technical BRIEF TECHNICAL BRIEF Motorola Wireless Broadband Technical Brief OFDM & NLOS Splitting the Data Stream Exploring the Benefits of the Canopy 400 Series & OFDM Technology in Reaching Difficult to Reach Locations
Table of Contents INTRODUCTION 3 WIRELESS ESSENTIALS 4 ISSUES IMPACTING RADIO PERFORMANCE 4 Fresnel Zone 4 Propagation Phenomena 5 THE CANOPY PLATFORM 6 Key Features 6 Longer Cyclic Prefix 6 High-Gain Antennas 6 Forward Error Correction (FEC) 6 Adaptive Modulation 7 GPS Synchronization 7 Software Defined Radios 7 PROVEN PERFORMANCE 7 Urban Line-of-Sight Scenario 7 Urban Non-Line-of-Sight Scenario 8 SUMMARY 9 MOTOROLA WIRELESS BROADBAND SOLUTIONS 9 RESOURCES 9 APPENDIX A: LIST OF ACRONYMS 10 LIST OF ILLUSTRATIONS List of Figures Figure 1. The Fresnel Zone 4 Figure 2. Canopy 400 Series in an Urban LOS Scenario 7 Figure 3. Canopy 400 Series Urban NLOS Scenario 8 List of Tables Table 1. Degradation Issues that Impact Radio Performance 5 Table 2. Results of Canopy 400 in an Urban LOS Scenario 8 Table 3. Canopy 400 Series in a NLOS Scenario 8
INTRODUCTION According to recent estimates, the world s population reached nearly seven billion people in 2007. Of that number, 1.15 billion were regular Internet users. By 2012, projections suggest that over 1.7 billion people worldwide will access the Internet. Increasingly, Internet users are accessing content and information on the web via wireless broadband networks. In an ideal world, customers seeking wireless broadband connectivity would be located in just the right location, in an environment that is free and clear of obstructions and interference. Unfortunately, this ideal world exists only in a limited number of places. Line-of-Sight (LOS) issues can, and very often do, inhibit a network operator s ability to service a given area that may be shrouded by trees, buildings or other impediments. In densely populated urban areas, LOS issues are even more complicated and difficult to resolve. As networks continue to expand and users connectivity demands accelerate, LOS issues will continue to become more exasperating. This technical brief provides a brief discussion of wireless essentials and the issues impacting radio performance. It also delves into the unique features of the Motorola Wireless Broadband Fixed Point-to-Multipoint Canopy 400 Series that enable it to operate quite effectively in areas where LOS issues require an innovative and highly effective solution.
Wireless Essentials Radio waves behave very much like light waves. On striking the surface of an object, they are partially reflected, partially absorbed and partially transmitted through the object. The relative extent to which this occurs depends on many factors including wavelength, angle of incidence and the material composition of the object. Like light waves, radio waves are reflected and diffracted as they pass obstacles and refracted as they pass through materials of varying density. When a radio wave is transmitted, these basic physical laws cause the signal to take a multitude of different paths toward the receiving station. Some of these paths lead to the receiver with sufficient signal strength to be detectable and demodulated into a meaningful data stream. In a Canopy or Canopy Advantage (Canopy 100 & 200 Series) network, a modulation technique called Frequency Shift Keying (FSK) is used for transmitting data in digital over a single carrier. FSK involves shifting the frequency to encode digital data on the single carrier. In the Canopy 400 Series, OFDM splits the radio carrier into multiple RF channels called subcarriers. Each subcarrier is said to be orthogonal or independent/unrelated to the other data streams. The user s data stream is split into multiple data symbols and encoded onto each subcarrier in parallel using one of several modulation schemes including Quaternary Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM). By doing so the data symbol time per subcarrier is longer than a corresponding data symbol time of a single carrier. OFDM uses a Fast Fourier Transform (FFT) technique that allows individual channels to maintain their orthogonality (or distance) to the adjacent channels. This orthogonality means that data symbols reaching the subcarrier receiver along different paths and arriving at different times are less likely to interfere with each other due to the longer symbol time. This increases the likelihood of a successful transmission especially in multipath reflective environments and translates into significant benefits for the network operator seeking to provide connectivity to locations that are Near Line-of-Sight (nlos) and Non-Line-of-Sight (NLOS). ISSUES IMPACTING RADIO PERFORMANCE The Canopy system operates in unlicensed or managed