EEC173B/ECS152C, Spring 2010

Wireless Channel Access- Challenges: Hidden/Exposed Terminals- Access Methods: SDMA, FDMA, TDMA,

CDMA Multimedia compression and communication

basics

Amit Pande, Computer Science, UC Davis

comments/ feedback: amit@cs.ucdavis.edu

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Medium Access Sublayer

Medium Access Control (MAC) Protocols

--- distributed algorithm that arbitrate access to a common shared channel among a population of users, i.e., determine when a node can transmit

Physical

Network

Medium Access Sublayer

Data Link

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Remarks on MAC Sublayer

MAC is not important on point-to-point links MAC is only used in broadcast or shared channel

networks- Only one can send successfully at a time- Two or more simultaneous transmissions => interference- How to share a broadcast channel?- Communication about “sharing” must use the channel

itself!

Examples: - Packet-switched Radio Network (Aloha)- Ethernet IEEE 802.3 (CSMA/CD)- Token Ring IEEE 802.5, FDDI (Token Passing)- Cellular, Satellite, Wireless LAN (MACAW)

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Motivation

Can we apply media access methods from fixed networks?

Example CSMA/CD- Carrier Sense Multiple Access with Collision

Detection- Send as soon as the medium is free, listen into

the medium if a collision occurs (original method in IEEE 802.3)

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Problems in wireless networks

Signal strength decreases proportional to the square of the distance

The sender would apply CS and CD, but the collisions happen at the receiver

It might be the case that a sender cannot “hear” the collision, i.e., CD does not work

Furthermore, CS might not work if, e.g., a terminal is “hidden”

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Hidden terminals- A sends to B, C cannot receive A - C wants to send to B, C senses a “free” medium (CS

fails)- Collision at B, A cannot receive the collision (CD fails)- A is “hidden” for C

Hidden Terminals

BA C

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Exposed terminals- B sends to A, C wants to send to another terminal (not A or

B)- C has to wait, CS signals a medium in use- But A is outside the radio range of C, waiting is not

necessary!- C is “exposed” to B

Exposed Terminals

BA C D

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Terminals A and B send, C receives- Signal strength decreases proportional to the square of the

distance- The signal of terminal B therefore drowns out A’s signal- C cannot receive A

If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer

Also severe problem for CDMA-networks - Precise power control needed!

Near and Far Terminals

A B C

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Taxonomy of MAC Protocols

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Controlled Access methods

SDMA (Space Division Multiple Access)- Segment space into sectors, use directed antennas - Cell structure

FDMA (Frequency Division Multiple Access)- Assign a certain frequency to a transmission channel

between a sender and a receiver- Permanent (e.g., radio broadcast), slow hopping (e.g.,

GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum)

TDMA (Time Division Multiple Access)- Assign the fixed sending frequency to a transmission

channel between a sender and a receiver for a certain amount of time

=> The multiplexing schemes presented before are now used to control medium access!

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FDD/FDMA – e.g., GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

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TDD/TDMA, e.g., DECT

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 µs

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CDMA (Code Division Multiple Access)

All terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel

- Each sender has a unique random number, the sender XORs the signal with this random number

- The receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function

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CDMA – Cont’d

Disadvantages:- Higher complexity of a receiver (receiver cannot just

listen into the medium and start receiving if there is a signal)

- All signals should have the same strength at a receiver

Advantages: - All terminals can use the same frequency, no planning

needed- Huge code space (e.g. 232) compared to frequency space- Interferences (e.g. white noise) is not coded- Forward error correction and encryption can be easily

integrated

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CDMA: An Example

Sender A - Sends Ad = 1, key Ak = 010011 (assign: “0“= -1, „1“= +1)

- Sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1) Sender B

- Sends Bd = 0, key Bk = 110101 (assign: “0“= -1, „1“= +1)

- Sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1) Both signals superimpose in space

- Interference neglected (noise etc.)

