EPL 657 Wireless
Networks
Some fundamentals:
Multiplexing / Multiple Access /
Duplex
Infrastructure vs Infrastructureless
Panayiotis Kolios
Recall: The ‘big’ picture ...
2
Modulations: some basics
Multiplexing / multiple access / duplexing (1)
Multiplexing / multiple access
Signals to/from different users share a common channel using
• time division methods (TDM/TDMA, CSMA),
• frequency division methods (FDM/FDMA),
• code division methods (CDMA), or
• space division (SDMA).
A combination of above is also often used
Multiplexing / multiple access / duplexing (2)
• Duplexing:
– The signals moving between two elements in opposite
directions can be separated using
• time division duplexing (TDD)
• frequency division duplexing (FDD)
• Code Division Duplexing (CDD)
Multiplexing
• Goal: multiple use of a shared medium
• Multiplexing in 4 dimensions: space (si)
time (t)
frequency (f)
code (c)
or even a combination
• Important: guard spaces needed!
• Selective receivers/filters required to obtain/extract signal intended for user
Time multiplex
A channel gets the whole spectrum for a certain amount of time
Advantages:
only one carrier in the medium at any time
throughput high even for many users
Disadvantages:
Precise Synchronization necessary
Can be complex
Can be inefficient
TDM (Time Division Multiplexing): channel
divided into N time slots, one per user;
inefficient with low duty cycle users and at
light load.
Example Channel Partitioning MAC protocols:
TDMA
TDMA: time division multiple access
• access to channel in "rounds"
• each station gets fixed length slot (length = packet
transmision time) in each round
• unused slots go idle
• example: 6-station LAN, 1,3,4 have packet, slots
2,5,6 idle
Frequency multiplex
Separation of the whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum for the whole time
Advantages:
no dynamic coordination necessary
works also for analog signals
Disadvantages:
waste of bandwidth if the traffic is distributed unevenly
inflexible
guard spaces
Selective filters required
Can be complex
FDM (Frequency Division Multiplexing):
frequency subdivided among N users,
each user takes one; inefficient with low
duty cycle users and at light load.
Example Channel Partitioning MAC protocols:
FDMA
FDMA: frequency division multiple access
• channel spectrum divided into frequency bands
• each station assigned fixed frequency band
• unused transmission time in frequency bands go idle
• example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
f1-f2
…
f11-f12
freq
uenc
y ban
ds e.g. Assigned
channel frequency
800 – 812 Mhz
Needs of each
user: 1Mhz
f1= 800 Mhz,
f2=801, Mhz ...
Time and frequency multiplex
• Combination of both methods
• A channel gets a certain frequency band for a certain amount of time
• Example: GSM
• Advantages:
better protection against tapping
protection against frequency selective interference
higher data rates compared to code multiplex
but:
precise coordination required
increased complexity, and inefficiency
Code multiplex
Each channel has a unique code
All channels use the same spectrum at the same time (spread the spectrum- each ‘bit’ is ‘expanded’ to many bits-a code, e.g logical bit ‘1' is expanded to 010011)
Advantages:
bandwidth efficient
no coordination and synchronization necessary
good protection against interference and tapping
Disadvantages:
lower user data rates
more complex signal regeneration
Implemented using spread spectrum technology
Example: Channel Partitioning (CDMA)
CDMA (Code Division Multiple Access) • unique “code” assigned to each user; i.e., code set partitioning
• used mostly in wireless broadcast channels (cellular, satellite, etc)
• all users share same frequency, but each user has own “chipping” sequence (i.e., code, ‘language’) to encode data
– encoded signal = (original data) X (chipping sequence)
– decoding: inner-product of encoded signal and chipping sequence
• allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)
• Note each user appears as interference to others!!!
Example: CDMA Encode/Decode
Example: CDMA two-sender interference
space division multiplex • Cell structure
– Implements space division multiplex: base station covers a certain transmission area (cell)
– Mobile stations communicate only via the base station
– Advantages of cell structures: higher capacity, higher number of users
less transmission power needed
more robust, decentralized
base station deals with interference, transmission area etc. locally
– Problems: fixed network needed for the base stations
handover (changing from one cell to another) necessary
interference with other cells
– Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - less for higher frequencies (e.g. UMTS)
Example: cell
• What is a Cell?
