Mobile Communications Structure of the Lecture

Transcrição

Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Mobile Communications
Structure of the Lecture
Application Layer
• Service location
• Adaptive applications
• New applications
Transport Layer
• Congestion and flow control
• Quality of Service
Network Layer
• Addressing, routing
• Device location
• Handover
Data Link Layer
• Medium access control
• Multiplexing
• Authentication
Physical Layer
• Frequencies, modulation
• Interferences, attenuation
• Encryption
Chapter 2
• Technical Basics: Layer 1
• Methods for Medium Access: Layer 2
Chapter 3
• Wireless Networks: Bluetooth, WLAN, WirelessMAN, WirelessWAN
• Mobile Networks: GSM, GPRS, UMTS
• Satellites and Broadcast Networks
Chapter 4
• Mobility on the network layer: Mobile IP, Routing, Ad-Hoc Networks
• Mobility on the transport layer: reliable transmission, flow control, QoS
• Mobility support on the application layer
Page 0
Chapter 2.2: Layer 2
Channels
• The whole assigned frequency band is divided somehow in areas (channels) which
are used for different transmissions. To avoid interference between the channels,
guard spaces are needed.
• Static schemes: to avoid collisions of several transmission attempts, a transmission
gets assigned a certain channel exclusively:
Channels ki
k1
k2
k3
k4
k5
k6
Multiplexing in space:
Frequency Division
Multiple Access (FDMA)
c
c
t
Frequency
s1
Frequency
Frequency
Code
f
s2
f
c
...
t
s3
Code Division
Multiple Access (CDMA)
Time Division
Multiple Access (TDMA)
...
t
Goal: multiple use of
a shared medium
Page 1
Multiplexing
Multiplexing in 4 dimensions:
• space (si)
• time (t)
• frequency (f)
• code (c)
• Channels in a frequency band
• Static medium access methods
• Flexible medium access
methods
...
1 2 3 4 1 2 Time
Time
Time
Each transmission is
Each transmission is assigned
All transmissions take place all
a certain sub-band of the whole assigned the whole frequency the time on the whole frequency
band for certain time slots
frequency band
band, but using a code
f
Page 2
Page 3
Frequency Multiplexing
k1
k2
k3
k4
Frequency Division Duplex (FDD)
k5
k6
c
f
t
Principle:
• Separate the whole frequency band
into sub-bands
• One sub-band corresponds to one
channel, assigned to one transmission
• Usable bandwidth depends on the
frequency band and the modulation
method
• Guard spaces between the sub-bands
are necessary to avoid interference of
neighbored channels
FDD is a standard (since early systems of mobile communication) to use frequency
multiplex for a duplex communication
Principle:
• For a duplex communication, two sub-bands are used, one for sending, the other
for receiving (uplink resp. downlink)
• On technical reasons to avoid interference between the sent and the received
signal in the antennas:
– a guard space between the uplink- and the downlink frequencies is needed
(e.g. 45MHz GSM-System)
– two separate antennas have to be used, or a “duplexer” is needed which
consists of two band filters which suppress the unwished signals
Downlink
Disadvantages:
• Waste of bandwidth if the traffic is
distributed unevenly
• Inflexible
Advantages:
• Easy to implement (Radio, TV)
• No coordination necessary
Page 4
890.2 MHz
...
X MHZ
Du
Au
Bu
Cu
...
Frequency
X+45 MHz
Page 5
Advantages:
• No guard spaces
• Throughput high even
for many users
• Low power consumption (deactivation
of sending/receiving device for
unused time slots)
124
123
122
200 kHz
1
124
123
122
Disadvantage:
• Precise synchronization
between all senders
• Guard times between
the time slots
1
t
Cd
Principle:
• A transmission is assigned the whole frequency band exclusively for the
duration of a certain time slot
20 MHz
915 MHz
Bd
Time Multiplexing
f
935.2 MHz
Ad
FDD/FDMA in GSM
960 MHz
Dd
Uplink
Page 6
k1
k2
k3
k4
k5
k6
c
f
t
Page 7
TDMA – Guard Times and Synchronization
TDMA - Guard Times and Synchronization
