Tutorial on Basic Link Budget Analysis

1 On Basic link Budget AnalysisAbstractAdvances in the state-of-the-art havemade wireless technology a morecompelling solution for many consumerapplications. This poses a problem tomany practicing engineers andtechnology managers who are unfamiliar with the relevantconcepts and terminology. To add to the confusion, FCCregulations require the use of Spread Spectrum techniquesfor most applications in the unlicensed paper is intended to provide the Basic conceptsnecessary to perform a top level Analysis of a wireless addition, other terms and concepts are briefly describedwith should help in understanding some of the systemdesign issues, including a brief description of SpreadSpectrum techniques. Two examples are given whichdemonstrate the influence of range, data rate, andmodulation technique on the radio has been a great deal of interest of late in theapplication of wireless technology in industrial, commercial,and even consumer environments.

2 Several vendors,including Intersil, offer products which comply with FCCregulations for unlicensed operation at Theseregulations permit radiated RF power of up to 1W whenspread spectrum modulation techniques are used. There aremany applications for which the complexities imposed by theuse of spread spectrum radios are more than offset by theinterference rejection properties and higher RF powerpermitted by FCC design of high speed wireless data links involvesmany factors and is well beyond the scope of this applicationnote. However, a top-level link Budget Analysis is a fairlystraightforward exercise. It is the first step an engineer willtake in order to determine the feasibility of any given link Budget calculation is also an excellent means foranyone to begin to understand the various factors whichmust be traded off to realize a given cost and level ofreliability for a communications application note describes a method for performing abasic link Budget Analysis .

3 This discussion is followed by twosimple examples. One example involves a short rangewireless link capable of 40kbits/s (kbps), which might besuitable to provide a laptop computer with wireless access toa nearby dial-up modem. The second example involves ahigh speed (2 Mbps), longer range link designed for WirelessUSB in a home environment. These examples willdemonstrate the effects of range, data rate, and modulationmethod on system OverviewPRISM is a Wireless Local Area Network (WLAN) chipset designed to meet the direct sequence spread spectrumphysical layer (radio) specifications of the WLAN standard. PRISM uses Differential Phase Shift Keying (DPSK)as the modulation scheme. The PRISM radio architectureprovides half duplex wireless RF communications for packetdata rate of 2 Mbps. PRISM provides 70mW of RF power at theantenna, which enables continuous data connectivity at up to400 feet indoors and 1000 feet outdoors.

4 For more details, seeIntersil Application Note AN9624 PRISM DSSS PC CardWireless LAN BasicsWhen evaluating a wireless link , the three most importantquestions to be answered are:1. How much radio frequency (RF) power is available?2. How much bandwidth is available?3. What is the required reliability (as defined by Bit ErrorRate, or BER)?In general, RF power and bandwidth effectively place anupper bound on the capacity of a communications link . Theupper limit in terms of data rate is given by Shannon sChannel Capacity Theorem:where:C = channel capacity (bits/s)B = channel bandwidth (Hz)S = signal strength (watts)N = noise power (watts)Note that this equation means that for an ideal system, the biterror rate (BER) will approach zero if the data transmissionrate is below the channel capacity. In the real world , thedegree to which a practical system can approach this limit isdependent on modulation technique and receiver NoiseFor all communications systems, channel noise is intimatelytied to bandwidth.

5 All objects which have heat emit RFenergy in the form of random (Gaussian) noise. The amountof radiation emitted can be calculated by:where:N = noise power (watts)k = Boltzman s constant ( x 10-23 J/K)T = system temperature, usually assumed to be 290KB = channel bandwidth (Hz)( )C = B * log2 (1 + S/N)( )N = kTBApplication NoteJune 1998 Authors: Jim Zyren and Al Petrick1-888-INTERSIL or 321-724-7143|Intersil and Design is a trademark of Intersil Corporation.|Copyright Intersil Corporation 2000 PRISM is a registered trademark of Intersil Corporation. PRISM logo is a trademark of Intersil is the lowest possible noise level for a system with agiven physical temperature. For most applications,temperature is typically assumed to be room temperature(290K). Equations 1 and 2 demonstrate that RF power andbandwidth can be traded off to achieve a given performancelevel (as defined by BER).

