Wireless & Personal Communication Systems – CSE5807 Lecture: 03 1 Wireless Personal Communications...

1Wireless & Personal Communication Systems – CSE5807Lecture: 03

Wireless Personal Communications Systems – CSE5807

Lecture: 03

Stephen Giles and Satha K. Sathananthan

School of Computer Science and Software Engineering

Monash University

Australia

Modified by Peter Granville August 2006

These slides contain figures from Stallings, and are based on a set developed by Tom Fronckowiak .


Access Technologies

Base station

Downlink / Forward ChannelUplink / Reverse Channel

• Responsible for interacting with the medium (wireless channel) to coordinate the successful operation of multiple terminals.

• Separation between uplink and downlink:- Frequency Division Duplexing (FDD):

Uplink and downlink channels use different carrier frequencies.

- Time Division Duplexing (TDD)Uplink and downlink channels use same frequency band but

different time slots.


Access Technologies

• Fixed-Assignment Access: Voice-oriented Networks– Frequency Division Multiple Access (FDMA)

– Time Division Multiple Access (TDMA)

– Code Division Multiple Access (CDMA)

• Random or Dynamic Access: Data-oriented Networks

– Aloha Techniques

– Carrier Sense Multiple Access Techniques (CSMA)


Access Technologies

ReservationCollision Resolution

e.g.: Pure Aloha

Fixed Allocation

e.g.: FDMA & TDMA

Dynamic Allocation

Use of round-robin ordering

e.g.: Token ring, token bus

Use of collision resolution with implicit reservation

e.g.: CSMA/CD


Frequency Division Multiple Access (FDMA)

• Based on Frequency Division Multiplexing (FDM).

• All users transmit signal simultaneously, and they are separated from one another from by their frequency of operation.

Time

FrequencyF1 F2

F3 F4F5

• Used in most of the First Generation (1G) cellular networks.– AMPS (Advanced Mobile Phone Service) system allocates 30 kHz of

bandwidth for each forward and reverse channel.


Frequency Division Multiple Access (FDMA)

• Adjacent channel interference – Use of guard bands..

– Power control.

• TDD and FDD:

Time

FrequencyF1 F2 F3 F1 F2 F3Uplink Downlink

Time

FrequencyF1 F2F3 F4

Uplink

Downlink

FDMA/FDD FDMA/TDD


Time Division Multiple Access (TDMA)

• Based on Time Division Multiplexing (TDM).

• A number of users share the same frequency band by taking assigned turns in using the channel.– Users are assigned different time slots.

– Synchronous – time slots pre-assigned to sources and fixed.

Frequency

TimeT1 T2T3 T4

T5


Time Division Multiple Access (TDMA)• Digital format (more flexible).

• Used in most of the Second Generation (2G) cellular networks.– GSM (Global System for Mobile Communications) allocates 8 time slots

within a 200 kHz carrier bandwidth.

• TDD and FDD:

Time

FrequencyF1 F1

Uplink Downlink

TDMA/FDD

T2

T3

T1

Downlink

Uplink

Time

FrequencyF1

TDMA/TDD


Code Division Multiple Access (CDMA)

• Based on Spread Spectrum technology.

• A number of users use the same band at the same time, and the users are differentiated by a code.

• Accommodate different bandwidth requirements and switching methods without any need for coordination.

• Each user contributes to interference => Power control

C3

C2

C1

Frequency

TimeCode


Code Division Multiple Access (CDMA)

• More capacity and better quality.

• Used in 2G and 3G cellular networks.

– 2G: IS-95

– 3G: IMT-2000

• Uses TDD and FDD

Uplink

Downlink

Frequency

Time

CDMA/FDD CDMA/TDD

TimeCode

Uplink Downlink Frequency


Random Access Methods• Allocating a single channel among competing users.

• Belongs to a sublayer of the data link layer, called the “Medium Access Control (MAC)” sublayer.

