tseng:1
Wireless Transmission Fundamentals(Dayem’s book, Chapter 4)
(Nico’s book, Chapter 2)
Electromagnetic Spectrum Wireless Propagation Models Digital Modulation Techniques Multiple Access Performance Issues Cellular and Ad Hoc Concepts Link Budget Analysis
tseng:2
Electromagnetic Waves predicted by British physicist James Maxwell in
1865, and observed by German physicist Heinrich Hertz in 1887
These waves are created by the movement of electrons and have the ability to propagate through space. using appropriate antennas, transmission and reception
of electromagnetic waves through space becomes feasible.
the speed of electron vibration determines the wave’s frequency.
Hertz: how many times the wave is repeated in 1 sec. (to honor Heinrich Hertz)
tseng:3
Wavelength and Amplitude l = wavelength, f = frequency, c = speed of
light
tseng:4
Electromagnetic Spectrum spectrum: range of electromagnetic
radiation band: spectrum parts
tseng:5
Radio Waves
HF band enables worldwide transmission: HF signals are reflected off the ionosphere and thus can
travel very large distances
tseng:6
Microwaves small wavelengths compared to radio waves easily attenuated by objects
tseng:7
Infrared emitted by very hot objects
such as human body (night vision applications) frequency depends on the temperature of the
emitting body line-of-sight, point-to-point
of no use outdoors (interfered by heat of sun) short-rang: 10 meters IrDA: Infrared Data Association
tseng:8
Microwave and Infrared Bands Most wireless networking traffic is in the
microwave frequency bands. some licensed, some unlicensed
Infrared: for short-range wireless communication
tseng:9
Spectrum Regulation ITU = Int’l Telecommunications Union
a worldwide spectrum regulation org. the world is split into 3 parts:
American continentEurope, Africa, and former Soviet unionrest of Asia and Oceania
Rules of assigning spectrum lottery auction comparative bidding
such as pricing, technology, etc.
tseng:10
Licensed Microwave Band Examples: cellular, paging, PCS Use of a license is typically in an order of
10 years. A company can’t have the license and not use
it. Bandwidth is regarded as a resource that the
public wants and needs.
tseng:11
Unlicensed Microwave Band Also on the same microwave band, but no
license required. To avoid interfering primary (licensed) users,
spreading spectrum is required. Two types:
FHSS: Frequency-hopping spread spectrumDSSS: Direct sequence spread spectrum
Also known as ISM band. industrial, scientific, and medical
tseng:12
Model of Wireless Propagation
Free space path loss Doppler shift Slow/fast fading Error modeling
tseng:13
Shannon’s Formula an upper bound on the bit rate W of any channel of
bandwidth H Hz:W = H log2(1 + S/N)S/N = signal to thermal noise ratio
However, in real world, the upper bound is difficult to achieve due to: free space path loss
proportional to r-2, where r is the distance between transmitter and receiver (sometimes at higher exponent)
Doppler shifta signal transmitter and receiver are moving relative to one
another slow/fast fading
tseng:14
Slow Fading
tseng:15
Definitions Reflection:
when an electromagnetic wave falls on an object with dimension very large compared to the wave’s wavelength
Scattering: when obstructed by objects with dimensions in
the order of the wavelength Diffraction (or shadowing):
when the wave falls on an impenetrable object in which case, the secondary waves are formed
behind the obstructing body
tseng:16
Fast Fading: Multipath Effect waves traveling along different paths may
be completely out of phase when they reach the antenna (thereby canceling each other)
tseng:17
Multipath propagation delay can degrade performance in indoor/outdoor environment. When the path length differences are short, the
effect is smaller. multipath fading is also referred as fast fading
When LOS (line of sight) exists, this kind of fading is known as Ricean Fading
When LOS does not exist, this kind of fading is known as Rayleigh Fading
tseng:18
Propagation Models We say that the relative strength
of signal x, P(x), to that of signal y, P(y), is D dB, if D = 10 log10(P(x)/P(y))
In free space, the average path loss (PL) at a distance of r is (in dB): PL(r) = PL(r0) +
10n log(r/r0) r0 = reference distance
(typically 1 Km for macrocells; and 100 m for microcells)
n = environmental factor (typically >= 2)
To take into account of the shadowing effect PL(r) = PL(r0) +
10n log(r/r0) + X
X = zero-mean Gaussian random variable with standard deviation
tseng:19
Digital Modulation Techniques
Binary Modulation Phase Shift Keying Minimum Shift Keying /4-Shifted QPSK
tseng:20
Basics Convert digital stream into the analog signal
A(t)cos(wt + ), where w = 2f. The characteristics in this formulation that
may be changed are: amplitude frequency phase
Ex: ASK = amplitude shift keying; FSK = frequency shift keying; PSK = phase shift keying
tseng:21
Most systems modulate the information onto a carrier centered in a (small) allocated spectrum.