frequencies. As such, the radio signals can be subjected to a high degree of interference from other wireless equipment operating in the same frequency bands as well physical obstructions that can impede the RF signal. This section provides an overview of these issues. Fresnel Zone The first step in considering the issues impacting radio performance begins with a discussion of the Fresnel zone. The set of reflected paths between two antennas is contained within the Fresnel zone. It is an ellipsoid zone between where the total path distance varies by multiples of half of the particular radio wavelength. Figure 1 depicts the Fresnel zone for three types of conditions 4 TECHNICAL BRIEF: Motorola Wireless Broadband, OFDM & NLOS
Line-of-Sight locations feature both visual line-of-sight and clear radio line-of-sight (Fresnel zone). In nlos locations, there is clear visual line-of-sight but the Fresnel zone is partially blocked. In NLOS locations, both visual and radio lines-of-sight are blocked. Taking into account the Fresnel zone and the capabilities of the broadband equipment is imperative when designing a network. Propagation Phenomena Determining the RF attenuation properties and locations of obstructions is required to characterize the true performance of any wireless broadband network. All networks are subject to unwanted impacts on the radio signal due to a number of degradations including reflection, diffraction and scattering. All of these degradations result in a receiver getting the same signal from different directions and in different phases, polarity and strength. Often the combined effect of these is termed multipath reception. In a network based on FSK, a direct line of sight is nearly always required as the short data symbol time allows the signal to be corrupted by a late arriving reflection. This contrasts sharply with an OFDM-based network that, in many cases, is able to overcome these multipath phenomena to transmit/receive data from Point A TYPE Free Space Loss Obstruction Path Loss Reflection DESCRIPTION As a radio signal travels through space, the waveform spreads out and begins to loose strength. As the waveform spreads, so does its power. This is a computation related to signal attenuation caused by distance and is fundamental to any analysis of path loss analysis. As a radio signal encounters a solid object a portion continues through and is attenuated based on the type of material and the frequency of the signal. Attenuation occurs very rapidly at 5 GHz. Radio waves can reflect off hills, trees, bridges, buildings, windows, and walls essentially anything. Reflections allow radio signals to bounce around objects to reach their targets and may vary in phase and strength from the original wave. Diffraction When an opening or an edge restricts a waveform, the waveform deviates from its path in what is known as diffraction. Waveforms bend as they pass through a narrow opening. Scattering If the radio signal reflection hits multiple facets, the signal is said to be scattered and therefore does not arrive at a radio receiver. In particular, right angle reflections send signal power in almost random directions. Polarity If the reflecting surface is at an angle, the effect is not only a change in direction but also a rotation in the signal s polarity. Selective Frequency Fading The destructive interference caused by multipath may occur only in part of channel bandwidth, with the resultant fading (signal attenuation) often different over a small frequency scale. Table 1. Degradation Issues that Impact Radio Performance 5 TECHNICAL BRIEF: Motorola Wireless Broadband, OFDM & NLOS
The Canopy Platform Motorola s Motorola Wireless Broadband Fixed portfolio offers products that support LOS, near Line-of-Sight (nlos) and Non-Line-of-Sight (NLOS) environments. In particular, the new Point-to-Multipoint Canopy 400 Series is uniquely capable of delivering outstanding performance even in the harshest of environments. The 400 Series makes use of OFDM technology that is capable of receiving multipath signals to reach around buildings and other obstructions serving previously unreachable users. The Canopy 400 Series products operate in the 5.4 GHz frequency band. In many parts of the world, this newly available spectrum gives operators 255 MHz of usable spectrum that s nearly twice the amount available in the 5.7 GHz band. The Canopy 400 Series offers a fast, cost-conscious way to enter this exciting and advantageous spectrum. The 400 Series operates in a 10 MHz channel using 256 FFT and delivers up to 21 Mbps throughput in a single sector. This smaller channel bandwidth provides relatively better interference immunity than a wider channel. It also provides the network engineer with many more options