Issues in Wireless Physical Layer

1
Issues in Wireless Physical Layer

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

2
Outline

• RF Spectrum Issues
• Wireless Channel Characteristics
• Combating Fading
• Diversity Techniques
• Transmit Diversity
• Multiple Access
• Power Control
• Co-channel Interference
• Ultra Wideband Techniques

3
Radio Frequency Spectrum

• Communication through electromagnetic wave
  propagation
• Frequency Spectrum
• Certain ranges of frequency
• Only certain frequency spectra are usable
• Limitations of atmospheric propagation effects
• Technology/Device limitations
• Regulatory issues
• Safety hazards
• Demand for spectrum far exceeds supply
• Efficient use of RF spectrum is important

4
RF Spectrum - Some Current systems

• 900 MHz Cellular Band
• GSM 890 - 915 MHz Uplink 935 - 960 MHz Downlink
• IS-54 824 - 849 MHz Uplink 869 - 894 MHz
  Downlink
• PDC 810 - 820 MHz and 1429 - 1453 MHz Uplink
• 940 - 960 MHz and 1477 - 1501 MHz
  Uplink
• IS-95 824 - 844 MHz Uplink 869 - 889 MHz
  Downlink
• 1800 MHz PCS Band
• 1850 - 1910 MHz Uplink 1930 - 1960 MHz Downlink
• DECT 1880 - 1900 MHz
• C, Ku, L and S-Bands for SATCOM
• C-band 5.9 - 6.2 GHz Uplink 3.7- 4.2 GHz
  Downlink
• Ku-band 14 GHz Uplink 12 GHz Downlink
• L-band 1.61 - 1.6265 GHz S-band 2.4835 - 2.5
  GHz

5
Unlicensed Radio Spectrum
Carrier wavelength
33 cm
12 cm
5 cm
26 MHz
83.5 MHz
200 MHz
5.35 MHz
902 MHz
2.4 GHz
5.15 GHz
928 MHz
2.4835 GHz

• Wireless LANs
• Cordless phones

• 802.11b
• Bluetooth
• Microwave Oven

• 802.11a

6
RF Spectrum

• Some forward looking developments
• 300 MHz BW in the 5 GHz band made available
• to stimulate Wireless LAN technologies and use
• Ultra wideband (UBW) technology
• 60 GHz band for high-speed, short-range
• communications

7
Physical Layer Tasks

• Wireless systems need to overcome one or more of
  the following distortions
• AWGN (receiver thermal noise)
• Receiver carrier frequency and phase offset
• Receiver timing offset
• Delay spread
• Fading (without or with LOS component)
• Co-channel and adjacent interference (CCI, ACI)
• Nonlinear distortion, intermodulation, impulse
  noise

8
Motivation for PHY Layer Advances

• Increase channel capacity (spectral efficiency) -
  higher average bit rate
• Increase Erlang Capacity - more users per square
  area
• Increase reliability
• Reduce Tx power
• Increase range
• Increase coverage

9
PHY Layer Advances
Erlang Capacity
Spectral Efficiency

• Transmit Diversity

Spatial Multiplexing
OFDM
Sectorisation
Link Adaptation
Space-Time Coding
Variable Bit-Rate

Voice Activity Detection
Transmit Diversity
Frequency Hopping
Receive Diversity
Smart Beam-forming
Turbo Coding
Interference Suppression
DS-CDMA
Fixed Beamforming
Power Control
Range (Power Efficiency)
Multi-user Detection
Dynamic Channel Selection
10
Wireless Channel Characteristics

• Free-space Transmission
• (
  )

Rx
Tx
11
Mobile Radio Channel

• Characterized by
• Free space (distance) loss
• Long-term fading (shadowing)
• Short-term fading (multipath fading)

12
Mobile Radio Channel
Short Term Fading
0.1 - 1 m (10 - 100 msecs)
Received Power
Distance Loss
Long Term Fading
10 - 100 m (1 - 10 secs)
Distance, d
13
Distance Loss

• In line-of-sight AWGN channels (AWGN Additive
  White
• 
  
  
  Gaussian Noise)
• distance loss , distance
  between Tx and Rx
• loss exponent is 2 (i.e., 20 dB/decade
  loss)
• In urban mobile radio channels
• loss exponent varies between 2.5 to 5.5
• 40 dB/decade loss (typ) Rx Signal power
• (Based on field measurements)
• Slowly varying compared
• to carrier wavelength
• Fwd Rev links impacted
• in the same way