6 Range and Path LossAnother key consideration is the issue of range. As radiowaves propagate in free space, power falls off as the squareof range. For a doubling of range, power reaching a receiverantenna is reduced by a factor of four. This effect is due tothe spreading of the radio waves as they propagate, and canbe calculated by:where:D = the distance between receiver and transmitter = free space wavelength = c/fc = speed of light (3 x 108 m/s)f = frequency (Hz)Equation 3 above describes line-of-sight, or free spacepropagation. Because of building obstructions such as wallsand ceilings, propagation losses indoors can be significantlyhigher. This occurs because of a combination of attenuation bywalls and ceilings, and blockage due to equipment, furniture,and even people. For example, a 2 x 4 wood stud wall withsheetrock on both sides results in about 6dB loss per has shown that line-of-sight propagation holds onlyfor about the first 20 feet.

7 Beyond 20 feet, propagation lossesindoors increase at up to 30dB per 100 feet (see Figure 1) indense office environments. This is a good rule-of-thumb , inthat it is conservative (it overstates path loss in most cases).Actual propagation losses may vary significantly depending onbuilding construction and and Fade MarginMultipath occurs when waves emitted by the transmittertravel along a different path and interfere destructively withwaves travelling on a direct line-of-sight path. This issometimes referred to as signal fading. This phenomenonoccurs because waves travelling along different paths maybe completely out of phase when they reach the antenna,thereby canceling each signal cancellation is almost never complete, onemethod of overcoming this problem is to transmit morepower. In an indoor environment, multipath is almost alwayspresent and tends to be dynamic (constantly varying).

8 Severe fading due to multipath can result in a signalreduction of more than 30dB. It is therefore essential toprovide adequate link margin to overcome this loss whendesigning a wireless system. Failure to do so will adverselyaffect amount of extra RF power radiated to overcome thisphenomenon is referred to as fade margin. The exactamount of fade margin required depends on the desiredreliability of the link , but a good rule-of-thumb is 20dB method of mitigating the effects of multipath is antennadiversity. Since the cancellation of radio waves is geometrydependent, use of two (or more) antennas separated by at leasthalf of a wavelength can drastically mitigate this problem. Onacquisition of a signal, the receiver checks each antenna andsimply selects the antenna with the best signal quality. Thisreduces, but does not eliminate, the required link margin thatwould otherwise be needed for a system which does notemploy diversity.

9 The downside is this approach requires moreantennas and a more complicated receiver method of dealing with the multipath problem is viathe use of an adaptive channel equalizer. Adaptiveequalization can be used with or without antenna diversity.( )L = 20 log10 (4 D / )FIGURE 1. ESTIMATED INDOOR propagation LOSSES 100 120 140 160 180 200 220 240 RANGE (FT)PATH LOSS (dB)FREE SPACEINDOORFIGURE 2. MULTIPATHMULTIPATH SIGNAL #2 BUILDINGSTRUCTUREDIRECT PATH SIGNALMULTIPATH SIGNAL #1 OFFICEFURNITURETXRXA pplication Note 98042-3 After the signal is received and digitized, it is fed through aseries of adaptive delay stages which are summed togethervia feedback loops. This technique is particularly effective inslowly changing environments such as transmission overtelephone lines, but is more difficult to implement in rapidlychanging environments like factory floors, offices and homeswhere transmitters and receivers are moving in relation toeach other.

10 The main drawback is the impact on system costand complexity. Adaptive equalizers can be expensive toimplement for broadband data spectrum systems are fairly robust in the presenceof multipath. Direct Sequence Spread Spectrum (DSSS)systems will reject reflected signals which are significantlydelayed relative to the direct path or strongest signal. This isthe same property which allows multiple users to share thesame bandwidth in Code Diversity Multiple Access (CDMA)systems. Frequency Hopping Spread Systems (FHSS) alsoexhibit some degree of immunity to multipath. Because aFHSS transmitter is continuously changing frequencies, itwill always hop to some frequencies which experience littleor no multipath loss. In a severe fading environment,throughput of an FHSS system will be reduced, but it isunlikely that the link will be lost completely. The performanceof DSSS systems in the presence of multipath is describedfurther in a separate section TechniqueModulation technique is a key consideration.