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 LIGHTRADIO HARMFUL RADIATION

VHF = VERY HIGH FREQUENCYUHF = ULTRA HIGH FREQUENCYSHF = SUPER HIGH FREQUENCY EHF = EXTRA HIGH FREQUENCY

4G CELLULAR56-100 GHz

3G CELLULAR1.5-5.2 GHz

1G, 2G CELLULAR0.4-1.5GHz

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 rateDetermined by data throughput requirementsSpeed calculation:

In point-to-point systems: RF data rate = (Dpl+Dao+Dro)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:

Dao would be 32 bits and Dro 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) + SNRmin

Nt is the thermal noise floor Ns is the system noise figure BW is the symbol rate SNRmin 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