mobile communications fitton 2004
TRANSCRIPT
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Digital Modulation:Current Wireless Techniques
Mike Fitton,[email protected]
[email protected] Altera Corporation
European Technology Centre
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Outline of Lecture
Personal communication system requirementsMultiple Access Techniques
Frequency Division Multiple Access Time Division Multiple Access Code Division Multiple Access
Wireless Technologies Coding Equalisation
OFDM Diversity and Diversity Combining Spread Spectrum
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Evolution of personal cellular communicationsEvolution of personal cellular communicationsEvolution of personal cellular communications
Availability of complementary wireless systems Short range: wireless PAN (Bluetooth) Medium range: wireless LAN, WiFi Longer range: WiMAX
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Multiple AccessMultiple Access
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Multiple Access Requirements
A wireless communications system employs a multiple access technique tocontrol the allocation of the network resources. The purposes of amultiple access technique are:
To provide each user with unique access to the shared resource: thespectrum.To minimise the impact of other users acting as interferers.To provide efficient use of the spectrum available.To support flexible allocation of resources (for a variety of services).
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Frequency Division Multiple Access (FDMA)
Each user is assigned a uniquefrequency for the duration of their call.Severe fading and interferencecan cause errors.Complex frequency planningrequired. Not flexible.Used in analogue systems, suchas TACS (Europe), and AMPS(USA).0
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7
0 1 2 3 4 5 6 7Frequency Slot
T i m e
S l o t
1 user shown
U s e r 1
U s e r 2
2 users shown
U s e r 3
3 users shown
I n c r e a s
i n g T
i m e
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Frequency HoppingCode Division Multiple Access (FH-CDMA)
Each user regularly hopsfrequency over the availablespectrum.Users are distinguished fromeach other by a unique hoppingpattern (or code ).Interference is randomised.Used in Bluetooth TM
0
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2
3
4
5
6
7
0 1 2 3 4 5 6 7Frequency Slot
T i m e
S l o t
I n c r e a s
i n g T
i m e
1 user shown
Usr 1
2 users shown
Usr 2
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Direct Sequence
Code Division Multiple Access (DS-CDMA)
All users occupy the samespectrum at the same time .The modulated signal is spread to a much larger bandwidth thanthat required by multiplying witha spreading code . Users aredistinguished from each other bya unique spreading code.Very flexible, but complex.Currently used in 3G and 2 ndgeneration IS-95
0
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0 1 2 3 4 5 6 7Frequency Slot
T i m e
S l o t
1 user shown
User 1User 2
2 users shown
User 3
3 users shown
CodeDomain
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Summary of Multiple Access Techniques:The Cocktail Party
To illustrate the nature of the multiple access techniques, consider anumber of guests at a cocktail party. The aim is for all the guests tohold an intelligible conversation. In this case the resource available is
the house itself.FDMA: each guest has a separate room to talk to their partner.TDMA: everyone is in the same room, and has a limited time to holdtheir conversation (so they must talk very quickly).FH-CDMA: the guests run from room to room to talk.DS-CDMA: everyone is in the same room, talking at the same time, but
each pair talks in a different language .
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Duplex Communication
Two way communication is called duplex (eg. for cellular radio). Oneway is called simplex (eg. for paging).The link from the base-station to mobile is the down-link . The link
from the mobile to base-station is the up-link .The up-link and down-link can exist simultaneously on differentfrequencies: Frequency Division Duplex (FDD).The up-link and down-link can exist on the same frequency at differenttimes: Time Division Duplex (TDD).
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Wireless technologiesWireless technologies
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Coding: Forward Error CorrectionCoding: Forward Error Correction
So far we have considered the uncoded case It is possible to apply redundancy (in time, frequency or space)
and exploit this to give error detection and error correction A simple example is a repetition code (1 111) There are many types of coding that can be used
Block code Convolution code (use current input and previous ones) Turbo codes: use two recursive systemic encoders, and two
decoders that are run iteratively)
Many more Coding requires an overhead (e.g with a rate _ code, the
information rate is half the transmission rate). May not beappropriate in all instances (e.g. in interference)
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Automatic Repeat Request (ARQ) Automatic Repeat Request (ARQ)
Detect an error in a packet, for example with a CyclicRedundancy Check (c.f. checksum).
Inform the transmitter of the problem (e.g. through failure toreturn an ACK, or using a NACK)
Transmitter then retransmits that packet Many different ARQ schemes are possible
ARQ is more appropriate for non-real time traffic (e.g. data), or isochronous traffic (where a limited number of retransmissions
are permitted) FEC is useful for real-time traffic (e.g. voice and real-time video)
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Equalisation
Frequency-selective fading arises due to time-dispersion in the multipathchannel. This type of wideband fading causes irreducible errors, unlessits effects are mitigated.Equalisation is employed to remove the harmful frequency-selectivefading. It acts as an adaptive filter, to produce an output signal with a flatfrequency response. Consequently, error-free transmission at high datarates is possible.