For avoiding overlapping of the signals of different senders, a precise
synchronization is necessary
Problem: Assume a central base station to which all mobile stations communicate
and which sends out synchronization signals. A mobile station near the base
station can start sending earlier as stations far away, because it receives the
synchronization signal earlier (speed of signal propagation)
Example: Distance mobile station
Example: slot duration in GSM: 577µs
=> guard time of 234µs causes a loss of 40% of transmission capacity!
base station 35 km
=> Duration of synchronization signal:
base station
But: decreasing efficiency, because within guard times no transmission is possible
mobile station:
Solution: Timing Advance: the base station measures the round-trip-times to a
mobile station and instructs stations far away to start sending earlier
=> Reduction of the guard time to 30µs
35 ⋅ 10 3 m
= 117 µs
3 ⋅ 10 8 m / s
round-trip-time: 234µs
Solution: insert a guard time between time slots to compensate the difference in
the round-trip-times
Slot i
Slot i+1
guard
time
Page 8
Time and Frequency Multiplexing
Time Division Duplex (TDD)
Principle:
Combination of FDMA and TDMA
• A channel gets a certain frequency sub-band for certain time slots
• Example: GSM
Advantages:
• Protection against tapping
• Protection against frequency
selective interferences
• High data rates,
adaptation to the traffic
amount is possible
k1
k2
k3
• Only a single carrier frequency
• Separation of downlink and uplink in terms of time
k4
k5
• Flexible: if you need more capacity in the downlink than in the uplink, just
spend larger time slots in the downlink part and give the uplink part
smaller time slots
k6
c
f
Downlink - 1 ms
Dd
• But: precise coordination t
is necessary
Page 9
Ad
2 ms
Bd
Cd
Uplink - 1 ms
Du
Au
Bu
Cu
time
variable sender/receiver separation
Page 10
Page 11
TDD/TDMA - Example DECT
FDD vs. TDD
FDD
TDD
(-) two carriers per connection,
separation between them
(-) efficient symmetry only possible
with two equal bandwidths of the
carriers
Flexibility
417 µs
1 2 3
11 12 1 2 3
Downlink
Uplink
(-) asymmetric connections need
different bandwidths
11 12
t
(-) Duplexer needed (antenna)
Complexity
(-) FDD “wastes” frequencies
(+) easy to implement
Efficiency and
Throughput
Page 12
(+) one carrier per connection
(+) efficient symmetry by using
equal uplink/downlink division
(+) efficient asymmetric connections
with variable borderlines
between uplink and downlink
(+) no duplexer needed
(+) TDD only needs small frequency
bands
(-) synchronization is needed
(+) high spectral efficiency with
QAM modulation
(+) high spectral efficiency with QAM
modulation
(+) two carriers enable a high
throughput in both directions
(+) high capacity
Page 13
Code Multiplex
Code Division Multiplex (CDM)
• For an efficient usage of the bandwidth, the whole frequency band is used by all
transmissions in parallel
• The transmissions are separated by codes
• The code realizes a signal spreading over the whole bandwidth. When choosing
codes, it must be guaranteed that they are “different enough” to separate the
transmissions (orthogonal codes)
• All stations use the same frequency and utilize the whole bandwidth of the
transmission channel simultaneously
• The signal is combined with a pseudo random number (XOR); each sender needs
a unique number
• The receiver is able to reproduce the signal if he knows the sender‘s random
number and performs a correlation function
• Bit rate ri << Bit rate rc (for optical systems: ri/rc ≤ 5000)
Advantages:
• Efficient bandwidth usage
• No coordination/synchronization needed between the stations if appropriate codes
are chosen (misleading – not really completely true)
• Good protection against tapping (military)
• Redundancies protect against interferences
Disadvantages:
• More complex signal regeneration
