1. UNIT II Wireless Medium Access Control
Prof. Dr.-Ing. Jochen H. Schiller MC

2. Spread spectrum technology
Universität Karlsruhe
Institut für Telematik
Spread spectrum technology
Mobilkommunikation
SS 1998
Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal using a special code
protection against narrow band interference
Direct Sequence Spread Spectrum (DSSS),
Frequency Hopping Spread Spectrum (FHSS)
signal
power
interference
spread signal
power
spread
interference
detection at
receiver f f
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

3. Effects of spreading and interference
Universität Karlsruhe
Institut für Telematik
Effects of spreading and interference
Mobilkommunikation
SS 1998
dP/df
dP/df
user signal
broadband interference
narrowband interference i)
ii) f f
sender
dP/df
dP/df
dP/df
iii)
iv) v) f f f
receiver
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

4. Spreading and frequency selective fading
Universität Karlsruhe
Institut für Telematik
Spreading and frequency selective fading
Mobilkommunikation
SS 1998
channel quality 2 1 5 6
narrowband channels 3 4
frequency
narrow band signal
guard space 2
frequency
channel quality 1
spread spectrum
spread spectrum channels
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

5. DSSS (Direct Sequence Spread Spectrum)
Universität Karlsruhe
Institut für Telematik
DSSS (Direct Sequence Spread Spectrum)
Mobilkommunikation
SS 1998
XOR of the signal with pseudo-random number (chipping sequence)
many chips per bit (e.g., 128) result in higher bandwidth of the signal tb
user data 1
XOR
User signal with 1MHz spreading with 11 bit chipping sequence || V
11 MHz spread spectrum signal tc
chipping
sequence 1 1 1 1 1 1 1 1 =
resulting
signal 1 1 1 1 1 1 1
tb: bit period
tc: chip period
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

6. Example Receiver – example 1 User data 01
Chipping sequence
Spread Signal
First bit – Sum of product = 0 (binary 0)
second bit – Sum of product = 11 (binary 1)
original data 01 can be recovered.
Receiver – example 2
Spread Signal
First bit – Sum of product = ?
second bit – Sum of product =?

7. DSSS (Direct Sequence Spread Spectrum)
Universität Karlsruhe
Institut für Telematik
DSSS (Direct Sequence Spread Spectrum)
Mobilkommunikation
SS 1998
spread
spectrum
signal
transmit
signal
user data X
modulator
chipping
sequence
radio
carrier
transmitter
User signal with 1MHz spreading with 11 bit chipping sequence – 11 MHz spread spectrum signal. Radio carrier with 2.4 GHz shift this signal to a carrier frequency.
correlator
lowpass
filtered
signal
sampled
sums
products
received
signal
data
demodulator X
integrator
decision
radio
carrier
chipping
sequence
receiver
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

8. Advantages and Disadvantages of DSSS
reduces frequency selective fading
in cellular networks
base stations can use the same frequency range
several base stations can detect and recover the signal
soft handover
Disadvantages
precise power control necessary

9. FHSS (Frequency Hopping Spread Spectrum)
Universität Karlsruhe
Institut für Telematik
FHSS (Frequency Hopping Spread Spectrum)
Mobilkommunikation
SS 1998
Discrete changes of carrier frequency
sequence of frequency changes determined via pseudo random number sequence
Two versions
Fast Hopping: several frequencies per user bit
Slow Hopping: several user bits per frequency
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

10. Advantages and Disadvantages of FHSS
frequency selective fading and interference limited to short period
simple implementation
uses only small portion of spectrum at any time
Disadvantages
not as robust as DSSS
simpler to detect

11. FHSS (Frequency Hopping Spread Spectrum)
Universität Karlsruhe
Institut für Telematik
FHSS (Frequency Hopping Spread Spectrum)
Mobilkommunikation
SS 1998 tb
user data 1 1 1 t f td f3
slow
hopping
(3 bits/hop) f2 f1 t td f f3
fast
hopping
(3 hops/bit) f2 f1 t
tb: bit period td: dwell time
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

12. FHSS (Frequency Hopping Spread Spectrum)
Universität Karlsruhe
Institut für Telematik
FHSS (Frequency Hopping Spread Spectrum)
Mobilkommunikation
SS 1998
narrowband
signal
spread
transmit
signal
user data
modulator
modulator
frequency
synthesizer
hopping
sequence
transmitter
narrowband
signal
received
signal
data
demodulator
demodulator
hopping
sequence
frequency
synthesizer
receiver
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

13. Medium Access Control (MAC) Layer
Data Link Layer
Logical Link Control Layer
MAC Layer
To establish a reliable point to point or point to multi-point connection between different devices over a wired or wireless medium.

