todd stockert ieee northcon 2002 october 23, 2002 [email protected][email protected]...

Todd StockertIEEE Northcon 2002October 23, 2002 [email protected]

Physical Layer and RF Testing Overview of Wireless LAN MODEMS

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Physical Layer and RF Testing Overview if Wireless LAN MODEMS23 October, 2002

Copyright Agilent Technologies 2002

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WLAN: What and Why

WHAT? • An extension to, or a replacement for, wired LANs that offers:

• Ease of installation• Flexibility• Scalability• Cost savings• Mobility

WHY?• Eliminate physical constraints of wires• Installing wires is expensive, time consuming, and disruptive



Topics

• Introduction to Wireless LAN Standards

• 802.11b PHYsical layer

• Overview of Spread Spectrum modulation

• 802.11b PPDU Structure

• 802.11a PHYsical layer

• Overview of OFDM

• 802.11a PPDU Structure

• Glance at 802.11g (Draft standard)

• Overview of WLAN Testing and Specification Measurements



Current WLAN Standards

Approved Standard Throughput

1996 ETSI HIPERLAN/ I 1.47, 23.5 MBit/ sec

1997 IEEE 802.11 1, 2 MBit/ sec

1999 IEEE 802.11b 1, 2, 5.5, 11MBit/ sec

1999 IEEE 802.11a 6 - 54 MBit/ sec

2000 ETSI HIPERLAN/ II 6 - 54 MBit/ sec

2002? IEEE 802.11g 1 - 54 MBit/ sec



IEEE 802.11b• Incorporates 802.11 1,2 Mbit/sec modes, adds

5.5 and 11 Mbit/sec modes

• Most successful WLAN standard to date

• “Wi-Fi” standard

• 2.4GHz ISM frequency band



IEEE 802.11b: 2.4GHz Difficulties

802.11b Burst

uWave Ovens

Discrete Tone

uWave OvenSplatter

FHSS(Cordless Phone, Bluetooth...)



802.11B: Modulation Formats• All modes incorporate “Spread Spectrum” modulation

• 802.11b extends 802.11 data rates to include 5.5 and 11 MBit/sec

• There are three rates to keep track of:Chip Rate: 11 MChip/sec -- always

Bit Rates: 1 MBit/sec DBPSK 11 chip Barker sequence

2 MBit/sec DQPSK 11 chip Barker sequence 5.5 MBit/sec QPSK, 4 8-chip CCK spreading 11 MBit/sec DQPSK, 64 8-chip CCK spreading

5.5 MBit/sec BPSK PBCC (optional) 11 Mbit/sec QPSK PBCC (optional)

Symbol Rates: 1 MHz (11/11) and 1.375 MHz (11/8)

• At all rates, the signal looks like an 11 MHz BPSK or QPSK waveform



Spread Spectrum Concepts: Simple Spreading 802.11B -- 1 MBit/Sec

Map

Barker Sequence+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1

1 Bit In

Data Phase Change(deg)

0 01 180

DBPSKFor each single input bit, there are two possible 11 chip sequences that can be transmitted

+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1–1, +1, –1, –1, +1, –1, –1, –1, +1, +1, +1

One sequence is simply the inverse of the other

11 Complex Chips Out



Spread Spectrum Concepts: Simple Spreading 802.11B -- 2 MBit/Sec

Map

Barker Sequence+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1

2 Bits In

For every two input bits, there are four possible 11 chip sequences that can be transmitted

+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1+j, –j, +j, +j, –j, +j, +j, +j, –j, –j, –j–1, +1, –1, –1, +1, –1, – 1, –1, +1, +1, +1–j, +j, –j, –j, +j, –j, – j, –j, +j, +j, +j


Data (Dibit) Phase Change (deg)

00 0

01 9011 18010 -90

DQPSK



802.11B -- 11 MBit/Sec ??

Map

Barker Sequence+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1

11 Bits In


D2048PSK?



