wireless communication session 5 modulation & multiple access
Post on 05-Nov-2021
2 Views
Preview:
TRANSCRIPT
1
Wireless Communication
Session 5
Modulation & Multiple Access
M. Daneshvar Farzanegan
Soourosh.blogfa.com
smdanesh@yahoo.com
IIT Madras5
Other Multiple Access Techniques
Multi-Carrier TDMA
DECT, PACS
Frequency Hopped Spread Spectrum
Bluetooth
CSMA/CA
IEEE 802.11 (1 or 2 Mbps standard)
DS-CDMA with Time Slotting
3GPP W-CDMA TDD (Time Division Duplex)
Packet Switched Air Interface is vital for high bit-rates
and high capacity (for data users) -- GPRS, DPRS, etc.
IIT Madras6
What is an OFDM System ?
Data is transmitted in parallel on multiple carriers that
overlap in frequency
IIT Madras7
FEC IFFT
DAC
Linear
PA
add cyclic extension
bits
fc
OFDM symbol
Pulse shaper
&
view this as a time to
frequency mapper
Generic OFDM Transmitter
Complexity (cost) is transferred back from the digital to the analog domain!
Serial to
Parallel
IIT Madras8
Add
Cyclic
Prefix
Serial/
Parallel
]0,[ns
]1,[ns
],[ Nns
Parallel/
SerialIFFT
]0,[nd
]1,[nd
],[ Nnd
OFDM Transmitter -- contd.
S/P acts as Time/Frequency mapper
IFFT generates the required Time domain waveform
Cyclic Prefix acts like guard interval and makes equalization easy (FFT-cyclic
convolution vs. channel-linear convolution)
1
0
2
],[1
],[N
k
N
kij
eknsN
ind
IIT Madras9
OFDM Receiver
Cyclic Prefix is discarded
1
0
2
],['1
],[N
i
N
ikj
eindN
knr
FFT
]0,[nr
]1,[nr
],[ Nnr
Parallel/
Serial
Serial/
Parallel
Remove
Cyclic
Prefix
]0,[' nd
]1,[' nd
],[' Nnd
FFT generates the required Frequency Domain signal
P/S acts like a Frequency/Time Mapper
IIT Madras10
AGC
fc
VCO
Sampler FFTError
gross offset
Slot &
fine offset
Freq. Offset
Estimation
Timing
Sync.
(of all tones sent in one OFDM symbol)
Generic OFDM Receiver
RecoveryP/S and
Detection
IEEE Symp./ IISc -2001 IIT Madras11
OFDM Basics
To maintain orthogonality where
• = sub-carrier spacing
• = symbol duration
If N-point IDFT (or FFT) is used
Total bandwidth (in Hz) =
= symbol duration after CP addition
fTs
1
f
sT
fNW
CPS TT
IIT Madras13
OFDM Basics -- contd.
If the Cyclic Prefix > Max. Delay Spread, then
the received signal after FFT, at the nth
tone for the kth OFDM block can be
expressed as
where
is additive noise
is channel frequency response
],[],[],[],[ knwknsknHknr
],[ knw
],[ knH
IIT Madras15
Sync Basis Functions(of equal height for single-ray channel)
Shape gets upset by
(a) Fine Frequency Offset
(b) Fading
IIT Madras16
OFDM -- PHY layer tasks
Signals sent thro’ wireless channels encounter one or moreof the following distortions:
additive white noise
frequency and phase offset
timing offset, slip
delay spread
fading (with or without LoS component)
co-channel interference
non-linear distortion, impulse noise, etc
OFDM is well suited for high-bit rate applications
IIT Madras17
Frequency Offset
Carrier recovery and tracking critical for OFDM
Offsets can be comparable to sub-carrier spacing in OFDM
Non-coherent detectors possible with differential coding
Residual freq. offset causes
constellation rotation in TDMA
loss of correlation strength over integration window in CDMA(thereby admitting more CCI or noise)
increased inter-channel interference (ICI) in OFDM
OFDM can easily compensate for gross freq. offsets (offsetswhich are an integral multiple of sub-carrier width)
IIT Madras18
Timing Synchronisation
Timing recovery (at symbol level) is easily achieved in OFDMsystems
Can easily overcome distortions from delay spread
Can employ non-coherent timing recovery techniques by introducingself-similarity
• => very robust to uncompensated frequency offsets
If cyclic prefix is larger than the rms delay spread, range of (equallygood) timing phases become available
• => robust to estimation errors
IIT Madras19
Slot and Timing Synchronization in OFDM
Example: 4 tones per slot (OFDM symbol)T
self-symmetry can be
exploited for non-
coherent timing recovery
zero tones
IFFT PA
T secs
t
IFFT PA
T secs
t
T/2 T
Traffic Slot
Preamble/Control Slot
IIT Madras20
Effect of Delay Spread
Typical rms delay spread in macro-cells
• Urban : 1-4 msecs, Sub-urban : 3-6 msecs
• Rural (plain, open country) : 3-10 msecs
• Hilly terrain : 5-15 msecs
TDMA requires equalization (even if rms delay spread is only20-30% of symbol duration)
higher bit-rates would imply more Inter-Symbol Interference (ISI)
therefore, equalization complexity increases with bit rate
IIT Madras21
Effect of Delay Spread -- contd. 1
Effect of delay spread on DS-CDMA is multi-fold
On the Uplink, the time diversity inherent in the delay spread canbe used to mitigate fading
On the Downlink, multipath delay spread upsets channelization(short) code orthogonality
Sectorisation vital in CDMA to reduce CCI on the Uplink
However, sectorisation reduces delay spread as well, therebyreducing the RAKE performance
IIT Madras22
Effect of Delay Spread in OFDM
Delay spread easily compensated in OFDM using :
Cyclic Prefix (CP) which is longer than the delay spread
Thereby, converting linear convolution (with multipath channel) toeffectively a circular convolution• enables simple one-tap equalisation at the tone level
However, the frequency selectiveness could lead to certain tones
having very poor SNR=> poor gross error rate performance
Data Payload CP
3.2msecs 0.8msecs
Example: IEEE 802.11 A (and also in HiperLAN)
IEEE Symp./ IISc -2001 IIT Madras23
Delay Spread Compensation in OFDM
Two basic ideas to combat freq. selectivity in OFDM
Feed-forward only techniques
• Temporal FEC and interleaving
• Transmit diversity and space-time coding
Feed-back based techniques (similar to approaches used inMulti-Carrier Modulation in the ADSL modems)
• Water-pouring (bit-loading)
• Pre-equalisation or pre-distortion
Sectorisation in macro-cell OFDM can help reduce delayspread
IIT Madras24
AGC
Sampler DFTError
-- Gross Freq. Offset
-- Channel Estimation
and Equalization
OFDM Receiver Algorithms -- Recap
RecoveryP/S and
Detection
Freq.
