The Internet Encyclopedia (Volume 3)

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REFERENCES 133

Communications, IEEE Transactions on Vehicular Tech-

nology, IEEE Communications Magazine, and other

sources. A number of Web sites containing a wealth of

information on some of the topics presented in this chap-

ter are also available at the National Institute of Stan-

dards and Technology Web site, http://w3.antd.nist.gov/

wctg/manet/wirelesspropagationbibliog.html (accessed

September 5, 2002), and the IEEE Communications Soci-

ety Web site, http://www.comsoc.org/pubs/pcm / (accessed

September 25, 2002).

APPENDIX: POWER UNITS

The power is normally expressed in decibel units. Power

(P 0 ) in milliwatts (mW) can be expressed in terms of deci-

bel units, dBm, as

P 0 (dBm)=10 log 10

[

P 0 (mW)

1 mW

]

. (A-1)

Thus, the power in dBm is an absolute measure of power

in mW. 10 mW power is 10 dBm, 1 W is 30 dBm, and 1μW

is –30 dBm. The unit dB, on the other hand, is the ratio of

two powers in identical units. For example, if the average

signal power isP 0 (mW) and the average noise power is

Pn(mW), the signal-to-noise ratio (S/N) can be expressed

as

(S/N)dB=10 log 10

[

P 0 (mW)

Pn(mW)

]

. (A-2)

Thus, signal-to-noise ratio expressed in dB carries infor-

mation on the strength of the signal relative to the noise.

If the signal-to-noise ratio is 0 dB, the signal power and

noise power are equal. If the signal-to-noise ratio is 20 dB,

the signal is 100 times stronger than the noise. If the

signal-to-noise ratio is –3 dB, the signal is only 50% of the

noise. Loss or attenuation can be expressed in dB units as

Loss(dB)=Transmit power(dBm)−Receive power(dBm).

(A-3)

If the transmitted power is 10 mW and the received

power is 1μW, one can calculate the transmission loss as

10 log 10 (10mW)−10 log 10 (1μW)= 10 −(−30)= 40 dB.

GLOSSARY

Base station (BS) A fixed station in a cellular system

that communicates with the mobile units within a cell

and may be located at the center (or the edge of the

cell). A BS has transmitting and receiving antennas

mounted on a tower and is the link between a mobile

unit and a mobile switching center.

Bit error rate (BER) The ratio of the number of bits

received in error to the total number of bits received.

Cell A geographic region served by a base station.

Doppler shift The upshift or downshift in frequency re-

sulting from the motion of the transmitter with respect

to the receiver or vice versa. If the relative speed isv,

the Doppler shift is

fd=

vcos(θ)

c

f 0 ,

where cis the speed of the electromagnetic wave

(3× 108 m/s), f 0 is the carrier frequency, andθis the

angle between the directions of the transmitter and re-

ceiver.

Hand set See mobile unit.

Inter symbol interference (ISI) The interference

caused by the overlapping of adjoining symbols or

pulses resulting in signal distortion. Viewed in the fre-

quency domain, existence of ISI implies that the chan-

nel cannot carry all frequencies with equal gain, with

gain decreasing as the frequencies go up.

Link budget The process of computing the maximum

transmission distance taking into account the trans-

mitted power, loss or attenuation, and power margin.

Macrocell, microcell, and picocell Microcell typically

will cover a few hundred meters while picocells are

formed when the base station antenna is located within

a building. The coverage of the picocell will be a few

meters or less. Macrocells typically cover a few kilome-

ters.

Mobile station (MS) See mobile unit.

Mobile unit (MU) A mobile unit is carried by the sub-

scriber. It may be handheld or vehicle mounted.

Outage Whenever the performance of the wireless sys-

tem does not reach the minimum acceptable levels, the

system goes into outage. For example, if a user requires

a minimum power ofPthto have an acceptable perfor-

mance, any time the received power goes belowPth, the

system goes into outage. The rate at which this happens

is the outage probability.

Power margin The excess power budgeted to account

for any effects other than the attenuation or the loss of

the signal. For example, if a user requires a thresh-

old power of Pth(dBm) to maintain acceptable per-

formance and M dB is the power margin to account

for fading, the user will require that the minimum ac-

ceptable powerPmin(dBm) at the receiver be set to

Pmin=Pth+M. This has the effect of reducing the avail-

able separation between the transmitter and receiver.

CROSS REFERENCES

See Radio Frequency and Wireless Communications;

Wireless Application Protocol (WAP); Wireless Communi-

cations Applications.

REFERENCES

Bertoni, H. L., Honcharenko, W., Maciel, L. R., & Xia,

H. H. (1994): UHF propagation prediction for wireless

personal communications.Proceedings of the IEEE, 82,

1333–1359.

Bultitude, R. J. C., Mahmoud, S. A., & Sullivan, W. A.

(1989). A comparison of indoor radio propagation

characteristics at 910 MHz and 1.75 GHz.IEEE Jour-

nal on Selected Areas in Communication, 7,20–30.