Medium Access Techniques

Block 4: Medium Access Techniques

Francisco J. Escribano

April 26, 2015

Francisco J. Escribano Block 4: Medium Access Techniques April 26, 2015 1 / 77

Table of contents

1 Motivation

2 FDMA

3 TDMA

4 SDMA

5 CDMA

6 OFDMA

7 Conclusions

8 References


Motivation

Motivation


Motivation

Sharing resources

Any communication has to be routed through a physical medium.◮ The air interface (radiocommunications).◮ A cable (optical, electrical), or waveguide.

Few communication contexts are designed for exclusive dedicated allo-cation of media resources.

In a normal situation, limited media resources have to be shared by anumber of concurrent communications [1, 2, 3].

MEDIUM

user 1

user 2user 3

user n

Figure 1: Competing for resources.


Motivation

Counteracting medium/channel impairments

Medium access is about...◮ Allocating transmission resources to communicating users.◮ Managing the actual transmission medium.

Medium access techniques are not independent from the characteristicsof the transmission medium.

Medium access techniques can have an impact on counteracting theimpairments of the medium/channel.Beware of terminology:◮ Medium: physical entity that physically bears the transmission.◮ Channel: logical entity, defined between the input and output of some

subsystem or device of the same hyerarchical level.

I

I

Figure 2: A cable.

MOD DEMbn

bn

channel

Figure 3: Binary channel.


Motivation

Medium Accesss & Multiple Access

Medium access is a general concept covering the actual managementof the transmission media.

When the transmission media are used concurrently by a number ofusers, competing about resources, we speak of multiple access.

However, both concepts are not exempt of certain ambiguity.Here we focus mainly on multiple access, as the particular context tocharacterize medium access techniques of particular interest.◮ E.g., modern mobile communication standards are developed around the

particular multiple access technique used in the radio interface.◮ The choice of the multiple access technique greatly determines the pos-

sibilities, performance and flexibility of the related standard.

?

Figure 4: Medium access.

?

? ?

? ?

Figure 5: Multiple access.


Motivation

Multiple Access vs. Multiplexing

On the other hand, multiple access and multiplexing are related con-cepts. Both can be grouped as medium access techniques.◮ They also offer some degree of ambiguity and even confusion.

Multiple access:◮ The access to the resources is performed on a de-centralized basis.◮ Each user meets the other ones directly in the medium.

Multiplexing:◮ A centralized entity handles the communications from different users.◮ These communications are organized into a frame by said entity.◮ Users meet at a higher hyerarchical level than the medium.

uplink

downlink

TDMATDM

Figure 6: Multiple access vs multiplexing in a mobile context.


Motivation

Duplexing

Along with the concepts of medium access, multiple access and multi-plexing, we find duplexing.Each two-way communication needs to provide for forward and back-ward communicating paths.◮ This is not needed in broadcasting, for example.

There are two main types of duplexing techniques:◮ FDD (Frequency Division Duplex), each path has a dedicated bandwidth

located at different frequencies. They are normally adjacent in spectrum.◮ TDD (Time Division Duplex), each path occupies the same bandwidth,

but performs transmission during given periods of time, when the othertransmission remains silent. This requires strict timing to avoid collisions.

f (Hz)

PSD

available bandwidth

forward

way

backward

way

Figure 7: FDD mode.

t (s)

Amplitude

TDD period

forward

way

backward

way

TDD period

forward

way

backward

way

Figure 8: TDD mode.


Motivation

PHY layer: the whole picture

Medium access completes the initial vision about the PHY layer.Recall:◮ Operations at the PHY are divided into blocks with specific tasks (e.g.

modulation), but this does not mean they are fully unrelated.◮ The MAC (Medium Access Control) sublayer is a subentity in the second

level, where the LLC (Logical Link Control) is also located.◮ Operations are performed just before the transducer that accesses the

medium, but they are controlled and coordinated from higher levels.

PHY

LLCMAC

medium

channel

encoder/

decoder

modulator/

demodulator

medium

access

Figure 9: PHY structure and adjacent layers.


Motivation

PHY layer example: LTE

Figure 10: PHY LTE structure (source IS-Wireless).


Motivation

PHY & MAC

We cover here the specific procedures of the PHY.

◮ How resources are physically distributed.

◮ Concurrent communications take a part of an available budget.

◮ The budget can be measured in terms of power, Rb, B, etc.

◮ The objective is to keep a minimum quality measurement target value(normally Pb or BER.).

Specific procedures of the MAC sublayers are out ot the scope of thiscourse.

◮ How the access is actually managed: access protocols.

◮ These are already part of the telematics discipline.


Motivation

PHY & MAC: example

Figure 11: MAC/PHY reception scheme in IEEE 802.11 (Wifi).


Motivation

Context

We are going to see how resources are shared and managed alongspecific signal axis.

◮ Frequency.◮ Time.◮ Space.◮ Spreading codes.

Though each technique favours a given axis, this does not mean theother axis do not play a role.

◮ Several techniques can take part in a given standard, in a complex way(e.g. Bluetooth).

◮ Some of the mentioned signal axis can be jointly managed in a givenaccess technique (e.g. OFDMA).


FDMA

Frequency Division Multiple Access


FDMA

Definitions

FDMA (frequency division multiple access) is based on dividing theavailable spectrum into dedicated intervals (channels).The total bandwidth is divided into intervals with given width.◮ The width can be uniform, or, in sophisticated schemes, non-uniform.