in selecting a relatively lower-noise channel and in setting up alternate Dynamic Frequency Selection (DFS) channels. Key Features The 400 Series employs the same innovative feature employed by the Canopy FSK-based products along with a host of new, dynamic features that help to provide longer-ranges and improved performance in nlos and NLOS conditions. These include: Longer Cyclic Prefix High-Gain Antennas Forward Error Correction (FEC) Adaptive Modulation GPS Synchronization Software Defined Radios Longer Cyclic Prefix A cyclic prefix is a repeat of the data at the end of the symbol at the beginning of the symbol. The purpose is to allow the various multipath signals to settle before the main data arrives at the receiver. The Canopy 400 series uses a one-fourth cyclic prefix that provides six microseconds of guard time at the beginning of each symbol. This is effectively the amount of time that a symbol can be delayed without interfering with the symbol coming down the path behind it. By this logic and within practical limitations, the longer the cyclic prefix the better multipath performance. The cyclic prefix employed here is longer than typical 802.11 based systems. High-Gain Antennas An antenna of this type employs a focused, narrow radio wave beamwidth. The narrow beam allows more precise targeting of the radio signal and thereby delivers a stronger, more focused signal. The Canopy 400 Series Subscriber Modules (SMs) and Access Points (APs) are equipped with high-gain antennas (17dBi) so that the link budget is 133 db at the lowest modulation and 114 db at 64QAM. This compares with 122 db for Canopy single carrier in the 5.4 GHz frequency band. Forward Error Correction (FEC) A technique used by the 400 Series receiver for correcting errors incurred in transmission over the communications channel without requiring retransmission of information by the transmitter. In other words, the Canopy 400 radios can recover data from the bit stream even if some bits have been altered during transmission due to interference from reflection, diffraction or scattering. Just as in a Canopy FSK-based network, data is segmented into short packets that are 2.5 milliseconds in duration. The difference is now these packets are coded to allow automatic error correction and then mapped onto OFDM subcarriers. 6 TECHNICAL BRIEF: Motorola Wireless Broadband, OFDM & NLOS
Adaptive Modulation The Canopy 400 Series employs three different modulation levels (QPSK, 16QAM and 64QAM). The actual throughputs per sector achieved are 7, 14 and 21 Mbps respectively. The radio continually assesses the link conditions and uses the highest speed modulation possible. GPS Synchronization Ensures that radios transmit and receive at identical times thereby reducing the amount of self-interference from adjacent sectors and towers. This is especially important in multipath and high modulation schemes. Through the use of GPS synchronization, the Canopy 400 Series can be collocated with Canopy FSK-based 100 & 200 Series modules. Software Defined Radios Like the Canopy 100 & 200 Series, the 400 Series is a software-defined radio. This radio architecture and its implementation permit extensive modifications should the need for some specific optimizations arise and the solutions enhancing performance become available. Over-the-air programmability ensures that the installed equipment will be able to benefit from the continuing development of the technology. PROVEN PERFORMANCE The Canopy 400 Series has demonstrated its value in urban areas where LOS issues have prohibited connectivity. Actual field results for two different scenarios are highlighted in this section. In each of the scenarios, Canopy 400 series APs and SMs were collocated with Canopy Advantage (200 Series) APs and SMs to compare performance. Overall, in LOS situations, the Canopy 400 series equipment provided higher throughput. In NLOS situations, the Canopy 400 series equipment was able to establish a connection where the 200 Series equipment was unable to connect. Urban Line-of-Sight Scenario In the urban LOS scenario, the APs were located on a downtown office building. The Canopy Outdoor SMs were located approximately 0.27 miles from the AP with a clear LOS between the modules. Figure 2 depicts the urban LOS scenario. BUILDING WHERE AP IS MOUNTED AP at 9 th floor in Downtown 0.27 miles Figure 2. Canopy 400 Series in an Urban LOS Scenario 7 TECHNICAL BRIEF: Motorola Wireless Broadband, OFDM & NLOS
