By-Jesmin Akhter

Associate ProfessorInstitute of Information Technology

Jahangirnagar University

Wireless NetworkPMIT-

2

Wireless LAN requirements:High capacity, short distances, full connectivity, broadcast capabilityThroughput: efficient use wireless mediumNumber of nodes: Hundreds of nodes across multiple cellsConnection to backbone LAN: Use control modules to connect to both

types of LANsBattery power consumption: Need long battery life on mobile stationsTransmission robustness and security: To protect Interference prone and

easily eavesdroppedLicense free operationHandoff/roaming: Move from one cell to anotherDynamic Configuration: Addition, deletion, and relocation of end

systems without disruption to users

3

The Institute of Electrical and Electronics Engineers (IEEE) is a Professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence. It has more than 400,000 members in more than 160 countries, about 51.4% of whom reside in the USA.

IEEE 802 Activities

4

IEEE 802 Activities· Wired

· 802.3: Ethernet· 802.17: Packet Ring (new)

· Wireless· 802.11: Wireless LAN

• Local Area Network· 802.15: Wireless PAN

– Personal Area Network (e.g. BluetoothTM)

· 802.16: WirelessMANTM

– Metropolitan Area Networks

5

There are several specifications in the 802.11 family:

IEEE has defined the specifications for WLAN, called IEEE 802.11, which covers physical and data-link layer. Public uses the term WiFi (Wireless Fidelity) for WLAN.

802.11 — applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) which uses 2 or 4 level FSK or direct sequence spread spectrum (DSSS) which uses BPSK or QPSK.

802.11a— an extension to 802.11 that applies to wireless LANs and provides up to 54-Mbps in the 5GHz band. 802.11a uses an orthogonal frequency division multiplexing (OFDM) scheme rather than FHSH  or DSSS. Here PSK and QAM modulation scheme is used.

6

802.11b (also referred to as 802.11 High Rate or Wi-Fi) — an extension to 802.11 that applies to wireless LANS and provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1-Mbps) in the 2.4 GHz band.

802.11g — applies to wireless LANs and is used for transmission over short distances at up to 54-Mbps in the 2.4 GHz bands.

802.11n — 802.11n builds upon previous 802.11 standards by adding multiple-input multiple-output (MIMO). The additional transmitter and receiver antennas allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity through coding schemes like Alamouti coding. The real speed would be 100 Mbit/s (even 250 Mbit/s in PHY level).

BPSK and QPSK modulation Modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted.Binary phase-shift keying BPSK is the simplest form of phase shift keying (PSK). It uses two phases which are separated by 180° . BPSK is able to transmit one bit per symbol. It only able to modulate at 1 bit/symbol and so is unsuitable for high data-rate applications.QPSK uses four points on the constellation diagram. Quadrature Phase Shift Keying QPSK transmits two bits per symbol. So QPSK can be used to double the data rate and still use the same bandwidth .The main goal of modulation today is to squeeze as much data into the least amount of spectrum possible. That objective, known as spectral efficiency, measures how quickly data can be transmitted in an assigned bandwidth.

7

8

A partial view of the 802.11 protocol stack is given in fig. below. The physical layer corresponds to the OSI physical layer fairly well, but the data link layer in all the 802 protocols is split into two or more sublayers.

In 802.11, the MAC (Medium Access Control) sublayer determines how the channel is allocated, that is, who gets to transmit next. Above it is the LLC (Logical Link Control) sublayer, whose job is to hide the differences between the different 802 variants and make them indistinguishable as far as the network layer is concerned.

The 802.11 Protocol Stack

Infrared with PPM scheme

FHSS (Frequency Hopping Spread Spectrum)

DSSS (Direct Sequence Spread Spectrum)

OFDM (Orthogonal Frequency Division Multiplexing)

HR-DSSS (High Rate DSSS)

9

The infrared option never gained market support.Infrared at 1 Mbps and 2Mbps operates at wavelength between

850nm and 950nm.

Infrared with PPM scheme

Spread Spectrum• Spread spectrum involves the use of a much wider BW than

actually necessary to support a given data rate. The result of using wider BW is to minimize interference and drastically reduce BER. It operates in 2.4GHz band at data rate of 1Mbps and 2Mbps.

