media access schemes - tu- · pdf filemaca, polling, etc. cdma theory and practice ... also...
TRANSCRIPT
TECHNISCHE UNIVERSITÄTILMENAU
Inte
grat
ed H
ard
and
Softw
are
Syst
ems
http
://w
ww
.tu-il
men
au.d
e/ih
sMedia Access Schemes
Motivationlimits of CSMA/CDhidden and exposed terminalsnear-far problem
SDMA, FDMA, TDMA overviewTDMA
Aloha, slotted AlohaDemand Assigned Multiple Access (DAMA)Multiple Access with Collision Avoidance: MACA, Polling, etc.
CDMA theory and practiceSAMA (Spread Aloha Multiple Access)Comparison
Wireless Internet 2Andreas Mitschele-Thiel 6-Apr-06
Media Access: Motivation
The problem: multiple users compete for a common, shared resource (medium)
Can we apply media access methods from fixed networks?
Example: CSMA/CDCarrier Sense Multiple Access with Collision Detection (IEEE 802.3)send as soon as the medium is free (carrier sensing – CS)listen to the medium, if a collision occurs stop transmission and jam (collision detection – CD)
Problems in wireless networkssignal strength decreases (at least) proportional to the square of the distancethe sender would apply CS and CD, but the collisions happen at the receiverit might be the case that a sender cannot “hear” the collision, i.e., CD does not workfurthermore, CS might not work if, e.g., a terminal is “hidden”
Wireless Internet 3Andreas Mitschele-Thiel 6-Apr-06
Hidden terminalsA sends to B, C cannot receive A C wants to send to B, C senses a “free” medium -> CS failscollision at B, A cannot receive C -> CD failsA is “hidden” for C
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 usebut A is outside the radio range of C, therefore waiting is not necessaryC is “exposed” to B
Motivation - hidden and exposed terminals
BA C
BA C
Wireless Internet 4Andreas Mitschele-Thiel 6-Apr-06
Terminals A and B send, C receivessignal strength decreases proportional to the square of the distancethe signal of terminal B therefore drowns out A’s signalC 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!
Motivation - near and far terminals
A B C
Wireless Internet 5Andreas Mitschele-Thiel 6-Apr-06
Access methods SDMA/FDMA/TDMA
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 receiverpermanent (e.g., radio broadcast), slow hopping (e.g. GSM), fasthopping (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 previously are now used to control medium access!
Wireless Internet 6Andreas Mitschele-Thiel 6-Apr-06
Communication link types
Each terminal needs an uplink and a downlink channel
Types of communication links:
Simplex unidirectional link transmission
Half DuplexBi-directional (but not simultaneous)
Duplex simultaneous bi-directional link transmission, two types:
Frequency division duplexing (FDD)Time division duplexing (TDD)
downlink
uplink
Wireless Internet 7Andreas Mitschele-Thiel 6-Apr-06
Duplex modes
Frequency Division Duplex (FDD)
Separate frequency bands for up- and downlink
+ separation of uplink and downlink interference
- no support for asymmetric traffic
Examples: UMTS, GSM, IS-95, AMPS
Fd
Fu
TdTu
TdTu
Time Division Duplex (TDD)
Separation of up- and downlink traffic on time axis
+ support for asymmetric traffic
- mix of uplink and downlink interference on single band
Examples: DECT, WLAN, UMTS (TDD)
Wireless Internet 8Andreas Mitschele-Thiel 6-Apr-06
FDD/FDMA - general scheme, example GSM
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz
Wireless Internet 9Andreas Mitschele-Thiel 6-Apr-06
TDD/TDMA - general scheme, example DECT
1 2 3 11 12 1 2 3 11 12
tdownlink uplink
417 µs
Wireless Internet 10Andreas Mitschele-Thiel 6-Apr-06
Mechanismrandom, distributed (no central arbiter), time-multiplexSlotted Aloha additionally uses time-slots, sending must always start at slot boundaries
Aloha
Slotted Aloha
TDMA: Aloha/slotted aloha
sender A
sender B
sender C
collision
sender A
sender B
sender C
collision
t
t
Wireless Internet 11Andreas Mitschele-Thiel 6-Apr-06
TDMA: Demand Assigned Multiple Access (DAMA)
Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution of packet arrivals and packet lengths)
Reservation can increase efficiency to 80%a sender reserves a future time-slotsending within this reserved time-slot is possible without collisionreservation also causes higher delaystypical scheme for satellite links (long round-trip-times)application to packet data, e.g. in GPRS and UMTS
Examples for reservation algorithms:Explicit Reservation (Reservation-ALOHA)Implicit Reservation (PRMA)Reservation-TDMA
Wireless Internet 12Andreas Mitschele-Thiel 6-Apr-06
TDMA: DAMA - Explicit Reservation
