IEEE 802.11 Physical Layer Operation and Measurement

Introduction

This application note is written for those who desire an understanding of the test system configuration and testing of Wireless LAN (WLAN) devices and some of the issues that arise in connection with it. Further detail on many of the topics covered herein may be found in the Appendices.

The use of wired Local Area Networks has become ever more commonplace, even in situations where only a few computers need to be connected together. Price reductions have helped stimulate use and so have easier system configuration and increasing robustness.

A number of applications can benefit from the removal of the cable connections needed by a fixed LAN. Remote database access is a good example – from warehouses to retail stores to college campuses. WLAN cards will also be used soon for public Internet access in certain "hotspots," such as airports and hotels, where users are largely stationary and need access to a variety of medium- and high-speed digital services.

The IEEE 802.11 Wireless LAN specification was written to extend the functionality provided by the IEEE 802.3 Wired LAN standard. A radio interface adds considerable complexity; however, advances in highly integrated radio circuitry have made it possible to bring the cost of wireless devices down to affordable levels.

The ETSI BRAN HiperLAN/2 is an alternative specification for WLAN, with more extensive services, but diminishing commercial support. Its radio frequency (RF) operates in a similar way to 802.11a, although the allocation of transmission time-slots is quite different. Increasing collaboration is now taking place between those involved in the two standards.

While wired LAN already uses numerous techniques to deal with multiple users who must access a central server, additional steps must be taken to deal with the vagaries of WLAN links. A WLAN link has many less-than-ideal transmission characteristics, such as the dependency of signal errors on physical position and the ability of nearby RF devices to "eavesdrop" or interfere.

Security is always an important issue in radio transmissions. Considerable effort is being made to ensure that security for WLAN is both adequate and straightforward to apply. This application note begins with a brief description of an IEEE 802.11 Wireless LAN system, emphasizing the radio or physical layer. Consistency at this level provides the basis for widespread device interoperability.

Comparisons are made to cellular radio systems to highlight the significant differences in the operation of the two links. Transmitter and receiver measurements needed to verify conformance with the IEEE specification are described, along with information on how to set up the Device Under Test (DUT) and the test equipment. Appropriate equipment from Keysight Technologies, Inc. is highlighted in Appendix A. Finally, Appendices B and E provide a wide range of reference and learning material.

Table of Contents

• Basic Concepts of IEEE 802.11 Wireless LAN
  • Use of Radio Carriers and Modulation
    
  • Anatomy of a WLAN Device
  • Time Division Duplex and Frame Structure
  • The Medium Access Control Layer
  • Establishing Contact
  • Exchanging Data: Two Methods
• PHY Layer (RF) Test Suite
• Transmitter Measurements
  • Test Conditions and Measurement Setup
  • Test Modes
  • Transmitter Power
  • Transmit Output Spectrum
  • Modulation Tests
  • Transmitter Bit Error and Packet Error Rates
• Timing Tests
  
  • Power vs. Time
  • Spectrogram Testing
  • Transmitter-Receiver, Receiver-Transmitter Turnaround Time
• Transceiver Spurious Tests
• Receiver Measurements
  • Test Conditions and Setup
    
  • Bit Error Rate
  • Receiver EVM Measurements
  • Frame Error Rate, Packet Error Rate
  • Minimum Input Sensitivity, Maximum Input Level.
  • Adjacent channel, Non-adjacent Channel Rejection
  • HiperLAN/2 Receiver Blocking Performance
  • Clear Channel Assessment, RSSI
• Power Supply Measurements
  

Basic Concepts Of IEEE 802.11 Wireless Lan

As the name implies, Wireless LAN was designed to extend the data transfer function of a Wired LAN. The heritage is important—standards that define how it works continue to evolve, but at its heart, WLAN is a system for transferring packets of digital data wirelessly and without error whenever an originating computer can send them. In this respect, it is like an asynchronous Bluetooth link but unlike asynchronous cellular voice connection, which is based on analog transmission. Transmissions take place after a device has first listened to make sure the channel is clear, a method called Carrier Sense Multiple Access/with Collision Avoidance (CSMA/CA). It is fundamentally different from the rigorous timeslot allocations used in cellular and cordless phones. This may cause confusion for engineers migrating from other technologies.