10

Spread Spectrum System• Input fed into channel encoder• Signal modulated using sequence of digits

– Spreading code/sequence– Typically generated by pseudonoise/pseudorandom number

generator– Generated by algorithm using initial seed

• Increases bandwidth significantly– Spreads spectrum

• Receiver uses same sequence to demodulate signal• Demodulated signal fed into channel decoder

General Model of Spread Spectrum System

Spread Spectrum SystemGains

• Immunity from various noise and multipath distortion– Including jamming

• Can hide/encrypt signals– Only receiver who knows spreading code can

retrieve signal• Several users can share same higher

bandwidth with little interference• Drastically reduce BER

Direct Sequence Spread Spectrum (DSSS)

• Each bit represented by multiple bits using spreading code• Spreading code spreads signal across wider frequency band

– In proportion to number of bits used– 10 bit spreading code spreads signal across 10 times bandwidth of 1

bit code• One method:

– Combine input with spreading code using XOR– Input bit 1 inverts spreading code bit– Input zero bit doesn’t alter spreading code bit– Data rate equal to original spreading code

• Performance similar to FHSS

14

DS spectrum spreading is accomplished by means of a two-input exclusive-OR gate where A is low-speed NRZ data and B is high-speed PN sequence.

NRZ (non-return-to-zero) refers to a form of digital data transmission in which the binary low and high states.

A Pseudo-random Noise (PN) sequence is a sequence of binary numbers, e.g. ±1, which appears to be random.

A

B

CNRZ Data

PN code

A

B

C

DS spreading

BABAC

DS Spectrum Spreading Technique

15

DS spectrum dispreading is a process of data recovery from the composite spread-spectrum signal. This is accomplished by means of another exclusive-OR gate where the composite data C is applied to one input and identical PN sequence is applied to second input. The output Y is a decomposed signal which is the original NRZ data.

C

B

ADS De-spreading

C

B

Y = A BCBCY

DS Spectrum Spreading Technique

16

A

B

C

B

Y=A

NRZ Data

PN code

BABAC BCBCY

BBABABBABAY )()(

AY

•  CDMA is a Direct Sequence Spread Spectrum system. The CDMA system works directly on 64 kbit/sec digital signals.

• In CDMA, each bit time is subdivided into m short intervals called chips. Typically there are 64 or 128 chips per bit. Each station is assigned a unique m-bit chip sequence.

• To transmit a 1 bit, a station sends its chip sequence. • To transmit a 0 bit, it sends the one's complement of its chip

sequence. • No other patterns are permitted. • Thus for m = 8, if station A is assigned the chip sequence

00011011, it sends a 1 bit by sending 00011011 and a 0 bit by sending 11100100. 

17

DS Spectrum Spreading Technique

•  Increasing the amount of information to be sent from b bits/sec to mb chips/sec can only be done if the bandwidth available is increased by a factor of m, making CDMA a form of spread spectrum communication.

•  In order to protect the signal, the chip sequence code used is pseudo-random. It appears random, but is actually deterministic, so that the receiver can reconstruct the code for synchronous detection. This pseudo-random code is also called pseudo-noise (PN).

18

DS Spectrum Spreading Technique

19

DS Spectrum Spreading Technique

CDMA example: sender encoding, receiver decoding

Wireless, Mobile Networks 6-20

CDMA encode/decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cmd0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = S Zi,m.cmm=1

M

M

Wireless, Mobile Networks 6-21

CDMA: two-sender interference

using same code as sender 1, receiver recovers sender 1’s original data from summed channel data!

Sender 1

Sender 2

channel sums together transmissions by sender 1 and 2

Code Division Multiple Access (CDMA)

• Multiplexing Technique used with spread spectrum• Start with data signal rate D

– Called bit data rate• Break each bit into k chips according to fixed pattern

specific to each user– User’s code

• New channel has chip data rate kD chips per second• E.g. k=6, three users (A,B,C) communicating with base

receiver R• Code for A = <1,-1,-1,1,-1,1>• Code for B = <1,1,-1,-1,1,1>• Code for C = <1,1,-1,1,1,-1>

CDMA Example

CDMA Explanation• Consider A communicating with base• Base knows A’s code• Assume communication already synchronized• A wants to send a 1

– Send chip pattern <1,-1,-1,1,-1,1>• A’s code

• A wants to send 0– Send chip[ pattern <-1,1,1,-1,1,-1>

• Complement of A’s code• Decoder ignores other sources when using A’s code to

decode– Orthogonal codes

Spreading Sequences• Spreading sequences are very important in the design

of spread spectrum communication• Two categories of Spreading Sequences

– PN sequences– Orthogonal codes

• FHSS systems– PN sequences most common

• DSSS CDMA systems– PN sequences– Orthogonal codes

PN Sequences• PN sequences are periodic but appear random within

one period• PN sequences are very easy to generate

– Generated using LFSR• PN sequences are easy to re-generate and synchronize

at the receiver• PN sequences have good random properties

linear-feedback shift register - LFSR

Linear-feedback shift register• A linear-feedback shift register (LFSR) is a shift register whose

input bit is a linear function of its previous state.• The most commonly used linear function of single bits is exclusive-

or (XOR). Thus, an LFSR is most often a shift register whose input bit is driven by the XOR of some bits of the overall shift register value.

