Author: Pearson approval IEEE 802.11 Standard wireless LANs given

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Condensed Review Spread Spectrum Techniques Band Systems
Author: Pearson
approval IEEE 802.11 Standard wireless LANs given WLAN industry needed boost. Manufacturers WLAN systems cooperating performing interoperability testing. Such testing providing assurance that 802.11 compliant equipment purchased from manufacturer will interoperate with 802.11 radios manufactured other OEMs. This important consideration managers desire WLAN technology provide mobility with connectivity their workforce.
data transmission rate when compared with that DQPSK, radios utilizing MBOK modulation format have achieved certification. This application note provides overview several frequency hopping direct sequence techniques used wireless data transmission band. Included discussion MBOK modulation technique able achieve Ethernet speeds over three separate band-limited channels within band. will shown that given increase waveform complexity MBOK modulation adds very little system complexity terms number components radio bill materials (BOM) cost, when compared 2Mbit/s radio with QPSK modulation.
November 1997 approved 802.11 Standard defines protocol compatible interconnection data communication equipment radio infrared interface local area network. radio implementation PHY, subject this paper, specifies either Frequency-Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (DSSS) modulation. frequency-hopping radios IEEE specifies minimum requirement 1Mbit/s data rate using two-level Gaussian frequency shift keying (2GFSK) modulation. optional rate 2Mbit/s supported using four-level Gaussian (4GFSK) modulation. Figure comparison signaling schemes 802.11 2GFSK 4GFSK. IEEE 802.11 specifies deviation factor, 0.32 nominal 2GFSK 0.45 nominal 4GFSK. direct sequence systems modulation formats data rates supported, basic access rate 1Mbit/s enhanced access rate 2Mbit/s. Both data rates utilize phase shift keying modulation with differential binary phase shift keying (DBPSK) used 1Mbit/s basic access rate differential quadrature phase shift keying (DQPSK) used enhanced access rate. These techniques, FHSS DSSS, constitute currently approved standard IEEE 802.11. Another modulation technique known M-ary Biorthogonal Keying (MBOK) been used achieve 5.5X improvement
FREQUENCY 2GFSK
Variable Data Rate Radio
Figure depicts system implementation 2.4GHz DSSS radio called PRISMdesigned operation unlicensed industrial, scientific medical (ISM) band. unlicensed band, mean that frequency band 2400MHz 2483.5MHz allocated wherein intentional, unintentional incidental radiator operated without individual license [1]. This international band also defined regulatory agencies Canada, Europe, Japan although countries minor differences allocated frequencies found. Table shows operating frequency ranges effect internationally 2.4GHz band [2].
TABLE BAND OPERATING FREQUENCIES LOWER UPPER CARRIER CARRIER FREQUENCY FREQUENCY LIMIT (GHz) LIMIT (GHz) 2.402 2.473 2.447 2.448
4GFSK
REGULATORY RANGE 2.400 2.4835 2.471 2.497 2.445 2.475 2.4465 2.4835
GEOGRAPHY North America Europe Japan Spain France
2.480 2.495 2.473 2.482
CENTER FREQUENCY h4/2
SYMBOL TIME INTERNAL TIME
FIGURE SIGNALING SCHEMES 2GFSK 4GFSK MODULATION
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1-888-INTERSIL 321-724-7143 Copyright Intersil Corporation 1999 PRISM® registered trademark Intersil Corporation. PRISM logo trademark Intersil Corporation.
