Post on 16-Mar-2018
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Wireless CommunicationsChapter 9. Multiple Access
Husheng Li
Dept. of Electrical Engineering and Computer Science
Fall, 2013
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Interference in Multiuser Systems
If two nodes transmit in the same time and the samefrequency band, their signal will collide. Signal becomesinterference.Multiaccess: to separate multiple signals.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Different Multiple Access Schemes
TDMA: Separating in time domainFDMA: Separating in frequency domainCDMA: mixed in both time and frequency domains;separating by codeCSMA: separated in time, but without scheduling
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Time Division Multiple Access
The time slots have been allocated before datatransmission.Synchronization, i.e. estimating timing information, is a keyproblem. Two types: decision-directed and non-decisiondirected.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Decision-directed Synchronization
Principle: assume the transmitted signal is known (e.g.pilot symbols);Expected received signal
s(t ; τ) =∑
k
s(k)g(t − kTs − τ).
Likelihood function:
L(τ) = exp[− 1
N0
∫T0
[r(t)− s(t ; τ)]2]
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Time Offset Estimation
The time offset τ maximizing the likelihood satisfies
∑k
s(k)d
dτzk (τ) = 0,
wherezk (τ) =
∫T0
r(t)g(t − kTs − τ)dt .
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Early-late Gate Synchronizer
Need not know the original signal.Principle: shift the time offset to match the correct one.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
History of CDMA
Mother of CDMA
Hedy Lamarr (1913-2000): a movie star in 1940s (sixhusbands). She has a star in the Hollywood Walk of Fame.On Aug. 11, 1942, US Patent 2,292,387 was issued toHedy Lamarr and Antheil. This is the early version offrequency hopping communications.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
History of CDMA
Frequency Hopping CDMA in WWII
Frequency hopping CDMA was widely used in World WarII.In each time slot, the transmit chooses one randomfrequency to transmit, for security.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
History of CDMA
Commercial CDMA Systems
IS-95 (Interim Standard 95) is the first CDMA based digitalcellular standard, pioneered by Qualcomm.Currently, there are two standards of CDMA sysetms:cdma2000 (3GPP2) and WCDMA (3GPP). Both are basedon direct-sequence CDMA.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Fundamentals of CDMA
Time Structure of DS-CDMA Systems
Each symbol contains N chips in time. N is calledspreading gain, or processing gain. Each chip could be 1or -1.Vector signal: r = xksk , where xk is scalar informationsymbol for user k and sk is an N-vector of the chips, calledspreading code (or signature waveform).
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Fundamentals of CDMA
Orthogonal Spreading Code
{s1,s2,s3,s4} =
1 1 1 11 1 −1 −11 −1 −1 11 −1 1 −1
Check sT
i sj = 0, if i 6= j .
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Fundamentals of CDMA
CDMA Receiver
Received signal:
r =K∑
k=1
xksk + n.
How to detect xk from the superimposed signal?For user k , the scalar output of a correlation detector isgiven by
zk = sTk r = xk + sT
k n.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Fundamentals of CDMA
Difference between coding and CDMA
Compared with uncoded system, channel coding usesmore time and energy to transmit one bit.CDMA system uses the same time and power as a narrowband system to transmit one bit. There is no coding here.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Fundamentals of CDMA
Spectrum Spreading
There are N changes within a symbol period, therefore itrequires approximately N times bandwidth. CDMA is awideband systemBetter resolution of multiple paths.Larger channel capacity (recall Shannon’s formula).
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Fundamentals of CDMA
Non-orthogonal Spreading Codes
In most practical CDMA systems, the spreading codes arenot orthogonal.Reason:
An orthogonal CDMA system can contain at most N users.More users will break the orthogonality.When multiple path fading exists, the orthogonality isbroken even if the spreading codes are orthogonal. (WHY?)The orthogonality is broken for asynchronous transmission.(WHY?)
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Fundamentals of CDMA
Matched Filter
Matched filter: for user k , we use
zk = sTk r = xk + sT
k n.
Multiple access interference (MAI):
Ik =∑i 6=k
sTi sk .
Signal-to-interference-plus-noise ratio (SINR) is given by
SINR =E[|sk |4
]E[|Ik |2
]+ σ2
n.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Fundamentals of CDMA
SINR
Signal-to-interference-plus-noise ratio (SINR) is given by
SINR =E[|sk |4
]E[|Ik |2
]+ σ2
n.
Asymptotic case: as K ,N →∞
SINR → 1β + σ2
n.
(Suppose all transmit powers are 1.)
