1. Department of Electrical and Computer Engineering
ECE 4331, Fall, 2009
                                                           
Zhu Han
Department of Electrical and Computer Engineering
Class 26
Nov. 19th, 2009

2. Outline Classes of MAC protocols Simplex and Duplex Channels FDMA TDMA
Term paper, only journal. For those who did not give me the title on time, I think you would not work for the term project, right? 
General Wireless System Architecture
Media Access Control
Classes of MAC protocols
Simplex and Duplex Channels
Coordinated MAC Schemes
FDMA
TDMA
Capacity of TDMA systems and which factors affect the capacity.
Spread Spectrum Access Methods
FHMA
Case study: Bluetooth
CDMA
Hybrid Spread Spectrum Schemes.
Case Study 2

3. 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 , GSM, etc. 3

4. Wireless System Architecture and Functions
Applications
TCP/IP
Neighbor Discovery and Registration,
Multicasting, Power Saving Modes, Address
Translation (IP-MAC), Routing, Quality of Services,
Subnet Security
Wireless Subnet
Controller
Wireless
Link Layer
(Layers 1 and 2 in ISO/OSI Network Reference Model)
Link Controller
Medium Access Control, MAC level Scheduling,
Link Layer Queueing, Link Layer Reliability – ACKs,
NACKs, ….
Transceiver
Frame Controller
Framing and frame synchronization, error control,
CRC, bit scrambling, widening, ….
Carrier frequency, channel bandwidth, carrier detect,
Captude detect, channel data rate, modulation,
Received signal strength (RSSI), transmit power,
Power control, …
Physical Radio 4

5. Medium Access Control We need to use this resource very efficiently
Wireless spectrum (frequency band) is a very precious and limited resource.
We need to use this resource very efficiently
We also want our wireless system to have high user capacity
A lot of (multiple) users should be able to use the system at the same time.
For these reasons most of the time, multiple users (or stations, computers, devices) need to share the wireless channel that is allocated and used by a system.
The algorithms and protocols that enables this sharing by multiple users and controls/coordinates the access to the wireless channel (medium) from different users are called MEDIUM ACCESS, or MEDIA ACCESS or MULTIPLE ACCESS protocols, techniques, schemes, etc…) 5

6. Wireless Media Access Control
Random Schemes (Less-Coordinated)
Examples: MACA, MACAW, Aloha, MAC,…
More suited for wireless networks that are designed to carry data: IEEE Wireless LANs
Coordinated Schemes
Examples: TDMA, FDMA, CDMA
More suited for wireless networks that are designed to carry voice: GSM, AMPS, IS-95,…
Polling based Schemes
Examples: Bluetooth, BlueSky,…
Access is coordinated by a central node
Suitable for Systems that wants low-power, aims to carry voice and data at the same time. 6

7. 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 7

8. 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 8

9. 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 R
Base
Station
Mobile
Station
Reverse
Channel
Forward
Channel
fc,R
fc,,F
frequency
Frequency separation
Frequency separation should be carefully decided
Frequency separation is constant 9

10. 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) B M
Base
Station
Mobile
Station
Slot number …
channel F R F R F R F R ….
Reverse
Channel
Forward
Channel
time Ti
Ti+1
Time separation 10

11. Duplexing – TDD versus 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… 11

12. Multiple Access - Coordinated
We will look now sharing the media by more than two users.
Three 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
Used in wideband systems. 12

13. Narrow- and Wideband Systems
Narrowband System
The channel bandwidth (frequency band allocated for the channel is small)
More precisely, the channel bandwidth is large compared to the coherence bandwidth of the channel (remember that coherence bandwidth is related with reciprocal of the delay spread of multipath channel)
AMPS is a narrowband system (channel bandwidth is 30kHz in one-way)
Wideband Systems
The channel bandwidth is large
More precisely, the channel bandwidth is much larger that the coherence bandwidth of the multipath channel.
A large number of users can access the same channel (frequency band) at the same time. 13