2-364 HFA3726 MODEM
(FILE# 4310)
HFA3424
(FILE# 4131)
QUADRATURE DEMOD
DESPREAD
DEMODULATE
Application Note 9820
HFA3624 RF/IF
(FILE# 4066)
LOW-PASS FILTERS
HFA3860A BASEBAND PROCESSOR
TX/RCV DATA HFA3840 SOFTWARE MEDIA ACCESS CONTROLLER USER-SUPPLIED
RFPA
SPREAD
HFA3925
(FILE# 4132)
POWER SWITCH
MODULATE/ ENCODE
CONTROL TEST
QUAD MODULATOR
DUAL SYNTHESIZER
HFA3524A
FIGURE 2.4GHz PROGRAMMABLE RADIO BAND (PRISM)
Application Note 9820
radio Figure features programmable data rate capability with high rates 4Mbit/s IEEE Standard 802.11 fallback rates 1Mbit/s. definition 1Mbit/s 2Mbit/s data rates utilize DPSK modulation. 4Mbit/s data rate mode also utilizes DPSK modulation double clock rate 2Mbit/s mode. higher data rates 5.5Mbit/s 11Mbit/s, radio Figure utilizes MBOK modulation. MBOK modulation will discussed following review some general topics spread spectrum techniques. Since spread spectrum very broad topic, intended) following discussion will somewhat constrained within context band regulations requirements IEEE 802.11 specification. married German munitions baron Fritz Mandl [5]. Beside chirp frequency hopping methods there time hopping, direct sequence hybrid combinations frequency hopping, time hopping, direct sequence modulations. Since their early uses during World spread spectrum techniques have mostly been used military secure, proof radios. Today, spread spectrum techniques have found their into many consumer industrial applications such phones, cordless telephones, wireless card readers, code scanners, other handheld portable appliances. Referring Figure note that with exception proprietary HFA3860A baseband processor medium access controller (MAC) chip, RF/IF front functional blocks available from multiple vendors like National Semiconductor, MA-Com, Works, Intersil Corporation others. widespread availability these components demonstrates that technology spread spectrum radio moved from esoteric, niche marketplace high volume, commercial off-the-shelf manufacturing.
Characteristics Spread Spectrum Signals
term spread spectrum used describe technique which bandwidth transmitted signal much wider than bandwidth information signal. This confused with conventional wideband which large deviation ratios tend spread spectrum signal. context definition pure spread spectrum mean only those techniques wherein spread function performed some signal other than information signal [3]. astute reader might question with today's overcrowded frequency spectrum, would anyone want spread signal bandwidth thereby using more precious frequency spectrum. answer evident when considers characteristics spread spectrum signals that made them pervasive military applications. These characteristics are: power spectral density information signal looks like noise eavesdroppers other radios. High immunity jamming interference. High resolution ranging. Possibility code division multiple access. Recognizing these benefits, 1985 made decision allow spread spectrum signals bands with power levels maximum [3]. allows three types spread spectrum signals band, Frequency (FH), direct sequence (DS) hybrid FH/DS signals. regulations have provision chirp spread spectrum bands.
Definition Orthogonal Signaling
basic problem digital communications reliably selecting actual transmitted signal from possible discrete signals. Here both transmitter receiver know signaling waveforms. Assume that have {Si(t)} possible signals where: with being period signal. Given that channel will corrupt signal superimposing additive white Gaussian noise (AWGN) receiver will observe signal:
where n(t) AWGN task receiver determine correct transmitted signal after been corrupted noise channel. task selecting correct transmitted signal presence noise typically accomplished correlation received signal with {Si(t)} possible signals. optimize process correlation, signal should possess property known orthogonality. Orthogonality implies that functions contained signal {Si(t)} independent disagreement with another. functions said orthogonal with another
Historical Background Spread Spectrum
early spread spectrum techniques traced back developments radar ranging techniques during World that time, Germany experimenting with pulse compression techniques that formed basis "chirp" spread spectrum systems [4]. With chirp spread spectrum carrier swept over wide band during given pulse interval. Chirp spread spectrum also known pulsed primarily used radar applications. Frequency-hopping also traces roots back World interesting that first patents method synchronizing frequency hopping transmitters receivers submarines held late actress Hedy Lamarr, Austrian once been 2-365
elsewhere
Therefore, optimize detection process, signal Si(t) chosen linear combination orthonormal, (i.e., unit energy orthogonal) functions such that:
(EQ.
Application Note 9820
Using integral operator unpredictable fashion which sequence generated. case code generator, shift register used generate sequence inclusion feedback loop which computes term first stage based previous terms. Because sequence ones zeros generated shift register deterministic repetitive, resulting random-like sequence designated pseudorandom [6]. will investigate some detail characteristics limitations signals band 2400MHz 2483.5MHz. Since difficult hopping synthesizer maintain phase coherence over wide hopping bandwidth, waveform widely used systems because relatively easy demodulate non-coherently. Therefore modulation assumed throughout following discussion because widespread systems. signal thought amplitude shift (ASK) signals [7]. this analogy refer Figure Figures have signals which represented mathematically
INFORMATION SOURCE DIGITAL MODULATOR FREQUENCY SYNTHESIZER CARRIER CODE GENERATOR FHSS SIGNAL
both sides yields:
Orthogonality will discussed later context both systems.