- Each node has a queue of frames to be transmitted.

- Nodes normally send new frames immediately, hoping for no interference from other nodes.

- How frames are transmitted when collision occur?

=> Pure Aloha

=> Slotted Aloha

=> Carrier Sense Multiple Access (CSMA)

=> CSMA/CD

=> CSMA/AD


Pure Aloha• Wireless access to a mainframe: “Alohanet”, invented and

demonstrated in 1970 in Hawaii.mainframe

::::

Stations/terminals

wireless (common channel)

• When stations are ready for transmission, they will transmit immediately hoping that other stations will not transmit at the same time.

• If two or more stations transmit at the same time, a transmission collision occurs and the receivers will not be able

to read the transmissions correctly.


Pure Aloha

• A sender will generally realize its transmission is unsuccessful, by not receiving an acknowledgement some time after the completion of its transmission. The sender then retransmits the same data frame again.

• Why use such an unreliable method?• Simple – a complicated mechanism will be more difficult and expensive to implement•Chances of transmission collision is small - if small number of terminals.

•Chances of collision increase with load, maximum utilization of channel is18%.


Slotted Aloha

• Time on the channel is organized into uniform slots whose size equals the frame transmission time.

• Central clock needed to synchronize all stations

• Stations when they are ready, can only transmit at the beginning of a timeslot. Thus, frames that do overlap will do so totally.

• Maximum utilization of channel increases to 37%

• Slotted Aloha with TDMA:- Also called Reservation-Aloha (R-Aloha)- Used in GSM for initial contact between MS and BS.


Carrier Sense Multiple Access (CSMA)

• If the signal propagation time is small compared to frame transmission time, a station should listen

• Carrier Sense - Listen to determine if another transmission is in progress before transmit.

• Reduces collisions - will only occur when two stations begin to transmit (almost) simultaneously

• Increased utilization of channel.

• non-persistent CSMA:When terminals sense a busy channel as they are ready, they will backoff for a while and return back to sense the channel some time later.


CSMA – What should a station do if medium busy ?

• non-persistent CSMA:1. If medium is idle, transmit; else go to 2

2. If medium is busy, wait an amount of time (the retransmission delay) drawn from a probability distribution; go to 1.

– Some capacity wasted

• Random delay reduces the probability of collisions

• 1-persistent CSMA: 1. If medium is idle, transmit; else go to 2

2. If medium is busy, continue to listen until channel is sensed idle; then transmit immediately (with p = 1)

– If two stations are waiting to transmit, a collision will occur.


CSMA – What should a station do if medium busy ?

• p-persistent CSMA:1. If medium is idle, transmit with probability p, and delay one time unit

with probability (1 – p). The time unit is equal to maximum propagation delay.

2. If medium is busy, continue to listen until the channel is idle, to go 1.

3. If transmission is delayed 1 time unit, after waiting, go to 1.


Carrier Sense Multiple Access/Collision Detection (CSMA/CD)

1. If the medium is idle, transmit; else go to 2.

2. If the medium is busy, continue to listen until channel is idle,then transmit immediately

3. If a collision is detected during transmission, transmit a briefjamming signal to assure that all stations know that there hasbeen a collision then cease transmission

4. After transmitting the jamming signal, wait a random amount of time, the backoff, then go to 1.


CSMA/CD Operation


CSMA/CD with Binary Exponential Backoff

Number of collisions Possible slots forexperienced in the past (c) retransmission

1 to 10 [n+1 to n+2c]11 to 16 [n+1 to n+210]

>16 =ABORT=

collision

n n+1 n+2 n+3 …

time

• For 1 to 10 retransmission attempts, mean value of random delay doubled.• Mean value remains same for 11 to 16

• Ethernet Protocol = 1-p CSMA/CD + BEB


CSMA/CD and Wireless LANs

• Collisions consume bandwidth - want to avoid.

