| Standard |
Type |
Year of Introduction |
Multiple Access |
Modulation |
Channel Bandwidth |
| E-TACS |
Cellular |
1985 |
FDMA |
FM |
25kHz |
| NMT-900 |
Cellular |
1986 |
FDMA |
FM |
12.5kHz |
| C-450 |
Cellular |
1985 |
FDMA |
FM |
20kHz/10kHz |
| GSM |
Cellular/PCS |
1990 |
TDMA |
GMSK |
200kHz |
| ERMES |
Paging |
1993 |
FDMA |
4-FSK |
25kHz |
| CT2 |
Cordless |
1989 |
FDMA |
GFSK |
100kHz |
| DECT |
Cordless |
1993 |
TDMA |
GFSK |
1.728MHz |
| DCS-1800 |
Cordless/PCS |
1993 |
TDMA |
GMSK |
200kHz |
Table1. Major Wireless Standard in Europe [1,2,3,4,5]
It seems that digital technologies win in the competition with analog technologies in the last 20 years,according to the history. Is it true? Let's
go in deep to these technologies.
A wireless communication system deals with two directions, a transmitting direction and a receiving direction.Normally,the size of the antenna
must be as large as one fourth of the wavelength of the signal to be transmited or received to get enough efficiency. For this reason, the original
signal (normally the voice) with a large wavelength must be transfered to a higher frquency (smaller wavelength) to downsize the antenna. At the
transmitting end, the original signal is imposed on a locally generated radio frequency (RF) signal called a carrier.This process is called modulation.
This carrier signal,along with the information signal imposed on it, is then radiated by the antenna.At the receiving end, the signal is picked up by
another antenna and fed into a receiver where the desired carrier with the imposed information signal is selected from among all of the other signals
impinging on the antenna.The information signal (e.g., voice) is then extracted from the carrier in a process referred to as demodulation.
The propagation of the signal in free space is not fluent. There may be some negative influence to the received signal.First,the signal becames
weaker and weaker during the propagation;Second,interference from some noise will distort the information signal;An then, some propagation
mechanisms such as reflection,diffraction and scattering,will distort the information signal too.It is even worse in mobile systems -- where one or both
of the terminals (transmitters and receivers) can move about -- due to an environment that changes dynamically from moment to moment. To get rid
of these problems, we need more precise models to describe the propagation, and more modulation technology to avoid distortion.
when we mention analog technology,we refer to electronic transmission accomplished by adding signals of varying frequency or amplitude to
carrier waves of a given frequency of alternating electromagnetic current.Broadcast and phone transmission have conventionally used analog technology.
Analog also connotes any fluctuating, evolving, or continually changing process. Analog is usually represented as a series of sine waves. The term
originated because the modulation of the carrier wave is analogous to the fluctuations of the voice itself.
One of the distinctive character betweent analog and digital wireless technology is their modulation method.For analog system, we usually use FM,
AM, DSB,SSB and VSB techniques.
FM: Frequency Modulation. It's the most popular analog modulation technique. In FM, the amplitude of the modulated carrier signal is
kept constant while its frequency is varied by the modulating message signal.Thus,FM signals have all their information in the phase
or quency of the carrier.
AM: Amplitude Modulation. There is a linear relationship between the quality of the received signal and the power of the received signal
since AM signals superimpose the exact relative amplitudes of the modulating signal onto the carrier.Thus,AM signals have all their
information in the amplitude of the carrier.
DSB: Double Sideband AM. This kind of AM removes the carrier spectrum from total signals to increase modulation efficiency.
SSB: Single Sideband AM. Since both sideband of an AM signal carry the same information,it is possible to remove one of them without
losing any information.SSB AM transmit only one of the sideband about the carrier,and hence occupy only half the bandwidth of
conventional AM systems.
Digital describes electronic technology that generates, stores, and processes data in terms of two states: positive and non-positive. Positive is
expressed or represented by the number 1 and non-positive by the number 0. Thus, data transmitted or stored with digital technology is expressed as
a string of 0's and 1's. Each of these state digits is referred to as a bit (and a string of bits that a computer can address individually as a group is a byte).
The modulation techniques of digital wireless signal are more complicated than those of analog signals.Here I just give you a list of them, for
further information please check the reference.
Linear Modulation Techniques:
BPSK : Binary Phase Shift Keying
DPSK : Differential Phase Shift Keying
QPSK : Quadrature Phase Shift Keying
Constant Envelope Modulation Techniques:
BFSK : Binary Frequency Shift Keying
MSK : Minimum Shift Keying
GMSK : Gaussian Minimum Shift Keying
Combined Linear and Constant Envelope Modulation Techniques:
MPSK : M-ary Phase Shift Keying
QAM : M-ary Quadrature Amplitude Modulation
MFSK : M-ary Frequency Shift Keying
Spread Spectrum Modulation Techniques:
DS-SS : Direct Sequence Spread Spectrum
FH-SS : Frequency Hopped Spread Spectrum
Because the frequency spectrum resource is not unlimited,and wireless backbone equipment is not free,users in a given area must contend for a
limited number of channels.So there comes the multiple access and duplexing techniques.
