Part 3. Multiple Access Methods - University of Hong …sdma/elec6040_2010/Part 3...p. 3 ELEC6040...
Transcript of Part 3. Multiple Access Methods - University of Hong …sdma/elec6040_2010/Part 3...p. 3 ELEC6040...
p. 1 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Part 3. Multiple Access Methods
p. 2 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Review of Multiple Access Methods• Aim of multiple access
– To simultaneously support communications between a base station and a number of users within a cell.
• TDMA (time division multiple access)– All users are time-synchronized. A user is assigned a time slot (a finite time
duration at a particular time) so that he or she can exclusively use the available frequency bandwidth to communicate. Other users are not allowed to transmit.
• FDMA (frequency division multiple access)– Users are assigned with different segments of the available frequency bandwidth.
A user has the exclusive right to use his/her allocated frequency bandwidth to communicate. Signals of other users are filtered by a bandpass filter.
• CDMA (code division multiple access)– Users are assigned with different codes and these codes have low correlation.
User signals are modulated by the assigned codes. The coded signals are detected at the receiver by correlations.
• SDMA (space division multiple access)
p. 3 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Illustration of TDMA, FDMA, CDMA and SDMASource: Rappaport’s Wireless Communications
TDMA
CDMA
FDMA SDMA
p. 4 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
FDMA
The number of channels that can be simultaneously supported is given by:
Bt is the total spectrum allocationBguard is the guard band allocated at the edge of the allocated spectrumBc is the channel bandwidth
2t guard
c
B BN
B−
=
p. 5 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
TDMA
TDMA Frame structure
Large overhead
p. 6 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
TDMA (cont.)
The number of TDMA channel slots can be provided is given by:
( )2t guard
c
m B BN
B−
=− m is the number of TDMA slots per channel− Bt is the total spectrum allocation− Bguard is the guard band allocated at the edge of the allocated spectrum− Bc is the channel bandwidth
Higher capacity
p. 7 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
TDMA (cont.)
Data transmission for users is not continuous, but occurs in bursts.
Unperceivable to users
p. 8 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Background of Spread-Spectrum Communications
• Frequency-hopping spread spectrum– Is a digital communication technique in which the carrier frequency of a signal
is varied in a (pseudo-)random fashion within a wideband channel.– Multiple access is supported as carrier frequencies of multiple users most likely
will not collide (can be done by design).– Examples: Bluetooth, an option of IEEE 802.11 wireless LANs (not popular)
• Direct-sequence spread spectrum– The signal is generated by multiplying the data with a (pseudo-)random
sequence so that the resultant rate (chip rate) is high, resulting in a wideband signal.
– Multiple access is supported as random sequences used by multiple users have low correlations so that the interference due to other-user signals is reduced (but not eliminated).
– Examples: IS-95, WCDMA, cdma2000
p. 9 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Direct-Sequence Spread Spectrum (DSSS)
Incoming symbol sequence
Spread spectrum signal
x
Chip
A Data Symbol
Spreading Sequence Periodic PN sequence
xData Symbol
Spreading Sequence
Spread SpectrumSignal
PulseShaping
Block diagram:
p. 10 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Spreading and Despreading
ModulationDataSymbols Channel Demodula-
tion+ OutputSymbols
Noise andInterference
Spreading Despreading
Spread-Spectrum Signal
Incoming data stream(Narrowband signal)
Spread-Spectrum Signal(Wideband signal)
x
High-ratespreading code
Spreading Despreading
Original data stream
x
Spreading code
p. 11 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multi-user Environment
Spreading Seq. 1
Info. Seq. 1
Spreading Seq. 2
Info. Seq. 2
Despreader
Spreading Seq. 1
Despreader
Spreading Seq. 2
Info. Seq. 1
Info. Seq. 2
p. 12 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
How DSSS Technique Reduces InterferenceRF Signals Correlator Outuput
Frequency
Spec
tral D
ensity
(dB)
signal
narrowbandinterference
Frequency
Spec
tral D
ensity
(dB)
desired signalafter correlation
narrowband interference(becomes wideband)
Proc
essi
ng G
ain
(dB
)
Proc
essi
ng G
ain
(dB
)Frequency
Spec
tral D
ensity
(dB)
desired signalother-user signal
Frequency
Spec
tral D
ensity
(dB) desired signal
after correlation
MAI (still wideband)
Proc
essi
ng G
ain
(dB
)
(1)
(2)
p. 13 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Direct-Sequence Spread Spectrum (Terminology)
• Spreading sequence (spreading code)– Is the sequence used to spread the data symbol.– A (pseudo) randomly generated sequence.– Has sharp autocorrelation peak but low autocorrelation sidelobe.– Examples: m-sequence, Gold sequences, Kasami sequences.