• Lower user data rates by coding the data
Signal 1
Bi trate ri
X
Code A
Code A
Bit rate rc
+
Bit rate rc
Mixing of the data streams
on one carrier frequency
Signal n
Bit rate ri
Page 14
X
X
X
Code B
Code B
Page 15
CDMA - Computation
CDMA – on Signal Level I
Sender A
• Sends Ad = 1, Code Ak = 010011 (set: „0“= -1, „1“= +1)
• Sent signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)
data A
Remark: the sequences of the form
“(- 1, +1, - 1, - 1, +1, +1)” are called
chip sequences
Receiver wants to receive transmission of A:
• perform code Ak bitwise (inner produkt)
(-2, 0, 0, -2, +2, 0) · Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6
Result is larger than 0, sent bit must have been a „1“
• Analogously for B
(-2, 0, 0, -2, +2, 0) · Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, thus a „0“
Ad
1
chip sequence A 0
1
0
1
0
0
1
0
0
0
1
0
1
1
0
0
1
1
data ⊕ code 1
0
1
0
1
1
1
0
0
0
1
0
0
0
1
1
0
0
Ak
As
signal A
Real systems use much longer chip sequences resulting in a larger distance
between single code words in code space
Page 16
Page 17
CDMA - on Signal Level II
CDMA - on Signal Level III
As
signal A
data B
0
code A
Sender B
• Sends Bd = 0, Code Bk = 110101 (set: „0“= -1, „1“= +1)
• Sent signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)
Both signals interfere in the air:
• As + Bs = (-2, 0, 0, -2, +2, 0)
1
1
0
0
data A
Bd
As + Bs
Bk
Ak
Bs
(As + Bs) * Ak
1
0
1
1
0
1
Ad
code B
chip sequence B 0
0
0
1 1
0
1
0
1 0
0
0
0
1 0
1
1
1
data ⊕ code 1
1
1
0 0
1
1
0
1 0
0
0
0
1 0
1
1
1
signal B
integrator
output
comparator
output
As + Bs
Page 18
Page 19
CDMA - on Signal Level IV
data B
CDMA - on Signal Level V
1
0
Bd
0
As + Bs
As + Bs
wrong
code K
Bk
(As + Bs) * K
(As + Bs) * Bk
integrator
output
comparator
output
1
0
integrator
output
comparator
output
0
Page 20
(0)
?
Page 21
CDMA - Example
CDMA - Example
Principle:
• Each station is assigned an 8 bit chip sequence *)
• For transferring a 1 it sends its chip sequence, for transferring a 0 it sends its complement
• Bipolar notation with 0 as -1 and 1 as +1
• All chip sequences are pairwise orthogonal,
i.e. if S and T are orthogonal chip sequences (S ≠ T), then holds:
1 m
S • T ≡ ∑ S i Ti = 0 , S • T = 0 ,
m i=1
On the receiver side:
• For “filtering out” the bit stream of a certain station, the receiver must know the
chip sequence of this station
• For extracting the bits of the station with chip sequence C from the received
sequence E, it computes E • C
• For example:
E = A +B +C
S•S =1
⇒ E • C = ( A + B + C) • C = A • C + B • C + C • C = 0 + 0 + 1= 1
Result of orthogonal codes
• If two or more stations are sending at the same time, their signals are added linear
Example transmissions: (in each case exactly one bit is transferred)
Chip sequences of four
stations:
- -1C
E1 = (-1 +1 -1 +1 +1 +1 -1 -1)
A: (- 1 - 1 - 1 +1 +1 - 1 +1 +1)
-11B+C
E2 = (-2 0 0 0 +2 +2 0 -2)
B: (- 1 - 1 +1 - 1 +1 +1 +1 - 1)
10- A +B
E3 = ( 0 0 -2 +2 0 -2 0 +2)
C: (- 1 +1 - 1 +1 +1 +1 - 1 - 1)
101A+B+ C
E4 = (-1 +1 -3 +3 -1 -1 -1 +1)
1 1 1 1 A + B + C + D E5 = (-4 0 -2 0 +2 0 +2 -2)
D: (- 1 +1 - 1 - 1 - 1 - 1 +1 - 1)
1101
*) simplified example, normally at least 10 bits
(0)
A + B + C + D E6 = (-2 -2 0 -2 0 -2 +4 0)
Ei = transferred chip sequence for case i
Page 22
For the six example transmissions one receives:
i.e. station transmits 1
E1 • C = (1 + 1 + 1 + 1 + 1 + 1 + 1 + 1) / 8 = 1
E 2 • C = ( 2 + 0 + 0 + 0 + 2 + 2 + 0 + 2) / 8 = 1
E3 • C = (0 + 0 + 2 + 2 + 0 − 2 + 0 − 2) / 8 = 0
E4 • C = (1 + 1 + 3 + 3 + 1 − 1 + 1 − 1) / 8 = 1
E5 • C = ( 4 + 0 + 2 + 0 + 2 + 0 − 2 + 2) / 8 = 1
E6 • C = ( 2 − 2 + 0 − 2 + 0 − 2 − 4 + 0) / 8 = −1
station transmits 1
station does not transmit anything
station transmits 1
station transmits 1
station transmits 0
Page 23
Spread Spectrum Technology
Effects of Spreading and Interference
Principle:
CDMA can also deal with one problem of radio transmission: frequency dependent
