EPL 657 Wireless Networks

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

frequency bands

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

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