• Cross-correlation– Is the correlation between two different spreading sequences.– For SSMA, cross-correlation should be small.– Examples of sequence sets with low cross-correlation: Gold sequences
• Processing gain– Is the ratio of the spread-spectrum signal bandwidth to the data-signal
bandwidth.– Is, in most cases, the number of chips per symbol.
p. 14 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
CDMA (1)
• CDMA (code division multiple access)– Is not specifically referred to the use of direct-sequence spread-spectrum
(DSSS) technique– But is most often implicitly used in mobile communications to indicate
the use of DSSS technique for multiple access.
• CDMA becomes attractive to mobile radio communications because the frequency reuse factor is 1. [c.f. reuse factor = 1/7 for many narrowband systems] The system capacity is increased.
• Different users use different spreading codes (i.e., spreading sequences) that are approximately orthogonal so that the receiver can decode each signal.
p. 15 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
CDMA (2)
• The power of multiple users at a receiver determines the noise floor after correlation. If a CDMA signal has a higher power, it will generate more interference to other users.
• Near-far problem: If the power is not controlled, the CDMA signal of a user near to the base station will overshadow those signals originated from a distance away.
• Power control is necessary for CDMA systems.
p. 16 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Features of CDMA Systems (extracted from Rappaport’s Wireless Communications)
• Many users of a CDMA system share the same frequency. ⇒ coexistence of multiple users on the same frequency band.
• CDMA has a soft capacity limit. Increasing the number of users in a CDMA system raises the noise floor. Thus, there is no absolute limit on the number of users in CDMA.
• The system performance gradually degrades (improves) for all users as the number of users increased (decreased).
• Inherent frequency diversity (multi-path diversity) can be exploited to mitigate the adverse effects of small-scale fading.
p. 17 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Features of CDMA Systems (cont.)
• Adjacent cells can use the same frequency. A mobile station at the boundary of two adjacent cells can simultaneously receive signals from the two base stations. It is a diversity effect and can be used to improve the performance for mobile stations at the boundary. The handoff process is called soft handoff.
• Adjacent cells use different sets of spreading codes but can be operated at the same carrier frequency. The frequency reuse factor is therefore one.
• Multiple-access interference (MAI) is a problem in CDMA systems. MAI occurs because the spreading sequences of all users are not exactly orthogonal, leading to interference to other users’ signals.
• The near-far problem occurs at a CDMA receiver if an undesired users has a high detected power as compared to the desired user.
p. 18 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multi-path Channel
T
p. 19 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
RAKE receiver to exploit multi-path diversity
Extract the signal arrived from the path with delay τ1.
Extract the signal arrived from the path with delay τ2.
Extract the signal arrived from the path with delay τ3.
Spreading code generated with a time delay τ1.
Path delays Path gains
Sourcr: T. Ojanpera and R. Prasad’s WCDMA: towards IP mobility and mobile Internet
p. 20 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
How RAKE receiver can exploit multipath diversity• Spreading sequence has spike-like autocorrelation function:
• A time shift more than or equal to one chip time yields very lowautocorrelation value.
• Therefore, if the difference between path delays τ1 and τ2 is greater than the chip time, the signals arrived from the two paths can be resolved and extracted.