Spread Spectrum Concepts: Simple Spreading 802.11B -- 5.5 and 11 MBit/Sec

4 Bits In

1 of 4 8-chip sequence

Selector

DQPSKRotate

8 Complex Chips Out

8 Bits In

1 of 64 8-chip sequence

Selector

DQPSKRotate

8 Complex Chips Out

d0-d3d0,d1

d2-d7

d0-d7

d0,d1

d2,d3



IEEE 802 Protocol Acronyms

*From Agilent 802.11b WLAN Signal Studio Software Product Note

Physical Medium Dependent(PMD)

PHY Convergence Layer Procedure(PLCP)

MAC sublayer Protocol Data Units (MPDU)

PHY Protocol Data Units (PPDU)

PLCP Service Data Units (PSDU).

*See IEEE802 standards for more detail



802.11B: Long PLCP PPDU Format




Structure of IEEE802.11b CCK Frame (= Burst)PLCP Preamble PSDUPLCP Header

144 bits Fn(LENGTH,SIGNAL) bits48 bits




PLCP Preamble• 144 usec, (72 for ShortPLCP)• 1MBit/s modulation• Scrambled 1’s + SFD• Signal detect, AGC,timing synchronization,frequency estimation.




PLCP Header•1MBit/s modulation (2MBit/s for ShortPLCP)•CCITT CRC-16 Protects these fields

SIGNAL

- - b2b3 - - - b7SERVICE

b2: Locked clocks 0: Not Locked 1: Lockedb3: Modulation Selection 0:CCK 1:PBCCb7: Length extension bit Solves LENGTH ambiguity

0x0A 1Mbps0x14 2Mbps0x37 5.5Mbps0x6E 11 Mbps

LENGTH

16bit Length of PSDU in µsec (Special Rounding)




PSDU• Format varies with data rate• Max symbols varies with data rate• MAC layer starts here.

•MAC Header•Payload Data•FCS



802.11B: Short PLCP PPDU Format (Optional)




Significant differences from 802.11b

• 5-6 GHz Frequency Bands (Less interference, more spectrum)

• OFDM Modulation

• 54 MBits/Sec in the same bandwidth

RF Characteristics

• More precise RF design (<1/2 the wavelength)

• Much Higher Peak-to-Average Power Ratio

• More Sensitive to Phase Noise

IEEE 802.11a



OFDM – Basic Concepts

• Slower symbol rate x multiple carriers = similar bits/sec/Hz

• Carrier spacing creates orthogonality.

• Less susceptible to:- single freq.

interference- multipath

dropouts- impulse noise

• IEEE 802.11a and HiperLAN/2

- 52 carriers- 250 kHz symbol

rate- 312.5 kHz

spacing- 18 MHz

bandwidth

. . . . . .



Generating OFDM

-2 +2

+1

0-1-3 +3.. ..-24

-25

+25

+26

+24

....-26

carrier number:

bits

encode, interleave,map onto

constellationload complex values

into frequency bins.29 + j.85

1011

do inverse FFT tocreate time waveform

add guard interval,clock out at 20

MSa/sectransmit as 1

symbol

repeat 52 times

repeat untildata

finished



OFDM vs. Single Carrier ModulationFrequency Domain View

1 carrier 52 carriers

BW = Sym(1+)

BW = #carriers x spacing

Adj Chan =Distortion

Adj Chan =Normal Rolloff

Carrier #0 always null



OFDM vs. Single Carrier ModulationFrequency Domain View

1 carrier 52 carriers

BW = Sym(1+)

BW = #carriers x spacing

Pilot Carriers• -21, -7, +7, +21• always BPSK• same amplitude, phase• used as reference for vector demod.



OFDM vs. Single Carrier ModulationTime Domain View

Data rate = 54 Mbits/sec @ ¾ coding = 72 Mbits/sec @ 64QAM = 12 MSym/sec

1 symbol = one point in time1 point in time = 1 symbol

SCM: OFDM:

Data rate = 54 Mbits/sec @ ¾ coding = 72 Mbits/sec @ 48 carriers= 1.5 Mbits/sec @ 64QAM = 250 kSym/sec1 symbol = 1 point in frequency and time1 point in time = ~meaningless

1 Sym = .083 usec 1 Sym = 4.0 usec

This is a sample;FFT(64 samples) gives64 freq bins (48 carriers + 4 pilots + 12 zeros)

This is a symbol = 6 bits



Structure of IEEE802.11a OFDM Frame (= Burst)

Short Training …Data NSIGNALData 1 Data 2Chan. Estimation

8 4 4 4 4 usec. . .