In the simplest case, each available channel is assigned to a user.

Once in possession of the channel, the user can transmit full duplexinformation without interruptions.

The receiver side will tune the frequency of the selected channel witha corresponding tuned filter.

f (Hz)

PSD

available bandwidth

f (Hz)

PSD

#2

Channel 2 tuned �lter

Figure 12: Sharing along the frequency axis.


FDMA

Characteristics

At transmission, spectral shaping has to be done very carefully to com-ply with strict spectral masks.

◮ The goal is to minimize co-channel interference.

At reception, strict filtering is needed to reject interferences and limitnoise.

◮ This requires high quality subsystems at the medium frontend (e.g. RFequipment).

Requirements are rendered less strict by including guard intervals be-tween channels.

◮ This reduces the total available data rate.◮ Assume a total bandwidth B, N equal channels, a Bg guard interval

bandwidth, a modulation (QAM or PSK) with k bits per symbol. B′ isthe usable total bandwidth, and Rb the available data rate per channel1.

B′ = B − (N − 1)Bg , Rb = kRs = k B′

2N = kB−(N−1)Bg

2N1

Beware! The relationship B-Rb has to be managed and calculated taking into account the modulation spectral efficiencyand the possible usage of pulse (spectral) shaping filters -e.g. our known RRC, Root Raised Cosine.


FDMA

Characteristics

Figure 13: PSD mask at the modulator output forEUTELSAT [4].

f (Hz)

PSD

B

B'/Ng��di��

Figure 14: FDMA bandwidth management.


FDMA

Characteristics

Figure 15: SPADE satellite system FDMA spectrum segmentation [5].


FDMA

Characteristics

From the point of view of channel assignment, FDMA could be ex-ploited following two principles:

◮ Fixed assignment.

◮ On-demand assignment.

From the point of view of multiplexing, FDMA could be exploited fol-lowing two principles:

◮ SCPC (Single Channel per Carrier), the bandwidth in a channel is as-signed to one communication.

◮ MCPC (Multiple Channels per Carrier), the bandwidth in a channel isassigned to multiple communications, multiplexed at a higher level.

FDMA could be implemented both in FDD or TDD modes, though themore natural way would be the first one.


FDMA

Characteristics

Advantages:

◮ Compatible with analog and digital modulations.

◮ Well known technology with a lot of background.

◮ Hardware is easy to build.

◮ Core network equipments are straightforward to implement.

◮ It does not suffer form the near-far problem of CDMA systems: powercontrol is less critical.

◮ It does not require strict timing, as in TDMA.

◮ Unhindered stream data transfers are possible due to the usage of dedi-cated channels.


FDMA

Characteristics

Disadvantages:

◮ Central stations with multiple channels are hard to mount.

◮ A lot of equipment may be required for each channel.

◮ It does not handle quite straightforwardly different applications or trafficflows.

◮ It poses difficulties to insert and handle signaling procedures associatedto each communication.

◮ It lacks flexibility: difficulties to enhance data quality.

◮ It requires high-performing filters in the radio hardware.


FDMA

Examples

FDMA in cellular networks has been used mainly in analog systems (1Gmobile systems):◮ NMT (Nordic Mobile Telephone).◮ AMPS (Advance Mobile Phone System) → ETACS 900 (Spain).◮ NAMTS (Nippon Advance Mobile Telephone System).

FDMA is also used for trunk networks:◮ APCO-25 (Association of the Public Safety Communications): used in

safety agencies in USA.

FDMA is as well an access technique traditionally exploited in satellitecommunications.◮ IRIDIUM project: provides mobile coverage worldwide.◮ Used in EUTELSAT satellite network [4].

Though other techniques were favoured with respect to FDMA along the pastdecades, we will see it coming back in a more sophisticated way (OFDMA).


TDMA

Time Division Multiple Access


TDMA

Characteristics

TDMA (Time Division Multiple Access) is based in dividing the timeaxis in periodic short time intervals (slots).

◮ This framework is clearly frame-oriented.◮ The slots are grouped into a frame structure that repeats periodically.

Each communication accesses thesame spectrum bandwidth duringthe assigned slot(s).

◮ More than one slot could be as-signed to a communication.

The transmission is performed indiscontinuous form for each commu-nicating system.

◮ Each slot transmission takes placeinto the form of (periodic) burstsor information packets.

f (Hz)

Amplitude

f�� f��

s�� t��

#� #�

t ��

PSD

B

m��!"$�%&m'��%

Figure 16: Ideal TDMA system frame/slot structure.


TDMA

Characteristics

Correct frame/slot timing requires strict synchronization at TX/RX.◮ In mobile systems, the base station and the mobile stations have to keep

some kind of centralized timing.◮ The base station records the timing advance of the mobile stations to

account for propagation delays.◮ Collision problems are minimized by using time guard intervals.◮ Propagation delays pose a limit to the area a base station can cover.◮ All this affects the uplink. In the downlink, there is only the base station’s

emitting frame: this is rather an instance of multiplexing (TDM).

Amplitude

d()( *+

+

,-(.d /0)1.2(l

34

t

Figure 17: TDMA frame arriving at the central station receiver.


TDMA

Characteristics

The TDMA frame requires training, control and synchronization infor-mation to be carried together with the data payload.◮ This control data normally occupies preambles or tails within a slot.◮ Assume a total bandwidth B, a modulation with k bits per symbol (M-

QAM or M-PSK), and Ns slots.◮ Assume Dc symbol periods are occupied by control data within each slot,

and Ds by proper information payload.◮ Rb is the available data bit rate per communication, Ts the slot period,

Tf the frame period, and Tg the guard interval per slot.

