Transcript Systematic Design of Space-Time Trellis Codes for Wireless

ECE 6332, Spring, 2014
Wireless Communications
Zhu Han
Department of Electrical and Computer Engineering
Class 24
April 21st, 2014
Multiple Access

How can we share a wireless channel:
– Results in Wireless Media Access Control Protocols


How we can change base stations: Results in Handoff
algorithms and protocols
How can we seamlessly support mobile applications over
wireless links:
– Results in mobility protocols like Mobile IP, Cellular IP, etc.

How can we design efficient transport protocols over wireless
links:
– Results in solutions like SNOOP, I-TCP, M-TCP, etc.

How different wireless networks/systems are designed?
– Bluetooth, IEEE 802.11, GSM, etc.
Duplexing

It is sharing the media between two parties.

If the communication between two parties is one way, the it is
called simplex communication.

If the communication between two parties is two- way, then it is
called duplex communication.

Simplex communication is achieved by default by using a single
wireless channel (frequency band) to transmit from sender to
receiver.

Duplex communication achieved by:
– Time Division (TDD)
– Frequency Division (FDD)
– Some other method like a random access method
Duplexing

Usually the two parties that want to communication in a duplex
manner (both send and receive) are:
– A mobile station
– A base station

Two famous methods for duplexing in cellular systems are:
– TDD: Time Division Duplex
– FDD: Frequency Division Duplex
Duplexing - FDD

A duplex channel consists of two
simplex channel with different
carrier frequencies
– Forward band: carries traffic
from base to mobile
– Reverse band: carries traffic
from mobile to base
F
M
B
Base
Station
Reverse
Channel
R
Mobile
Station
Forward
Channel
fc,,F
fc,R
Frequency separation
Frequency separation should be carefully decided
Frequency separation is constant
frequency
Duplexing - TDD

A single radio channel (carrier frequency) is
shared in time in a deterministic manner.
– The time is slotted with fixed slot length
(sec)
– Some slots are used for forward channel
(traffic from base to mobile)
– Some slots are used for reverse channel
(traffic from mobile to base)
Mobile
Station
Base
Station
Slot number
0
1
2
3
4
5
channel
F
R
F
R
F
R
Reverse
Channel
M
B
6
F
7
…
R
….
Forward
Channel
Ti+1
Ti
Time separation
time
Duplexing – TDD versus FDD

FDD
– FDD is used in radio systems that can allocate individual radio
frequencies for each user.

–
–
–
–

For example analog systems: AMPS
In FDD channels are allocated by a base station.
A channel for a mobile is allocated dynamically
All channels that a base station will use are allocated usually statically.
More suitable for wide-area cellular networks: GSM, AMPS all use FDD
TDD
– Can only be used in digital wireless systems (digital modulation).
– Requires rigid timing and synchronization
– Mostly used in short-range and fixed wireless systems so that
propagation delay between base and mobile do not change much with
respect to location of the mobile.

Such as cordless phones…
What is Full Duplex

A full duplex system allows communication at the same time
and frequency resources.
: Signal of interest
: Self interference
Fig. 5 Full duplex communication

Advantages
– Double spectral efficiency

Same time & same frequency band
– Cost little

Based on the existing MIMO radios
One Example Full Duplex
ECE 4371 Fall 2008
Multiple Access - Coordinated

We will look now sharing the media by more than two users.

Major multiple access schemes
– Time Division Multiple Access (TDMA)

Could be used in narrowband or wideband systems
– Frequency Division Multiple Access (FDMA)

Usually used narrowband systems
– Code Division Multiple Access (CDMA)

Used in wideband systems.
– Orthogonal Frequency Division Multiple Access (OFDMA)
– Spatial Division Multiple Access (SDMA)
Multiple Access - Coordinated
OFDMA
ECE 4371 Fall 2008
Multiple Access - Coordinated
ECE 4371 Fall 2008
Case Study: Bluetooth – Scatternet and FHSS
Piconet
S
S
M2
Piconet
S
FHSS
Piconet can be combined
into scatternets.
Red slave acts as a
bridge between two
piconets.
S
FHSS
M1
S
S
Each piconet uses FHSS with different
hopping sequences (masters are different).
This prevents interference between piconets.
Cellular Systems and MAC
Cellular System
Multiple Access Technique
AMPS
FDMA/FDD
GSM
TDMA/FDD
USDC (IS-54 and IS-136)
TDMA/FDD
PDC
TDMA/FDD
CT2 Cordless Phone
FDMA/TDD
DECT Cordless Phone
FDMA/TDD
US IS-95
CDMA/FDD
W-CDMA
CDMA/FDD
CDMA/TDD
cdma2000
CDMA/FDD
CDMA/TDD
Random Access: Pure Aloha
Algorithm:
A mobile station transmits immediately whenever is has data.
It then waits for ACK or NACK.
If ACK is not received, it waits a random amount of time and retransmits.
Ignoring the propagation delay between mobiles
and base station:
B
Ack/Nack
Data
M3
M1
M2
The time difference between the time
a mobile send the first bit of packet and the
time the base station receives the last bit of
the packet is given by 2T.
T = C/P
T: packet time.
C: channel data rate (bps)
P: packet length (bits)
During this 2T period of time, the packet may collide
with someone else packet.
Contention for Aloha
Slotted Aloha
CSMA: Carrier Sense Multiple Access

Aloha does not listen to the carrier before transmission.