fading can wipe out narrow band signals for duration of the interference
• Use of a bandwidth which is much larger than the one of the modulated signal
• Spread of the signal over the complete bandwidth using a pseudo random sequence
power
Original signal of a sender: Usage of
a narrow frequency band
f
power
power
interference
spread
signal
power
detection at
receiver
f
Spreading of the signal to a broad frequency band;
at the same time, the signal power is distributed
signal
spread
interference
f
power
f
During transmission,
narrowband and
broadband interference
occur
That means: spread a narrow band signal into a broad band signal using a special
code to protect against narrow band interference
Page 24
f
Page 25
Effects of Spreading and Interference
Spreading and Frequency-selective Fading
channel
quality
power
The receiver reconstructs the “disturbed” signal; as a
result the narrowband interference is spreaded
1
2
5
3
6
4
f
frequency
narrow band
signal
power
Using a band pass filter, those parts of the signal can
be removed which are outside the bandwidth of the
original signal
channel
quality
1
spread
spectrum
Page 26
narrowband channels:
Signal quality depends on
carrier frequency!
guard space
f
User signal
Broadband interference
Narrowband interference
2
2
2
2
2
spread spectrum channels:
Uniform quality for all
transmissions
frequency
Page 27
Spread Spectrum Technology
Direct Sequence
Original signal
+1
-1
Frequency
1 chip
1 code word
+1
-1
Code sequence
+1
-1
Spread signal
Power
Advantages:
• Several signals can be transferred without coordination within the same bandwidth at
the same time
• Small susceptibility for effects of the multipath transmission: due to the high
bandwidth in any case only a small part of the frequency spectrum is affected, so that
the typical signal weakenings are weaker than with narrow-band systems
• Small influence of environmental disturbances
• Existence of transmissions (and as a result their decoding) is difficult to detect (of
special relevance for military systems)
Power
1 bit
Frequency
• Division of the signal into redundant information units (chips), the transmitter sends
several (at least 10) bits for one bit of information
• Both, sender and receiver must know the chip sequence (code)
Procedures:
• Direct Sequence Spread Spectrum (DSSS)
• Frequency Hopping Spread Spectrum (FHSS)
• Spreading, i.e. distribution of the chips over a large bandwidth
• For other users, the transmission appears to be background noise
• Re-establishment of the original, possibly disturbed signal is possible due to the
redundancy
Page 28
Frequency Hopping
Principle:
• Doubled modulation:
a. Modulation of the data to a spread wide-band signal
b. Modulation of this signal to the carrier frequency
• The receiver processes the inverted procedure with identical chip sequences
• Integration over the bit period
60
Frequency
40
20
Frequency
Hopping signal
0
user
data
X
modulator
chipping
sequence
radio
carrier
transmitter
2
3
4
5
6
7
8
Time
Power
Frequency
1
transmit
signal
Original signal
Power
Direct Sequence
spread
spectrum
signal
Page 29
Frequency
correlator
received
signal
lowpass
filtered
signal
demodulator
radio
carrier
• Carrier frequency is changed in certain time intervals in accordance to a code
sequence (synchronous change of the frequency by sender and receivers)
products
X
integrator
• Frequency hops of the signal in fixed times of approx. 20-400 ms
• Collisions are possible, if two or more senders use by coincidence the same
frequency. Therefore suitable codes must be used.
chipping
sequence
sampled
sums
receiver
• Interference are limited to short periods, simple implementation
• Not as robust as DSSS, easier to tap
decision
data
Page 30
Page 31
Frequency Hopping
Frequency Hopping
Two variants:
• Fast change (fast hopping): several frequencies per bit