- As + Bs = (-2, 0, 0, -2, +2, 0) Receiver wants to receive signal from sender A

- Apply key Ak bitwise (inner product)

• Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6• Result greater than 0, therefore, original bit was “1“

- Receiving B

• Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. “0“

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Another Example: CDMA on signal level (1)

data A

key A

signal A

data key

key sequence A

Real systems use much longer keys resulting in a larger distance between single code words in code space.

• For this example, ‘0’ is high, ‘1’ is low.

1 0 1

10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1

01 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Ad

Ak

As

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CDMA on signal level (2)

signal A

data B

key B

keysequence B

signal B

As + Bs

data key

1 0 0

00 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1

11 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bd

Bk

Bs

As

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CDMA on signal level (3)

Ak

(As + Bs) * Ak

integratoroutput

comparatoroutput

As + Bs

data A

1 0 1

1 0 1 Ad

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CDMA on signal level (4)

integratoroutput

comparatoroutput

Bk

(As + Bs) * Bk

As + Bs

data B

1 0 0

1 0 0 Bd

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comparatoroutput

CDMA on signal level (5)

wrong key K

integratoroutput

(As + Bs) * K

As + Bs

(0) (0) ?

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Implications for High-Speed Wireless Data

Controlled multiple access performs well with continuous stream traffic but inefficient for bursty traffic

Complexity: frequency division < time division < code division

Multiple data rates- Multiple frequency bands- Multiple time slots- Multiple codes

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Comparison SDMA/TDMA/FDMA/CDMA

Approach SDMA TDMA FDMA CDMAIdea segment space into

cells/sectorssegment sendingtime into disjointtime-slots, demanddriven or fixedpatterns

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Terminals only one terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directedantennas

synchronization inthe time domain

filtering in thefrequency domain

code plus specialreceivers

Advantages very simple, increasescapacity per km²

established, fullydigital, flexible

simple, established,robust

flexible, less frequencyplanning needed, softhandover

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),synchronizationdifficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA

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Video coding & communications

state-of-the-art

24Application Scenario

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Terms

Sampling RGB – YUV

- Sampling- 4:4:4- 4:2:2- 4:2:0

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Steps

Prediction Model

Spatial Model

Entropy Coding- Huffman coder- Arithmetic Coder

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Overview of H.264 / AVC

Latest Video coding standard Basic design architecture similar to MPEG-x or H.26x Better compression efficiency

- Upto 50% bitrate reduction from the preceding video codec standard

Subjective quality is better Advanced functional element Wide variety of applications such as video

broadcasting, video streaming, video conferencing, D-Cinema, HDTV.

Layered structure - consists of two layers: Network Abstraction Layer (NAL) and Video Coding Layer (VCL); supports 4:2:0 chroma sampling picture format including QCIF and CIF formats

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H.264/AVC Profiles Profiles and Levels for particular applications

- Profile : a subset of entire bit stream of syntax, different decoder design based on the Profile

• Four profiles : Baseline, Main, Extended and High

Streaming Video Extended

Digital Storage Media

Television Broadcasting Main

Video Conferencing

Videophone Baseline

Applications Profile

Studio editing High

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Specific coding parts for the Profiles

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H.264 Encoder (contd.)

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4G Mobile BWA networks

Design QoS oriented New techniques

aimed to improve the reliability and the efficiency

Enable the delivery of mobile video-on-demand services to

mobile users

Enable the delivery of mobile video-on-demand services to

mobile users

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New techniques

Explicit QoS support Frequency selective scheduling

with OFDMA Adaptive Modulation and Coding

with new modulations available Enhanced data protection

mechanisms (H-ARQ) Multiple Input Multiple Output Relay nodes

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Open issues

Channel variability Bandwidth and resource management:

- Call admission control- Scheduling algorithms (QoE oriented)

eMBMS (Multimedia Broadcast Multicast Service) vs unicast

Multi-layer approach

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Scalable video coding and wireless

Scalable video coding can deal with some of the issue:

- Dynamic channel condition- High error channels

Others issues:- Heterogeneous clients

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Thanks

Amit Pande

Got questions?

www.cs.ucdavis.edu/~amit