Frequency Hopping Spread Spectrum (FHSS)
frequency hopping system, carrier caused jump around pseudorandom fashion under control synthesizer that driven pseudonoise (pn) code generator. Figure illustrates concept.
elsewhere
elsewhere
waveform Figure linear waveforms Figures Thus waveform represented mathematically
Binary Binary
FIGURE BLOCK DIAGRAM FREQUENCY HOPPING SYSTEM
Where: with signal, bandwidth signal depends modulation index. Figure shows typical magnitude spectrum carrier frequency deviation Having described waveform graphically mathematically refer Figure typical spectrum band Signal. Figure shows ideal line spectra channels defined IEEE 802.11 North America most Europe. center frequencies channels defined IEEE 802.11 1MHz steps beginning 2.402GHz ending 2.480GHz (see IEEE 802.11 channels center frequencies France, Spain, Japan). This compliance with Part regulation specifying least hopping frequencies FHSS systems operating 2400MHz 2483.5MHz band. minimum rate governed regulatory authorities specified terms maximum dwell time 400ms channel. This equates minimum rate hops/s. minimum 802.11 6MHz North America Europe (including France Spain) 5MHz minimum Japan.
FIGURE FREQUENCY HOPPING SIGNAL
Figure information signal used digital modulator modulate carrier signal typically using modulation. modulated carrier signal then hopped over very wide bandwidth compared bandwidth information signal. hopping sequence generated code generator which sets synthesizer output. code generator generates what appears random sequence ones zeros, truly random. understand why, consider random process flipping coin. assign logical occurrence head logical zero occurrence tail, then sequence ones zeros produced flipping coin random process
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Application Note 9820
ASK1
FIGURE
ASK2
FIGURE
FIGURE
ASK1 ASK2
FIGURE WAVEFORM COMPONENTS
500kHz
20dB
FIGURE MAGNITUDE SPECTRUM [19]
FIGURE IDEAL LINE SPECTRA FREQUENCIES WITHIN NORTH AMERICAN BAND
2402MHz 2480MHz
1MHz
FIGURE BINARY MODULATION WITHIN CHANNEL 2438MHz
2438MHz
FIGURE
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Application Note 9820
Figure operating channel 2438MHz expanded show carrier made deviate binary frequency shift keying (FSK) modulation. IEEE 802.11 compliant FHSS systems, carrier deviation defined 1Mbps 2Mbps data rates. Table shows symbol encoding versus carrier deviations these systems, along with calculated modulation indices. Given small modulation indices Table spectrum will narrowband, having most significant sidelobe. These systems transmit entire packet data each middle packet. While hopping, carrier turned off. This method transmission known slow frequency hopping because there many bits transmitted hop. contrast, fast frequency hopping system hopping chip rate higher than rate. time from channel another specified IEEE 802.11 which states that operating channel center frequency must settle within 60kHz nominal center frequency maximum 224µs. Once completed carrier settled nominal frequency channel, system must reacquire 802.11 signal. Consequently, assist receiver with acquiring signal 802.11 radio transmitter will send preamble header which allows receiver sync with transmitter. Figure shows composition 802.11 packet.
TABLE IEEE 802.11 CARRIER DEVIATION 2Mbps DATA RATES MODULATION SYMBOL CARRIER DEVIATION 1MBIT/S, 2GFSK fCLK -1/2 fCLK 2MBIT/S, 4GFSK NOTES: Deviation factor 0.32 nominal; deviation factor 0.45 fCLK 1MSymbol/s. fCLK fCLK -1/2 fCLK -3/2 fCLK 0.216 0.072 0.072 0.216 0.16 0.16 MODULATION INDEX
Frequency Hopping Collisions
When signals being broadcast same channel signals interfere with another. With multiple radios speak probability collision. collision occurs when radios same channel interfere with each other. probability such event depends upon number hopping channels number active, collocated radios. Since dwell time (time spent channel) typically less than half second, collision will unnoticed. number radios increases, collisions become more frequent effective data throughput noticeably degraded.