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)
synchronisation: 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
Aloha reserved Aloha reserved Aloha reserved Aloha
collision
t
Wireless Internet 13Andreas Mitschele-Thiel 6-Apr-06
TDMA: DAMA – Packet Reservation (PRMA)
Implicit reservation (PRMA - Packet Reservation MA):a certain number of slots form a frame, frames are repeatedstations compete for empty slots according to the slotted aloha principleonce 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 sendcompetition for this slots starts again as soon as the slot was empty in the last frame
1 2 3 4 5 6 7 8 time-slot
frame2 A C A B A
frame3 A B A F
frame4 A B A F D
frame5 A C E E B A F Dt
frame1 A C D A B A FACDABA-F
ACDABA-F
AC-ABAF-
A---BAFD
ACEEBAFD
reservations
collision at reservation attempts
ACEEBAFD
New successful reservation attempts are in bold letters
Wireless Internet 14Andreas Mitschele-Thiel 6-Apr-06
TDMA: DAMA - Reservation-TDMA
Reservation Time Division Multiple Access every frame consists of N mini-slots and x data-slotsevery 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 round-robin sending scheme (best-effort traffic)
N mini-slots N * k data-slots
reservationsfor data-slots
other stations can use free data-slotsbased on a round-robin scheme
e.g. N = 6 stations, k = 2 data slots per station
Advantage: (small) guaranteed bandwidth with small latency for each station
Disadvantages: fixed number of stations (mini slots); global coordination
Wireless Internet 15Andreas Mitschele-Thiel 6-Apr-06
TDMA: Multiple Access with Collision Avoidance (MACA)
Motivation: deal with hidden terminals without a base station (central controller)
MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance
RTS (request to send): a sender requests the right to send from a receiver with a short RTS packet before it sends a data packetCTS (clear to send): the receiver grants the right to send as soon as it is ready to receiveall other stations listen to the signal
Signaling packets containsender addressreceiver addresspacket size
Collision may occur during transmission of RTS signal only but this is small compared to the data transmission
Variants of this method can be found in IEEE 802.11 (DFWMAC – Distributed Foundation Wireless MAC)
Wireless Internet 16Andreas Mitschele-Thiel 6-Apr-06
MACA avoids the problem of hidden terminalsA and C want to send to BA sends RTS firstC waits after receiving CTS from B
MACA avoids the problem of exposed terminalsB wants to send to A, C to another terminalB sends RTS, A replies with CTSC does not receive CTSfrom A
=> C concludes that it is not within receiving range of AC can start its transmission
Disadvantage:overhead where data packets are small
TDMA: MACA examples
A B C
RTS
CTSCTS
A B C
RTS
CTS
RTS
Wireless Internet 17Andreas Mitschele-Thiel 6-Apr-06
TDMA: MACA variant: DFWMAC in IEEE 802.11
idle
wait for the right to send
wait for ACK
sender receiver
packet ready to send; RTS
time-out; RTS
CTS; data
ACK
RxBusy
idle
wait fordata
RTS; RxBusy
RTS; CTS
data; ACK
time-out ∨incorrect data; NAK
ACK: positive acknowledgementNAK: negative acknowledgement
RxBusy: receiver busy
time-out ∨NAK;RTS
Simplified state machine
Wireless Internet 18Andreas Mitschele-Thiel 6-Apr-06
TDMA: Polling mechanisms
If one terminal can be heard by all others, this “central” terminal (e.g. 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, round-robin, random, reservation-based)
Example: Randomly Addressed Pollingbase station signals readiness to all mobile terminalsterminals ready to send can now transmit a random number withoutcollision 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 received random numbers (collision if two terminals choose the same address) the base station acknowledges correct packets and continues polling the next terminalthis cycle starts again after polling all terminals of the list
Application to Bluetooth and 802.11 (possible access function)
Wireless Internet 19Andreas Mitschele-Thiel 6-Apr-06
TDMA: ISMA (Inhibit Sense Multiple Access)
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 stopsthe base station signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach)mechanism used, e.g. for CDPD (AMPS)
Wireless Internet 20Andreas Mitschele-Thiel 6-Apr-06
CDMA access method
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 numberthe receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function
Advantages: all terminals can use the same frequency, less planning neededhuge code space (e.g. 232) compared to frequency spaceinterference (e.g. white noise) is not codedforward error correction and encryption can be easily integrated
