Wireless LANs: Physical Layer Standards and Comparisons

1
Wireless LANsPhysical Layer
Standards and Comparisons

• Dr. A. Chockalingam
• Assistant Professor
• Indian Institute of Science, Bangalore-12
• achockal_at_ece.iisc.ernet.in
• http//www.ece.iisc.ernet.in/achockal

2
Current Wireless Systems
Wireless Systems
LANs, Ad-Hoc NWs
Broadband Fixed wireless
Cellular NWs
Cordless systems - low power - local area
Wireless LANs - high speed - local area
Ad-Hoc NWs Bluetooth, HomeRF
Cellular/PCS/LEO - high power - wide area

• Evolving from primarily
• data-centric towards some
• voice capability
• Strong networking focus
• Limited mobility

• Evolving from primarily
• voice-centric towards data
• capability at high speeds
• Strong PHYsical layer focus
• Mobility

3
Wireless LANs

• Extension to or alternative for Wired LANs
• Uses radio frequency (RF) or infrared (IR)
  technology
• Combines data connectivity with user mobility
  (mostly in-building environments - not wide area
  cellular mobility!)
• Can serve vertical markets - manufacturing
  floors, warehouses, shopping malls, stock
  exchange halls, etc.
• Of late, academia and home networking enthusiasts
  increasingly use low cost WLANs
• wireless, yet no licensing, no service provider,
• no bills to pay (for the wireless segment)

4
WLAN Configuration

• Client - Access Point Configuration

Laptop / PC
Multiple access wireless channel
Wlan card
Clients
Server / NW resources / other clients

Laptop / PC
Wlan card
Access Point
Wired LAN
Wireless interface
Laptop / PC
Wlan card

• Single Access Point serves several (typ. 10 to
  50) wireless clients
• Access points range - 500 ft to 1000 ft (typ.)

5
WLAN Issues

• Range and Coverage
• Throughput
• Compatibility with the Existing Network
• Interference and Coexistence - Use Spread
  Spectrum!
• Licensing Issues
• Security
• Battery Life for Mobile Platforms

6
WLAN Physical Layer

• Spread Spectrum
• Direct Sequence SS (DSSS)
• Frequency Hopped SS (FHSS)
• Orthogonal Frequency Division Multiplexing (OFDM)

7
Spread Spectrum

• A transmission technique in which
• transmission bandwidth (W) gtgt signal bandwidth (R)

Information Signal
Transmitted Signal
Spread Spectrum Transmitter
freq.
R
W
freq.
Processing Gain W/R gtgt 1
8
Spread Spectrum Origins

• SS technique has been used in military comm.
  primarily to meet two objectives
• overcome effects of strong Intentional
  Interference (Anti-jamming Capability)
• hide the signal from the eavesdropper -
  covertness (Low Probability of Intercept)
• Both the above goals can be met by spreading
• the signals spectrum to make it virtually
• indistinguishable from background noise

9
Non-military Applications of SS

• Multiple Access
• Code Division Multiple Access (CDMA)
• Improved reliability in multipath fading
  environment
• RAKE receiver in cellular CDMA receivers
• Accurate Position Location
• Global Positioning System (GPS) range and
  location determination using SS signals
  transmitted by several satellites
• Maintain low flux density of transmission
• e.g., in VSATs and WLANs

10
Spread Spectrum Types

• Direct Sequence SS (DS-SS)
• Frequency Hopped SS (FH-SS)
• Basic ingredient of both DS-SS and FH-SS
• is Pseudo-Noise (PN) Sequences
• Both types of SS are used in WLANs and
• Ad-Hoc Networks

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DS-SS

• DS-SS signal is obtained by multiplying the
  information bits with a wideband PN signal

Information Bits
Carrier Modulation
Tb
PN Signal
Information Bits
t
Tb N Tc
Tc
N Processing Gain
PN Signal
t
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DS-SS Receiver
Received DS-SS Signal
Rx. Data Output
Integrator
Sampler
Local carrier
Local chip-rate clock
Local PN Seq. Gen.
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FH-SS