PREAMBLE HEADER DATA
SYNC FRAME DATA WORD DATA ERROR DATA WORD TIMING LENGTH RATE CHECK WORD 1-4095 BITS BITS BITS BITS BITS BYTES BITS
FIGURE COMPOSITION IEEE 802.11 PACKET
Capacity FHSS System
have reviewed some characteristics FHSS system. foregoing discussion means been exhaustive. intention give reader flavor FHSS systems work. final note FHSS systems; will investigate regulations practically limit maximum data rate achievable system. begin with statement Shannon's Capacity Theorem communication channel. following theorem taken directly from Shannon's paper "Communication Presence Noise", published 1949 [12]. Theorem: average transmitter power suppose noise white thermal noise power band sufficiently complicated encoding systems possible transmit binary digits rate:
Associated with each packet bits overhead synchronization error checking. Assume desired transmit large file 1Mbyte size. Since 802.11 standard specifies maximum data packet 4095 bytes, system must fragment 1Mbyte file into smaller packets. Typical packets bytes 3200 bits. Since preamble header always transmitted 1Mbit/s, another 128µs consumed with reacquiring signal. Thus MBPS, takes 3200µs 128µs 3.33ms transmit preamble, header first packet. Thus 400ms dwell, about packets transmitted. Together hopping reacquisition represent about overhead large packets. Packets lost interference multipath effects will overhead resulting lower effective data throughput rates.
with small frequency errors desired. possible encoding method send higher rate have arbitrarily frequency errors. Shannon's expression channel capacity represents theoretical upper limit data rate where arbitrarily small probability error achieved with coding [4]. practical systems difficult transmit near capacity limit because system complexity increases proportion complexity coding scheme; and, randomness system noise will tend limit number discrete subdivisions signal that distinguished reliably. consider implications Shannon's capacity theorem place limitations signaling speed FHSS system. Part 15.247 code states that
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Application Note 9820
frequency hopping systems, maximum 20dB bandwidth hopping channel 1MHz. This means that sidelobes must attenuated 20dB within 500kHz carrier center frequency (see Figure well known rule thumb bandwidth signal given Carson's rule [8], which states that: 2[fD+fM] where: bandwidth frequency deviation frequency modulating signal maximum frequency deviation 0.16MHz assuming rate 1Mbit/s, width spectrum generated from modulation would 2[0.16 1]MHz
particular communication schemes. Here energy joules, one-sided noise spectral density units watts/Hz. expression related signal-to-noise ratio bandwidth utilization efficiency ratio defined where system bandwidth rate. Thus:
-B/R
Figure shows curves Error Rate (BER) versus energy utilization /N0) non-orthogonal 2GFSK 4GFSK signaling.
1E+0 1E-1 1E-2 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 (dB) 2FSK 4FSK
2.32MHz other words, Carson's rule held this case, information contained spectrum would spread across 2.32MHz bandwidth. Since information contained sidelobes, this example shows that data rate severely cramped available 1MHz bandwidth FCC. fact, 1MHz bandwidth cramped make orthogonal signaling impossible. Here's why. M-ary signals, distance between pair signals, i.e., dissimilarity signals, measured correlation coefficient gamma between signals. turns that form
FIGURE PROBABILITY ERROR NON-ORTHOGONAL SIGNALING
function with minimum frequency separation between symbols being 1/2T orthogonality where symbol interval formal proof this statement found reference [21]). 802.11 system, this equates
500KHz
achieve higher data rates given occupied bandwidth, Shannon said encode data that single symbol represent many bits data. binary signaling binary FSK, rate symbol rate equal. double rate letting symbol represent dibit, i.e., bits. Referring Figure seen that compared 2GFSK binary signaling, 4GFSK system requires much higher signal strength given performance, i.e., about more Thus, high packing coding schemes, approach becomes impractical prohibitively high signal-to-noise ratios required reliable transmission constrained bandwidth allowed. Consequently, despite favorable characteristics such relatively complexity cost, systems expected find application broad horizontal markets such enterprise computing where Ethernet speeds norm.