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 (power control)
Wireless Internet 21Andreas Mitschele-Thiel 6-Apr-06
CDMA principle
Code 0
Code 1
Code 2
Σ
data 0
data 1
data 2
Code 0
Code 1
Code 2
data 0
data 1
data 2
sender (base station) receiver (terminal)
Transmission viaair interface
Wireless Internet 22Andreas Mitschele-Thiel 6-Apr-06
CDMA by example
Source 2
Source 1
data stream A & B
Code 2
Code 1
spreading
Source 2 spread
Source 1 spread
spreaded signal
Wireless Internet 23Andreas Mitschele-Thiel 6-Apr-06
CDMA by example
Sum of Sources Spread
+
overlay of signals
Sum of Sources Spread + Noise
transmission and distortion (noise and interference)
Despread Source 2
Despread Source 1
decoding and despreading
Wireless Internet 24Andreas Mitschele-Thiel 6-Apr-06
CDMA in theory
Sender A
sends Ad = 1, key Ak = 010011 (i.e. -1 1 -1 -1 1 1)
sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)
Sender B
sends Bd = 0, key Bk = 110101 (i.e. 1 1 -1 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“
Wireless Internet 25Andreas Mitschele-Thiel 6-Apr-06
CDMA on signal level I
data A
key A
signal A
data ⊕ key
keysequence A
Real systems use much longer keys resulting in a larger distancebetween single code words in code space
1 0 1
10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 101 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0
Ad
Ak
As
Wireless Internet 26Andreas Mitschele-Thiel 6-Apr-06
CDMA on signal level II
signal A
data B
key Bkey
sequence 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 1 111 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1
Bd
Bk
Bs
As
10-1
Wireless Internet 27Andreas Mitschele-Thiel 6-Apr-06
CDMA on signal level III
Ak
(As + Bs) * Ak
integratoroutput
comparatoroutput
As + Bs
data A
1 0 1
1 0 1 Ad
10-1
1
-1
10-1
Wireless Internet 28Andreas Mitschele-Thiel 6-Apr-06
CDMA on signal level IV
integratoroutput
comparatoroutput
Bk
(As + Bs) * Bk
As + Bs
data B
1 0 0
1 0 0 Bd
10-11
-110-1
Wireless Internet 29Andreas Mitschele-Thiel 6-Apr-06
comparatoroutput
CDMA on signal level V
wrongkey K
integratoroutput
(As + Bs) * K
As + Bs
(0) (0) ?
Assumptionsorthogonality of keysneglectance of noiseno differences in signal level => precise power control
10-11
-1
10-1
Wireless Internet 30Andreas Mitschele-Thiel 6-Apr-06
MotivationAloha has a very low efficiencyCDMA 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
Spread Aloha Multiple Access (SAMA)
1sender A0sender B
01
t
narrowband
send for a shorter periodwith higher power
spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“)
Problem: find a chipping sequence with good characteristics• good autocorrelation for ϕ=0• low autocorrelation for cases where phase ϕ differs from 0
11
collision
Wireless Internet 31Andreas Mitschele-Thiel 6-Apr-06
Comparison SDMA/TDMA/FDMA/CDMA
Approach SDMA TDMA FDMA CDMA Idea segment space into
cells/sectors segment sending time into disjoint time-slots, demand driven or fixed patterns
segment the frequency band into disjoint sub-bands
spread the spectrum using orthogonal codes
Terminals only one terminal can be active in one cell/one sector
all terminals are active for short periods of time on the same frequency
every terminal has its own frequency, uninterrupted
all terminals can be active at the same place at the same moment, uninterrupted
Signal separation
cell structure, directed antennas
synchronization in the time domain
filtering in the frequency domain
code plus special receivers
Advantages very simple, increases capacity per km²
established, fully digital, flexible
simple, established, robust
flexible, less frequency planning needed, soft handover
Dis-advantages
inflexible, antennas typically fixed
guard space needed (multipath propagation), synchronization difficult
inflexible, frequencies are a scarce resource
complex receivers, needs more complicated power control for senders
Comment only in combination with TDMA, FDMA or CDMA useful
standard in fixed networks, together with FDMA/SDMA used in many mobile networks
typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse)
higher complexity, typically integrated with FDMA
Wireless Internet 32Andreas Mitschele-Thiel 6-Apr-06
References
Jochen Schiller: Mobile Communications (German and English), Addison-Wesley, 2000(most of the material covered in this chapter is based on the book)
Ramjee Prasad, Marina Ruggieri: Technology Trends in Wireless Communications, Artech House, 2003