1.800 1.802 1.804 1.806 1.808 1.810-40
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-10
0
Frequency (GHz)
(i) Channel (Frequency Domain)
T rms = 2.67
s
1.810
-40
-30
-20
-10
0
1.800 1.802 1.804 1.806 1.808
Frequency (GHz)
(ii) Forward Filter (Frequency Domain)
Noise EnhancingAmplification
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Linear Transversal Equaliser
The linear transversal equalisation (LTE) is one of the simplest forms of
equaliser.The tap coefficients (C1 to Cn) are adapt to suit the current channelconditions. Normally this adaptation is done on a training sequence.In the presence of severe amplitude and phase distortion, the requiredinverse filter tends to result in an unacceptable degree of noiseamplification.
T T T
Z k
r(t-T) r(t-2T) r(t-nT)INPUT
r(t)
C0
DECISIONDEVICE
e k TRAININGSEQUENCE
+
+
++
+
-
OUTPUT
r(t)
ERROR
Forward Filter
C1 C2 Cn
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Decision Feedback Equaliser The equaliser output signal is thesum of the outputs of thefeedforward and feedback sections of the equaliser.The forward section similar tothe LTE
Decisions made from the outputof the equaliser are now feedback through a second filter.If these decisions are correct,the ISI caused by these symbolscan be cancelled without noiseenhancement
However, errors made in harddecisions are fedback throughthe equaliser and can causeerror propagation
T T T
C C Cn-1 n-2 0
Zk
r(t+[n-1]T) r(t+[n-2]T) r(t)INPUT
r(t+nT)
Cn
DECISIONDEVICE
ek TRAININGSEQUENCE
+
+
++
+
-
OUTPUT
ERROR
Forward Filter
TTT
X k ^X k-1
^
b1 b2 bm
Xk-2^
-
- -
+
Feedback Filter
Xk-m^
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Equalisers (cont.)Maximum Likelihood Sequence Estimation (MLSE or Viterbi equaliser ) is a more complex alternative to LTE or DFE, but hasgood performance and is often used in GSM.
Equaliser training for LTE, DFE and Channel Estimator withMLSE
LMS Gradient (less complex)RLS (Kalman) algorithm (fast but computationally expensive)
Training algorithm selectionConvergence speedComplexityRobustness to Channel VariationsNumerical Stability
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Orthogonal Frequency Division Multiple Access(OFDM)Orthogonal Frequency Division Multiple Access(OFDM)
Equalisation is required when the channel time dispersionbecome significant wrt the symbol period
Alternatively, lengthen the symbol period (reduce the data rate)until time dispersion is no longer a problem
Reduce the throughput? Divide the input into multiple streams and use them to modulate
multiple carriers Multicarrier
OFDM is a method of implementing Multicarrier with optimalthroughput and spacing of the carriers
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OFDM overviewOFDM overview
Input data,period T ip
N
pointIFFT
M o d ul a t i on
or d er m
N modulatedparallel streams,symbol period
Tip /m.N
Guard
interval
Upconvert,amplify and
transmit
SyncRemoveGuardinterval
NpointFFT
P/SOutput
data
N parallelstreams, period
Tip /N
S/P
Downconversion
Estimate andremove chan
effects.Demodulate
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Transmitted Spectrum in OFDMTransmitted Spectrum in OFDM
A comb of carriers is produced, each one running at a baudrate of R data /m.No_carriers
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Effect of the wireless channelEffect of the wireless channel
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The carriers are spread over the fades in the frequencydomain, producing frequency diversity .- This can be exploited with e.g. coding
Effect of the wireless channelEffect of the wireless channel
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OFDM advantages and disadvantagesOFDM advantages and disadvantages
For: The system is robust to channel time dispersion and exploits the nature
of the wideband channel (frequency diversity) Complex equalisation is not required Very high data rates can be achieved
Can be applied as multiple access (OFDMA)
Against: Accurate synchronisation required There is an overhead associated with immunity to time dispersion the
Guard Interval High peak-to-mean power ratio linear amplifier required
Limited range and unit speeds (e.g. WLAN) More complex than some alternatives (c.f. 802.11a vs 802.11b)
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Diversity
Diversity: the provision of two or more uncorrelated (independent)fading paths between transmitter and receiver.Performance improvement results as it is unlikely that all the diversitypaths will be poor at the same time. Consequently, the probability of outage is reduced.Methods for generating uncorrelated paths for diversity combininginclude time, frequency, polarisation, angle, and space diversity.
