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


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)


Illustration of TDMA, FDMA, CDMA and SDMASource: Rappaport’s Wireless Communications

TDMA

CDMA

FDMA SDMA


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−

=


TDMA

TDMA Frame structure

Large overhead


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


TDMA (cont.)

Data transmission for users is not continuous, but occurs in bursts.

Unperceivable to users


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


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:


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


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


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)


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.


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.


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.


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.


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.


Multi-path Channel

T


RAKE receiver to exploit multi-path diversity

Extract the signal arrived from the path with delay τ1.



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


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


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


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


Development on Spread Spectrum TechniquesSource: T. Ojanpera and R. Prasad, WCDMA: Towards IP Mobility ..., Table 1.1


OFDM (Orthogonal Frequency Division Multiplexing)


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


Introduction (2)Realization of parallel transmission

Multicarrier Transmission

serial to parallel converter


Multipath Channels (1)

Terrestrial Mobile Radio Communication– Multipath channels– Transmitted

signals arrive at the receiver in various paths

Illustration of multipath transmission



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



Illustration of multipath interference

Transmitter

Receiver

Multipath channel



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


f

Signal Bandwidth ~100MHz

frequency selective fading channel

the signal experiences a frequency selective fading channel


Interpretation of multipath interference in frequency domain

Multipath channel

time freq.

f



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


Multicarrier Transmission

frequency selective fading channel

f

flat fading channel


Interpretation in frequency domain


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.


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


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


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−

=

= ∑


( )( )

( ) ( )( )

( )

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:


OFDM Basics (6)

0

T

∫

0

T

∫

0

T

∫

0

T

∫


Cyclic Prefix of OFDM (1)OFDM symbol

interference from the previous symbol

OFDM symbol

received symbol

received symbolno interference

from the previous symbol


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



( )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:


( ) ( ) ( )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


( ) ( )m m md r m MH d= =

Recovered data on the mth sub-carrier:

One-step equalization in frequency domain

Multipath signals



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


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


Flexibility of OFDMAs a multiple access scheme

for user1 for user2 for user3 for user4

simple, flexible, make full use of the whole bandwidth


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


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


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


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.

part 3. multiple access methods - university of hong …sdma/elec6040_2010/part 3...p. 3 elec6040...

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