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
Medium Access Sublayer
Medium Access Control (MAC)
Protocols
Network
Data Link
Physical
Medium Access
Sublayer
--- distributed algorithm that arbitrate
access to a common shared channel
among a population of users, i.e.,
determine when a node can
transmit
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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)
4
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

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
A
B
C
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Exposed Terminals

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
A
B
C
D
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Near and Far Terminals

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
A


B
C
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!
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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
960 MHz
935.2 MHz
124
200 kHz
1
20 MHz
915 MHz
890.2 MHz
124
1
t
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TDD/TDMA, e.g., DECT
417 µs
1 2 3
downlink
11 12 1 2 3
uplink
11 12
t
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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
1
0
Ad
1
key A
key
sequence A
data  key
0
1
0
1
0
0
1
0
0
0
1
0
1
1
0
0
1
1
1
0
1
0
1
1
1
0
0
0
1
0
0
0
1
1
0
0
signal A
Ak
As
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.
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CDMA on signal level (2)
As
signal A
data B
key B
key
sequence B
data  key
signal B
1
0
Bd
0
0
0
0
1 1 0
1 0
1 0
0
0
0
1 0
1
1
1
1
1
1
0
1 0
1
0
0
0
1 0
1
1
1
0
1
0
Bk
Bs
As + B s
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CDMA on signal level (3)
data A
1
0
1
1
0
1
Ad
As + B s
Ak
(As + Bs)
* Ak
integrator
output
comparator
output
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CDMA on signal level (4)
data B
1
0
0
1
0
0
Bd
As + B s
Bk
(As + Bs)
* Bk
integrator
output
comparator
output
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CDMA on signal level (5)
As + B s
wrong
key K
(As + Bs)
*K
integrator
output
comparator
output
(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
Idea
SDMA
segment space into
cells/sectors
Terminals
only one terminal can
be active in one
cell/one sector
Signal
separation
cell structure, directed
antennas
TDMA
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
all terminals are
active for short
periods of time on
the same frequency
synchronization in
the time domain
FDMA
segment the
frequency band into
disjoint sub-bands
CDMA
spread the spectrum
using orthogonal codes
every terminal has its all terminals can be active
own frequency,
at the same place at the
uninterrupted
same moment,
uninterrupted
filtering in the
code plus special
frequency domain
receivers
Advantages very simple, increases established, fully
simple, established,
robust
inflexible, antennas
Disadvantages typically fixed
inflexible,
frequencies are a
scarce resource
flexible, less frequency
planning needed, soft
handover
complex receivers, needs
more complicated power
control for senders
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA
capacity per km²
Comment
only in combination
with TDMA, FDMA or
CDMA useful
digital, flexible
guard space
needed (multipath
propagation),
synchronization
difficult
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
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Video coding & communications
state-of-the-art
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Application 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
Profile
Baseline
Main
Applications
Video Conferencing
Videophone
Digital Storage Media
Television Broadcasting
Extended
Streaming Video
High
Studio editing
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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 videoon-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
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