Tf = (Dc +Ds)Ns

B/2 + NsTg , Ts = TfNs

= 2(Dc +Ds)B + Tg ,

Rb = k DsTf

= k Ds2(Dc+Ds)Ns/B+Ns Tg

The binary rate allowed by the available bandwidth B is esentially di-vided into the Ns communications (plus some reduction due to controloverhead and guard periods).

The instantaneous TX bit rate at each burst is nominally Rbi= k B

2 .


TDMA

Characteristics

Figure 18: GSM TDMA frame [6]. See that there are burst types that carry no information (further decrease in capacity). Theynevertheless serve for synchronization purposes.


TDMA

Characteristics

Figure 19: INTELSAT TDMA frame [7]. Reference bursts RB1 and RB2 further decrease capacity. They are useful to keepEarth stations time aligned.


TDMA

Characteristics

Increasing the capacity of the TDMA system (higher number of com-munications) means a corresponding increase in the instantaneous bitrate.◮ This would lead to increasing the bandwidth used, or the modulation

efficiency (which could worsen the Eb/N0 and increase BER).

There is a limit in the maximum number of simultaneous communica-tions (Ns) due to propagation limits.◮ This could be traded off with the help of the guard period Tg , but at

the cost of higher overhead and higher latency.

It is also important to consider power limits, and maximum allowedlatency.◮ In practical cellular systems, the limitation for Ns is around 10 per radio-

channel (of bandwidth B).

A TDMA system can work straightforwardly in TDD or FDD modes.

Due to the discrete nature of the TDMA system, it can only be usedfor digital communications.


TDMA

Characteristics

Advantages:

◮ Simpler multichannel central station.It only requires a single transceiver for Ns channels.

◮ Higher versatility.The number of time slots assigned to a communication may be adaptedto specific requirements.

◮ Easier channel signaling.The control information may be introduced in some bits inside a burst(e.g. a header field), or be carried through some specific dedicated slots.

◮ Higher spectral performance.◮ Good for handling medium to high traffic volumes.◮ It can transmit in a single frequency using TDD in a natural way.

This avoids the extensive usage of a number of frequency mixers ormedium-specific filters (e.g. RF filters).


TDMA

Characteristics

Disadvantages:

◮ Higher access complexity.

It requires a tight time synchronization to avoid collisions.

◮ Inherent frame size limit.

If the frame is too big, the terminal has to store a lot of information.

◮ Higher communication delay.

Due to the need of information buffering, the communication sufferssome delay.

◮ The information must be digital.

In case of analog sources, it has to be sampled (quantization noise).

◮ Broadband TDMA communications may be severely affected by fre-quency selective fading.


TDMA

Examples

TDMA is being used in 2G mobile systems:

◮ GSM worldwide (used along with frequency hopping techniques) [6].

It is also used in trunk networks:

◮ TETRA (Terrestrial Trunked Radio): used in safety agencies mainly, inEurope.

Proposed as well in wired standards:

◮ ITU-T G.hn: for high speed LAN over existing home wiring [8].

TDMA constitutes one of the access possibilities for satellite commu-nications:

◮ Used as one of the available implementations of the IRIDIUM satellitemobile system.

◮ Used in the INTELSAT satellite network system [7].

TDMA is being recycled together with FDMA in a joint time-frequency scheme asOFDMA.


SDMA

Space Division Multiple Access


SDMA

Characteristics

SDMA (Space Division Multiple Access) is traditionally based in di-viding a particular coverage area into discrete sectors (or cells).◮ In theory, the sectors are non-overlapping.◮ Communications are centralized somehow inside each sector (central or

base station: intra-cell management).◮ Communications use different parameters in neighboring cells to ensure

low or no interference (this requires coordination at a higher level: inter-cell management).

◮ Normally, this is ensured by using different frequency bands.

f

f

f

f

Figure 20: 4 frequencies cell group (cluster).


SDMA

Characteristics

This technique is characteristic of cellular systems.◮ Multiple access is not managed at the user level, but for groups of users.◮ Inside the cell, other concurrent medium access technique can be used

(e.g. TDMA in GSM networks).◮ This technique is more about planning, architecture, coverage and de-

signing protocols (e.g. for handoffs) than about proper medium access.In urban areas, a single base station placement could serve severalcontiguous cells (S).◮ This is called sectorization, and normally S = 3.

f

f

ff1

f2

f3

Figure 21: Sectorized cells with S = 3.


SDMA

Characteristics

The whole coverage area is tiled into cells, that group into clusters.◮ This is the so-called frequency planning for a cellular network.◮ The area is covered by replicating the cluster structure along the space.◮ The larger the cluster, the farther the cells with same frequencies.

The problem is the need for more spectrum allocation → tradeoff.◮ (N , S · N) reuse pattern: N groups of S sectorized cells.

N · S total B frequency bands, inside which xDMA is employed.

A1

A2

A3

B1

B2

B3

C1

C2

C3

A1

A2

A3

B1

B2

B3

C1

C2

C3

A1

A2

A3

B1

B2

B3

C1

C2

C3

A1

A2

A3

B1

B2

B3

C1

C2

C3

A1

A2

A3

B1

B2

B3

C1

C2

C3

A1

A2

A3

B1

B2

B3

C1

C2

C3

Figure 22: (3, 9) reuse pattern.