• The initial value of the LFSR is called the seed, and because the operation of the register is deterministic, the stream of values produced by the register is completely determined by its current (or previous) state. Likewise, because the register has a finite number of possible states, it must eventually enter a repeating cycle. However, an LFSR with a well-chosen feedback function can produce a sequence of bits that appears random and has a very long cycle.

• Applications of LFSRs include generating pseudo-random numbers, pseudo-noise sequences, fast digital counters, and whitening sequences. Both hardware and software implementations of LFSRs are common. 29

30

Linear-feedback shift register

A 4-bit Fibonacci LFSR with its state diagram. The XOR gate provides feedback to the register that shifts bits from left to right. 

31

16-bit Fibonacci LFSR 

Linear-feedback shift register

32

In case of FHSP, spread spectrum is achieved by frequently jumping from one carrier frequency to another; thus if there is interference or performance degradation at a given frequency, it only affects a small fraction of transmission.

The amount of time spent at each frequency, the dwell time, is an adjustable parameter but must be less than 400 ms.

Operates at 2.4 GHz band at data rate of 1Mbps and 2Mbps

Frequency Hopping Spread Spectrum

33

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme that transmits data over a number of orthogonal subcarriers. A conventional transmission uses only a single carrier modulated with all the data to be sent.

OFDM breaks the data to be sent into small chunks, allocating each sub-data stream to a sub-carrier and the data is sent in parallel orthogonal sub-carriers. As illustrated in Figure 1, this can be compared with a transport company utilizing several smaller trucks (multi-carrier) instead of one large truck (single carrier).

Fig.1 Single carrier vs. multi-carrier transmission

Orthogonal Frequency Division Multiplexing IEEE 802.11a and 802.11g

34

OFDM Versus FDM

35

1. It elongates the symbol period so that the signal is more robust against intersymbol interference caused by channel dispersions and multipath interference.

2. It divides the entire frequency band into narrow bands so that it is less sensitive to wide-band impulse noise and fast channel fades.

3. Splitting the channel into narrowband channels enables significant simplification of equalizer design in multipath environments.

OFDM offers many advantages over single-carrier modulations:

36

4. Different modulation formats and data rates can be used on different subcarriers depending on the noise level of individual subbands (the symbol periods are kept the same). In serial transmission, certain types of noise (such as timevarying tone interference) may cause an entire system to fail; the parallel OFDM system can avoid this problem by adaptively reducing the data rate of the affected subbands or dropping them.

5. OFDM can be implemented digitally using an inverse discrete Fourier transform and discrete Fourier transform (IDFT/DFT) pair (via the efficient fast algorithm IFFT/FFT pair), which greatly reduces the system complexity.

37

A baseband OFDM transmission model is shown in Figure 3. It basically consists of a transmitter (modulator, multiplexer and transmitter), the wireless channel, and a receiver (demodulator).

tfje 02

U

U

Tm

mT

)1(

...mNca 1

ˆ

S/PΣ x(t)

x0(t)

x1(t)

xNc-1(t)

Channel

r(t)

mN

mmC

aaa 110 ,...,,

ma0

ma1

mNca 1

tfje 12

tfj Nce 12

tfje 02

tfje 12

tfj Nce 12

………

U

U

Tm

mT

)1(

...

U

U

Tm

mT

)1(

...

ma0ˆ

ma1ˆFig.3

38

S/P IDFT

0

0

……

A0

A1

AN-1

P/S

mN

mmC

aaa 110 ,...,,

ma0

ma1

mNca 1

………

………………

D/A Channel

DFTA/D S/P

r0

r1

rN-1

……………… …

…

………

ma0ˆ

ma1ˆ

ma0ˆ

mNC

a 1ˆ

Useless

P/S

mN

mmC

aaa 110 ˆ,...,ˆ,ˆ

Fig.4 Transmitter and receiver by using FFT processing

r(t)

x(t)

Thank You

39