14. Narrow- and Wideband Systems
Narrowband Systems
Could be employing one of the following multiple access and duplexing schems
FDMA/FDD
TDMA/FDD
TDMA/TDD
Wideband systems
Could be employing of the following multiple access and duplexing schemes
CDMA/FDD CDMA/TDD 14

15. Cellular Systems and MAC
Multiple Access Technique
AMPS
FDMA/FDD
GSM
TDMA/FDD
USDC (IS-54 and IS-136)
PDC
CT2 Cordless Phone
FDMA/TDD
DECT Cordless Phone
US IS-95
CDMA/FDD
W-CDMA
CDMA/TDD
cdma2000 15

16. Frequency Division Multiple Access
Individual radio channels are assigned to individual users
Each user is allocated a frequency band (channel)
During this time, no other user can share the channel
Base station allocates channels to the users B
f1,F
f2,F
fN,F
f1,R
f2,R
fN,R … M M M 16

17. Features of FDMA An FDMA channel carries one phone circuit at a time
If channel allocated to a user is idle, then it is not used by someone else: waste of resource.
Mobile and base can transmit and receive simultaneously
Bandwidth of FDMA channels are relatively low.
Symbol time is usually larger (low data rate) than the delay spread of the multipath channel (implies that inter-symbol interference is low)
Lower complexity systems that TDMA systems. 17

18. Capacity of FDMA Systems
Frequency spectrum allocated for FDMA system …
Guard
Band
channel
Guard
Band
Bt : Total spectrum allocation
Bguard: Guard band allocated at the edge of the spectrum band
Bc : Bandwidth of a channel
AMPS has 12.MHz simplex spectrum band, 10Khz guard band, 30kHz
channel bandwidth (simplex): Number of channels is 416. 18

19. Time Division Multiple Access
The allocated radio spectrum for the system is divided into time slots
In each slot a user can transmit or receive
A user occupiess a cyclically repeating slots.
A channel is logically defined as a particular time slot that repeats with some period.
TDMA systems buffer the data, until its turn (time slot) comes to transmit.
This is called buffer-and-burst method.
Leaky bucket
Requires digital modulation 19

20. TDMA Concept
Downstream Traffic: Forward Channels: (from base to mobiles) 1 2 3 … N 1 2 3 …. N …
Logical forward channel to a mobile
Base station broadcasts to mobiles on each slot
Upstream Traffic: Reverse Channels: (from mobile to base) 1 2 3 … N 1 2 3 …. N …
Logical reverse channel from a mobile
A mobile transmits to the base station in its allocated slot
Upstream and downstream traffic uses of the two different carrier frequencies. 20

21. TDMA Frames
Multiple, fixed number of slots are put together into a frame.
A frame repeats.
In TDMA/TDD: half of the slots in the frame is used for forward channels, the other is used for reverse channels.
In TDMA/FDD: a different carrier frequency is used for a reverse or forward
Different frames travel in each carrier frequency in different directions (from mobile to base and vice versa).
Each frame contains the time slots either for reverse channels or forward channel depending on the direction of the frame. 21

22. General Frame and Time Slot Structure in TDMA Systems
One TDMA Frame
Preamble
Information
Trail Bits
Slot 1
Slot 2
Slot 3 …
Slot N
Guard
Bits
Sync
Bits
Control
Bits
Information
CRC
One TDMA Slot
A Frame repeats in time 22

23. A TDMA Frame
Preamble contains address and synchronization info to identify base station and mobiles to each other
Guard times are used to allow synchronization of the receivers between different slots and frames
Different mobiles may have different propagation delays to a base station because of different distances. 23

24. Efficiency of a Frame/TDMA-System
Each frame contains overhead bits and data bits.
Efficiency of frame is defined as the percentage of data (information) bits to the total frame size in bits.
bT: total number of bits in a frame
Tf: frame duration (seconds)
bOH: number of overhead bits
Number of channels in a TDMA cell:
m: maximum number of TDMA users supported in a radio channel 24