CSMA listen to the carrier before transmission and
transmits if channel is idle.

Detection delay and propagation delay are two important
parameters for CSMA
– Detection delay: time required to sense the carrier and
decide if it is idle or busy
– Propagation delay: distance/speed_of_ligth. The time
required for bit to travel from transmitter to the receiver.
ECE6331
CSMA Variations

1-persistent CSMA:
– A station waits until a channel is idle. When it detects that the channel
is idle, it immediately starts transmission

Non-persistent CSMA:
– When a station receives a negative acknowledgement, it waits a
random amount of time before retransmission of the packet altough the
carrier is idle.

P-persistent CSMA
– P-persistent CSMA is applied to slotted channels. When a station
detects that a channel is idle, it starts transmission with probability p in
the first available timeslot.

CSMA/CD
– Same with CSMA, however a station also listen to the carrier while
transmitting to see if the transmission collides with someone else
transmission.


Can be used in listen-while-talk capable channels (full duplex)
In single radio channels, the transmission need to be interrupted in order
to sense the channel.
CSMA vesrsus Aloha
• Comparison of the channel utilization versus
load for various random access protocols.
20
MACA – Medium Access with Collision Avoidance

CSMA protocols sense the carrier, but sensing the carrier
does not always releases true information about the
status of the wireless channel
– There are two problems that are unique to wireless
channels (different than wireline channels), that makes
CSMA useless in some cases. These problems are:


Hidden terminal problem
Exposed terminal problem.
Hidden Terminal Problem
C’s cell
A’s cell
A
B
C
Hidden
terminal
• A is transmitting to B.
• C is sensing the carrier and detects that it is idle (It can not hear A’s
transmission).
• C also transmits and collision occurs at B.
• A is hidden from C.
Exposed Terminal Problem
B’s cell
A
B
C’s cell
C
D
Exposed
terminal
• B is transmitting to A. C is hearing this transmission.
• C now wants to transmit to D. It senses the existence of carrier signal and
defers transmission to D.
• However, C can actually start transmitting to D while B is transmitting to A,
• Since A is out of range of C and C’s signals can not be heard at A.
• C is exposed to B’s transmission.
MACA Solution Concept
Ali, lets talk! I
am available.
Can
Can, I want to
talk to you!
Can, I want to
talk to you!
Biltepe
Mountain
Ali
Veli
MACA Protocol

When a station wants to transmit data
– It sends an RTS (Ready-to-Send) packet to the intended
receiver


The RTS packet contains the length of the data that needs to be
transmitted
Any station other than the intended recipient hearing RTS defers
transmission for a time duration equal to the end of the
corresponding CTS reception
– The receiver sends back CTS (Clear-to-Send) packet back
to sender if it is available to receive.


The CTS packet contains the length of the data that original sender
wants to transmit
Any station other than the original RTS sender, hearing CTS defers
transmission until the data is sent.
– The original sender upon reception of the CTS, starts
transmitting.
Solution for Hidden Terminal Problem
A is transmitting to B.
C’s cell
A’s cell
X
RTS(n)
X defers transmission
until expected CTS
reception time by RTS
sender.
A
RTS(n)
CTS(n)
Data(n)
B
CTS(n)
C
C defers transmission
for duration of n bytes of
data transmission. Node A
is no longer hidden from C
effectively.
Waiting time of node X is much smaller than waiting time of node C.
Solution for Exposed Terminal Problem
B is transmitting to A
B’s cell
A
RTS(n)
CTS(n)
Data(n)
B
C’s cell
RTS(n)
C
RTS(m)
D
CTS(m)
Data(m)
• C defers transmission upon hearing B’s RTS until B could get CTS from A.
• After that C can start transmission to D. For that it first sends an RTS.
• C is not longer exposed to the data transmission of B.
CSMA/CA Collision Avoidance
RTS/CTS is used to reserve channel for
the duration of the packet transmission. This prevents
Access
Point hidden and exposed terminal
problems
Mobile
RTS
CTS
DATA
ACK
ACK is required to understand if the packet
is correctly received (without any collisions ) at the
receiver.
Ethernet does not require ACK to be sent, since the
transmitter can detect the collision on the channel
(cable) without requiring an explicit feedback from the
receiver.
A wireless transmitter can not detect collision,
because:
1) Transmit power is much larger than the received
power: received signal is regarded as noise (not
collision).
2) There could be a hidden terminal
Ethernet CSMA/CD
Only possible in wired or full duplex
CSMA/CD (carrier sense multiple access with collision
detection) media access protocol is used.
Data is transmitted in the form of packets.
Sense channel prior to actual packet transmission.
Transmit packet only if channel is sensed idle; else,
defer the transmission until channel becomes idle.
After packet transmission is started, the node monitors
its own transmission to see if the packet has
experienced a collision.
If the packet is observed to be undergoing a collision,
the transmission is aborted and the packet is
retransmitted after a random interval of time using
Binary Exponential Backoff algorithm.
ECE 4371 Fall 2008
Ethernet Standard
ECE 4371 Fall 2008