-- Fine Freq. Offset
-- Timing Estimation
IIT Madras25
Conventional
OFDM
Frequency Domain Equalization
-- Conventional OFDM
DFTFrequency
Domain
Equaliser
Remove
CPRx
Algos.
Detection
& P/S
IDFTAdd
CPTx
Mod.
Symbol
Mapping
& S/P
IIT Madras26
Tx -- low-complexity, TDMA
Rx -- implements SC-FDE;
Linear Equaliser or DFE
Frequency Domain Equalization
-- Single Carrier FDE (SC-FDE)
DFTFrequency
Domain
Equaliser
Remove
CPRx
Algos.DetectorIDFT
Add
CP
(of symbols)
Tx
Mod.
Symbol
Mapping
to permit FDE
IIT Madras27
TDE + FDE
for OFDM
Time & Frequency Domain Equalization
-- for OFDM in large delay spread channels
DFTFrequency
Domain
Equaliser
Remove
CPRx
Algos.
Detection
& P/S
IDFTAdd
CPTx
Mod.
Symbol
Mapping
& S/P
Time-
Domain
Equaliser
IIT Madras28
Fading and Antenna Diversity
Short-term fading exhibits spatial correlation
• Two antennas, spaced l/4 meters or greater apart, fade
independently
• Spatial diversity combining can mitigate fading
– Switch diversity (least complex, modest improvement)
– Selection diversity
– Equal gain combining
– Maximal ratio combining (most complex, optimal)
TDMA, CDMA, and OFDM systems will invariably require
antenna diversity to overcome fading
IIT Madras29
Fading and Channel Estimation
Use of midamble in GSM and EDGE to avoid channel tracking
within the slot duration
Unlike in TDMA and OFDM, fading affects not only signal
quality, but also system capacity in DS-CDMA
• Fast closed-loop power control required which can track
short-term fading
• For RAKE combining, multipath delays and gains are
required to be estimated and tracked
– By using orthogonal signaling, IS-95 uplink does not need gain
estimation, but requires delay estimation
In OFDM systems, the long symbol duration makes channel
estimation and tracking very important
IIT Madras30
Channel Estimation in OFDM -- Example
Traffic slots may contain a few equally spaced tones for phase correction(due to residual freq. offset, phase noise, fading)
Control slot may also contain MAC messages
Frame (say, 4 slots)
Control +
Training SlotTraffic Slot 1 Traffic Slot 3Traffic Slot 2
Phase
Correction
Tones
Training
Tones
(for channel
identification)
MAC message
(broadcast)
Control +
Training Slot
IIT Madras31
Fading Compensation in OFDM
OFDM using a FDE, observes only “flat” fading at the sub-
carrier level
Fading manifests as ICI terms in the Frequency Domain
In OFDM Phy Layer, two basic ways to reduce ICI
Reduce OFDM symbol duration (increase sub-carrier width)– 802.16 has FFT sizes ranging from 256 to 4096
• Transmit pulse shaping can reduce ICI
– (by providing excess “time-width”)
IIT Madras32
Other PHY Issues in OFDM
High peak-to-average ratio of the signal envelope
Linear Power Amp., with 5-8dB back-off required (costly)
To support mobility (fast fading) it will require
More training tones per symbol and also in every slot
Tx diversity and/or ST coding support
Exploit time, frequency, and space diversity / processing
IIT Madras33
Phy Layer Issues in Macro-cell OFDM
Macrocells will require larger cyclic extensions / prefix
Microcells may not be economical during initial deployment
GPS locked base stations required
To control ACI from neighbor BS sites (at cell edge)
CCI can be estimated / controlled only if it is tone-aligned
Strict power control required may be required on uplink
To minimize cross-talk between tones of different users
sharing the same OFDM symbol (time slot)
To avoid uplink power control
• allocate only one user per uplink slot
• or, make uplink a pure TDMA (not OFDM)
top related