• Slow change (slow hopping): several bits per frequency
Principle:
• Vary the carrier frequency in discrete levels:
Level sequences are determined by pseudo random sequence
Receiver must use identical sequence
• Two categories regarding the number of transferred bits per level:
Max. one bit: Fast Frequency Hopping
Several bits: Slow Frequency Hopping
user
data
spread
signal
narrowband
signal
modulator
transmitter
received
signal
Data
narrowband
signal
data
modulator
demodulator
frequency
synthesizer
frequency
synthesizer
demodulator
receiver
hopping
sequence
hopping
sequence
Page 32
Space Division Multiple Access (SDMA)
Frequency Planning
• Space Multiplexing means: base stations serve a certain space (cell) only,
different base stations have a distance large enough to supply different regions
• Mobile stations communicate only with the base station in range
• Advantages of a cell structure:
The same frequencies can be used in different cells for different users, i.e.
higher capacity, higher number of users possible
Less transmission power
More robust against break-down
Propagation of signals is (relatively) easy to handle
• Problems:
Fixed network needed for connecting base stations
Handover (changing from one cell to another) necessary
Interference with neighbored cells at cell borders – avoid using the same
frequencies in neighbored cells!
• Cell sizes from 300 m in cities to 35 km on the country side for GSM – even less
for higher frequencies, e.g. 10 – 30 m for Wireless LAN
Page 33
Page 34
Frequency reuse only with a certain distance between the base stations
Standard model using 7 frequencies:
k3
k5
k4
k2
k6
k1
k3
An area in which all frequencies are used, is called cluster
• Fixed frequency assignment:
Certain frequencies are assigned to a certain cell
Problem: different traffic load in different cells
k5
k4
k7
k1
k2
• Dynamic frequency assignment:
Base station chooses frequencies depending on the frequencies already
used in neighbor cells
More capacity in cells with more traffic
Assignment can also be based on interference measurements
Page 35
Frequency Planning
f3
f3
f2
f1
f1
f1
f3
f2
f1
f3
• CDM systems: cell size depends on current load
• Additional traffic appears as noise to other users
• If the noise level is too high users drop out of cells
f3
f2
f3
f2
Cell Breathing
f1
f3
3 cell cluster
f2
f3
f2
f3
f5
f4
7 cell cluster
f6
f1
f3
f2
f2
f2
f
f1 f
f
h2 1 f3
h2 1 f3
3
h
h
g2 1 h3 g2 1 h3
g2
g1
g1
g1
g3
g3
g3
f5
f4
f7
f1
f3
f2
f6
f7
f2
f5
f2
3 cell cluster with
3 sector antennas
Page 36
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
Channel Access
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
Page 37
Comparison of SDMA/TDMA/FDMA/CDMA
Approach
Idea
Page 38
• FDMA and simple TDMA are in general to inflexible for data communication
• CDMA is very complex
• DSSS and FHSS do no access control, but increase the robustness of a
transmission
• Maybe we could use proofed mechanisms from data communication in fixed
networks instead of or in combination with static mechanisms?
Let’s try for Ethernet: CSMA/CD
• Simply send if the medium is free, recognize if a collision occurs
Problem in wireless networks
• The signal strength decreases at least quadratic with the distance
• CS/CD are used by the sender, but collisions occur at the receiver
• Possibly, the sender can’t recognize this collision, i.e. CD fails
• Furthermore, CS can fail if a terminal is to far away (Hidden Station)
Page 39
Hidden Station and Exposed Station
Further Problem with CSMA/CD:
Power Control
Hidden Station
• 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
Terminals A and B send, C receives
• The signal strength decreases
proportionally with the square
of the distance
• Therefore, the signal from terminal
B drowns out A’s signal
• C is not able to receive A
• Precise power control needed!
Exposed Station
• B sends to A, C wants to send to D