25. TDMA GSM: 30% overhead DECT: 30% overhead IS-54: 20% overhead.
TDMA Efficiency
GSM: 30% overhead
DECT: 30% overhead
IS-54: 20% overhead.
TDMA is usually combined with FDMA
Neighboring cells be allocated and using different carrier frequencies (FDMA). Inside a cell TDMA can be used. Cells may be re-using the same frequency if they are far from each-other.
There may be more than one carrier frequency (radio channel) allocated and used inside each cell. Each carrier frequency (radio channel) may be using TDMA to further multiplex more user (i.e. having TDMA logical channels inside radio channels)
For example: GSM uses multiple radio channels per cell site. Each radio channel has 200KHz bandwidth and has 8 time slots (8 logical channels). Hence GSM is using FHMA combined with TDMA. 25

26. Contemporary TDMA Systems
GSM
(Europa)
IS-54
(USA)
PDC
(Japan)
DECT
(European Cordless)
Bit Rate
270.8 Kbps
48.6 Kbps
42 Kbps
1.152 Mbps
Bandwidth
200 KHz
30 KHz
25 KHz
1.728 MHz
Time Slot
0.577 ms
6.7 ms
0.417 ms
Upstream slots
per frame 8 3 12
Duplexing
FDD
TDD
Efficiency of Time Slots
73 %
80 %
67 %
Modulation
GMSK
p/4 DQPSK
Adaptive Equalized
Mandatory
Optional
None 26

27. Features of TDMA
Enables the sharing of a single radio channel among N users
Requires high data-rate per radio channel to support N users simultaneously.
High data-rate on a radio channel with fixed bandwidth requires adaptive equalizers to be used in multipath environments (remember the RSM delay spread s parameter)
Transmission occurs in bursts (not continues)
Enables power saving by going to sleep modes in unrelated slots
Discontinues transmission also enables mobile assisted handoff
Requires synchronization of the receivers.
Need guard bits, sync bits. large overhead per slot.
Allocation of slots to mobile users should not be uniform.
It may depend on the traffic requirement of mobiles.
This brings extra flexibility and efficiency compared to FDMA systems. 27

28. Capacity of TDMA Systems
Capacity can be expressed as
System Capacity (the capacity of the overall system covering a region)
Depends on:
Range of cells
Whether the system can support macro-cells, micro-cells or pico-cells.
Cell Capacity
Depends on the radio link performance between a base-station and mobiles:
The lowest C/I (carrier-to-interence) ratio the system can operate for example quality of transmission. This in turn depends on the speech coding technique, desired speech quality, etc.
Data-rate over the channel which depends modulation efficiency (bits_per_second/Hz) and channel bandwidth.
The frequency re-use factor 28

29. Spread Spectrum Access
SSMA uses signals that have transmission bandwidth that is several orders of magnitued larger than minimum required RF bandwidth.
Provides
Immunity to multipath interference
Robust multiple access.
Two techniques
Frequency Hopped Multiple Access (FHMA)
Direct Sequence Multiple Access (DSMA)
Also called Code Division Multiple Access – CDMA 29

30. Frequency Hopping (FHMA)
Digital muliple access technique
A wideband radio channel is used.
Same wideband spectrum is used
The carrier frequency of users are varied in a pseudo-random fashion.
Each user is using a narrowband channel (spectrum) at a specific instance of time.
The random change in frequency make the change of using the same narrowband channel very low.
The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized.
Rate of hopping versus Symbol rate
If hopping rate is greather: Called Fast Frequency Hopping
One bit transmitted in multiple hops.
If symbol rate is greater: Called Slow Frequency Hopping
Multiple bits are transmitted in a hopping period
GSM and Bluetooth are example systems 30

31. Capacity of CDMA Systems
Uplink Single-cell System Model
Assumptions
Total active users Ku
The intra-cell MAI can be
modeled as AWGN
Perfect power control is assumed
Random sequences
User 2
...
User 1
User k . .
...
User n
User Ku 31