• FH-SS signal is obtained by hopping the carrier
  frequency over a set of frequencies
• hopping pattern specified by a PN Sequence

Tb Bit duration Th Duration of 1 hop
Information Bits
f1, f2, , fn
PN Seq. Generator
Frequency Synthesizer
Th lt Tb Fast Frequency Hopping Th gt Tb Slow
Frequency Hopping
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FH-SS Receiver
Rx. Data Output
Received FH-SS Signal
Demodulator
f1, f2, , fn
Frequency Synthesizer
Time Sync
Local PN Seq. Gen.
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OFDM

• Available system BW is divided into a number of
  (N) narrower sub-bands
• Input data stream is divided into N sub-streams
  and the sub-streams are allowed to modulate the N
  sub-carriers
• bit interval in the sub-stream is increased by a
  factor of N
• Channel fading becomes frequency non-selective
  (flat) than frequency selective
• since symbol period is increased, delay spread
  becomes a fraction of the symbol period

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OFDM Transmitter
BPF
Serial Data Stream
BPF
Serial to Parallel Converter
M-QAM Encoder
BPF
BPF
17
OFDM Receiver
BPF
BPF
Parallel to Serial Converter
Output Data
M-QAM Demodulator
BPF
BPF
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WLAN Frequency Bands

• Unlicensed spread spectrum systems operating in
  ISM bands
• 902 - 908 MHz, 2.4 - 2.5 GHz, 5.8 - 5.9 GHz
• Tx. Power lt 1 W
• Range lt 1000 ft
• Licensed systems in the 18 - 19 GHz band
• Tx. Power lt 250 mW
• Range 50 - 150 ft
• Infrared range
• Wave length 800 - 900 nm
• Range 50 to 150 ft

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Standards

• IEEE 802.11 series
• IEEE 802.11
• IEEE 802.11a
• IEEE 802.11b
• IEEE 802.11g
• Bluetooth
• HomeRF
• IEEE 802.15 - Wireless PAN
• IEEE 802.16 - Wireless MAN
• HIPERLAN 2

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IEEE 802.11
Media Access Control (MAC)
the protocol
FHSS PHY
IR PHY
DSSS PHY
the transmitter receiver
DSSS Direct Sequence Spread Spectrum FHSS
Frequency Hopped Spread Spectrum IR Infrared
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802.11 - DS-SS PHY

• 2.4 GHz band
• About 80 MHz license free BW in 2.4 - 2.4835 GHz
  band (US and Europe)
• 2.471 - 2.497 GHz band (In Japan)
• 1 Mbps and 2 Mbps data rate
• Processing gain at least 10 dB
• 11 chip Barker sequence used for spreading
• sequence -1,1,-1,-1,1,-1,-1,-1,1,1,1
• Autocorrealtion function is 0 or -1 except at
  origin
• 1 symbol 11 chips, Transmission rate 11 MHz
• Modulation
• Differential BPSK (for 1 Mbps)
• Differential QPSK (for 2 Mbps)

22
802.11 - Barker Code

• From Coding point of view, Barker code can be
  viewed as a linear block code over the set of
  integers modulo 4, Z40,1,2,3
• Consider
• k x n 1 x 11 repetition generator matrix
• G 1,1,1,1,1,1,1,1,1,1,1,1
• and the cover vector B 2 0 2 2 0 2 2 2 0 0 0
• The four Barker codes for 2 Mbps case is given by
  
• C m. G B c1,c2,c3,c4,c5,c6,c7,c8,c9,c10,c1
  1 mod 4, where m the message symbol in Z4
• C0 (for m0) 2 0 2 2 0 2 2 2 0 0 0
• C1 (for m1) 3 1 3 3 1 3 3 3 1 1 1
• C2 (for m2) 4 0 4 4 0 4 4 4 0 0 0 0 0 0 0
  0 0 0 0 0 0 0
• C3 (for m3) 5 3 5 5 3 5 5 5 3 3 3 1 3 1 1
  3 1 1 1 3 3 3