Tx
1
2
m /2
...
......
...
1
2
m ...
distance
0
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+10
P o w e r
A
B
C
A C
/2
(i) Space Diversity (ii) Power Variation with Distance
B
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Diversity combiningDiversity combining Switched combining: the current branch is
used until a metric fails a certain threshold(e.g. Received Signal Strength Indicator) Cheap and simple, but not ideal
Selection combining: the most appropriatebranch is always selected. Slightperformance advantage over switch
diversity. All diversity branches must be analysed RSSI is not ideal unduly affected by
interference
Equal Gain Combining: simply co-phaseand sum all branches Multiple receive chains are required
Maximal Ratio Combining: each branch isco-by its signal-to-noise ratio. Optimal performance Requires multiple receive chains and S/N
calculation
1
C o ph a s i n g & S
ummi n
M
a1
2
a 2
a M
Detector
Output
ia = 1 for Equal Gain Com
NOTE:
MRC
(for EGC a i=1)
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Wireless technologies:Spread SpectrumWireless technologies:Spread Spectrum
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What is Spread Spectrum?What is Spread Spectrum?What is Spread Spectrum?
NarrowbandMessage
NarrowbandMessage
WidebandChannel
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Classification of Spread Spectrum Systems:Frequency Hopping Classification of Spread Spectrum Systems:Classification of Spread Spectrum Systems:Frequency Hopping Frequency Hopping
Frequency Hopping (FH) Narrow band message signal is modulated with a
carrier frequency which is rapidly shifted The hop frequency is indicated by a spreading
function. This spreading function is also available at the
receiver and enables it to retune to the correctchannel for each hop.
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The effects of frequency hopping The effects of frequency hopping The effects of frequency hopping
Carrier 2Carrier 1
inherent frequency diversity Interference diversity
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Hop rates in an FH systemHop rates in an FH systemHop rates in an FH system
Fast frequency hopping Data symbol spread over several hop frequencies Symbol diversity
Very resistant to jamming and interference, often used in militarysystems
Slow frequency hopping Several data symbols on each hop frequency Codeword diversity with interleaving
More likely to have successful retransmission with ARQ Less complex
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Current FH systemCurrent FH systemCurrent FH system
Bluetooth Wireless Personal Area Network. Robust to interference (ISM band). Maximise likelihood of successful retransmissions. 1,600 hops/second.
Based on IEEE 802.11 WLAN specifications. Frequency Hopped Spread Spectrum is a candidate system
for Wireless Local Loop. The GSM specification includes the possibility of full or
limited frequency hopping. FH randomises the interference observed and eases frequencyplanning.
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Classification of Spread Spectrum Systems:
Direct Sequence (DS)
Classification of Spread Spectrum Systems:Classification of Spread Spectrum Systems:
Direct Sequence (DS)Direct Sequence (DS)Direct Sequence (DS)
Secondary modulation in the form of pseudo-noise is appliedto an already modulated narrowband message, therebyspreading the spectrum .
At the receiver, the incoming waveform is multiplied by anidentical synchronised spreading waveform in order torecover the message.
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Direct Sequence Spread SpectrumDirect Sequence Spread SpectrumDirect Sequence Spread Spectrum
d(t)
c(t) c(t)f c
s(t)
f c
NarrowbandMessage
WidebandPseudo random
noise
MessageEstimate
Up conversionto fixed carrier
frequency
Down conversionWideband
Pseudo randomnoise
Spreading De-Spreading
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Data and spreading modulationData and spreading modulationData and spreading modulation
Data modulation Uplink: generally BPSK (data only) or QPSK (data on I and control
information on Q) Downlink: QPSK (half channels on I and half on Q)
Spreading modulation (called secondary modulation ) Choice depends processing gain required, available bandwidth
(normally BPSK or QPSK). Certain schemes are more tolerant to amplifier non-linearities For PSK modulated signal it is assumed that at least a bandwidth of
at least 88% of the chipping rate must be transmitted (3dB point) MSK can be utilised to confine the power spectral density
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Spreading CodesSpreading CodesSpreading Codes
Maximal length sequences good auto- and cross-correlation small code set
Gold codes and Kasami sequences are derived fromM-sequences with similar correlation properties, anda larger code set.
Offsets in a long code (e.g. an m-sequence) can beemployed if the mobiles are synchronised (as is usedin IS95).