tseng:22
tseng:23
Binary Modulation Scheme Amplitude Shift Keying (ASK):
using ON/OFF to represent 1/0 “keying”: like a telegraph key
Frequency Shift Keying (FSK): 1/0 represented by two different frequencies
separated by some distance
tseng:24
Binary Phase Shift Keying Binary Phase Shift Keying (BPSK)
use alternative sine wave phases to encode bits simple to implement very robust, used extensively in satellite
communications
tseng:25
Quarternary Phase Shift Keying QPSK:
multi-level modulation: 2 bits per symbol more spectrally efficient, more complex
receiver
tseng:26
Differential PSK (DPSK) 1 = changing the phase relative to the
previous symbol by some amount 0 = having the same phase as the previous
symbol adv: self-clocked
tseng:27
/4-Shifted QPSK coding by bit pairs varying the phase of the
current bit pair to the phase of the previous bit pair by a multiple of /4
example: 10 10 01 (Fig. 2.27)
(i.e., -/4, -/4, +5/4)
tseng:28
Hybrid of PSK + ASK QAM = Quadrate Amplitude Modulation
mixture of PSK and ASK 3 bits at a time
tseng:29
Multiple Access
defining how nodes in a wireless network to share a common medium
tseng:30
Objectives MAC layer is to define how a user access a
channel when he needs one. Random access: ALOHA and CSMA Ordered access: Token bus and Token Ring Deterministic access: FDMA, TDMA, and
CDMA Combinations: TDMA-over-FDMA, TDD-
CDMA, and TDMA/CSMA
tseng:31
FDMA frequency division multiple access
** NMT = nordic Mobile Telephony
tseng:32
TDMA time division multiple access
tseng:33
CDMA code division multiple access
each station has a “station code” each bit is encoded by station code
code 1 is mapped to 1code 0 is mapped to -1
tseng:34
ALOHA A type of packet-radio network. The first well-known wireless network as
well as network system. Very simple, but not efficient!
Variations: pure-ALOHA: whenever desired, send the
packet slotted-ALOHA: further divide time axis into
slots
tseng:35
CSMA Before sending, sense the carrier.
tseng:36
Persistent and Non-persistent CSMA Persistent CSMA:
when the medium is busy, a station can persistently wait for the medium to become idle, and then transmit with a probability p
This is called 1-persistent or p-persistent CSMA.
Non-persistent CSMA: A station can stop monitoring the wireless
medium, and listen to the medium again at predefined time.
tseng:37
Hidden-Node Problem CSMA has the following problem:
when two nodes are too far away, carrier sensing is difficult
tseng:38
CSMA/CA CA = collision avoidance
sender first does carrier sense sender broadcasts RTS (request to send) to
receiver receiver broadcasts CTS (clear to send) to
sender then send data packet
Q: Is CSMA/CD possible in wireless network?
tseng:39
tseng:40
Ordered MAC Techniques Can a token-ring or token-bus protocol be
applied to a wireless network?
Problems: mobility (nodes joining or leaving the ring) token loss
tseng:41
Comparison and Summary Random access: CSMA
under light load: fast response time under heavy load: throughput declines simplicity
Deterministic protocols: TDMA, FDMA guaranteed bandwidth larger average delay small delay variance
Hybrid: CSMA/TDMA adaptive, higher overhead
tseng:42
Spread Spectrum
FHSS DSSS
tseng:43
Spread Spectrum Technology Spread spectrum must be used in ISM band. Two major technologies:
Frequency Hopping SS (FHSS) Direct Sequence SS (DSSS)
Located at the PHY of the network stack:
tseng:44
FHSS Most Wireless LANs
use the ISM bands as secondary users. They must use SS in
order not to interfere with the primary users.