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802.11 - Barker Code

• QPSK mapping produces the signal vector
• C -gt X x1, x2, x3, x4, x5, x6,x7, x8, x9,
  x10, x11
• using the mapping
• Code symbol, Ci Signal, Xi
• 0 1j
  
• 1 -1j
• 2 -1-j
• 3 1-j
• E.g., For m00,0, C1 2 0 2 2 0 2 2 2 0 0 0
• x1 -1-j, 1j, -1-j, -1-j, 1j, -1-j, -1-j,
  -1-j, 1j, 1j, 1j
• For m311, C3 1 3 1 1 3 1 1 1 3 3 3
• x1 -1j, 1-j, -1j, -1j, 1-j, -1j, -1j,
  -1j, 1-j, 1-j, 1-j

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802.11 - Barker Code

• Asymptotic Coding Gain (ACG) of a coded (C)
  system relative to the uncoded (U)system
• For 2 Mbps, ,
• For 1 Mbps,

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802.11 - DS-SS PHY

• Synchronization 128 bits of Preamble SYNC (all
  1s)
• Frame Error Check CRC-16
• Data scrambling, No FEC
• Tx Power
• 1mW (min), 1000mW (max)
• Tx power control Max 4 PC levels, Min lt 100
  mW
• Tx power-up ramp
• RF carrier suppression gt 15 dB
• Receiver sensitivity
• min. -80 dBm (8 FER with 1024 byte frames), max
  -4 dBm
• RSSI, ED threshold, Carrier sense, CCA

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802.11 - FHSS PHY

• 2.4 GHz band
• Hopping set size
• 79 frequencies in increments of 1 MHz (US,
  Europe)
• 23 (Japan), 27 (Spain), 35 (France)
• Hopping sequence
• Pseudo-random hopping pattern (22 patterns)
• Hop rate governed by the MAC (min rate
  regulatory)
• 2.5 hops/sec in US
• Channel Switching/Settling time
• freq to settle within /- 60 KHz of nominal
  center freq within 224 microsec
• Tx Power
• 10 mW (min), lt 100 mW

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802.11 - FHSS PHY

• Data rates
• 1 Mbps (mandatory) and 2 Mbps (optional)
• Modulation
• 2 GFSK for 1 Mbps (1 Fcfd, 0 Fc-fd, Fc center
  freq, fd peak deviation)
• 4 GFSK for 2 Mbps
• BT product 0.5
• No FEC

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802.11a

• Overcrowding in the 2.4 GHz band
• 802.11b, Bluetooth, HomeRF, microwave ovens
• So 802.11a is defined in the 5 GHz band
• Unlicensed National Information Infrastructure
  band
• 300 MHz of interference free spectrum
• 5.15 to 5.35 GHz, 5.725 to 5.825 GHz
• Data rates
• 6, 9, 12, 18, 24, 36, 48, 54 Mbps
• 6,12,24 Mbps mandatory
• Uses OFDM technology
• 52 orthogonal subcarriers (4 used for Pilot)
• 20 MHz sliced in to 300 KHz subchannels

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802.11 - Data Rates

• BPSK/QPSK, 16 QAM, 64 QAM modulation
• Rate 1/2, 2/3, 3/4 Convolutional code

Data Rate (Mbps)
Code Rate
Coded bits per subcarrier
Coded bits per OFDM symbol
Data bits per OFDM symbol
Modulation
1
24
48
6
BPSK
1/2
9
48
BPSK
1
36
3/4
12
QPSK
1/2
2
48
96
18
2
QPSK
3/4
96
72
24
1/2
4
192
16-QAM
96
4
3/4
144
36
16-QAM
192
6
64-QAM
2/3
48
288
192
64-QAM
54
288
6
216
3/4
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802.11a OFDM

• Input bit steam is divided in to groups of 1, 2,
  4, 6
• this grouping depends on the rate/modulation
• These groups of bits are mapped to complex
  numbers depending on BPSK/QPAK/QAM
• This stream of complex numbers is divided into
  groups of 48
• Send each complex number in this group on a
  separate subcarrier
• Channel estimation, frequency offset
• training symbols

