 |
|
|
|
Mobile Phones - The Basics
Written by T.Farley and Mark VanderHoek
X. Appendix
I. Introduction
Cellular radio provides mobile telephone service by employing a network
of cell sites distributed over a wide area. Cell sites incorporate a radio
transceiver to manage, send and receive traffic from the mobile phones
in its area, a tower and its antennas, and a link to a distant switch
called an MTSO. This mobile telecommunications switching office places
calls from land based telephones to wireless customers, switches calls
between cells as mobile phones travel across cell boundaries, and authenticates
wireless customers before they make calls.
Cellular uses a principle called frequency reuse to greatly increase
customers served. Low powered mobile phones and radio equipment at each
cell site permit the same radio frequencies to be reused in different
cells, multiplying calling capacity without creating interference. This
spectrum efficient method sharply contrasts with earlier mobile systems
that used a high powered, centrally located transmitter, to communicate
with high powered car mounted mobile phones on a small number of frequencies,
channels which were then monopolized and not re-used over a wide area.
Complex signaling routines handle call placements, call requests, handovers,
or call transfers from one cell to another, and roaming, moving from one
carrier's area to another. Different cellular radio
systems use frequency division multiplexing (analog), time division multiplexing
(TDMA), and spread spectrum (CDMA) techniques. Despite different operating
methods, AMPS, PCS, GSM, E-TACS, and NMT are all cellular radio. That's
because they all rely on a distributed network of cell sites employing
frequency re-use. Is your head spinning yet? Take it easy. Let's ease
into this cellular discussion by discussing some history first.
II. History
United States cellular planning began in the mid
1940s-after World War II, but trial service did not begin until 1978,
and full deployment in America not until 1984. This delay must
seem odd compared to today's furious pace of wireless development, but
there were many reasons for it. Limited technology, Bell System ambivalence,
and government regulation limited radio-telephone progress.
As the vacuum tube and the transistor made possible the early telephone
network, the wireless revolution began only after low cost microprocessors,
minature circuit boards, and digital switching became available. And while
AT&T personnel built the finest landline telephone system in the world,
Bell System management never truly committed to mobile phone telephony.
The U.S. Federal Communications Commission also contributed to the delay,
stalling for decades on granting more frequency space. This limited the
number of mobile phone customers, and thus prevented any new service from
developing since serving those few customers would not make economic sense.
But in Europe, Scandinavia, Britain, and Japan, where state run telephone
companies operated without competition, and where regulatory interference
was minor, cellular came at the same time or later, not sooner than in
America. It remains a question, then, on what the biggest factor limiting
cellular development truly was.
Although theorized for years before, Bell Laboratories' D.H. Ring articulated
the cellular concept in 1947 in an unpublished paper. W.R.Young, writing
in The Bell System Technical Journal, said Ring' s paper stated
all of cellular's elements: a network of small geographical areas called
cells, a low powered transmitter in each, traffic controlled by a central
switch, frequencies reused by different cells and so on. Young states
that from 1947 Bell teams "had faith that the means for administering
and connecting to many small cells would evolve by the time they were
needed." While cellular waited to evolve, a more simple system was
used for mobile telephony, a technology that, as it finally matured, originated
some practices that cellular radio later employed.
On June 17, 1946 in Saint Louis, Missouri, AT&T and Southwestern
Bell introduced the first American commercial mobile radio-telephone service.
It was called simply Mobile Telephone Service or MTS. Car drivers used
newly issued vehicle radio-telephone licenses granted to Southwestern
Bell by the FCC. These radios operated on six channels in the 150 MHz
band with a 60 kHz channel spacing, twice the size of today's analog cellular.
Bad cross channel interference, something like cross talk in a landline
phone, soon forced Bell to use only three channels. In a rare exception
to Bell System practice, subscribers could buy their own radio sets and
not AT&T's equipment.
