Pmit lecture 03_wlan_wireless_network_2016

By-
Jesmin Akhter
Associate Professor
Institute of Information Technology
Jahangirnagar University
Wireless Network
PMIT-

2
Wireless LAN requirements:
High capacity, short distances, full connectivity, broadcast capability
Throughput: efficient use wireless medium
Number of nodes: Hundreds of nodes across multiple cells
Connection to backbone LAN: Use control modules to connect to both
types of LANs
Battery power consumption: Need long battery life on mobile stations
Transmission robustness and security: To protect Interference prone and
easily eavesdropped
License free operation
Handoff/roaming: Move from one cell to another
Dynamic 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

Gains
• 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
A
DS De-spreading
C
B
Y = A
BCBCY 

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

• 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

19
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
.cm
d0 = 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 0
channel
output
slot 1
channel
output
channel output Zi,m
sender
code
data
bits
slot 1 slot 0
d1 = -1
d0 = 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 0
channel
output
slot 1
channel
outputreceiver
code
received
input
Di = SZi,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 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
• FHSS systems
– PN sequences most common
• DSSS CDMA systems
– PN sequences

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
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

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

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).
tfj
e 02

 U
U
Tm
mT
)1(
...m
Nc
a 1
ˆ 
S/P
Σ x(t)
x0(t)
x1(t)
xNc-1(t)
Channel
r(t)
m
N
mm
C
aaa 110 ,...,, 
m
a0
m
a1
m
Nca 1
tfj
e 12
tfj Nc
e 12 
tfj
e 02
tfj
e 12
tfj Nc
e 12  
…
…
…

 U
U
Tm
mT
)1(
...

 U
U
Tm
mT
)1(
...
m
a0ˆ
m
a1ˆ
Fig.3

38
S/P IDFT
0
0
…
…
A0
A1
AN-1
P/S
m
N
mm
C
aaa 110 ,...,, 
m
a0
m
a1
m
Nca 1
…
…
…
…
…
…
…
…
…
D/A Channel
DFT
A/D S/P
r0
r1
rN-1
…
…
…
…
…
… …
…
…
…
…m
a0ˆ
m
a1ˆ
m
a0ˆ
m
NC
a 1ˆ 
Useless
P/S
m
N
mm
C
aaa 110 ˆ,...,ˆ,ˆ 
Fig.4 Transmitter and receiver by using FFT processing
r(t)
x(t)

Pmit lecture 03_wlan_wireless_network_2016

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (8)

Similaire à Pmit lecture 03_wlan_wireless_network_2016

Similaire à Pmit lecture 03_wlan_wireless_network_2016 (20)

Plus de Chyon Ju

Plus de Chyon Ju (6)

Dernier

Dernier (20)

Pmit lecture 03_wlan_wireless_network_2016