32. Capacity of CDMA Systems
Coarse estimate of the reverse link (uplink) capacity
Assumptions:
Single Cell.
The interference caused by other users in the cell can be modeled as AWGN.
Perfect power control is used, i.e. the received power of each user at the base station is the same.
If the received power of each user is Ps watts, and the background noise can be ignored (ex: microcells), then the total interference power (MAI) at the output of the desired user’s detector is
where Ku is the total number of equal energy users in the cell. Suppose each user can operate against Gaussian noise at a bit-energy-to-noise density level of Eb/Io. Let W be the entire spread bandwidth, then the interference spectral density can be expressed as: 32

33. Case Study - Bluetooth
Uses Frequency Hopping in cell (piconet) over a 79 MHz wideband radio channel.
Uses 79 narrowband channels (carrier frequencies) to hop through.
Freq (f) = 2402+k MHz, k = 0,...,78
Channel spacing is 1 MHz (narrowband channel bandwidth)
Wideband spectrum width = 79 MHz.
Hopping Rate = 1600 Hops/Second
Hopping sequence is determined by Bluetooth Hardware address and Clocks that are syncrozied between sender and receiver
79 MHZ 1 2 3
..... 77 78
79-Hop System
1 MHZ
A hop sequence could be: 7,1,78,67,0, 56,39, 33

34. Case Study: Bluetooth – Piconet and FHSS
Each node is classified as master or slave.
Master defines a piconet (a cell). Maximum 7 slaves can be connected to
a master. Master coordinates access to the the media.
All traffic has to go over master.
Slaves can not talk to each-other
directly.
Picocell S
Range = 10m
Raw Data-rate: 1 Mbps/piconet
Radio channel used by devices in
a piconet is 79MHz channel, which
Is frequency hopped: hopping
though 789 channels.
Hoprate = 1600 hops/sec
FHSS M S S
All slaves and the master hops according to the same hopping sequence.
The hopping sequence is determined by the clock and BT_address of the master. 34

35. Case Study: Bluetooth – Scatternet and FHSS
Piconet S S
Piconet can be combined
into scatternets.
Red slave acts as a
bridge between two
piconets. M2
Piconet S
FHSS S
FHSS M1
Each piconet uses FHSS with different
hopping sequences (masters are different).
This prevents interference between piconets. S S 35

36. Case Study: Bluetooth - Media access in a piconet
Inside a piconet, access to the
frequency hopped radio channel
is coordinated using time
division multiple access: TDMA/TDD.
Slot duration = 1/1600 sec = 625ms
Piconet S1
FHSS M
In an even slot, master transmits to a
slave.
In an odd slot, the slave that is addressed
in the previous master-to-slave slot transmits. S3 S2
…..
M-S1
S1-M
M-S2
S2-M
M-S3
S3-M
M-S1
S1-M ……
slot time=625ms 36

37. 802.11
2.4G G, G
802.11a/g, OFDM, b: CDMA

38. Channel
11, 5.5, 2, 1Mbps

39. Channelization scheme
channels

40. 802.11 Application Presentation Session Transport Network Data Link
802.11a/g: 54, 48, 36, 24, 18, 6Mbps
802.11e -MAC Enhancements-Security/QoS
802.11f- Inter-Access Point Protocol
802.11h- Spectrum Managed 5Ghz
802.11i- Enhanced Security (TKIP and 802.1x)
p- vehicular
n- MIMO
Application
Presentation
Session
Transport
Network
Data Link
Physical
ISO
OSI
7-layer
model
Logical Link Control
Medium Access (MAC)
Physical (PHY)
IEEE 802
standards

41. Wireless hotpot planner
Wireless valley

42. Design Procedure

43. Future WIFI

44. Signal to Noise Ratio at home

45. Fail of Iridium Satellite System
The system was originally to have 77 active satellites, and as such was named for the element iridium, which has atomic number 77.
Too few users per square miles, cost too much.
Chapter 11 bankruptcy on August 13, 1999
In 70s, 12 calls per NYC.