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Orthogonal Spreading CodesOrthogonal Spreading CodesOrthogonal Spreading Codes
Walsh and Hadamard sequences zero correlation between codes when aligned cross-correlation non-zero when time shifted fixed spreading factor (codes of different length are not
orthogonal) Orthogonal Variable Spreading Factor (OVSF) codes
permit orthogonal codes for different rate services
Both types of code lose orthogonality when shifteddue to channel dispersion e.g. 40% loss of orthogonality in a large macrocell
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Processing Gain in Direct SequenceProcessing Gain in Direct SequenceProcessing Gain in Direct Sequence
freq
freq
Data
Speading WaveformC h an
WD
WSS
Processing Gain, PG =W
SS
WD
R C
R D
=T
D
TC
=
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Processing Gain in Direct SequenceProcessing Gain in Direct SequenceProcessing Gain in Direct Sequence
freq
C h ann el
freq
freq
Sync SpeadingSequence
NarrowbandJammer
NoiseJammer
WantedSignal
WantedSignal
Despread Signals
J S
Data
Jammer
Eb
N0
STD
J/R C
=R
CS
R D
J= = PG S/J
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Multi-User DS/SS System - CDMAMulti-User DS/SS System - CDMAMulti-User DS/SS System - CDMA
S 1S 2S j
S M
n(t)
Users Channel Receiver for jth user
V j
1Tb
Tb
0m j
PG/1)E-(M N N b0'
0+=
0
b
0 b'0
b
NE
PG1)(M
1
NE NE
+
=
= b
0
b
'
0
E N
E NPGM
M
'0
b N
E1EfficiencyBandwidth
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Theoretical CDMA Capacity Theoretical CDMA Capacity Theoretical CDMA Capacity
DS-CDMA capacity is inversely proportional to theenergy per bit per noise power density which istolerated
A standard DS-CDMA system is interference limited by intra-cell interference Therefore increase capacity by:
voice activity detection antenna sectorisation adaptive antennas interference cancellation
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The Multipath Environment TheThe MultipathMultipath Environment Environment
The received signal is made up of a sum of attenuated, phase-shifted and time delayed versions of the transmitted signal.
Propagation modes include diffraction, transmission andreflection.
abcExcess Delay
acb
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Path diversity in the multipath environment Path diversity in thePath diversity in the multipathmultipath environment environment
Path diversity can be exploited by separating out the multipathcomponents, co-phasing and summing them.
Number of paths resolved (Lm) depends on the total multipathdelay (Tm) and the chip period (Tc)
Excess Delay
acb
1+C
mm
T
T L
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RAKE receiver RAKE receiver RAKE receiver
One method of realising path diversity is with a RAKE and abank of correlators
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Coherent RAKE receiver structureCoherent RAKE receiver structureCoherent RAKE receiver structure
A RAKE receiver can also be visualised as a matched filter
(which resolves the propagation paths) and a channelestimation filter (to recover coherent channel information)
* Reproduced from Adachi et al in IEEE Comms magazine September 1997
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W-CDMA in UMTSW-CDMA in UMTSW-CDMA in UMTS
W-CDMA is used in FDD mode in UMTS On the downlink it is possible to use orthogonal
spreading codes to reduce interference. A scramblingcode is used to separate the cells
On the uplink, low cross correlation codes are usedto separate the mobiles. A single mobile can usemulti-code transmission: each service is mappedonto several bearers, each of which is spread by an
orthogonal code.
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..m1 m2 mnm0
TD-CDMA (UMTS TDD mode)TD-CDMA (UMTS TDD mode)TD-CDMA (UMTS TDD mode)
There are a number of time slots, and a number of codes in each time slot. For example 16 time slots and8 or 9 codes in UMTS TDD mode.
Time M a g n
i t u
d e
C o d
e L a
y e r
10 11 12 1n
..00 01 02 0n
m codesn time slots
Codes are orthogonal on DL UL codes must either be synchronised or some form of multiuser
detection used in BS
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Why do I need to know how my radio works?Back to our first questionsQ: Whats the difference between WiFi modes is 802.11a better than802.11b?
A: 802.11a uses OFDM and therefore can achieve a higher datarate
Q: Bluetooth is cheap, why cant I use it for everything? A: Bluetooth is good for short-range, cable replacement. Data rate,
range, and services might be limitedQ: Why is my wireless link giving me poor performance? Can I just
increase the transmit power to improve things? A: It could be noise, interference or the effects of the wireless
channel. Increasing transmit power may not solve the problem,e.g. diversity might be appropriate to combat the wirelesschannel
Q: What can we expect from the future of wireless communications?Will it provide ubiquitous, pervasive connectivity?
A: Multiple-Input Multiple-Output techniques, Ultrawideband,Multicarrier CDMA, ad hoc mesh networks, and?
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Thank youThank you