SDMA

Characteristics

Planning a cellular network could be far more complicated.◮ Depending on the terrain, presence of buildings, etc., a given area cannot

be usually covered with equal-sized cells and clusters.◮ On certain cases (e.g. CDMA-based networks), no frequency planning

is needed (but... spreading codes planning).

SDMA is also about managing power inside the cell.As technology evolves, SDMA is refining its possibilities.◮ With the help of the smart antennas concept, a base station can accu-

rately locate a mobile (direction of arrival, DoA, + timing advance).◮ This information can be used to direct the TX/RX radiation patterns,

save power and refine planning possibilities.

timing advance

timing advance + DoA

Figure 23: Omnidirectional vs. smart antennas usage.


SDMA

Examples

SDMA is extensively used in cellular networks.

◮ It is used along with TDMA and frequency planning in GSM (2G mobilestandard).

◮ It is used along with CDMA (and no frequency planning) in UMTS (3Gmobile standard).

◮ It is used along with OFDMA and frequency planning in LTE (4G mobilestandard).

Planning in LTE is nevertheless far more complex than in GSM.

In collaboration with the smart antennas concept, SDMA is also usedin satellite networks (e.g. INTELSAT IV-A).

◮ With the help of dual-beam receive antenna, it can provide coveragesimultaneously to two Earth different regions.

A related concept of SDM (Spatial Division Multiplexing) is used incabled systems.

◮ The space multiplexing concept translates here into using different cablesfor different communications.


CDMA

Code Division Multiple Access


CDMA

Spread Spectrum techniques

CDMA (Code Division Multiple Access) is based in assigning code

sequences with appropriate properties to concurrent communications.

The concurrent transmissions are performed at the same time and oc-

cupying the same spectrum.

Interferences are not avoided by managing the time or frequency axisas in FDMA or TDMA.How can the information in the medium be recovered?◮ The answer lies the usage of spread spectrum techniques.

f (Hz)

Amplitude

frame period frame periodt (s)

PSD

B

#1

f (Hz)

Amplitude


PSD

B

#2

f (Hz)

Amplitude


PSD

B

#3

medium

Figure 24: CDMA: an inherently interference dominated technique.


CDMA


t (s)

t (s)

t (s)

Tb Tb

Tc

sequence period

+A

+1

+A

−A

−1

−A

d(t)

c(t)

d(t) · c(t)

Figure 25: d(t): binary polar data sequence. c(t): spread-ing code sequence. d(t) · c(t): spread data sequence.

−1.5 −1 −0.5 0 0.5 1 1.5

x 106

−10

0

10

20

30

40

50

Normalized frequency

Pow

er S

pect

ral D

ensi

ty

Figure 26: Red: PSD of the binary polar data sequence.Blue: PSD of the spread data sequence with spreading fac-tor 128. RRC pulse shaping is used with rolloff factor 0.5.

There are N = Tb/Tc chips per symbol period: this is the spread factor.

See how the power of the resulting sequence is basically the same as the power

of the original one, but it is spread over a larger bandwidth (N× spread).


CDMA


Assume two binary polar data sequences with period Tb:

d1(t) =∑

∞

n=0b1,n · Π

(

t−nTbTb

)

, d2(t) =∑

∞

n=0b2,n · Π

(

t−nTbTb

)

Assume two spreading sequences with length N chips, and Tc = Tb/N .

c1(t) =∑

N−1

k=0c1,k · Π

(

t−kTcTc

)

, c2(t) =∑

N−1

k=0c2,k · Π

(

t−kTcTc

)

The signal at the output of an AWGN channel (noise w(t)) would be

r(t) = d1(t − t1) ·∑

∞

n=0c1 (t − nTb − t1) + d2(t − t2) ·

∑

∞

n=0c2 (t − nTb − t2) + w(t).

The despreading will be done by means of a correlator.

Receiver i :∫ (n+1)Tb

nTb

r(t) · ci (t − nTb − ti ) dt → bi,n

d(t)

c(t)

d(t) ·∑

∞

n=0c (t − nTb )

Figure 27: Spread spectrum modulator.

D5678

c(t)

r(t)

∫

Tb

0(·)dt

bn

Figure 28: Spread spectrum demodulator.


CDMA


0 5 10 15 20 25−0.2

0

0.2

0.4

0.6

0.8

1

1.2

τ

Aut

ocor

rela

tion

Figure 29: Rc1(τ), N = 4096 chips.

0 5 10 15 20 25 30 35 40 45−0.2

0

0.2

0.4

0.6

0.8

1

1.2

τ

Cro

ss c

orre

latio

n

Figure 30: Rc1,c2(τ), N = 4096 chips.

When c1(t), c2(t) have the periodic correlation properties of the figures:∫

(n+1)Tb

nTb

r(t) · c1 (t − nTb − t1) dt = b1,n + η; η = ǫ · Rc1,c2(t1 − t2) +

∫

(n+1)Tb

nTb

w(t) · c1 (t − nTb − t1) dt

ǫ·Rc1,c2 (t1 − t2) is a small interference term, and η can be modeled as AWGN.


CDMA

Spread Spectrum sequences

For a set of spread spectrum sequences {ci (t)}Mi=1 to be useful, they

have to meet certain properties:

◮ Rci (τ) ≈ δ (τ).◮ Rci ,cj (τ) ≈ 0, i 6= j.◮ They have to look like noise (pseudorandom or pseudonoise sequences).