– Cell is the Basic Union in The Mobile Telecommunications System • defined as the area where radio coverage is given by one base station.
– A cell has one or several frequencies, depending on traffic load. • Fundamental idea: Frequencies are reused, but not in
neighboring cells due to interference.
Example: cell planning (capacity, power, etc...)
• Cell splitting – Decrease transmission
power in base and mobile
– Results in more and smaller cells
– Reuse frequencies in non-contiguous cell groups
– Example: ½ cell radius leads 4 fold capacity increase (BUT higher infrastructure costs)
• Cell sectoring – Directional antennas
subdivide cell into 3 or 6 sectors
– Might also increase cell capacity by factor of 3 or 6
Example: Different Types of Cells
Duplex
• Frequency Division Duplex (FDD): Uplink and downlink
transmissions use two separated radio frequencies in
different frequency bands. A pair of frequency bands with
specified separation is assigned for the system.
• Time Division Duplex (TDD): Uplink and downlink
transmissions are carried over same radio frequency by
using synchronized time slots that divide the physical
channel into transmission and reception part. Information
on uplink and downlink are transmitted reciprocally.
• Code Division Duplex (CDD): Uplink and downlink
transmissions are carried over the same radio frequency
and time using orthogonal signal sequences (different
codes).
Example Radio Access
FDMA/FDD (as in 1st Generation Wireless)
• Access is FDMA: Frequency Division Multiple Access
• The 1st generation mobile system uses FDMA only. Example: AMP in USA
• Duplex is FDD: Frequency Division Duplex
• The FM channels are paired with an uplink and a downlink channel for each user.
Frequency
Uplink Downlink
Example Radio Access
TDMA (as in 2nd Generation wireless)
• GSM, a 2nd generation mobile system, uses 8
time slots in TDMA mode for each 200 kHz carrier.
Carriers are derived from frequency division over
the licensed frequency band (FDMA)
Time
Frequency
Note: Capacity in GSM is doubled by using alternate time slots to
support 16 channels
Example Radio Access
TDMA (as in 2nd Generation wireless)
• IS-136 TDMA or DAMP (Digital AMP) is the
American TDMA system with 3 time slots over a
30kHz carrier
• TDMA6 provides 6 channels by alternating the 3
time slots
Example Radio Access
TDMA/FDD (as in 2nd Generation wireless)
• GSM and IS-136 TDMA are TDMA/FDD
Time
Frequency
Example Radio Access
CDMA (as in 2nd Generation wireless)
• IS-95, a 2nd generation mobile system, uses
CDMA
Time
Frequency
Code
Sequences
User A
User B
User E
User C
User D
Example Radio Access
CDMA/FDD (as in 2nd Generation wireless)
• IS-95 is CDMA/FDD
Time
Frequency
Code
Sequences
Example Radio Access
2nd Generation
• Going from analog to digital and to CDMA makes
more efficient use of the scarce radio
resources (and expensive frequency spectrum
license), and hence helps to lower the price.
Example Radio Access
Wideband CDMA (3G)
• WCDMA allocates 10 ms (38,400 chips) frames to
users. The data rate for a user may change from
frame to frame (using variable length CDMA
codes).
Time
Frequency
10ms, 38,400
chips per frame User A User B
User E
User C
User D
Variable data rate
Example Radio Access
Wideband CDMA UTRA/FDD
• Separate carriers for
– Uplink
– downlink
Time
Frequency
Network
IEEE 802.15.1 WPAN (Bluetooth)
IEEE 802.15.4 LR-WPAN (ZigBee)
IEEE 802.11 WLAN (WiFi)
IEEE 802.16 WMAN (WiMAX)
Multiplexing / MA / duplexing
TDMA / TDD
CSMA/CA
CSMA/CA
TDM/TDMA (down/uplink) / TDD or (semi-duplex) FDD
Examples Multiplexing / multiple access / duplexing
Multiple Access protocols (MAC)
• Share access (time) on the common channel
– single shared broadcast channel
– two or more simultaneous transmissions by nodes
cause interference
• only one node can send successfully at a time, therefore need
multiple access protocols
• distributed algorithm that determines how nodes share
channel, i.e., determine when node can transmit
• communication about channel sharing must use channel itself!
Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. When one node wants to transmit, it can send at rate R.
2. When M nodes want to transmit, each can send at
average rate R/M
3. When more than one send at the same time, then
‘collision’
4. Fully decentralized:
– no special node to coordinate transmissions
– no synchronization of clocks, slots
5. Simple
MAC Protocols: a taxonomy
Three broad classes:
• Channel Partitioning
– divide channel into smaller “pieces” (time slots, frequency, code, space)
– allocate piece to node for exclusive use
• Random Access (e.g. Ethernet)
– access when data available to send (random)
– channel not divided, allow collisions
– “recover” from collisions
• “Taking turns” (e.g. Token ring)
– tightly coordinate shared access to avoid collisions
A popular wireless MAC:
CSMA/CA (Collision Avoidance)
Recall in wired Ethernet:
CSMA/CD: carrier sensing, deferral if busy
– collisions detected within short time
– colliding transmissions aborted, reducing channel
wastage
• collision detection:
– easy in wired LANs: measure signal strengths,
compare transmitted, received signals
– difficult in wireless LANs: receiver shut off while
transmitting
• in wireless CSMA/CA (Collision Avoidance) – more
later
Infrastructure /
Infrastructureless
networks
Sensor networks and VANETs are another form of
infrastructureless network, with many similarities to ad-hock
infrastructure vs. ad-hoc networks (WLAN)
infrastructure
network
ad-hoc network
AP AP
AP
wired network
AP: Access Point
Infrastructure-based networks (WLAN)
• Infrastructure networks provide access to other networks.
• Communication typically takes place only between the wireless nodes and the access point, but not directly between the wireless nodes.
• The access point does not just control medium access, but also acts as a bridge to other wireless or wired networks.
• Several wireless networks may form one logical wireless network:
– The access points together with the fixed network in between can connect several wireless networks to form a larger network beyond actual radio coverage.
Infrastructure-based networks (WLAN)
• Network functionality lies within the access point (controls network flow), whereas the wireless clients can remain quite simple.
• Use different access schemes with or without collision.
– Collisions may occur if medium access of the wireless nodes and the access point is not coordinated (e.g. DCF: CSMA/CA).
– If only the access point controls medium access, no collisions are possible (e.g. PCF).
• Useful for quality of service guarantees (e.g., minimum bandwidth for certain nodes)
• The access point may poll the single wireless nodes to ensure the data rate.
• Infrastructure-based wireless networks lose some of the flexibility wireless networks can offer in general:
– E.g. they cannot be used for disaster relief in cases where no infrastructure is left.
Infrastructureless
• No need of any infrastructure to work
– greatest possible flexibility
• Each node communicate with other nodes, so no access point controlling medium access is necessary (autonomous operation).
– The complexity of each node is higher • implement medium access mechanisms, forwarding data
• Nodes within an ad-hoc network can only communicate if they can reach each other physically
– if they are within each other’s radio range
– if other nodes can forward the message
Infrastructureless (Ad Hoc Networks)
• Some Features (typically)
– Lack of a centralized entity
– Network self-organization
– All the communication is carried over the wireless
medium
– Rapid mobile host movements possible
– Multi-hop routing
– Power and computing power may be constrained
Infrastructureless (Sensor Networks)
• Some Features – Many similarities to ad-hock networks – power and
computing power constrained
– Large number of sensors (application dependant)
– Limited wireless bandwidth
– Limited battery power
– Low energy use
– Efficient use of the small memory
– Data aggregation
– Network self-organization
– Collaborative signal processing
– Querying ability
VANET Networks:
• VANETS: Vehicle Ad-hock Networks
• Some Features
– Many similarities to ad-hock networks – power and
computing power not necessarily as constrained as in
sensor or ad-hoc networks
– Large number of mobile nodes (cars)
– Network self-organization
– Topology dictated by road system