• Collision detection requires a station to be able to send data and receive collision signals at the same time, requires more complex electronics in wireless interface card.

• Collisions may not be detected - a station may be hidden from in a wireless environment due to natural obstacles such as mountains or artificial obstacles such as buildings

• The distance between stations in a wireless environment can be great – signal fading could prevent a station hearing a collision


Carrier Sense Multiple Access/Collision Avoidance

(CSMA/CA)

• CSMA with Collision Avoidance.• An idle channel must be detected before a transmission can take place.• A receiver must reply with a short ACK to inform the sender that the transmission is successful so that unnecessary retransmission will not occur.


Spread Spectrum• Occupies a much wider bandwidth.

• Two types:– Frequency Hopping Spread Spectrum (FHSS)

– Direct Sequence Spread Spectrum (DSSS)

• More robust to interference and interception.– First used in military communication applications and later adopted for

commercial use.

– Superior performance over traditional radio signaling over multipath channel.

• CDMA system offers more flexibility, superior performance and more capacity over FDMA/TDMA based systems.– Adopted for 3G Cellular systems.

• Can be overlaid with other systems operating in the same band.– WLAN operating in unlicensed band.


Spread Spectrum

Frequency

PowerNarrowband Signal – High peak power

(Traditional Signal)

Spread Spectrum Signal – Low peak power


Spread Spectrum• What can be gained from apparent waste of spectrum?

– Immunity from various kinds of noise and multipath distortion.

– Can be used for hiding and encrypting signals.

– Several users can independently use the same higher bandwidth with very little interference.


Frequency Hoping Spread Spectrum (FHSS)

• Signal is broadcast over seemingly random series of radio frequencies.– A number of channels allocated for the FH signal.

– Width of each channel corresponds to bandwidth of input signal.

• Signal hops from frequency to frequency at fixed intervals.– Transmitter operates in one channel at a time.

– Bits are transmitted using some encoding scheme.

– At each successive interval, a new carrier frequency is selected.


Frequency Hoping Spread Spectrum (FHSS)

• Large number of frequencies used => Spread Spectrum.

• Channel sequence dictated by spreading code.

• Receiver, hopping between frequencies in synchronization with transmitter, picks up message.

• Advantages:– Eavesdroppers hear only unintelligible blips.

– Attempts to jam signal on one frequency succeed only at knocking out a few bits.

– Results in a system that is quite resistant to jamming.

• Jammer must jam all frequencies.

• With fixed power, this reduces the jamming power in any one frequency band.


Direct Sequence Spread Spectrum (DSSS)

• Each bit in original signal is represented by multiple bits in the transmitted signal.

• Spreading code spreads signal across a wider frequency band. – Spread is in direct proportion to number of bits used.

• One technique combines digital information stream with the spreading code bit stream using exclusive-OR.


Code-Division Multiple Access (CDMA)


Code-Division Multiple Access (CDMA)

• Break each bit into k chips.– Chips are a user-specific fixed pattern

• Chip data rate of new channel = kD where D = rate of original data signal.

• If k=6 and code is a sequence of 1s and -1s

– For a ‘1’ bit, A sends code as chip pattern

• <c1, c2, c3, c4, c5, c6>

– For a ‘0’ bit, A sends complement of code

• <-c1, -c2, -c3, -c4, -c5, -c6>

• Receiver knows sender’s code and performs decoding.

• <d1, d2, d3, d4, d5, d6> = received chip pattern

• <c1, c2, c3, c4, c5, c6> = sender’s code

665544332211 cdcdcdcdcdcddSu


CDMA Example

• User A’s code = <1, –1, –1, 1, –1, 1>

– To send a 1 bit use chip pattern <1, –1, –1, 1, –1, 1>

– To send a 0 bit use chip pattern <–1, 1, 1, –1, 1, –1>

• User B’s code = <1, 1, –1, – 1, 1, 1>

– To send a 1 bit = <1, 1, –1, –1, 1, 1>

• Base Station receives chip pattern and applies A’s code

– (A’s code) x (received chip pattern)

• User A ‘1’ bit: 6 ==> 1

• User A ‘0’ bit: -6 ==> 0

• User B receives chip pattern and applies its code, result: 0 ==> unwanted signal; ignored


Required Reading

• W. Stallings, “Wireless Communications and Networks” Prentice-Hall, 2000.