Installed high above Southwestern Bell's headquarters at 1010 Pine Street,
a centrally located antenna transmitting 250 watts paged mobiles when
a call was for them. Automobiles responded not by transmitting to the
headquarters building but to a scattering of receiving sites placed around
the city, usually atop neighborhood central switching offices. That's
because automobiles used lower powered transmitters, of course, and could
not always get a signal back to the middle of town. These central offices
relayed the voice traffic back to the manually operated switchboard at
the HQ where calls were switched. So, although the receiver sites were
passive, merely collectng calls and passing them on, they did presage
the cellular network of distributed, interactive cell sites.
One party talked at a time with MTS. You pushed a handset button to
talk, then released the button to listen. This eliminated echo problems
which took years to solve before natural, full duplex communications were
possible. Transmitting and receiving frequencies were different, offset
from each other to prevent interference. Operators placed all calls so
a complex signaling routine wasn't required. The Bell System was not interested
in automatic dial up and call handling until decades later, instead, independent
wireless companies or Radio Common Carriers, pioneered these techniques.
On March 1, 1948 the first fully automatic radiotelephone service began
operating in Richmond, Indiana, eliminating the operator to place most
calls. The Richmond Radiotelephone Company bested the Bell System by 16
years. AT&T didn't provide automated dialing for most mobile phones
until 1964, lagging behind automatic switching for wireless as they had
done with landline telephony. (As an aside, the Bell System did not retire
their last cord switchboard until 1978.) Most systems, though, RCCs included,
still operated manually until the 1960s. In 1964 the Bell System introduced
Improved Mobile Telephone Service or IMTS, a replacement to the badly
ageing Mobile Telephone System. It worked in full-duplex so people didn't
have to press a button to talk. Talk went back and forth just like a regular
telephone. It finally permitted direct dialing, automatic channel selection
and reduced bandwidth to 25-30 kHz. Operating details foreshadowed analog
cellular routines, the complexity of which we will see soon enough. Here's
how AT&T described automatic dialing:
Control equipment at the central office continually chooses an idle
channel (if there is one) among the locally equipped complement of channels
and marks it with an "idle" tone. All idle mobiles scan these
channels and lock onto the one marked with the idle tone. All incoming
and outgoing calls are then routed over this channel. Signaling in both
directions uses low-speed audio tone pulses for user identification and
for dialing.
In January,1969 the Bell System employed frequency reuse in a commercial
service for the first time. On a train. From payphones. As we've mentioned
before, frequency re-use is the defining principle or concept of cellular.
"Delighted passengers" on Metroliner trains running between
New York City and Washington, D.C. "found they could conveniently
make telephone calls while racing along at better than 100 miles an hour."
Six channels in the 450 MHz band were used again and again in nine zones
along the 225 mile route. A computerized control center in Philadelphia
managed the system. The main elements of cellular were finally coming
into being, and would result in a fully functional system in 1978.
Let's not dismiss early radio systems too quickly, especially since
we need to contrast them with cellular radio, to see what makes cellular
different. IMTS or the Improved Mobile Telephone System (and its variants)
is still around, serving isolated and rural areas not well covered by
cellular. Cellular service may be in 90% of urban areas, but it only reaches
30% to 40% of the geographical area of America. Most IMTS equipment operates
in the UHF band. Again, it uses a centrally located transmitter and receiver
serving a wide area with a relatively few frequencies and users. Only
larger areas will have additional receiving sites like in Saint Louis.
Most areas allow you to dial out directly from your car, however, there
are still places where the operator comes up on frequency to place the
call for you. A single customer can drive 25 miles or more from the transmitter,
however, only one person at a time can use that channel.
This limited availability of frequencies and their inefficient use were
two main reasons for cellular's development. The key to the system is,
to be stupidly and offensively repetitive, the concept of frequency reuse.
It is the chief difference between IMTS and cellular. In older mobile
phone services a single frequency serves an entire area. In cellular,
that frequency is used again and again. More exactly, a channel is used
again and again, a radio channel being a pair of frequencies, one to transmit
on and one to receive.
Now, since we are defining cellular so much, let's look at the terminology
and structure of cells.
|
|
Home
3
(3G)