A CDMA system uses a set of such sequences.

◮ Each concurrent communication is assigned a sequence from the set.◮ On reception, the rest of concurrent communications are small contri-

butions to the received noise.

Unless codes are truly orthogonal and they add synchronously in a frame.

N defines the so-called processing gain, PG = 10 log10 (N).

◮ The larger N , the higher the bandwidth required, and the lower thepower spectral density per communication.

◮ PG is a measurement of the improvement made in Eb/N0 when usingspread spectrum: the correlator recovers the signal, and filters the noise.


CDMA

Types of spread spectrum

The example seen is an instance of direct-sequence (DS) spread spectrum.◮ It can be straightforwardly applied to any kind of digital modulation.

Managing the frequency axis: frequency-hopping (FH) spread spectrum.◮ The symbols are transmitted by hopping along a set of frequency slots {∆fi } over the available bandwidth B.◮ Several hops per symbol period Ts : fast hopping.◮ Several symbols per hop: slow hopping.

Managing the time axis: time-hopping (TH) spread spectrum.◮ The symbols are transmitted by hopping along a set of time slots {∆ti } at each frame period TF .

In FH and TH, the spreading sequence determines a pseudorandom hopping pattern.

f (Hz)

PSD

fi fj

Figure 31: FH scheme.

t

Amplitude

frame period

ti tj

Figure 32: TH scheme.


CDMA

Spread Spectrum properties

α1 · s (t − t1)

α2 · s (t − t2)

α3 · s (t − t3)

Figure 33: Multipath in a wireless channel.

0 500 1000 1500 2000 2500

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

τ

Cro

ssco

rrel

atio

n

Figure 34: Crosscorrelation of received signal with spread-ing sequence.

In presence of multipath, a spread spectrum system can synchronizeitself with any of the arriving paths, e.g.

r(t) = α1 · s (t − t1) + α2 · s (t − t2) + α3 · s (t − t3) + w(t)

The crosscorrelation with the original spread sequence will givetree distinctive peaks, as shown in the figure.


CDMA

Rake receiver

A rake receiver with m fingers can individuate and combine up to m paths.◮ It can counteract multipath effects on the received signal.◮ The delays τ1, τ2 , ... τm are calculated with the help of the correlation properties of the SS sequence.◮ There are two steps: acquisiton and tracking.◮ The amplitude and phase compensation per finger, αi , φi , allow to constructively combine the resulting data

(e.g. following a maximal ratio combining criterion, which maximizes total SNR).

9: ;<=:;>?@9:A

c(t)

c(t)

c(t)

r(t)

Σ

τ1

τ2

τm

α1 , φ1

α2 , φ2

αm , φm

∫

Tb

0(·)dt

∫

Tb

0(·)dt

∫

Tb

0(·)dt

Figure 35: Rake receiver.


CDMA

Characteristics of CDMA

A CDMA system works by assigning suitable spreading codes from agiven set to concurrent communications.

◮ All the communication frames share the same spectrum and operate atthe same time, interfering each other.

The spreading factor can be different for different communications.

◮ This allows the assignment of different data rates / quality for differentusers.

CDMA permits the random and asynchronous usage of the medium.

◮ There is no need for a global synchronization system.◮ But, if all the frames are managed synchronously, it is possible to use

orthogonal spreading codes (e.g. OVSF2 codes in UMTS donwlink.)

In a cellular system with SDMA, CDMA does not require frequencyplanning.

◮ Neighboring cells use different sets of codes.

2 Orthogonal Variable Spreading FactorFrancisco J. Escribano Block 4: Medium Access Techniques April 26, 2015 48 / 77

CDMA


A CDMA cellular system can perform soft handoffs.

◮ The neighbor base stations track and pick up the data from a movinguser at the same time.

◮ This happens during some transition interval, and then handoff is per-formed without interruptions.

◮ Call drops are minimized with respect to TDMA based cellular systems.

soft hando

Figure 36: Soft handover scheme.


CDMA


A CDMA cellular system can perform softer handoffs.◮ A base station, with the help of a rake receiver, can track more than one

path from a single communication.◮ Depending on the quality of the different paths, the base station can

discard, attenuate or favour some of them after despreading.◮ Reconfiguring the path balance can be done seamlessly as in the soft

handoff case.

softer hando

a)

SNR

SNR

Figure 37: Softer handover scheme.


CDMA


A CDMA system is a system mainly limited by interference.◮ Each new communication adds to the background level of interference

(residue left after despreading).◮ This translates into a poorer overall Eb/N0 as the number of concurrent

communications increase.The worst problem related to interference is the so-called near-far prob-lem.◮ A system transmitting very close to the common receiver station can

effectively mask any other farther communications, as it translates intoa high degree of interference.

Figure 38: Near-far problem illustration.


CDMA


CDMA and interference concerns:◮ This means that CDMA needs to implement a sophisticated centralized

power control system.◮ This power control system manages the power transmitted by each user.◮ The power and interference buget in a cell is reconfigured as soon as

a significant change is identified (new user, communication drop, largescale mobility, near-far issue...).

◮ CDMA offers soft capacity limits: there is not a definite number ofmaximum possible simultaneous users.

◮ CDMA suffers from the breathing problem: the coverage of a cell caneffectively shrink or grow depending on the overall load.

Figure 39: Breathing in CDMA.