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802.11b

• 2.4 GHz band
• Supports 5.5 Mbps and 11 Mbps
• Backward compatible DSSS transmission to 802.11
• Only PHY changes to 802.11. MAC remains
  unmodified
• Enhancements to MAC (QoS) IEEE 802.11e
• 11b Highly successful
• millions of 11b compliance devices
• Complementary Code Shift Keying (CCK)
• Packet Binary Convoltuional Code (PBCC)
• high performance mode (optional)

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802.11b CCK-11/5.5

• CCK can be considered as a block code
  generalization of the lower rate Barker Code
• For CCK-11, the code is an (n8,k4) linear block
  code over Z4
• At 11 Mbps rate, 8 bits (4 - Z4 symbols) are
  encoded via the k x n 4 x 8 generator matrix
• 1 1 1 1 1 1 1 1
• 1 1 1 1 0 0 0 0
• 1 1 0 0 1 1 1 1
• 1 0 1 0 1 0 1 0
• using the matrix equation
• C M. G B c1,c2,c3,c4,c5,c6,c7,c8 mod 4
• the cover vector B 0 0 0 2 0 0 2 0
• M m1,m2,m3,m4, mj in Z4, represents 8 bits of
  information

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802.11b CCK-11/5.5

• CCK-11 For m10000, C 0 0 0 2 0 0 2 0
• gt X 1j, 1j, 1j, -1-j, 1j, 1j,
  -1-j, 1j
• CCK-5.5
• At 5.5 Mbps rate, 4 bits are encoded via the k x
  n 3 x 8 generator matrix
• 1 1 1 1 1 1 1 1
• 2 2 2 2 0 0 0 0
• 2 0 2 0 2 0 2 0
• using the matrix equation
• C M. G B c1,c2,c3,c4,c5,c6,c7,c8 mod 4
• the cover vector B 1 0 1 2 1 0 3 0
• M m1,m2,m3, m1 in Z4, m2 in Z2, m3 in Z2,
  represents 4 bits of information

34
802.11b CCK11/5.5

• ACG for CCK 2 (i.e., 3 dB)
• CCK codes can be compactly represented in terms
• of trellis

35
802.11b PBCC

• Packet Binary Convolutional Code
• PBCC-11 Es/No required for PER of 10-2 4.3 dB
• CCK-11Es/No required for PER of 10-2 7.8 dB
• PBCC-11 performs 3.5 dB better than CCK-11

Scrambled Data In
BPSK/QPSK signal
BPSK (5.5) / QPSK (11) Cover Map
(y0, y1)
BCC Rate-1/2 Encoder (64-state)
C
M
S
256 bits Pseudo-random Cover Sequence
Cover Code
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802.11b PBCC

• Cover Code Map
• Mapping from BCC outputs to PSK constellation is
• determined by a pseudo-random cover sequence
  as
• follows

S 1
S 0
QPSK Mode (y1y0) (1 bit per symbol)
10
00
01
00
11
10
11
01
0
S 1
0
S 0
BPSK Mode (1/2 bit per symbol)
1
1
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802.11g

• 2.4 GHz band
• Motivated by work done by Alantro Communications
  (now part of TI)
• Goal was to maintain backward compatibility to
  11b, but rates in excess of 20 Mbps
• PBCC-22
• Uncoded QPSK Es/No 13.5 dB
• PBCC-22 with 8 PSK Es/No 8.5 dB

38
HomeRF

• Industry Association of 80 companies from
  Computer, Telecom, Consumer Electronics
  industries
• Wireless networking within home - very last 150
  ft
• Enable a new class of mobile consumer devices -
  draw from content of Internet and Home PC
• Developed a open specification for voice data
  NW
• Shared Wireless Access Protocol (SWAP)
• Leverages PC-industry infrastructure around
  Internet, TCP/IP, and Ethernet
• Adds a standard way to connect to PSTN for voice
  telephony
• Broad backing of major corporate stakeholders in
  NW