8





Short Training Seq.• 8 uSec length• Every 4th carrier,equal amplitude/phase• Signal detect, AGC,timing synchronization,coarse freq. estimation.

. . .





Channel Estimation• 8 uSec length• Every carrier, equalamplitude and phase.• Channel equalization,fine freq. estimation.

. . .





SIGNAL Symbol• 4 uSec length• Always BPSK.• Describes this frame’s rate, length.

These parametersare read from signalunder test.

. . .





Data Symbols• 1 symbol = 4 uSec length 1 FFT 52 carriers (48 + 4) 52 constellation dots• Format varies• Coding varies• Max 4096 octetsper frame.• MAC layer starts here.3/4

2/3

3/4

1/2

3/4

1/2

3/4

1/2

Coding Rate

216

192

144

96

72

48

36

24

Bits per Symbol

Mod. Format

Data Rate

64QAM54

64QAM48

16QAM36

16QAM24

QPSK18

QPSK12

BPSK9

BPSK6 Mbits/sec

= 48 x 6 x .75

. . .



IEEE 802.11a Payload Structure

Address 4Address 2Address 3SequenceControlAddress 1Frame

ControlDuration/

ID

Frame ChkSequenceUser DataTail Bits Pad Bits

2 2 6 6 6 2 6 bytes

40-4061 bytes

MAC Header

. . .

. . .



IEEE802.11a versus HiperLAN/2

Similarities:• 5-6 GHz frequencies• 18 MHz bandwidth• 52 subcarriers• 250 kHz symbol rate• BPSK, QPSK, 16/64QAM• 6-54 MB/sec

Differences:• More choice of data services• Preamble contents• MAC contents• Sync procedure• Length of Guard Interval



IEEE 802.11g• Mandatory:

• All mandatory 802.11b modes

• 802.11b ShortPLCP packet format

• All mandatory 802.11a modes using 802.11b channels

• Goals:

• Puts 802.11a rates into 2.4GHz band

• Creates 1-54 Mbit/sec variable rate WLAN standard

• Completely backward compatible to 802.11b

• Many implementation choices

CCK-OFDM, PBCC-22, PBCC-33



802.11G: Rates and ModulationType Rates (MBit/ sec) Modulation Type Specified

802.11b Mandatory 1, 2, 5.5, 11 DBPSK-Barker,DQPSK-Barker, CCK

Mandatory

802.11b Optional 5.5, 11 PBCC Optional

802.11a Mandatory 6, 12, 24 OFDM Mandatory

802.11a Optional 9, 18, 36, 48, 54 OFDM Optional

CCK-OFDM 6, 12, 24 OFDM + 802.11bPreamble

Optional

CCK-OFDM Optional 9, 18, 36, 48, 54 OFDM + 802.11bPreamble

Optional

PBCC-22 22 PBCC-22 (8PSK) Optional

PBCC-33 33 PBCC-33 (8PSK) Optional



802.11G: Draft CCK-OFDM Packet formats

Long PLCP PPDU Packet

Short PLCP PPDU Packet

LongPLCP

OFDM LONG SYNCOFDM SIGNALOFDM PSDUSignal Extension

PSDU

8 µsec 8 µsec Nsym x 4 µsec 6 µsec

192 µsec

ShortPLCP

OFDM LONG SYNCOFDM SIGNALOFDM PSDUSignal Extension

PSDU

8 µsec 8 µsec Nsym x 4 µsec 6 µsec

96 µsec



WLAN Measurements• IEEE Specification Measurements

• Verify compliance of OEM components• Grade components for different price-points• Carry design-for-test from simulation to prototype to

production

• Transient capture• Troubleshoot conversation traffic• Detect sources of interference• Determine subtle DSP or analog impairments• Analyze DUT without test mode requirements

• Multi-domain measurements• Analysis in Demodulation/Time/Frequency domains

required• Most impairments have preferences for where they can

be diagnosed• Interactions exist between PvT/Spectral

Mask/Modulation Quality



IEEE802.11b Transmitter Testing

Measurement Specified by the 802.11B Standard

18.4.6.8 – TX spurious

18.4.7.1 – TX power level

18.4.7.3 – TX spectrum mask

18.4.7.4 – TX center freq. tolerance

18.4.7.5 – Symbol clock freq. tolerance

18.4.7.6 – TX power-time mask

18.4.7.7 – TX carrier leakage

18.4.7.8 – TX modulation accuracy



802.11B: TX Spectral Mask



802.11B: TX power-time mask : Power-Up Ramp

2 usec



802.11B: Power-Down Ramp



EVM: A Model For Signal Errors

Ideal Signal Generator

Error Signal Generator(e.g. noise, distortion, spurious, phase noise)