FHSS: send info in different frequencies on different time slots.
Hopping Pattern In each time slot, the
occupied frequencies are separated by some distance to avoid interference.
tseng:45
FHSS is different from FDM (frequency division multiplexing).
Example: (Fig. 4.7) In the 2.4 GHz band of ISM, we have a space
of 80 MHz. (2400~2483MHz) A typical bandwidth of the information signal
is 1 MHz.Maximum occupancy is 1MHz regulated by FCC.
One time slot = 0.1 sec.
tseng:46
Primary vs. Secondary Users In FHSS, a typical power limit is 1 watt. For primary users, the power limit is much larger.
So the interference from FHSS will not be noticeable primary users.
For FHSS secondary user, when there is 1 primary user there will be a
throughput loss of 1/80 = 1.25%; when there are 2 primary users there will be a
throughput loss of 2/80 = 2.5%. fig 4.8
tseng:47
primary user
primary user
tseng:48
DSSS The input data stream is transferred to a
chip stream that is x times higher by XOR. a chip is 0 or 1, but is called so to distinguish
from a bit example: x = 11, 13, 15, 16 chips/bit
tseng:49
The frequency spectrum is spread out and the spectral energy is x times lower. It’s so low that primary users are not
interfered.
tseng:50
Comparison of Interference Degradation due to existence of
interference: FHSS: linear to the level of interference DSSS:
degraded by half after a certain point (since it typically occupies 50% of the bandwidth)
won’t work after a certain level
tseng:51
tseng:52
tseng:53
Link Budget Analysis
“Tutorial on Basic Link Budget Analysis” Application Note, June 1998, AN9804.1, Intersil Co.
tseng:54
Communication Basics When evaluating a wireless link, there are 3
most important questions to be answered: How much radio frequency (RF) power is
available? How much bandwidth is available? What is the required reliability?
evaluated by BER (bit error rate)
tseng:55
Link Budget Example 1 Wireless Link to Modem
required rate: 40 Kbps (28.8 Kbps plus framing, overhead, checksum)
range: 5 meters BER: 10-6
tseng:56
Choices of Technology: 900 MHz
2.4GHz and 5GHz are not selected since the required rate is low.
no spread spectrumsince low transmission power is sufficient for 5
meters Orthogonal FSK
simplicity: two separated frequencies (one for “1” and the other for “0”)
separated by 40 kHz (called “orthogonal” since frequency-separation/bit-rate = 1)
tseng:57
Link Budget Example 2 Wireless USB
required data rate = 2 Mbps (1.408 Mbps plus framing, overhead, and checksum)
range = 30 meters BER = 10-6
tseng:58
Selection of Technologies: ISM band in 2.4 GHz (with 83MHz of band to
use) DSSS spreading to support long distance
transmissionwill occupy 2 x 11 = 22 MHz of bandwidth due to
spreading DQPSK (differential quadrature phase shift
keyed) modulation
tseng:59
Performance Increasing Techniquesfor Wireless Networks
antenna diversity coding power control
tseng:60
Diversity definition:
to send multiple copies of the same information signal through several channels
goal: to combat fading in wireless channels
example: time, frequency, antenna
tseng:61
Antenna Diversity also known as space diversity method
a set of array elements (also referred to as branches), spaced sufficiently apart from each other
usually 2 elements can combat multipath fading
because multipath fading is usually independent at distances in the order of channel’s wavelength
tseng:62
Example a 2-branch diversity system
a number of algorithms have been proposed to reconstruct the original transmission
ex: pick the strongest signal from one of the antennas
tseng:63
Smart Antennas multi-antennas that change in order to adapt
to the conditions of wireless channels can focus toward the receivers can focus to the transmitters also known as beamforming
Already available for several years not widely used due to costs
tseng:64
Coding Parity check Hamming code Cyclic redundancy check (CRC)
tseng:65
Power Control properly tuning the transmission power to
reduce coverage and interference
tseng:66
Summary What have we discussed?
Electromagnetic Spectrum Wireless Propagation Models Digital Modulation Techniques Multiple Access Performance Issues Cellular and Ad Hoc Concepts Link Budget Analysis Performance Improvement Techniques