39
HomeRF Origins
DECT Uses TDMA Good for Voice
IEEE 802.11 Uses CSMA/CA Good for Data
SWAP TDMA CSMA/CA Good for Voice Data
Optimized for small networks (in home)
Simplified radio protocol to reduce cost
40
HomeRF - SWAP

• SWAP handles voice like DECT, but
• frequency hopping
• 20 msec frames (2 x 10 msec) DECT frames
• interleaved up and down links
• retransmission (single)
• SWAP handles data like 802.11, but
• Relaxed PHY layer specs to reduce cost
• Beacons to manage isochronous traffic
  (reservation for voice)
• Simplified protocol (no roaming)
• Windows 2000 will fully support SWAP

41
HomeRF - PHY / MAC Features

• PHY
• 100 mW (nominal) Tx. Power
• 2 FSK modulation (4 FSK in 802.11)
• Min. Rx. Sensitivity of -76 dBm. Cost effective
  filters
• Use MAC to reduce PHY cost
• Makes single-chip integration simpler
• MAC
• easy integration with Ethernet
• excellent integration with TCP/IP protocols
• supports broadcast, multicast
• Extensive power management for ultra-portable
  devices
• Basic and Enhanced Data Security

42
Bluetooth

• Towards ubiquitous wireless connectivity
• Five promoters
• Ericsson, Nokia, IBM, Toshiba, Intel
• over 650 adopters
• Design Goals
• low cost
• low energy consumption
• robust operation
• high aggregate capacity
• flexible usage, global usage
• multimedia support

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Bluetooth

• 2.402 - 2.48 GHz band
• Frequency Hopped Spread Spectrum
• 79 frequencies of 1 MHz
• Hop rate 1600 hops/sec)
• Master-Slave configuration (1 master, 7 slaves)
• Time Division Duplexing (Master Tx in even slots
  and Slave Tx in odd slots)
• 625 microsec slots. T_HOP T_SLOT
• Data Rate 1 Mbps
• Gaussian shaped binary FSK with BT 0.5,
  modulation index 0.28 to 0.35
• Max. freq. Deviation 140 to 175 KHz

44
Bluetooth

• Tx power 1 mW - 100 mW
• Power Control
• min. step size 2 dB
• max. step size 8 dB
• RSSI
• For BER0.001,
• receiver sensitivity -70 dBm or better
• Co-channel interference resistance 11 - 14 dB
• Mix circuit (voice) packet (data) switching
• Rate 1/3 or 2/3 FEC for DATA
• CVSD (Continuous Variable Slope Delta modulation)
  for voice

45
Bluetooth - Physical Link Definition

• Synchronous Connection Oriented (SCO) Link
• Circuit switching
• Symmetric, synchronous services
• slot reservation at fixed intervals
• Asynchronous Connectionless (ACL) Link
• Packet switching
• (A)symmetric, asynchronous services
• polling access scheme

46
Bluetooth - Data Rates (kbps)
Asymmetric
Symmetric
Type
Downlink
Uplink
DM1
108.8
108.8
108.8
172.8
172.8
DH1
172.8
54.4
258.1
387.2
DM3
86.4
DH3
390.4
585.6
36.3
DM5
286.7
477.8
DH5
57.6
433.9
723.2
H No coding
47
IEEE 802.16

• IEEE 802.16 Projects IEEE 802.16 Projects
• Air Interface (PHYs with common MAC)
• P802.16 10- 66 GHz
• P802.16a 2- 11 GHz
• Licensed bands only
• P802.16b 5- 6 GHz
• License- exempt (WirelessHUMAN TM )
• Coexistence
• IEEE 802.16.2 (10- 66 GHz)
• P802.16.2a amendment w/ 2- 11 GHz licensed

48
HiperLAN-2

• 5.4 GHz frequency band
• Supports rates up to 54 Mbps
• Uses OFDM
• Each channel supports 23.5294 Mbps raw (effective
  rate is 20 Mbps after overheads)
• Max. Tx. Power 1 W. Range 30 to 150 m
• GMSK modulation with BT product 0.3
• Mobility lt 36 km/h
• Supports sync, async, and real-time traffic
• Supports QoS