Autocorrelation peak
Autocorrelation sidelobe
One chip time
Shift
Auto
corr
elat
ion
p. 21 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Performance of SSMA Systems (1)
• Assumptions:– BPSK is used.– AWGN channel (not multi-path fading channel) is considered.– Perfect power control is assumed so that the power levels of all users are
the same.– Rectangular chip waveform is used.– Random sequences are used as spreading sequences.– Gaussian approximation is used to approximate MAI so that the resultant
BER is only an approximate one.
• Derivation– See Rappaport’s Wireless Communications
p. 22 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Performance of SSMA Systems (1)• The bit error probability is
where– N is the processing gain of the system– K is the number of users in the system– Eb is the energy per bit– N0 is the noise spectral density
Observations:– A higher number of users degrades the performance.– A higher processing gain reduces more on the MAI.– Irreducible BER, which is the BER that cannot be reduced even if the signal power
is increased, occurs and is given by
P Q KN
NEb
b=
−+
RSTUVW
FHG
IKJ
−1
3 20
1 2
P Q KN
NE
Q NKb E N
bE N
b
b
0
0
13 2
31
01 2
→∞
−
→∞
=−
+RST
UVWFHG
IKJ
=−
FHG
IKJ
p. 23 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Development on Spread Spectrum TechniquesSource: T. Ojanpera and R. Prasad, WCDMA: Towards IP Mobility ..., Table 1.1
p. 24 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM (Orthogonal Frequency Division Multiplexing)
p. 25 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
IntroductionOFDM is a multi-carrier transmission scheme– transform high-speed serial transmission to low-speed parallel
transmission– increase symbol duration, robust to multipath interference
serial transmission
1 second 10 bits transmitted in 1 second, data rate: 10bits/s, bit duration:
1/10s
parallel transmission
bit duration: 1/10s
10 bits transmitted in parallel, bit
duration: 1s, total data rate: 10bits/s,
data rate per channel: 1bit/s
different bit duration,
same data rate
p. 26 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Introduction (2)Realization of parallel transmission
Multicarrier Transmission
serial to parallel converter
p. 27 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (1)
Terrestrial Mobile Radio Communication– Multipath channels– Transmitted
signals arrive at the receiver in various paths
Illustration of multipath transmission
p. 28 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (2)
Measurement of multipath channel
time
impulse signal
Multipath Channel
Channel impulse response– τmax is the maximum delay spread– T is the data symbol duration– When T< τmax, frequency selective fading channel– Multipath interference
Desired signal interfered by τmax/ T previous signals
Transmitter
ReceiverT
1/T
T→0
p. 29 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (3)
Illustration of multipath interference
Transmitter
Receiver
Multipath channel
p. 30 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (4)
Example: Broadband transmission, 100MHz– Single carrier systems: DS-CDMA, chip duration (T) about 10ns– Urban area, Microcell (<1km): τmax =1us
0path 0
path 10
path 100(1us/10ns)
0
100
–Each chip influenced by 100 previous chips–Serious multipath interference, difficult to recover the desiredsignal at the receiver
τmax =1us
T=10ns
p. 31 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
f
Signal Bandwidth ~100MHz
frequency selective fading channel
the signal experiences a frequency selective fading channel
Multipath Channels (5)
Interpretation of multipath interference in frequency domain
Multipath channel
time freq.
f
p. 32 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multipath Channels (6)
Example: Broadband transmission, 100MHz– Parallel processing– Multicarrier system with 1000 subcarriers, T about 10us– Urban area, Microcell (<1km): τmax 1us
no delay
–Each data influenced by approximately 0.1 previous data symbol
1000
delayed version 1000
T=10us
τmax=1us
p. 33 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Multicarrier Transmission
frequency selective fading channel
f
flat fading channel
Multipath Channels (7)
Interpretation in frequency domain
p. 34 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (1)
Conventional Frequency Division Multiplexing
f2/T Δf >=2/T
filter at the receiver
Orthogonal Frequency Division Multiplexing (OFDM)
f2/T
Δf=1/T
Better spectrum efficiency
Data symbolT
time
2/Tf
freq.
