RF Considerations for wireless communications

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21 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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RF Considerations for
wireless communications

Jose Antonio Echenique

Agenda


Introduction to wireless communications


Wireless link implications


Medium: the radio spectrum



The three main parameters that define radio

frequency:


Over
-
the
-
air data rate


Receive Sensitivity


Transmit power



Other Considerations



Barriers to Future Growth

Introduction


Wireless Communication System: Any electrical
communication system that uses a naturally occurring
communication channel, such as air, water, earth.



Examples:


Sonar


Broadcast: Radio, TV, pagers, satellite TV, etc.


Two Way: walkie talkie, cell phones, satellite phones,


Wireless Local Area Networks, etc.

Wireless link implications


Communications channel is natural (air)


poor quality: fading, shadowing, weather, etc.



Medium regulated by governments


frequency allocation, licensing, etc.



Security issues

Medium: the radio spectrum


Wireless communications use the electromagnetic
spectrum, which is regulated by government institutions
such as the Federal Communications Commission
(FCC).



Regulations specify what bands of frequency can be
used for different applications. For instance: FM radio
has 88
-
108MHz and AM radio has 540
-
1600KHz .

Medium: the radio spectrum

SOURCE:
JSC.MIL

SOUND

LIGHT

RADIO

HARMFUL RADIATION

VHF = VERY HIGH FREQUENCY

UHF = ULTRA HIGH FREQUENCY

SHF = SUPER HIGH FREQUENCY

EHF = EXTRA HIGH FREQUENCY

4G CELLULAR

56
-
100 GHz

3G CELLULAR

1.5
-
5.2 GHz

1G, 2G CELLULAR

0.4
-
1.5GHz

Frequency and wave length:



= c/f :wave length

, speed of light c


3x108m/s, frequency f

RF CONSIDERATIONS


The three main parameters that define radio

frequency:


Over
-
the
-
air data rate


Receive Sensitivity


Transmit power



Range is a result of these three RF parameters and can be
used to define them


RF CONSIDERATIONS


Over
-
the
-
air data rate


Determined by data throughput requirements


Speed calculation:


In point
-
to
-
point systems:


RF data rate = (D
pl
+D
ao
+D
ro
)x(1+rt)/time


In a multipoint application, unless a polling scheme
or time
-
division multiple access (TDMA) scheme is
used, the calculation is more complicated.

RF CONSIDERATIONS


Over
-
the
-
air data rate (Example)


Assume a remote unit needs to send 1000 bytes of
payload data in a response to a 2
-
byte access point
command every 75 milliseconds:


D
ao
would be 32 bits and D
ro

80 bits


The total amount of data for both transmissions must occur in
75 milliseconds

RF data rate =
[((16b+32b+80b)+(8000b+32b+80b))x1.1]/0.075sec


= 120.853 Kb/sec

RF CONSIDERATIONS


Receive Sensitivity


Indicates the level of signal strength that must be
present to correctly receive data at a specified bit
-
error rate.


Receive Sensitivity = Nt + Ns + 10log(
BW
) + SNR
min


Nt is the thermal noise floor


Ns is the system noise figure


BW is the symbol rate


SNR
min

is the minimum signal
-
to
-
noise
-

ratio required for a
given bit
-
error rate

RF CONSIDERATIONS


Transmit power


It is usually driven by regulatory and power
-
consumptions considerations


For example, FCC allows up to 1 W of transmit power in
the United States in the 2.4 Ghz band

Other Considerations


Antenna Selection



Directionality


Omni (360 degree coverage) directional


Directional (limited range of coverage)



Gain


More gain means more coverage



Polarization

Other Considerations


Modulation Techniques



Amplitude Shift Keying (ASK):


very simple


low bandwidth requirements


very susceptible to interference



Frequency Shift Keying (FSK):


needs larger bandwidth




Phase Shift Keying (PSK):


more complex


robust against interference

1

0

1

t

1

0

1

t

1

0

1

t

Range Depends On...


Frequency


Transmit power


Radio sensitivity


Processing gain from access technique and redundancy


Interference effects

Barriers to Future Growth


Irreducible size of antennas


Rising level of RF emissions
-


interference problems and safety concerns


Finite spectrum


Lack of standards and interoperability of
hardware

THANK YOU …

Reference:


http://www.ce
-
mag.com/archive/02/Spring/cutler2.html


“Unlicensed Wireless Data Communications, Part II:
Specifying RF Parameters” by
Tim Cutler