CDMA


Assume a unique service that requires at least a SNR after despreading of (Eb/N0)min.The power received is Pk for k-th communication, for a total L active communica-tions.

The scheme uses BPSK modulation and DSSS.

The chip rate is fixed for the system, Rch, while the service offers a Rb data rate,with a so-called activity factor3 0 < ν ≤ 1: Rch/ (Rb · ν) is the effective PG.

The total interference + noise affecting the communication at the receiver is IT .(

Eb

N0

)

min≤ Pk ·Rch

Rb ·ν·(IT −Pk ) , Lk = ITPk

NFR =(

1 −∑L

k=1 Lk

)

−1

Lk is the load factor, and NFR, the total near-far resistance factor, which measuresthe quality of the service inside the cell. An NFR of 5 is the upper practical limit.

This kind of calculation has to be extended over all the possible services, and forthe uplink and downlink separately. The final result allows the adjustement of thepower and interference bugdet of the cell.

3E.g. 1 for data, 0.67 for voice.


CDMA


Service Rate Profile Speed Ul EbN0 Dl EbN0

type (kpbs) kind (Km/h) (dB) (dB)Voice 8 Interior A 3 4.8 6.7Voice 8 Pedestrian A 3 4.8 6.8

RT Data 64 Interior A 3 2.3 1.9RT Data 64 Pedestrian A 3 2.4 1.9RT Data 64 Vehicular A 120 3.8 3.7

NRT Data 144 Vehicular A 120 3.0 2.9NRT Data 384 Pedestrian A 3 0.4 0.1

Table 1: Example of quality parameters for different services in UMTS [9].

RT: real time; NRT: non-real time.

Chip rate in UMTS is Rch = 3.84 Mcps.

Calculations should also take into account modulation effiency and channel codingrate. The real scope is providing a limited BER for each service/user.


CDMA

Characteristics

Advantages:

◮ Capacity is CDMA’s biggest asset. It can accommodate more users perMHz of bandwidth than any other technology, excepting OFDM.

◮ Has no built-in limit to the number of concurrent users (soft capacity).◮ Consumes less power: able to produce a communication of reasonable

quality with lower signal levels.◮ Soft hand-off reduces the likelihood of dropped calls.◮ High flexibility to accommodate different services (different spreading

factors).◮ It can resist narrowband jammers (interferers).◮ It can counteract multipath (rake receiver) and better resist frequency-

selective fading.


CDMA

Characteristics

Disadvantages:

◮ Undesirable effects linked to the usage of spread spectrum: breathing,near-far problem.

◮ Because CDMA towers interfere with themselves, they are normally in-stalled on much shorter towers.

◮ CDMA may not perform well in rugged terrain due to the lower towerheight.

◮ Strict need of a centralized and burdensome power control system.

◮ Difficult-to-manage coverage, quality, power, interference tradeoffs.

A CDMA-based system offers high degree of flexibility, but at the costof more complicated hardware, methods and protocols.


CDMA

Examples

CDMA is being used in mobile systems, mainly in DS mode:

◮ IS-95 (also known as CdmaOne), 2G American standard.◮ UMTS (Universal Mobile Telecommunications System), 3G standard: it

employs WCDMA (Wideband-CDMA) [9].◮ Cdma2000, 3G American standard [10].

Used as well in wired standards:

◮ MC-CDMA (Multi-Carrier CDMA), an alternative to known ADSL stan-dards; it is in fact a variant of OFDM, combined with FH spread spectrum[11].

CDMA constitutes one of the access possibilities for satellite commu-nications:

◮ Used in the Globalstar satellite phone network [12].

CDMA as a standalone access technique is not so important nowadays, butspread spectrum is extensively exploited in many state-of-the-art developments.


OFDMA

Orthogonal Frequency Division

Multiple Access


OFDMA

Characteristics

OFDMA (Orthogonal Frequency Division Multiple Access) is based inplugging the modulated symbols into different subcarriers by means ofthe inverse discrete Fourier transform (IDFT) [13].

◮ Let’s assume a sequence of baseband modulated symbols {Xn} (ASK,PSK, QAM, etc.), where Xn = X I

n + j · XQn .

◮ We perform an IDFT of order N over each block of N such modulatedsymbols.

vk = 1N

∑N−1n=0 Xne

j 2πknN , k = 0, · · · , N − 1

The new sequence vk can be seen in the spectral domain as comprisinga group of N orthogonal subcarriers, e

j2πkn/N .

◮ Orthogonality:∑N−1

n=0 ej2πkn/N

e−j2πmn/N = 0 for k 6= m.

The n-th subcarrier ej2πkn/N delivers the modulated data given by the

n-th symbol Xn along a duration k of N samples (subcarrier mapping).


OFDMA

Characteristics

S/P

+

subcarrier

mapping

P/S DACIDFT

modulated

symbols

OFDM

symbols

I channel

Q channel

output

frame

-

cos (2πfc t)

sin (2πfc t)Σv(t)

X0

X1

X2

XN−1

v0

v1

v2

vN−1

Figure 40: OFDM transmitter.