40 dB
40 dB/decade
40 dB
1 km
10 m
100 m
14
Shadowing

• Signals are blocked by obstacles (e.g., bridges
  buildings, trees, etc)
• Shadow loss variation - typ
• log-normally distributed
• (Std Dev of distribution 4 to 12 dB)
• Slowly varying compared
• to carrier wavelength
• Fwd Rev links impacted
• in the same waybri

15
Multipath Propagation
Base Station
Tx. signal
Rx. signal
Channel
Path 1
Path 2
Impulse Response
Path n
Mobile
Frequency Response
16
Multipath (Short term) Fading

• Time-varying impulse response
• Fluctuations in received signal amplitude
  (typically Rayleigh distributed)
• Time spread
• Doppler Spread
• Fade variations are fast
• Rev link fading independent
• of Fwd link fading

Signal Strength
Rev link fade
Fwd link fade
time
17
Key Multipath Parameters

• Delay / Frequency Characterization
• Delay spread,
• Coherence BW,
• Time variations
• Coherence time,
• Doppler BW,

18
Delay Spread / Coherence BW

• Autocorrelation function of
• If we let , gives the
  average
• power output of the channel as a function of

Autocorrelation
FT
Max. Delay Spread
FT Pair
Coherence Bandwidth
19
Delay / Frequency Characterization

• Delay Spread
• range of differential delay between different
  paths
• jitter in Rx time of the signal, long echoes
• results in Inter-Symbol Interference (ISI).
• Need equalization to combat ISI (in unspread
  systems)
• Provides time Diversity in spread systems (RAKE
  Combining in CDMA)
• Coherence BW
• BW over which fade remains constant or have
• strong amplitude correlation

20
Delay / Frequency Characterization

• Frequency non-selective fading
• Coherence BW gt Signal BW
• Frequency selective fading
• Coherence BW lt Signal BW

21
Time Variations

• Coherence Time
• Time over which fade remains constant or have
• strong amplitude correlation
• Coherence time gt symbol time Slow fading
• Coherence time lt symbol time Fast fading
• Doppler BW
• frequency shift on the carrier frequency due to
  relative motion between Tx and Rx
• depends on user velocity and carrier wavelength
• Note

22
Doppler Bandwidth
mobile velocity
carrier wavelength
carrier frequency
For MHz,
m
Km/h,
Hz

• Larger Doppler Bandwidth necessitates
• Larger power control control update
• rates in CDMA
• Faster converging algorithms when
• adaptive receivers are employed

23
Effect of Fading
Fading
AWGN
Non-fading AWGN Channel falls
exponentially with increasing SNR
Fading Channel falls linearly with
increasing SNR
24
Combating Fading Effects

• Diversity techniques
• Provide the receiver with multiple fade replicas
  of the same information bearing signal
• Assume independent diversity branches
• If denote the probability that the
  instantaneous SNR is below a given threshold on a
  particular diversity branch
• Then, the probability that the the instantaneous
  SNR is below the same threshold on diversity
  branches is

25
SISO to MIMO

• Single Input Single Output (SISO)
• LOS point-to-point links
• Single Input Multiple Output (SIMO)
• Receiver diversity
• Multiple Input Single Output (MISO)
• Transmit diversity
• Space time transmission
• Multiple Input Multiple Output (MIMO)
• Multiple transmitting and multiple receiving
  antennas

26
Receive Diversity Techniques

• Several methods by which receive diversity can be
  achieved include
• Space diversity
• Time diversity (coding/interleaving can be viewed
  as a efficient way of time diversity)
• Frequency diversity (multiple channels separated
  by more than the coherence BW)
• Multipath diversity (obtained by resolving
  multipath components at different delays)
• Angle/Direction diversity (directional antennas)
• Macro diversity

27
Receive Diversity Combining

• Method by which signals from different diversity
  branches are combined
• Predetection Combining
• Postdetection combining
• With ideal coherent detection there is no
  difference between pre- and postdetection
  combining
• With differentially coherent detection, there is
  a slight difference in performance

28
Receive Diversity Combining

• Maximal Ratio Combining (MRC)
• For BPSK
• Equal Gain Combining (EGC)
• Selection Combining (SC)
• 
  where
• Generalized Selection Combining (GSC)
• Switch and Stay Combining (SSC)

29
Diversity Performance
Fading (L1)
L2
AWGN
L3
L4
Average SNR

• Diversity gain is maximum when the diversity
  branches are
• uncorrelated.
• Correlation between diversity branches reduces
  diversity gain
• Diversity gain is greater for Raleigh fading
  than for Ricean

30
Transmit Diversity

• Issue Receive diversity at the mobile is
• difficult because of space
  limitations
• Using multiple transmit antennas at the
• base station with a single receive at the
• mobile can give same diversity benefits
• Tx. Diversity schemes
• with feedback from the mobile
• without feedback from the mobile