p. 35 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (2)Why could sub-carrier spacing be Δf=1/T?– Received signal:
– Sub-carrier down-conversion:
22
00 for can be obtained as long as is an integerji
T j f tj f te e dt i j f Tππ − = ≠ Δ ⋅∫
( )1
2
0
i
Mj f t
ii
y t d e π−
=
= ∑
( ) ( )
( )( )
( )
1 12 22 2
0 00 0
21
0
12
12
j ji
M MT Tj f t j f tj f t j f i j Tj i i
i i
j i j fTM
j iii j
T i jr y t e dt d e e dt d e i j
j f i j
ed T dj f i j
π ππ π
π
π
π
− −− − Δ −
= =
− Δ−
=≠
=⎧⎪= = = ⋅ −⎨ ≠⎪ Δ −⎩
−= ⋅ + ⋅
Δ −
∑ ∑∫ ∫
∑
– The minimum sub-carrier spacing Δf=1/T!
M is the total number of sub-carriers, di is the data signal transmitted on the ith sub-carrier
Interference from other sub-carriers
p. 36 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (3)Basic structure of OFDM systems
Seria
l-to-
Para
llel C
onve
rter
0
T
∫
0
T
∫
0
T
∫
Oscillators are analog devices: expensiveM up- and down- conversion: complicated
p. 37 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (4)
IFFT and FFT can be employed to realize the M sub-carrier up-and down-conversion– Digitalize the analog signal by sampling – Sample rate: fs=MxΔf, duration: Ts=1/fs
( )1
2
0
i
Mj f t
ii
s t d e π−
=
= ∑ t=nTs
( ) ( )( )
1 1 122 2
10 0 0
ii s
s
M M Mj i fn M ff i fj f nT j in M
s i i iT M fi i i
s nT d e d e d eππ π− − −
Δ ⋅Δ= Δ= ⋅Δ
= = =
= ⎯⎯⎯⎯⎯→ =∑ ∑ ∑
IFFT of di
M sub-carrier up-conversion
( )1
2
0
i
Mj f t
ii
s t d e π−
=
= ∑
M-point IFFT of di
( )1
2
0
Mj in M
ii
s n d e π−
=
= ∑
p. 38 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
( )( )
( ) ( )( )
( )
12
01
2
0
21 1 12
20 0 0
101
Mj in M
ii
s n d eMj mn M
nn
j i mM M Mj i m n M m
i i j i m Mi n i
y m s e
Md i mey m d e di me
π
π
ππ
π
−
=
=−−
=
−− − −−
−= = =
∑= ⎯⎯⎯⎯⎯⎯→
=⎧−⎛ ⎞= = = ⇒⎨⎜ ⎟ ≠−⎝ ⎠ ⎩
∑
∑ ∑ ∑ FFT
OFDM Basics (5)
Receiver
M-point FFT of s(n)
( ) ( )1
2
0
Mj mn M
my m s n e π
−−
=
= ∑
M sub-carrier down-conversion
( ) ( ) 2
0m
T j f ty m s t e dtπ−= ∫
Proof:
p. 39 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
OFDM Basics (6)
0
T
∫
0
T
∫
0
T
∫
0
T
∫
p. 40 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Cyclic Prefix of OFDM (1)OFDM symbol
interference from the previous symbol
OFDM symbol
received symbol
received symbolno interference
from the previous symbol
p. 41 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Cyclic Prefix of OFDM (2)
• Cyclic prefix is introduced to combat the inter-OFDM symbol interference (ISI) caused by the multipath channel.
ISI due to multipath channel falls into the cyclic prefix; OFDM data symbols not affected by ISI. ⇒ Adverse effects of ISI are eliminated.