frequency separation needed orthogonal signaling. conserve bandwidth comply with regulations, carrier deviation 802.11 system deliberately restricted nominal ±160KHz. Consequently, since orthogonal signaling possible, system will typically frequency discriminator convert frequency deviations into voltages demodulate signal. We've seen restricted bandwidth system dictates non-orthogonal signaling small modulation indices. From Shannon's theorem evident that bandwidth traded signal power vice versa. Thus, given bandwidth, information rate increased expense higher signal power measured case discrete signaling FSK, used compare relative efficiency 2-369
Direct Sequence Spread Spectrum (DSSS)
non-coherent detection waveform, FHSS radio less complex marginally less costly than DSSS radio. result, FHSS systems found many vertical market applications where data rate transmission palatable. will review some attributes
Application Note 9820
DSSS systems show direct sequence systems logical choice wireless Ethernet computing. Like direct sequence spread spectrum (DSSS), also uses code spread signal. term direct sequence spread spectrum appropriate since with this technique information signal directly modulated with code sequence. rate sequence called chip rate. IEEE 802.11 standard specifies Barker codes chip sequence used DSSS systems. Barker codes named after Ronald Barker first used them frame sync markers matched filter that used digital signals [9]. Barker codes known possess good aperiodic correlation properties [10], which simply means that non-repetitive behavior code matched filter correlator easily identify location Barker code sequence bits. same properties that make Barker codes good frame sync markers also make them good codes spreading despreading signals. Table contains complete listing known Barker codes 10].
TABLE LISTING KNOWN BARKER CODES CODE LENGTH +++-+ +++-++++-+-++++++-++-+-+ BARKER SEQUENCE
b(t)
b(t-1) LATCH
DATA STREAM
DIFFERENTIAL ENCODER DQPSK
11-BIT BARKER CODE
FIGURE PRISM RADIO DIFFERENTIAL ENCODER CODE SPREADING CIRCUIT
function performs what called modulo-2 addition digital data stream. Recall from basic computer logic that function truth table Table
TABLE TRUTH TABLE FUNCTION (a,b)
effect modulo-2 addition invert code each transition data stream spread bandwidth information signal. timing diagram Figure illustrates effect modulo-2 addition code. time invariant matched filter correlator receive section HFA3860A used despread DPSK signal. time invariant property means that time shift correlator input results same time shift output.
Differential Encoding
Notice differential encoder Figure Since IEEE 802.11 specifies DPSK modulation DSSS systems, baseband data differentially encoded transmitter subsequent demodulation differential decoder receiver. With DPSK modulation, information conveyed phase difference between adjacent signal elements transmitted signal. Thus necessary have coherent phase reference receiver demodulate DPSK signal. trade-off reduced system complexity differential higher error rate (BER) given signal-to-noise ratio because noise perturbs phase reference along with information signal. HFA3860A baseband processor Figure uses coherent demodulation data improved performance differentially encoded signals. Figure shows performance curve modes baseband processor. From Figure that 1Mbit/s data rate, about 11.5dB needed error rate 10-4 bit-error/s.
Table shows, list known Barker codes limited eight sequences. their relatively short length, Barker codes convenient when fast code synchronization requirement. Other coding schemes available when longer codes needed. Barker sequence code length used spread IEEE 802.11 DSSS waveform. code changes phase times symbol which means carrier changes phase times single symbol. Intersil PRISM radio Figure uses simple exclusive (XOR) gate sequence with differential encoded digital data shown Figure
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Application Note 9820
INFORMATION DATA STREAM
CHIP BARKER SEQUENCE
RESULTING MODULO-2 SEQUENCE
DATA TRANSITION INVERTS SEQUENCE
FIGURE RESULTING WAVEFORM MODULO-2 ADDITION INFORMATION
Processing Gain Jamming Margin
that we've seen encoded DSSS signal directly modulated code generator, let's consider effect this modulation. Figure shows typical power spectrum DSSS signal before after spreading. Notice that spectrum shape envelope expressed
receiver spread signal again modulo-2 added sequence. This effectively collapses spread signal original bandwidth amplitude while simultaneously spreading noise unwanted interfering signals. bandpass filter then reject most unwanted signal noise power. this spreading/despreading mechanism which processing gain achieved DSSS system. Processing gain important figure merit used DSSS systems readily determined Equation
Processing Gain INFO (EQ.
direct modulation (modulo-2 addition sequence with encoded baseband signal) effectively spreads signal over much wider bandwidth. main lobe bandwidth Figure function modulation waveshape code rate. general rule thumb DSSS systems that null null bandwidth chip rate. Thus using 11-bit Barker code chip rate Mcps null null bandwidth spread signal 22MHz. This allows three non-overlapping DSSS channels band.