The modulated data is blockwise processed and mapped into subcarri-ers, the OFDM4 symbols are converted into an analog signal v(t) withsymbol period T , and transferred to a carrier frequency fc .

v(t) =∑N−1

n=0 Xnej 2πnt

T , 0 ≤ t < T

The subcarrier spacing 1/T guarantees the orthogonality.4

Orthogonal Frequency Division Multiplexing


OFDMA

Characteristics

−15 −10 −5 0 5 10 1510

−3

10−2

10−1

100

101

Normalized frequency

Pow

er s

pect

ral d

ensi

ty

Figure 41: OFDM spectrum.

time

frequency

subcarr

iers

modulated symbols

T

1T

Figure 42: The time/frequency divided OFDM frame.

The subcarriers overlap, as a difference with a normal FDM(A) scheme.

Data can be separated due to the orthogonality of the subcarriers.

The OFDM frame is time/frequency divided, and can be flexibly man-aged.


OFDMA

Characteristics

S/P P/SBCE

FGT+

subcarrier

demapping

modulated

symbols

OFDM

symbols

I channel

Q channel

LHI

LHI

JKM

JKM

JKM

JKM

to demodulator

and channel decoder

from

channel cos (2πfc t)

sin (2πfc t)

X0

X1

X2

XN−1

v0

v1

v2

vN−1

Figure 43: OFDM receiver.

An OFDM receiver can ideally retrieve the modulated symbols overeach subcarrier with the help of the DFT.

Xn =∑N−1

k=0 vke−j 2πnk

N , n = 0, · · · , N − 1

Transform operations are in reality implemented as FFT (fast Fourier

transform) instances.

The RX performs frequency equalization (FEQ) over each subcarrier.


OFDMA

Characteristics

The IDFT/DFT pair decouples a large bandwidth into a number ofnarrowband subcarriers.◮ Assume N subcarriers, with OFDM symbol period T , the total band-

width is B = N+1T = (N + 1)Rs .

The effect of the channel passband Hc (f ) can be compensated at eachsubcarrier with the help of an FEQ block with a single coefficient.◮ The attenuation can be considered constant over the narrowband sub-

carrier data.◮ This reduces the amount of linear distortion in the received signal.

PSD PSDcNOPPnQ RPSUt

cNOPnel

cNOPPnQ VUWSUt

fff

|Hc (f )|2

Figure 44: Effects of linear (amplitude) distortion.


OFDMA

Characteristics

An OFDM frame requires to invest some resources to work properly:

◮ Cyclic prefix: the ending data is repeated at the beginning of a block, inorder to ease synchronization, during a Tg guard interval.

◮ Pilots: some frequencies at given time intervals are reserved to transmitknown pre-defined sequences, so that the channel can be identified forequalization purposes.

XY Z[\[Xn

Tg T

Figure 45: Cyclic prefix in an OFDM symbol periodTsym = T + Tg .

time

frequency

subcarr

iers

modulated symbols

PilotsT

1T

Figure 46: Pilot usage in an OFDM frame.


OFDMA

Characteristics

The OFDM frame can easily accomodate an OFDMA scheme.◮ A given communication can be assigned a number of subcarriers during

given symbol intervals.◮ The frequency hopping scheme fits quite naturally into an OFDM frame.◮ All this flexibility comes at the cost of more refined protocols and sig-

naling overhead.

time

frequency

subcarr

iers

modulated symbols

PilotsT

1T

Figure 47: Frequency/symbol assignment along an OFDMA frame.


OFDMA

Characteristics

Example of the OFDM frame in the LTE downlink [14].◮ Note the presence of the pilots and intervals dedicated to control, syn-

chronization, signaling, etc.

Figure 48: LTE downlink frame (source IS-Wireless).


OFDMA

Characteristics

An OFDM frame is not practical for an uplink, due to the differentpropagation delays.◮ In LTE, the system resorts to SC-FDMA (single carrier FDMA), where a

group of frequencies are assigned as a block (localized mapping), insteadof being divided into subcarriers.

◮ This is implemented by performing a previous DFT before the IDFT inthe transmitter.

S/P P/S ]A^

p_`abDFT

esubcarrier

mapping

modulated

symbols

SC-FDMA

symbols

I channel

Q channel

output

frame

-point

IDFT

hj

N- cos (2πfc t)

sin (2πfc t)Σv(t)

Figure 49: SC-FDMA transmitter (where M < N).


OFDMA

Characteristics

Example of the SC-FDMA frame in the LTE uplink [14].

◮ Note the differences with respect to the OFDM downlink frame.

Figure 50: LTE uplink frame (source IS-Wireless).


OFDMA

Characteristics

Assume an OFDM scheme with basic period T , a cyclic prefix withduration Tg , and N useful subcarriers.

The spectral efficiency of the modulated data is k bits per symbol.

Assume Np pilot frequencies (out of N) reserved each Nd symbol pe-riods.

The data rate offered will be:

Rs = NT+Tg

· Nd −1Nd

+N−Np

T+Tg· 1

Nd

Rb = k · Rs

Note how the usage of the CP and pilot frequencies affect the overallrate.

All this have to be taken into account when calculating the effectiveEb/N0 and BER.


OFDMA

Characteristics

Advantages:

◮ OFDM can easily be adapted to severe channel conditions without com-plex equalization.

◮ Robust to narrow-band co-channel interference.◮ Robust to inter-symbol interference and fading caused by multipath prop-

agation.◮ High spectral efficiency.◮ Efficient implementation (FFTs are highly optimized in hardware).◮ Low sensitivity to time synchronization errors.◮ Tuned sub-channel receiver filters are not required (unlike conventional

FDMA).◮ Enables the use of single frequency networks (as in CDMA).◮ High flexibility to accomodate different rates/services into subcarriers.