14. Motivation Example CSMA/CD Problems in wireless networks
Carrier Sense Multiple Access with Collision Detection
send as soon as the medium is free, listen into the medium if a collision occurs.
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”
Prof. Dr.-Ing. Jochen H. Schiller MC

15. Hidden and exposed 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
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, therefore waiting is not necessary
C is “exposed” to B A B C
Prof. Dr.-Ing. Jochen H. Schiller MC

16. 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
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! A B C
Prof. Dr.-Ing. Jochen H. Schiller MC

17. Access methods SDMA/FDMA/TDMA
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
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 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
Prof. Dr.-Ing. Jochen H. Schiller MC
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 15

18. FDMA example GSM f t 960 MHz 200 kHz 935.2 MHz 915 MHz 890.2 MHz 124 1
Prof. Dr.-Ing. Jochen H. Schiller MC

19. TDMA – DECT(Digital Enhanced Cordless Telecommunications)1 2 3 11 12 1 2 3 11 12 t
downlink
uplink
Prof. Dr.-Ing. Jochen H. Schiller MC

20. Aloha/slotted aloha Mechanism Aloha Slotted Aloha
random, distributed (no central arbiter), time-multiplex
Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries
Aloha
Slotted Aloha
collision
sender A
sender B
sender C t
collision
sender A
sender B
sender C t
Prof. Dr.-Ing. Jochen H. Schiller MC

21. DAMA - Demand Assigned Multiple Access
Channel efficiency only 18% for Aloha, 36% for Slotted Aloha
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
Examples for reservation algorithms:
Explicit Reservation according to Roberts (Reservation-ALOHA)
Implicit Reservation (PRMA)
Reservation-TDMA
Prof. Dr.-Ing. Jochen H. Schiller MC

22. MACA (Multiple Access with Collision Avoidance)
It 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
Prof. Dr.-Ing. Jochen H. Schiller MC

23. MACA examples MACA avoids the problem of hidden terminals
A and C want to send to B
A sends RTS first
C waits after receiving CTS from B
MACA avoids the problem of exposed terminals
B wants to send to A, C to another terminal
now C does not have to wait for it cannot receive CTS from A A C
RTS
CTS
CTS B A C
RTS
RTS
CTS B
Prof. Dr.-Ing. Jochen H. Schiller MC

24. packet ready to send; RTS
MACA variant
sender
receiver
idle
idle
packet ready to send; RTS
data;
ACK
RxBusy
time-out;
RTS
wait for the
right to send
RTS;
CTS
time-out 
data;
NAK
ACK
time-out 
NAK;
RTS
CTS; data
wait for
data
wait for ACK
RTS; RxBusy
ACK: positive acknowledgement
NAK: negative acknowledgement
RxBusy: receiver busy
Prof. Dr.-Ing. Jochen H. Schiller MC

25. Access method in CDMA (Code Division Multiple Access)
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Access method in 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
Prof. Dr.-Ing. Jochen H. Schiller MC
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller 28

26. Advantages and Disadvantages
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
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

27. What is a “good” code for CDMA?
Good autocorrelation
The absolute value of the inner product of a vector multiplied with itself should be large.
(+1,-1,+1,+1,-1,+1,+1,+1,-1,-1,-1)
Value is large
Orthogonal to other codes
Two vectors are called orthogonal if their inner product is 0.
(2,5,0) and (0,0,17)
(2,5,0) * (0,0,17) = =0
(1,2,3) and (4,2,-3) are almost orthogonal

28. CDMA Sender A Sender B Both signals superimpose in space
sends Ad = 1, key Ak = (assign: “0”= -1, “1”= +1)
sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)
Sender B
sends Bd = 0, key Bk = (assign: “0”= -1, “1”= +1)
sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)
Both signals superimpose in space
C = 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 = = 6
result greater than 0, therefore, original bit was “1”
receiving B
Be = (-2, 0, 0, -2, +2, 0)  Bk = = -6,
i.e. “0”
Prof. Dr.-Ing. Jochen H. Schiller MC

29. B’s strength is 5 times A’s strength
C = As +5 * Bs
= (-1, +1, -1, -1, +1, +1) + 5 * (-1, -1, +1, -1, +1, -1)
= (-6,-4,+4,-6,+6, -4)
Ae ?
Be?

30. CDMA on signal level I Ad 1 1 1 1 1 1 1 1 1 1 Ak 1 1 1 1 1 1 1 1 As
data A Ad 1 1
key A
key
sequence A 1 1 1 1 1 1 1 1 Ak
data  key 1 1 1 1 1 1 1 1 As
signal A
Real systems use much longer keys resulting in a larger distance
between single code words in code space.
Prof. Dr.-Ing. Jochen H. Schiller MC

31. CDMA on signal level II As Bd 1 1 1 1 1 1 1 1 1 Bk 1 1 1 1 1 1 1 1 1 1
signal A As
data B Bd 1
key B
key
sequence B 1 1 1 1 1 1 1 1 Bk 1 1 1 1 1 1 1 1 1 1
data  key Bs
signal B
As + Bs
Prof. Dr.-Ing. Jochen H. Schiller MC

32. CDMA on signal level III
data A Ad 1 1
As + Bs Ak
(As + Bs) * Ak
integrator
output
comparator
output 1 1
Prof. Dr.-Ing. Jochen H. Schiller MC

33. CDMA on signal level IV Bd 1 1 data B As + Bs Bk (As + Bs) * Bk
As + Bs Bk
(As + Bs) * Bk
integrator
output
comparator
output 1
Prof. Dr.-Ing. Jochen H. Schiller MC

34. CDMA on signal level V (0) (0) ? As + Bs wrong key K (As + Bs) * K
integrator
output
comparator
output
(0)
(0) ?
Prof. Dr.-Ing. Jochen H. Schiller MC

35. Comparison SDMA/TDMA/FDMA/CDMA
Prof. Dr.-Ing. Jochen H. Schiller MC