• C has to wait – CS signals that the
medium is in use
• A is outside the radio range of C –
waiting is not necessary!
• C is “exposed” to B
A
B
C
B
C
But: is it possible to design variants for wireless networks?
A
B
C
D
Page 40
Page 41
Aloha/Slotted Aloha
DAMA - Demand Assigned Multiple Access
Mechanism
• Random, distributed (no central arbiter), time-multiplex
• Slotted Aloha additionally uses time slots, sending must always start at
slot boundaries
Aloha
A
• Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson
distribution for packet arrival and packet length)
• Reservation can increase efficiency to 80%
A sender reserves a future time slot
Sending within this reserved time slot is possible without collision
Reservation also causes higher delays
Typical scheme for satellite links
Collision
Sender A
Sender B
Sender C
• Examples for reservation algorithms:
Explicit Reservation according to Roberts (Reservation-ALOHA)
Implicit Reservation (PRMA)
Reservation-TDMA
t
Slotted Aloha
Collision
Sender A
Sender B
Sender C
t
Page 42
Page 43
Access Method DAMA: Explicit Reservation
Access Method DAMA: PRMA
Explicit Reservation (Reservation Aloha):
• Two modes:
ALOHA mode for reservation:
competition for small reservation slots, collisions possible
Reserved mode for data transmission within successful reserved slots (no
collisions possible)
• It is important for all stations to keep the reservation list consistent at any point in
time and, therefore, all stations have to synchronize from time to time
Implicit reservation (PRMA - Packet Reservation MA):
• A certain number of slots form a frame, frames are repeated
• Stations compete for empty slots according to the slotted aloha principle
• Once a station reserves a slot successfully, this slot is automatically assigned to
this station in all following frames as long as the station has data to send
• Competition for this slots starts again as soon as the slot was empty in the last
frame
Reservation Time slot:
ACDABA-F
Collision
AC-ABA-A---BAF-
Aloha
Reserved
Aloha
Reserved
Aloha
Reserved
A---BAFD
Aloha
Page 44
frame 2
A C
frame 3
A
B A F
frame 4
A
B A F D
frame 5
A C E E B A F D
A B A
collision at
reservation
attempts
t
Page 45
MACA – Collision Avoidance
Reservation Time Division Multiple Access
•Every frame consists of N mini-slots and x data-slots
•Every station has its own mini-slot and can reserve up to k data-slots
using this mini-slot (i.e. x = N * k).
•Other stations can send data in unused data-slots according to a roundrobin sending scheme (best-effort traffic)
reservations
for data-slots
1 2 3 4 5 6 7 8
A C D A B A
F
Access Method DAMA: Reservation-TDMA
N mini-slots
frame 1
N * k data-slots
MACA (Multiple Access with Collision Avoidance) uses short signaling packets for
collision avoidance
•RTS (request to send): a sender request the right to send from a receiver with a
short RTS packet before it sends a data packet
•CTS (clear to send): the receiver grants the right to send as soon as it is ready to
receive
Signaling packets contain
•sender address
•receiver address
•packet size
e.g. N=6, k=2
Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed
Foundation Wireless MAC)
other stations can use free data-slots
based on a round-robin scheme
Page 46
Page 47
MACA – Examples
MACA avoids the problem of hidden stations
• A and C want to
send to B
• A sends RTS first
• C waits after receiving
CTS from B
Polling Mechanisms
• If one terminal can be heard by all others, this “central” terminal (a.k.a. base
station) can poll all other terminals according to a certain scheme
Now all schemes known from fixed networks can be used (typical mainframe terminal scenario)
RTS
CTS
A
MACA avoids the problem of exposed terminals
• B wants to send to A, C
to some other terminal
• C does not wait unnecessarily
because it cannot receive
CTS from A
A
• Example: Randomly Addressed Polling