FDMA:
The simplest and most straightforward method is known as frequency division multiple access (FDMA). With FDMA, the available
spectrum is divided into non-overlapping slots in the frequency dimension or domain. These frequency slots or channels are then
put into a pool and assigned to users on either a manual or automated basis for the duration of their particular call[3].FDMA
technique has been applied to almost every modern wireless communication systems,no matter analog system or digital system.
TDMA:
Time Division Multiple Access(TDMA) systems divide the radio spectrum into time slots, and in each slot only one user is allowed to
either transmit of receive. In TDMA systems, each user occupies a cyclically repeating time slot,so a channel may be thought of as
particular time slot that reoccurs every frame,where N time slots comprise a frame.TDMA systems transmit data in a noncontinuous way
for any user.This implies that, unlike in FDMA systems which accommodate analog FM,digital data and digital modulation must be used
with TDMA.
CDMA:
Code Division Multiple Access(CDMA) is both a modulation and an access technique that is based upon the spread-spectrum concept [3].
In CDMA systems,the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal.All users
in a CDMA system use the same carrier frequency and may transmit simultaneously. Each user has its own pseudorandom codeword which
is approximately orthogonal to all other codewords. The receiver performs a time correlation operation to detect only the specific
desired codeword. So CDMA is also a digital technique.It claims eight to fifteen times the capacity of analog[4].
3.Who is the winner?
After introducing so many details of analog and digital wireless technology,we can make sure that digital technology has more advantages than analog
system ,but it's also much more complicated than analog system. What's the result of applying these technologies to real systems? Let's make a comparison
of the real systems:
|
|
Analog
|
Digital
|
| standard
|
EIA–553 (AMPS)
|
IS–54 (TDMA + AMPS)
|
| spectrum
|
824 MHz to 891 MHz
|
824 MHz to 891 MHz
|
| channel bandwidth
|
30 kHz
|
30 kHz
|
| channels
|
21 CC/395 VC
|
21 CC / 395 VC
|
conversations per
channel
|
1
|
3 or 6
|
| subscriber capacity
|
40 to 50 conversations per cell
|
125 to 300 conversations per cell
|
| TX/RCV type
|
continuous
|
time shared bursts
|
| carrier type
|
constant phase variable frequency
|
constant frequency variable phase
|
mobile/base
relationship
|
mobile slaved to base
|
authority shared cooperatively
|
| privacy
|
poor
|
better—easily scrambled
|
| noise immunity
|
poor
|
high
|
| fraud detection
|
ESN plus optional password (PIN)
|
ESN plus optional password (PIN)
|
Table2. Comparison of two systems with analog and digital technology[4]
Digital system ! Although some users still choose to use analog systems,and analog systems will not disappear in a short time, but that's mainly
because of nontechnical reasons(such as the price). In the near future, all the drawbacks of digital system will be overcomed, and more and more new
services such as email, internet access, short message, video phone,etc., will be available by applying digital technology.
The digital wireless technology still represents the future.
[1] Comcast Cellular Communications, Inc.,Digital FAQ., 1999, [refered 8.10.1999]
<http://www.comcastcellular.com/store/digital/DigitalFAQ.htm>
[2] Farley,Tom ,Digital Wireless Basics, 1997, [refered 8.10.1999]
<http://www.privateline.com/PCS/PCS.htm>
[3] Hatfield,Dale N. ,Technological Trends In Wireless Telecommunications, 5.11.1996, [refered 8.10.1999]
<Http://tap.gallaudet.edu/Hatfield_a.htm>
[4] International Engineering Consortium ,Cellular Communications Tutorial, 1999, [refered 8.10.1999]
<http://www.webproforum.com/cell_comm/index.html>
[5] WirelessAdvisor.com Corp., Analog, Digital -- What's the Difference?, 1999, [refered 8.10.1999]
<http://wirelessadvisor.com/analog-cellular.cfm>
- Digital Mobile Phones
- A introduction of digital wireless communication systems and mobile phones in different country.
- Wireless Communication Standards (North-American)
- This document provides a sorted index into TIA Standards for cellular and PCS communication.
- CDMA vs TDMA
- A brief introduction to the difference between CDMA and TDMA systems.
- Wireless Telecom Glossary
- A collection of frequently used terms in wireless communication field.
|