OFDMA

Characteristics

Disadvantages:

◮ High sensitive to Doppler shift.Difficulties to manage reception under mobility conditions.

◮ Sensitive to frequency synchronization problems (carrier offset).Dedicated algorithms and extra redundancy are needed to compensatefor this, adding to the complexity and the loss of efficiency of the scheme.

◮ Tones on the edges have to be spared to comply with spectral masks.Side lobes do not fall steep enough in standard OFDM.

◮ Inefficient transmitter power consumption.An OFDM signal has a large Peak-to-Average Power Ratio (PAPR).An HPA (High Power Amplifier) works best near its saturation point.HPA + high PAPR require operation in the linear region.Unavoided nonlinear distortion creates spectral leakages.SC-FDMA solves in part this issue (single carrier scheme!).


OFDMA

Examples

In the wireless broadcast domain, OFDM and OFDMA find extensive usage.

◮ Digital audio systems EUREKA 147, Digital Radio Mondiale, HD Radio,T-DMB and ISDB-TSB.

◮ The terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T.

In two-way wireless comms, OFDM and OFDMA are becoming dominant.

◮ Pre-4G and 4G systems: the IEEE 802.16 or WiMax Wireless MANstandard and the 3GPP LTE (Long Term Evolution) standard [14].

◮ The IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) stan-dard.

◮ IEEE 802.11a and 802.11g Wireless LANs.◮ The Flash-OFDM cellular system.◮ Some Ultra wideband (UWB) systems.◮ Point-to-point (PtP) and point-to-multipoint (PtMP) wireless applica-

tions.

In cable systems, we can find also important examples.

◮ ADSL and VDSL broadband access via telephone network copper wires.◮ Power line communications (PLC).


Conclusions

Conclusions


Conclusions

Conclusions

Recall the definitions and ambiguities in the medium access domain.

◮ Medium access, multiple access, multiplexing, duplexing.

The different methods put the stress over a given signal axis.

◮ Frequency, time, space, code.

The classification is not exclusive.

◮ Different methods can be used at the same time (e.g. SDMA and TDMAin GSM).

◮ Putting the stress over one given axis does not imply that the systemforgets about the rest.

Current trend goes towards the assimilation of different techniques ona single system.

◮ This leads to a progressive increase in management complexity (recallLTE).

Medium access is performed at the PHY level, but naturally involveshigher communication layers, as it is controlled by the MAC sublayer.


References

References


References

Bibliography I

[1] M. K. Simon and M.-S. Alouini, Digital Communications over Fading Channels. New Jersey: John Wiley & Sons, Inc.,2005.

[2] A. Goldsmith, Wireless Communications. New York: Cambridge University Press, 2005.

[3] E. Krouk and S. Semenov, Modulation and coding techniques in wireless communications. Chichester: Wiley, 2011.

[4] Eutelsat Company Web Site, “SMS QPSK/FDMA System Specification,” EESS 501 G Issue 3. [Online]. Available:http://www.eutelsat.com/files/contributed/satellites/pdf/eess501.pdf

[5] A. Khanifar, Satellite Communication Systems. Boca Raton: CRC Press LLC, 2000.

[6] 3rd Generation Partnership Project, “Physical layer on the radio path,” Technical Specification Group GSM/EDGE RadioAccess Network. [Online]. Available: http://www.3gpp.org/DynaReport/45001.htm

[7] A. S. Oei, R. J. Colby, R. Parthasarathy, and A. L. Stimson, “Alignment, testing and maintenance principles in theintelsat tdma/dsi network,” International Journal of Satellite Communications, vol. 3, no. 1-2, pp. 161–166, 1985. [Online].Available: http://dx.doi.org/10.1002/sat.4600030118

[8] ITU-R.G.9960, “Unified high-speed wireline-based home networking transceivers - System architecture and physical layer spec-ification,” International Telecommunication Union. [Online]. Available: http://www.itu.int/rec/T-REC-G.9960/en

[9] 3G TS.25.212 V3.3.0 (2000-06), “Multiplexing and Channel Coding (FDD),” Technical Specification Group Radio AccessNetwork, 3rd Generation Partnership Project.

[10] 3GPP2 C.S0002-D Version 1.0, “Physical Layer Standard for cdma2000 Spread Spectrum Systems, Revision D,” ThirdGeneration Partnership Project 2 (3GGP2).

[11] K. Fazel and S. Kaiser, Multi-Carrier and Spread Spectrum Systems: From OFDM and MC-CDMA to LTE and WiMAX.New York: John Wiley & Sons, 2008.


http://www.eutelsat.com/files/contributed/satellites/pdf/eess501.pdf

http://www.3gpp.org/DynaReport/45001.htm

http://dx.doi.org/10.1002/sat.4600030118

http://www.itu.int/rec/T-REC-G.9960/en

References

Bibliography II

[12] F. Dietrich, P. Metzen, and P. Monte, “The Globalstar cellular satellite system,” Antennas and Propagation, IEEE Transac-tions on, vol. 46, no. 6, pp. 935–942, 1998.

[13] Y. S. Cho, J. Kim, W. Y. Yang, and C. G. Kang, MIMO-OFDM Wireless Communications with MATLAB. Singapore:John Wiley & Sons, 2010.

[14] 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation,” TechnicalSpecification Group Radio Access Network, 3rd Generation Partnership Project.


Medium Access Techniques

Engineering

Transcript of Medium Access Techniques