1.E-02 1.E-03 1.E-04 1.E-05
where: BW(ss) bandwidth after spreading R(INFO) baseband information data rate. Applying Equation HFA3860A 2Mbit/s mode, obtain processing gain
22MHz 10.4dB 2Mbits/s
Although processing gain defined Equation useful figure merit easily obtainable, does tell whole story. Another figure merit called jamming margin takes into account internal system losses signal noise ratio measured demodulated output receiver. uses jamming margin method measuring processing gain requires that DSSS transmitters have processing gain least 10dB measured this method. purposes processing gain calculated from Equation
(EQ.
1.E-06
Where:
1.E-07
(S/N)0 theoretical signal noise ratio required maintain normal operation relative nominal error rate. maximum jammer-to-signal ratio detected BER; also known jamming margin.
FIGURE Eb/N0 PERFORMANCE MODES
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Application Note 9820
SIGNAL BEFORE SPREADING
JAMMER GETS SPREAD WHEN SIGNAL GETS DESPREAD SIGNAL AFTER SPREADING
11MHz MAIN LOBE NULL-TO-NULL BANDWIDTH 11MHz
FIGURE POWER SPECTRUM DSSS BEFORE AFTER SPREADING
LSYS cumulative systems losses filtering, synchronization, tracking, etc. solve Equation jamming margin follows:
(EQ.
maximize system utilization desired keep spread rate same that 802.11 order maintain least three non-overlapping channels band. This minimum necessary co-located networks because allows frequency reuse. Figures illustrate concept frequency reuse co-located networks.
Now, given BPSK signal with (S/N)0 9.6dB LSYS assuming minimum allowed 10dB, Equation yields system jamming margin -1.6dB. Consequently, would expect system operate reliably with interfering signal more than -1.6dB above desired data signal. This meaning system jamming margin.
22MHz
22MHz
22MHz
2400MHz
30MHz
30MHz
2483.5MHz
Choice MBOK Modulation
selection particular modulation technique involves making trade-offs among various constraints conflicting goals. communications engineer generally will attempt Maximize spectral efficiency (bits/Hz) Minimize power required Maximize system utilization Minimize system cost Trading among these four criteria indeed case choosing MBOK modulation high data rate radio Figure First all, desired that high rate radio backward compatible with IEEE 802.11 basic access enhanced access rates 1Mbit/s 2Mbit/s. Thus radio capable using 802.11 preamble header signal acquisition then rate switching high data rate. Conveniently, 802.11 protocol already accommodates rate changing. rate switching offers added benefit allowing radio downshift lower, more robust data rates high multipath environments such might found large open areas like supermarket, factory floor.