The Canopy Advantage APs and SMs demonstrated 14 Mbps aggregate throughput while the Canopy 400 series APs and SMs demonstrate 19 Mbps aggregate throughput. Table 2 details the results achieved. DESCRIPTION CANOPY (400 SERIES) CANOPY ADVANTAGE (200 SERIES) SM Rx (dbm) -56-44 AP Rx (dbm) -59-54 Aggregate 19 Mbps 14 Mbps Throughput (Mbps) Table 3. Canopy 400 Series in a NLOS Scenario Urban Non-Line-of-Sight Scenario In the urban NLOS scenario, the conditions were very similar. The AP was located on a downtown office building in the same location. The Canopy Outdoor SM was located approximately 0.27 miles from the AP. In this scenario, however, an eight-story brick office building blocked the LOS path between the SM and AP. Figure 3 shows an environmental scan of the area..27 miles AP at 9 th floor in Downtown AP SM Figure 3. Canopy 400 Series Urban NLOS Scenario In this scenario, the Canopy Advantage AP and SM were not, as expected, able to connect with one another. In sharp contrast, the Canopy 400 series AP and SM demonstrated 6.1 Mbps aggregate throughput. Table 3 highlights the results. DESCRIPTION CANOPY (400 SERIES) CANOPY ADVANTAGE (200 SERIES) SM Rx (dbm) -82 AP Rx (dbm) -83 No Registration Aggregate Throughput (Mbps) 6 Mbps Table 3. Canopy 400 Series in a NLOS Scenario 8 TECHNICAL BRIEF: Motorola Wireless Broadband, OFDM & NLOS
Summary For enterprises, municipalities and service providers, the new Canopy 400 Series provides a powerful, proven solution for reaching previously unreachable users and subscribers. Using OFDM technology, the 400 Series delivers improved nlos and NLOS coverage to extend broadband coverage to urban areas where obstructions have limited system expansion. Motorola Wireless Broadband Wireless Broadband Solutions Motorola is one of the most trusted resources for wireless communication solutions around the world. With more than 75 years of RF innovation and leadership, Motorola is one of the world s most experienced wireless communication companies. The Motorola Wireless Broadband Fixed Point-to-Multipoint Canopy solutions are part of the Motorola Wireless Broadband portfolio a comprehensive portfolio of wireless broadband solutions and services that provides high-speed connectivity, enabling a broad range of applications in a host of environments. The Motorola Wireless Broadband portfolio also includes Motorola Wireless Broadband WiMAX, Motorola Wireless Broadband Mesh and Motorola Wireless Broadband Indoor solutions for public and private networks. Resources Additional information is available from a host of resources including: Canopy User Community at http://motorola.canopywireless.com/support/community. This resource facilitates communication with other users and with authorized Canopy experts. Available forums include General Discussion, Network Monitoring Tools, and Suggestions. Canopy Knowledge Base at http://motorola.canopywireless.com/support/knowledge. This resource facilitates exploration and searches, provides recommendations, and describes tools. Available categories include General (Answers to general questions provide an overview of the Canopy system.) Product Alerts Helpful Hints FAQs (frequently asked questions) Hardware Support Software Support 9 TECHNICAL BRIEF: Motorola Wireless Broadband, OFDM & NLOS
Tools APPENDIX A: List Of Acronyms AP db EIRP FFT FSK FSL FZ Hz IFFT LOS NLOS nlos OFDM QAM QPSK Rx SC SM Tx Access Point Decibel Equivalent Isotropic Radiated Power Fast Fourier Transform Frequency Shift Keying Free Space Loss Fresnel Zone Hertz Inverse Fast Fourier Transform Line-of-Sight Non Line-of-Sight Near Line-of-Sight Orthogonal Frequency Division Multiplexing Quadrature Amplitude Modulation Quaternary Phase Shift Keying Receiver Single Carrier Subscriber Module Transmitter 10 TECHNICAL BRIEF: Motorola Wireless Broadband, OFDM & NLOS
10 TECHNICAL BRIEF: Wind Loading of Canopy Network Elements, Issue 2
motorola.com Part number Motorola Wireless Broadband- TB. Printed in USA 08/08. About Motorola Wireless Broadband: Motorola s industry leading portfolio of reliable and cost effective wireless broadband solutions provide and extend coverage both indoors and outdoors. The Motorola Wireless Broadband portfolio offers high-speed connectivity systems that support voice, video and data solutions enabling a broad range of applications for both fixed and mobile public and private networks. With Motorola s One Point Wireless Suite of innovative software solutions, customers can now design, deploy and manage their broadband networks at lower installation costs that maximize up-time and reliability. MOTOROLA and the Stylized M Logo and Symbol and the Symbol Logo are registered in the US Patent & Trademark Office. All other product or service names are the property of their respective owners. Motorola, Inc. 2008. All rights reserved. For system, product or services availability and specific information within your country, please contact your local Motorola office or Business Partner. Specifications are subject to change without notice.