Transmitter

Ideal(t)

error(t)

actual(t)

Ideal(t)Act

ual(t

) Error(t)

Magnitude Error(t)

Error Vector Magnitude(t)

Phase Error(t)I

Q

Carrier Leakage



IEEE802.11b Modulation Accuracy

1000..1:35.0))(( nnVMax err



IEEE802.11a Transmitter Testing

Measurement Specified by the 802.11A Standard

17.3.9.1 – TX power level

17.3.9.2 – TX spectrum mask

17.3.9.3 – TX spurious

17.3.9.4 – TX center freq. tolerance

17.3.9.5 – Symbol clock freq. tolerance

17.3.9.6.1 – TX carrier leakage

17.3.9.6.2 – TX spectral flatness

17.3.9.6.3 – TX constellation error



802.11A: Measuring Center Frequency Leakage With Time Gating, Band Power, C/N Marker

Time Gating selects justthe short sync sequence.

Band Power measures powerduring the short sync.

C/N marker computes leakage at carrier #zero.



802.11A: Measuring Modulation Quality



802.11A: Modulation Quality

Data Rate (Mbit/sec) Relative Constellation Error (dB) EVM (%RMS)6 -5 56.29 -8 39.8

12 -10 31.618 -13 22.324 -16 15.836 -19 11.248 -22 7.954 -25 5.6



802.11A: Measuring Peak-to-Average StatisticsWith Time Gating, CCDF Plot

CCDF results reveal the P.A. designer’s nightmare.

Time Gating selects justthe data portion of theframe (no preamble).

Use appropriate trigger setup, delay, time length, etc.



802.11A: Channel Freq Response (Spectral Flatness)Equalizer-based

Display as Amplitude Response,Phase Response,Impulse Response,or Group Delay.



802.11A: Transmit Spectral Mask



IEEE802.11 Receiver TestingMeasurement Specified by the 802.11A Standard (Packet Error Ratio (PER) testing)

•17.3.10.1 Receiver minimum input level sensitivity•17.3.10.2 Receiver adjacent channel rejection•17.3.10.3 Receiver non-adjacent channel rejection•17.3.10.4 Receiver maximum input level

Measurements for Receiver Troubleshooting•Quality of the test signal used for RX Sensitivity Testing•Receiver Spurious•Receiver Phase Noise•Receiver EVM (distortion, noise, etc)•Measurements at RF, IF and IQ Baseband

Measurement Specified by the 802.11B Standard (Frame Error Ratio (FER) testing)

•18.4.8.1 Receiver minimum input level sensitivity•18.4.8.2 Receiver maximum input level•18.4.8.3 Receiver adjacent channel rejection



IEEE802.11 Receiver Testing802.11B : Uses FER (Frame Error Ratio)802.11A : Uses PER (Packet Error Ratio)•DUT uses demodulated CRC(11b) or FCS(11a) to determine failures.•Ratio of unsuccessful frames to total is the FER or PER•Requires software testpoint in receiver•802.11B Provides Test Mode to simplify legal transmit signals (Currently no 802.11A provision exists, but probably should).

“Golden” TX Signal Level Adjust

Interfering TX

DUT

Test Software

Signal Level Adjust

May substitute Source Instrument for this block (e.g. E4438C).



Summary• Wireless LAN Standards - Interoperability is

key to success. IEEE 802.11 standards are leading the way.

• 802.11b PHY

• Described the mandatory Spread Spectrum modulation formats

• Explored the 802.11b PHY packet structure

• 802.11a PHY

• Described some details about OFDM modulation

• Explored the 802.11a PHY packet structure

• Took a glance at 802.11g (Draft standard)

• Discussed WLAN testing methods and 802.11b/a Specification measurements



WLAN Measurement Setup

todd stockert ieee northcon 2002 october 23, 2002 [email protected][email protected]...

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