>> Chapter 2.5 and Chapter 7

Optional Reference

• K. Pahlavan and K. Krishnamurthy “Principles of Wireless Networks”, Prentice-Hall, 2002.


The following slides are for interest ONLY


Spreading Sequences• Spreading Sequence Categories:

– PN sequences:• M-sequences – Constructed by using shift registers.

• Gold sequences – Constructed by using two m-sequences.

– Orthogonal codes:• Walsh codes

• Variable-Length Orthogonal codes

• For FHSS systems– PN sequences most common

• For DSSS systems not employing CDMA– PN sequences most common

• For DSSS CDMA systems– PN sequences

– Orthogonal codes


PN Sequences• PN generator produces periodic sequence that appears to be

random.

• PN Sequences: – Generated by an algorithm using initial seed.

– Sequence isn’t statistically random but will pass many test of randomness.

– Sequences referred to as pseudorandom numbers or pseudonoise sequences.

– Unless algorithm and seed are known, the sequence is impractical to predict.


Orthogonal Codes• Orthogonal codes

– All pairwise cross correlations are zero.

– Fixed- and variable-length codes used in CDMA systems.

– For CDMA application, each mobile user uses one sequence in the set as a spreading code.

• Provides zero cross correlation among all users.

• Types:– Welsh codes

– Variable-Length Orthogonal codes


Typical Multiple Spreading Approach• Spread data rate by an orthogonal code (channelization

code).– Provides mutual orthogonality among all users in the same

cell.

• Further spread result by a PN sequence (scrambling code).– Provides mutual randomness (low cross correlation) between

users in different cells.


Orthogonal Frequency Division Multiplexing (OFDM)

• Transmission of Data in Parallel on Multiple Subcarriers (or Multiple Frequency Bands).

• Orthogonal Subcarriers.



• Advantages:

• Resistance to Multipath fading

• Spectral Efficiency

• Easy Implementation using FFT chips



• Adopted in several wireless communication system standards.

– Digital Audio Broadcasting (DAB)

– Digital Video Broadcasting (DVB)

– Wireless LAN

• IEEE802.11a and IEEE802.11g (Wi-Fi)

• HiperLAN-1 and HiperLAN-2

– Fixed Broadband Wireless Access Networks

• IEEE802.16 (Wi-Max)


OFDM and Multi-Access Technology• Multicarrier-CDMA (MC-CDMA):

– OFDM and CDMA

– Multiple users are separated by CDMA codes.

– It is being considered for 4G Cellular Networks.

• Orthogonal Frequency Division Multiplexing Access (OFDMA).

– Multiple users are assigned group of subcarriers.

– Used in IEEE802.16 networks.


Ultra Wide Band (UWB) Transmission

• Transmission of narrow pulses (order of few tenths of a nanosecond) with low power.

– Very wide band (several GHz).

– Can coexist with existing radio systems (in licensed band too).

• Robust to interference and multipath fading.

• Suitable for short-range communication.

– Is being considered for Wireless Personal Area Networks (WPAN).

– May replace Bluetooth?


Non-Preemptive Multiple Access (NPMA)

• Used in HiperLAN-1.• Listen before talk (similar to CSMA/CA).• Uses the characteristics of Slotted-Aloha and TDMA.• If a terminal sense a medium free for a certain duration, then it immediately transmits.• If the channel is busy, the terminal goes through three phases when the channel become available.

- Prioritization phase- Contention phase- Transmission phase

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