FIGURE 13A. ILLUSTRATION NON-OVERLAPPING CHANNELS BAND
FIGURE 13B. ILLUSTRATION FREQUENCY REUSE CELL PLANNING
Figure shows three DSSS channels occupying band. Figure having minimum three channels band allows network planner implement cellular network frequency reuse. Each
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Application Note 9820
adjacent cell assigned different channel therefore interference between adjacent cells minimized dimensional network Figure 13B. This same concept used cellular phone networks. course adding more channels within band would allow increased system utilization allowing network planner more users unit area smaller cells. drawback adding more channels must contend with Shannon's capacity law. narrowing channel bandwidth order more channels within band, reduce channel capacity. this case desired increase data rate factor least over basic access rate decreasing channel bandwidth conflicted with this goal. addition, bandwidth reduction would make difficult impossible meet 10dB processing gain requirement since processing gain related spread rate. above considerations search conducted modulation technique that: energy efficient, i.e., high signal energy hertz. Would allow minimum three non-overlapping channels band. Could achieve high data throughput rates through efficient coding. After running simulations number candidate waveforms, MBOK modulation chosen offering best combination necessary characteristics meeting above requirements while minimizing system complexity cost. First take matrix where:
definition:
Similarly:
Walsh Functions
stated earlier that orthogonal signaling could used optimize detection process digital communications system. That detector designed that makes fewest errors average signal possesses orthogonality property. class functions that true orthogonality Walsh functions. Walsh functions have been known since 1923 advantageous because they assume only values therefore easily generated digital circuits. gate used modulate baseband information with Walsh function. turns that Walsh function simply column taken from Hademard matrix. Hademard matrix symmetric, square matrix composed ones zeros, with dimension that power two. Hademard matrices defined recursively
Notice Hademard matrix that rows columns mutually orthogonal, that rows number columns which they agree equal number columns which they disagree. Similarly, pick columns number rows which they agree equal number rows which they disagree. Matrix represents eight Walsh functions. these functions codes used spreading baseband information signal because orthogonality, detection receiver optimized.
MBOK Implementation Baseband Processor
purpose this discussion 11Mbit/s mode (QMBOK) described. Refer HFA3860A [15] data sheet (AnswerFAX Doc. #4488) description 5.5Mbit/s mode. have seen Hademard matrix used generate orthogonal code words called Walsh functions. let's digital encoder HFA3860A uses these functions modulate input data stream (see Note First eight orthogonal Walsh functions stored correlator banks, serial data stream comes into baseband processor partitioned into 4-bit nibbles. 4-bit nibbles used 11Mbit/s mode, channel channel. Each nibble channels partitioned into bits magnitude sign data. magnitude bits channels independently select one-of-eight 8-bit Walsh functions.
NOTE: actual Walsh functions used HFA3860A modified insure zero member. HFA3860A data sheet details.
Let's build 8-bit Walsh functions from Hademard matrices.
sign bits modulo-2 added 8-bit Walsh functions channels. Modulo-2 addition sign nature biorthogonal signaling. With biorthogonal signals there sets mutually orthogonal signals, sets mutually
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Application Note 9820
orthogonal; instead they antipodal another. already familiar with antipodal signaling type used BPSK QPSK modulations 1Mbit/s 2Mbit/s modes HFA3860A. Thus modulo-2 addition sign generates sets antipodal signals channel channel. signals within each mutually orthogonal. have seen bits data encoded into single symbol HFA3860A. chipping rate 8-bit Walsh functions 11Mchip/s order keep spread bandwidth same that 1Mbit/s 2Mbit/s modes. Thus same occupied bandwidth QMBOK modulation packs times much data QPSK modulation. Since Walsh functions bits length chipping rate 11Mchip/s, symbol rate 1.375MS/s, i.e., 11Mbit/s bits/symbol 1.375MS/s. Figure depicts constellation diagram QMBOK waveform. detect which signal sent matching correlating Walsh functions received signal. Figure block diagram process.
BIGGEST PICKER y(t)
1(t)
r(t)
2(t)
8(t)
FIGURE BLOCK DIAGRAM RECEIVER CORRELATOR FUNCTION
correlators multiply-accumulate architecture. serial data corrupted noise enters each correlator multiplied Walsh functions added accumulator. correlator outputs integrated over symbol period, sampled dumped "select largest" "biggest picker" circuit. output correlator with correct Walsh function will peak response signal. outputs other correlators peak receiver knows which signal transmitted. Thus, output correlator
y(t)
FIGURE CONSTELLATION DIAGRAM MBOK SIGNALLING
similar fashion BMBOK mode (5.5Mbit/s) packs times much data into symbol BPSK modulation 1Mbit/s mode. this point astute reader might question: given that requires minimum processing gain 10dB, does MBOK waveform meet requirement with 8-bit Walsh spreading function? answer this question lays fact that MBOK waveform possesses types gain, i.e., processing gain coding gain. have already reviewed processing gain. Coding gain arises from fact that MBOK modulation about 1.6dB better performance than BPSK modulation. other words, signal-to-noise ratio necessary make symbol decision about 1.6dB lower than that necessary make individual decisions. Together inherent MBOK coding gain processing gain exceed 10dB requirement.
where r(t) n(t) consequently
y(t)
y(t)
since n(t) uncorrelated with Walsh function, integration with left with integral transmitted signal multiplied with Walsh function.