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Fundamentals of CDMA
Rake Receiver
Working for multipath fading. Each finger corresponds to apath. Achieving diversity.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Power Control
Near-far Problem
Because of inter-user interference, the user far from thebase station may be killed by a closer user.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Power Control
Power Control
The user closer to the base station should transmit withless power and the user farer away from the bast stationshould transmit with larger power.Requirement of power control is usually expressed as (R.Yate, 1995)
p ≥ I(p),
where p = (p1, ...,pK ) is the vector of transmit power of Kusers, I(p) represents the vector of effective interference.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Power Control
SINR
We consider K users and 1 base station. The channel gainfrom user k to base station is denoted by hk and the noisepower at base station m is denoted by σ2
n.The SINR of user k is given by pkµk , where
µk =hk∑
i 6=k Pihi + σ2n.
When the required SINR is γk for user k , the effectiveinterference can be written as
I(p)k =γk
µk.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Power Control
Power Control Algorithm
The power control is carried out in an iterative way(proposed by R. Yates)1
p(t + 1) = I(p(t))
Theorem: If I(p) is feasible, for any initial powerconfiguration, the power control algorithm converges to aunique fixed point.
1R. Yates, “A framework for uplink power control in cellular radio systems”,IEEE Journal on Selected Areas in Communications, Vol.13, No.7,Sept.1995.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Introduction
OFDM
OFDM: Orthogonal Frequency Division MultiplexingOFDM was born in 1960s in Bell Labs.Life begins at forty (John Lenon).Now OFDM is the fundamental signaling technique for 4Gcellular systems (UWB, LTE, WiMAX).
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Introduction
Big Picture of OFDM
The data is divided into many streams and transmitted onmany subcarriers (also called tones).
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Introduction
Features of OFDM
No need for equalization (no ISI). Each tone looks like ascalar channel. High speed.Good structure for resource allocation.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Tranceiver of OFDM
Transmitter
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Tranceiver of OFDM
Receiver
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Tranceiver of OFDM
OFDM Symbol and Chip
Each OFDM symbol (in time) transmits a block ofinformation bits. It contains N chips (in time) (similar toCDMA).
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Tranceiver of OFDM
Signal in Time Domain
Suppose the transmitted signal is x(n) (n is time index)and channel response is h(n). The received signal is
y(n) = (x⊗
h)(n)⊗means circular convolution:
(x⊗
h)(n) =N−1∑k=0
x(k)h(n − k)N ,
where h(k)N = h(mod(k ,N)).
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Tranceiver of OFDM
Property of Circular Convolution
Recall that circular convolution in time means multiplicationin frequency domain (please check your textbook of digitalcommunications)y = (x
⊗h) means Yi = XiHi , where Y ,X ,H are the
Discrete Fourier Transform of y , x ,h.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Tranceiver of OFDM
Property of Circular Convolution
Recall that circular convolution in time means multiplicationin frequency domain (please check your textbook of digitalcommunications)y = (x
⊗h) means Yi = XiHi , where Y ,X ,H are the
Discrete Fourier Transform of y , x ,h.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Tranceiver of OFDM
Why Use Frequency Domain?
If the information symbols are in time domain, how torecover x from y = x
⊗h? Need complicated equalization.
If the information symbols are in frequency domain, it isvery easy to recover X from Y : Xi =
YiHi
.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Tranceiver of OFDM
Information Flow
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Tranceiver of OFDM
Cyclic Prefix
Due to delay spread in the channel, there exists interferenceacross different OFDM symbols (ISI).
We set a portion of the OFDM symbol as prefix, whose length islarger than the delay spread. (suppose the length of prefix is L.)
The prefix should be the same as the end of the OFDM symbol,thus forming a circulation:
x = (xN−L, .., xN−1, x0, x1, ..., xN−L, ..., xN−1) .
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Introduction
Why Use Random Access?
In TDMA, CDMA or OFDMA, the channels (time slots forTDMA, spreading code for CDMA and subcarrier forOFDMA) need to be distributed to the transmitters by acenter.In a general network, there is no center or the users mayemerge or quit, it is different to schedule the transmissionin a stationary way.In random access, there is no fixed channel assignment.The access is probabilistic and the signals are separatedby probability.