Base station signals readiness to all mobile terminals
Terminals ready to send can now transmit a random number without collision
with the help of CDMA or FDMA (the random number can be seen as dynamic
address)
The base station now chooses one address for polling from the list of all
random numbers (collision if two terminals choose the same address)
The base station acknowledges correct packets and continues polling the next
terminal
This cycle starts again after polling all terminals of the list
CTS
B
C
RTS
RTS
CTS
B
C
Page 48
Page 49
ISMA (Inhibit Sense Multiple Access)
Other Mechanisms with Busy Tone
Current state of the medium is signaled via a “busy tone”
• The base station signals on the downlink (base station to terminals) if the
medium is free or not
• Terminals must not send if the medium is busy
• Terminals can access the medium as soon as the busy tone stops
• The base station signals collisions and successful transmissions via the busy
tone and acknowledgements, respectively (media access is not coordinated
within this approach)
Page 50
Collision avoidance by Out-of-Band Signaling
• Use an additional channel for signaling information
Busy Tone Multiple Access (BTMA)
• Each station hearing an ongoing transmission on the data channel sends a “busy
tone” on the additional control channel
• All terminals in the range of 2 hops of a sending terminal will wait
• No hidden stations, but many exposed stations
Receiver initiated Busy Tone Multiple Access (RI-BTMA)
• Only the receiver sends “busy tone”
• Nearly no exposed stations, but the busy tone only can be sent when the receiver
has decoded the transmission request
Wireless Collision Detect (WCD) Protocol
• Combination of BTMA and RI-BTMA: two types of “busy tones”
• First like BTMA: terminals send a busy tone “collision detect”
• After recognizing a transmission request, the receiver sends a “feedback-tone”, the
other terminals stop the “collision detect” busy tone
Page 51
SAMA - Spread Aloha Multiple Access
Conclusion
• Aloha has only a very low efficiency, CDMA needs complex receivers to be able
to receive different senders with individual codes at the same time
• Idea: use spread spectrum with only one single code (chipping sequence) for
spreading for all senders accessing according to aloha
Layer 1
• Common modulation techniques GMSK, QPSK, QAM for all wireless networks
• Signal propagation and robustness depend on the frequency band
Collision
Sender A
Sender B
1
0
0
1
1
narrow band
1
send for a
shorter period
with higher power
Spread the signal, e.g. using the chipping sequence 110101 („CDMA without CD“)
t
Layer 2
• Static access methods TDMA/FDMA/CDMA: suitable for voice transmission
because fixed capacities can be assigned
• Dynamic access methods for data communication exist in several variants: with
reservation, using special signaling packets, polling, busy tones on additional
channels, …
• Additionally: CDMA techniques (DSSS, FHSS) can increase the robustness of a
transmission – this can be combined with dynamic access methods!
Problem: find a chipping sequence with good characteristics!
Page 52
Page 53

Mobile Communications Structure of the Lecture

Transcrição

Documentos relacionados

Layer 1 – Physical Layer Transmission Media Twisted Pair

Wireless Broadband Networks 802.16

Mobile Communications

Mobile Communications

Chapter 2 - Informatik 4

History of Optical Disks Basic Technology Basic

Limitations of Object-Based Middleware Components

On the Way to Today`s Internet ARPANET ARPANET

Metropolitan Area Networks Resilient Packet Ring

Data Encryption Standard (DES) General Structure of

das institut für südasien-, tibet- und

Transfer and Control Protocols Standards of ITU H.261

Name of Module: Biometric Identification Credit Points (ECTS):3

Wireless Ethernet Wireless LANs: Design Goals

Mangamarkt Anmeldung