MBOK System Complexity Cost
11Mbit/s radio Figure same number components, fits same PCMCIA card Intersil's existing 2Mbit/s PRISM radio. HFA3860A baseband processor same footprint that HFA3824 2Mbit/s baseband processor both processors same Lead TQFP package. 5.5X increase data rate performance cost penalty overall radio Figure modest when compared 2Mbit/s radio.
Detection MBOK Signal
transmitter generates MBOK signal imposes waveform channel. other channel receiver must detect signal. let's correlator receiver uses orthogonality property signal detect correct signal. Since there eight Walsh functions (and their inverses), bank sixteen correlators, eight each channels used
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Application Note 9820 Conclusions
efficient modulation technique known MBOK been described wireless data transmission. been shown that when used direct sequence spread spectrum radio MBOK modulation technique capable achieving: Ethernet data rates Greater than 10dB processing gain Three non-overlapping channels band. Relatively system complexity cost. Frenzel, Louis Principles Electronic Communications Systems, Glencoe, McGraw-Hill, copyright 1998. Spilker, James Jr., Digital Communications Satellite, Prentice-Hall, copyright 1977. [10] Ziemer, Tranter W.H., Principles Communications, Systems, Modulation, Noise, John Wiley Sons, copyright 1995. [11] Panter, Philip Modulation, Noise Spectral Analysis Applied Information Transmission, McGrawHill, copyright 1965. [12] Shannon, Claude Communications Presence Noise, Proceedings IRE, Vol. 10-21, Jan. 1949. [13] Andren, Carl, 11Mb/s Modulation Techniques, Proceedings Sixth Annual Wireless Symposium, Penton Publishing, copyright 1998. [14] Andren, Carl, 2.4GHz, 11Mb/s Baseband Processor 802.11 Applications, Proceedings Sixth Annual Wireless Symposium, Penton Publishing, copyright 1998. [15] Intersil Corporation, HFA3860A Data Sheet, AnswerFAX Document #4488. [16] Couch, Leon Modern Communications Systems, Principles Applications, Prentice-Hall, copyright 1995. [17] Schumacher, Paul Understanding Basics Spread-Spectrum Communications, Part Microwaves magazine, 1993. [18] Fakatselis, Petrick, System Considerations Spread-Spectrum Designs, Wireless Design Development magazine, April 1995, Volume Number [19] Stremler, Ferrel Introduction Communication Systems, Edition, Addison-Wesley Publishing copyright 1982. [20] Cooper, McGillen, Modern Communications Spread Spectrum, McGraw-Hill, copyright 1985. [21] Proakis, John Salehi, Masoud, Communications Systems Engineering, Prentice Hall, copyright 1994.
Such radios have been certified operation band. shown frequency hopping radios operating band will have difficulty achieving Ethernet speed limited bandwidth waveform. This application note means been inclusive treatise subject spread spectrum communications. Important subjects like carrier tracking, symbol timing synchronization multipath effects were covered. interested reader refer References more information this broad subject.
References
Intersil documents available internet, site http://www.intersil.com/ Intersil AnswerFAX (321) 724-7800. Code Federal Regulations, Part IEEE 802.11, Wireless Medium Access Control (MAC) Physical Layer (PHY) Specifications, Institute Electrical Electronic Engineers, November, 1997. Dixon, Robert Spread Spectrum Systems with Commercial Applications, Edition, John Wiley Sons, Inc., copyright 1994. Simon, Marion Omura, Scholtz, Robert Levitt, Barry Spread Spectrum Communications Handbook, McGraw-Hill copyright 1994. Scientific American magazine, April 1998, Utlaut, William Spread Spectrum, Principles Possible Application Spectrum Utilization Application, IEEE Communication Society Magazine, September 1978. Stremler, Ferrel Introduction Communication Systems, Edition, Addition-Wesley Publishing Company, copyright 1982.
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Author: Pearson approval IEEE 802.11 Standard wireless LANs given

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