31
Transmit Diversity
Tx
Rx
32
Spatial Multiplexing

• Use N Tx antennas and M Rx antennas (N lt M)
• by sending N symbols at a time

Rx
Tx
Channel Matrix
33
Co-channel Interference

• Frequencies reused in different cells to increase
  capacity
• Reuse Distance
• Minimum distance between cells using
• same frequencies
• Cell Radius
• Reuse Ratio

34
Co-channel Interference

• S/I Signal-to-Interference Ratio
• For same size cells, co-channel interference
  (CCI)
• becomes a function of and
• Increasing reduces CCI
• path loss exponent (4 typ) No. of
  co-channel cells
• S/I required 18 dB (typ) gt cluster size N gt
  6.49
• For 7-cell reuse (N 7), S/I 18.7 dB

35
Co-Channel Interference

• In FDMA/TDMA CCI determines the reuse distance
• In CDMA, CCI affects the number of users
• supported by a BS
• CCI can be reduced by
• Sectorization
• Power Control
• Discontinuous Transmission
• Frequency Hopping
• Multiuser detection

36
Multiple Access

• FDMA
• AMPS
• TDMA
• GSM, EDGE, DECT, PHS
• CDMA
• IS-95, WCDMA, cdma2000
• OFDM (can be viewed as a spectrally efficient
  FDMA)
• 802.11a, 802.11g, HiperLAN, 802.16

37
OFDM
Tones
Carriers
Power
Frequency
Time
Time-slots
38
DS-CDMA vs OFDM
Tx. signal
Rx. signal
Channel
CDMA attempts to exploit time-diversity
through RAKE receiver
Impulse Response
OFDM attempts to exploit frequency-diversity
by frequency slicing
Frequency Response
39
RAKE Receiver
H(f)
Carrier
L-Parallel Demodulators
90
H(f)
40
RAKE Finger
nTc
H(f)
Carrier
Initial timing from searcher
Pilot Sequence Despreader
Pilot Seq Tracking Loop (Early-Late Gate)
90
nTc
H(f)
41
Power Control

• To combat the effect of fading, shadowing and
  distance losses
• Transmit only the minimum required power to
  achieve a target link performance (e..g, FER)
• Minimizes interference
• Increases battery life
• FL Power Control
• To send enough power to reach users at cell edge
• RL Power Control
• To overcome near-far problem in DS-CDMA

42
Power Control

• Types of Power Control
• Open Loop Power Control
• Closed loop Power Control
• Open Loop Power Control (on FL)
• Channel state on the FL is estimated by mobile
• RL Transmit power made proportional to FL channel
  Loss
• Works well if FL and RL are highly correlated
• which is generally true for slowly varying
  distance and shadow losses
• but not true with fast multipath Rayleigh fading
• So open loop power control can effectively
  compensate for distance and shadow losses, and
  not for multipath fading

43
Power Control

• Closed Loop Power Control (on RL)
• Base station measures the received power
• Compares it with the desired received power
  (target Eb/No)
• Sends up or down command to mobile asking it to
  increase or decrease the transmit power
• Must be performed fast enough a rate (approx. 10
  times the max. Doppler BW) to track multipath
  fading
• Propagation and processing delays are critical to
  loop performance

44
Ultra wideband (UBW) Techniques

• Impulse Radio Tx (Marconis century old radio tx)
  has now emerged under the banner ultrawideband
• Reason
• mature digital techniques
• practicality low power impulse radio
  communications
• UWB
• Tx and Rx of ultra-short (sub-nanosecs)
  electromagnetic energy impulses (or monocycles
  with few zero crossings)
• FCCs definition of UWB
• BWs greater than 1.5 GHz or
• or BWs greater than 25 of the center frequency
  measured at 10 dB down points

45
UWB

• Modern UWB radio is characterized by
• very low effective radiated power (sub-mW
• range)
• extremely low power spectral densities and
• wide bandwidths (gt 1GHz)
• EIRP lt -41.25 dBm/MHz, with restrictions in bands
  below 960 MHz, between 1.99 and 10.6 GHz

46
UWB

• Ways of generating signals having UWB
  characteristics
• TM-UWB
• Time modulated impulse stream
• DS-UWB
• continuous streams of PN-coded impulses (resemble
  CDMA signaling)
• employ a chip rate commensurate with the emission
  center frequency
• TRD-UWB
• employs impulse pairs that are differentially
  polarity encoded by the data

47
UWB Capabilities

• High spatial capacity
• High channel capacity and scalability
• Robust multipath performance
• Very low transmit power
• Location awareness and tracking