Examples The FLEX5 is capable of adding a number of channel impairments. The following examples show how to add some of these channel impairments as well as their impact on the AP-to-AP performance. Rayleigh Fading Rayleigh fading is commonly referred to as fast fading. A Rayleigh modulated signal is caused by scattering from man-made and natural obstacles, such as buildings and trees, in the local geographical area (within a few hundred wavelengths of the receiver). It is formed by a large number of these scattered (reflected) signals combining at the receiver. Each of these signals has a random phase and amplitude at the receiver due to the reflections and difference in distance traveled. Refer to Figure 4 for a graphical representation of Rayleigh fading for a typical AP installation. Figure 4. Example of a Rayleigh Fading Environment Configuration The Rayleigh fading function of the FLEX5 emulates both constructive and destructive interference, and therefore affects signal strength. The typical range for constructive interference is 0 to +10 db, while the typical range for destructive interference can range from 0 to 50 db. To use Rayleigh fading: 1 From the TASKIT GUI, click on the Modulation tab. 2 Select the path(s) in question, and then choose the Rayleigh option from the pull down menu. 3 Set the Fading Velocity. Since the AP is a stationary device, choose a value as close to 0 as possible. 4 Select a Power Spectrum Shape and choose a distribution. Classic 6dB is the default value. 5 The FLEX5 is now impairing the input signal based on the parameters specified. Spirent Communications Application Note #43 9

12 Channel Impairments Examples Figure 5 shows the impact of applying Rayleigh modulation to the DUT. Note: Depending on the output signal level of the desired signal, additional output attenuation may be required for the impairments to have a discernable effect. Transfer Delay Packet Rate Bandwidth Figure 5. Rayleigh Fading Traffic Impairment Example As the fader is impairing the link, notice the relationship between Transfer Delay and Packet Rate. As the packet rate declines, the delay increases. This occurs because the transmitter must retransmit each packet that is not successfully received. Therefore, more transmissions take more time, meaning increased latency. Notice that Packet Rate and Bandwidth utilization are directly related. 10 Spirent Communications Application Note #43

13 Channel Impairments Examples Rician Fading Rician fading is formed by the sum of a Rayleigh distributed signal and a Line-Of-Site (LOS or direct path) signal, where the LOS signal is typically subjected to a static frequency shift (static Doppler). A fading environment typically associated with Rician fading is one where a single strong direct path reaches the receiver at roughly the same delay as multipath from local scatterers. The Rician fading function of the FLEX5 is very similar to the Rayleigh fading function. There are two additional variables, however; the LOS arrival angle and the K factor. The LOS specifies the relative location of the direct path with respect to the faded spectrum by changing the static Doppler shift. The K variable controls the ratio of LOS signal to the scattered signal. Therefore the impacts of simulating a Rician fade model can be analyzed by selecting the Rician modulation option, and programming the desired LOS arrival angle and K factor. Delay Spread Relative path delay is a phenomenon where individual signal paths from the transmitter to the receiver arrive at different times. The net effect of the arrival time difference is to spread the signal in time. In a digital system this will cause received symbols to overlap resulting in inter-symbol interference. The amount of relative path delay varies with terrain and application. In an indoor application, delays could be in the 10s of nanoseconds, where 10 ns is approximately 10 feet. In outdoor applications, delays of 10 microseconds or less are typical (1 microsecond is about 100 feet). The Delay Spread function of the FLEX5 allows a different delay setting for each path in each channel. The specified delay is in addition to the nominal delay within the system. To use the Delay Spread function: 1 From the TASKIT GUI, select the Delay tab. 2 Enter the desired delay into each channel, or using the Channel Set Up (Graphical Display) select the Delay tab, and click on the Spread button. This feature will prompt the user for an Initial Delay, t and a Delay, d. The spread option will produce the following delay spread: path n = t + (n 1)*d. Spirent Communications Application Note #43 11

14 Figure 6 shows the impact of applying Delay Spread to the DUT. In the chart shown, the fader is impairing traffic, but the results are very different. Unlike fading, delay spread provides a near-constant impairment, i.e., there is no constructive or destructive interference. The result is that latency increases significantly whereas packet rate and bandwidth remain close to their non-impaired levels. Transfer Delay Packet Rate Bandwidth Figure 6. Delay Spread Traffic Impairment Example Phase Shift A static phase shift is a result of the random distance between the transmitter and receiver. This distance is very rarely going to be an integer number of carrier wavelengths; a noninteger value will result in a static phase shift on the signal path. The Phase Shifting function of the FLEX5 allows the fading emulator to independently vary the phase of any path from 0 to 360 in 0.1 increments. Relative phase changes in the desired signal indicate 1, 2, or more bits of the data stream depending on the link rate. Therefore, introducing phase shifts within the desired signal will enable the FLEX5 to introduce multiple bit errors into the data stream. However, since the receivers are only interested in relative phase changes, using the FLEX5 to introduce a single phase shift will produce a negligible result. In order to seriously

15 Conclusion impact the performance of the AP through phase shifts, the Dynamic Environmental Emulation (DEE) mode of the FLEX5 must be used. Using the DEE mode of the FLEX5, the two phase shift states were configured. The first state produces 90 phase shifts, while the second state produces no impairments. Each state lasts for 1.0 second. Figure 7 shows the impact of alternating between these two different phase shift states. The chart shows that the fader is oscillating between transmitting a phase shifted channel and transmitting a clean channel. Again, packet rate and latency are inversely proportional. The phase shifting introduces symbol errors into the radio transmission. Since the receiver will not recognize corrupted packets, the transmitter must send them again, and therefore increase the latency. Transfer Delay Packet Rate Bandwidth Figure 7. Alternate Signaling Chart Display Conclusion The FLEX5 RF Channel Emulator is a powerful tool for emulating accurate and repeatable multipath fading for various WLAN applications. When combined with traffic generation and analysis tools it provides a complete solution for analyzing various WLAN performance characteristics. Spirent Communications Application Note #43 13

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