Multipath signals
p. 42 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Cyclic Prefix of OFDM (3)
( )1
2
0
, 0, , 1M
j in Mi
i
s n d e n Mπ−
=
= = −∑
Transmitted signal:
( ) ( )1
0
L
ll
H n h n lδ−
=
= −∑
Why CP could help to avoid ISI? Mathematical interpretation
( ) ( )12
0, 0, , 1, , 1g
Mj i n L M
i g gi
s n d e n L M Lπ−
−
=
= = − + −∑
⎯⎯⎯⎯⎯→add cyclic prefix
Multipath Channel
( ) ( ) ( ) ( )( )
( ) ( ) ( )
0 1 1
1
0
1 1L
L
ll
y n h s n h s n h s n L
h s n l s n H n
−
−
=
= + − + + − −
= − = ⊗∑
Received signal:
p. 43 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
( ) ( ) ( )1 1
2 2
0
gg
g
m
M L Lj n L m M j ml M
m ln L l
H
r m y n e Md h eπ π+ − −
− − −
= =
⎛ ⎞= = ⎜ ⎟
⎝ ⎠∑ ∑
FFT after discarding cyclic prefix:
Channel response on the mth sub-carrier
Cyclic Prefix of OFDM (4)
( ) ( )m m md r m MH d= =
Recovered data on the mth sub-carrier:
One-step equalization in frequency domain
Multipath signals
p. 44 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Cyclic Prefix of OFDM (5)
Power efficiency–total transmission power fixed–the cyclic prefix: no new data information–the system power efficiency is degraded
Definition of power efficiency: g
g
LM L
γ =+
How to improve power efficiency– reduce Lg: if Lg is shorter than the maximum channel delay,
there will be ISI– increase M: the bandwidth is fixed, larger M, narrower sub-
bands, the system is vulnerable to inter-carrier interference caused by fast fading or frequency synchronization error
p. 45 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Cyclic Prefix of OFDM (6)Summary of CP
–As long as the length of CP is no less than the maximum channel delay, there is no ISI and data symbols can be recovered by using a simple one-step equalization in frequency domain
–Since the CP reduces the power efficiency of the system, the length of cyclic prefix is generally set to about 20% of the whole OFDM symbol length.
• Example: IEEE 802.11a & HIPERLAN/2• Data symbol length =3.2µs• Cyclic prefix = 800ns [can absorb a channel dispersion of 800ns]• Total length =4µs
p. 46 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Flexibility of OFDMAs a multiple access scheme
for user1 for user2 for user3 for user4
simple, flexible, make full use of the whole bandwidth
p. 47 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Flexibility of OFDM (2)
Easy to adapt to channel conditions
Assume the channel condition is known at the transmitter (realized by feedback)
sub-channel in good condition
1.0
high-level modulations
such as 64QAM
sub-channel in deep fading (bad
condition)
low-level modulations
such as QPSK
p. 48 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Flexibility of OFDM (3)
Multiuser Diversity
A combination of the former two: channel conditions of all usersare known to the transmitter
user3
user2
user1
for user3 for user2 for user1
p. 49 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
PAPR
PAPR: Peak-to-Average Power Ratio
– Definition:
– OFDM symbol is a sum of sinusoids
– When the number of sub-carriers Mis large, PAPR is high
– RF amplifiers: limited linear range, distort OFDM signals
( )( ){ }
2
2
maxPAPR=
s n
E s n
Signal Power in one OFDM symbol duration
Source: http://www.ece.uvic.ca/~agullive/defence.pdf
p. 50 ELEC6040 Mobile Radio Communications, Dept. of E.E.E., HKU
Summary of OFDM
• Advantages– Easy to mitigate the adverse effects of channel dispersion by the use of cyclic
prefix.– Low-complexity implementation based on FFT/IFFT.– Support high-rate transmission at a low implementation cost.
• Disadvantages– High peak-to-average power ratio, so that highly linear power amplifiers are
required at the transmitters in order to avoid intermodulation interference.– The use of cyclic prefix reduces transmission efficiency. Some power is wasted
by transmitting cyclic prefix, which are redundant.
• Good reference: R. van Nee and R. Prasad, OFDM for Wireless Multimedia Communications, Boston: Artech-House, 2000.