Advantage: no channel scheduling;Drawback: possible collision and less efficient; whenthroughput when the number of nodes is large.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Introduction
Types of Random Access
Aloha: the first technology of random accessCarrier sensing multiple access with collision detection(CSMA-CD): used in EthernetCarrier sensing multiple access with collision avoidance(CSMA-CA): used in WiFi (IEEE 802.11)
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Aloha
History of Aloha Aloha
Proposed by Abramson in 1970 at the University of Hawaii.The idea was to use low-cost ham radio-like systems tocreate a computer network linking the far-flung campusesof the University.It can be categorized into pure and slotted Aloha systems.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Aloha
Basic Protocol of Pure Aloha
Transmission: when a user has data, it transmitsimmediately. It is possible that some other user istransmitting.Random recession: if the transmission fails (due totransmission collision), the user waits for a random time τand then retransmits. The random time τ satisfies theexponential distribution:
p(τ) = αe−ατ
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Aloha
Basic Protocol of Slotted Aloha
The time is slotted. The transmissions can only begin atthe head of each time slot.The waiting time is discrete, measured by time slots.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Aloha
Performance Analysis of Pure Aloha
Let Tp be the time required to send a packet. Then, if thereis another packet transmitting within the vulnerable region(2Tp), the transmission will fail.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Aloha
Performance Analysis of Pure Aloha
Assume that the arrival of packets satisfies a Poissondistribution and averagely λ packets arrive within a unittime. The probability of k packets arrives within time periodt is given by
P(k) =(λt)ke−λt
k !The probability that there is no packet within a period of2Tp is given by e−2G, where G = λTp. This is also theprobability of successful transmission.The average number of successfully transmitted packets(throughput), S, is given by
S = Ge−2G.
The maximum of S is 0.184 when G = 0.5.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Aloha
Performance Analysis of Slotted Aloha
For slotted Aloha, the vulnerable time is Tp.Using the same argument, for slotted Aloha, S is given by
S = Ge−G.
The maximum of S is 0.368 when G = 1.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Aloha
Comparison of Pure and Slotted Aloha
Slotted Aloha achieves better performance.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
CSMA
Carrier Sensing Multiple Access (CSMA)
Principle of CSMA: sense the carrier before transmission;if finding some other transmitter is transmitting, it waits fora random time and then transmit. A transmitter transmitsonly when the channel is detected idle.Two versions:
collision detection (CD): a transmitter is able to detectsignal collision;collision avoidance (CA): use RTS and CTS to clear hiddennode and exposed node (will explain later).
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
CSMA
Nonpersistent CSMA/CD
Rule of transmission (from Proakis’ book):If the channel is idle, the user transmits a packet;If the channel is sensed busy, the user schedules thepacket transmission at a later time according to some delaydistribution. At the end of the delay interval, the user againsenses the channel and repeats.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
CSMA
1-persistent CSMA/CD
Rule of transmission (from Proakis’ book):If the channel is idle, the user transmits a packet;If the channel is sensed busy, the user schedules thepacket transmission at a later time according to some delaydistribution. At the end of the delay interval, the usertransmits with probability 1.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
CSMA
p-persistent CSMA/CD
Rule of transmission (from Proakis’ book):If the channel is idle, the user transmits with probability p ordelay by τ seconds with probability 1− p;If at time τ , the channel is still sensed idle, the above isrepeated. if a collision happens, the users rescheduleretransmission of the packets according to some delaydistribution;If at time τ , the channel is still busy, the user waits until itbecomes idle and repeat the above steps.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
CSMA
Comparison of Aloha and CSMA/CD
0.01-persistent CSMA looks the best. (Then, why considerother types of CSMA?)
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
CSMA
Hidden Node and Expose Node
Hidden node: A is transmitting. C wants to transmit to Bbut is out of the sensing range for A, then C transmits andcollides with A at B.Expose node: B is transmitting, then C dare not transmit,but D is actually out of the interference range of B.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
CSMA
Collision Avoidance
When A wants to transmit, it sends a request-to-transmit(RTS) to B. Neighboring nodes then know A intends totransmit.When B agrees to receive, it sends a clear-to-transmit(CTS) to A. Neighboring nodes will keep silent.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Interference Limited and Thermal Limited
Thermal limited: noise is too strong and SNR is too small.Example: in a one-to-one communication system, thetransmitter and receiver are too far away from each other;or in a multiuser system that has very little interference.Interference limited: noise is negligible and interferencedominates; it is useless to simply increase transmit power.Example: CDMA system.
Multiple Access TDMA CDMA OFDM Random Access Tackling Interference
Two Approaches
Interference Average: consider interference as noise and allow collision; but theinterference is averaged. (we can also call it interference diversity). Example:symbol rate hopping OFDM
Interference Avoidance: avoid collision. Example: CDMA/CA.