Pages in This Article (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)
Next page -->
(Page 2) Cellular Telephone Basics
cont. . .
- With cellular radio we use a simple hexagon to represent a complex object: the geographical area covered by cellular radio antennas. These areas are called cells. Using this shape let us picture the cellular idea, because on a map it only approximates the covered area. Why a hexagon and not a circle to represent cells?
- When showing a cellular system we want to depict an area totally covered by radio, without any gaps. Any cellular system will have gaps in coverage, but the hexagonal shape lets us more neatly visualize, in theory, how the system is laid out. Notice how the circles below would leave gaps in our layout. Still, why hexagons and not triangles or rhomboids? Read the text below and we'll come to that discussion in just a bit.
- Notice the illustration below. The middle circles represent cell sites. This is where the base station radio equipment and their antennas are located. A cell site gives radio coverage to a cell. Do you understand the difference between these two terms? The cell site is a location or a point, the cell is a wide geographical area. Okay?
- Most cells have been split into sectors or individual areas to make them more efficient and to let them to carry more calls. Antennas transmit inward to each cell. That's very important to remember. They cover a portion or a sector of each cell, not the whole thing. Antennas from other cell sites cover the other portions. The covered area, if you look closely, resembles a sort of rhomboid, as you'll see in the diagram after this one. The cell site equipment provides each sector with its own set of channels. In this example, just below , the cell site transmits and receives on three different sets of channels, one for each part or sector of the three cells it covers.
Is this discussion clear or still muddy? Skip
ahead if you understand cells and sectors or come back if
you get hung up on the terms at some later point. For most of
us, let's go through this again, this time from another point
of view. Mark provides the diagram and makes some key points
"Most people see the cell as the blue hexagon, being
defined by the tower in the center, with the antennae pointing
in the directions indicated by the arrows. In reality, the cell
is the red hexagon, with
the towers at the corners, as you depict it above and I illustrate
it below. The confusion comes from not realizing that a cell
is a geographic area, not a point. We use the terms 'cell' (the
coverage area) and 'cell site' (the base station location) interchangeably,
but they are not the same thing."
Click here if you want
an illustrated overview of cell
Mark goes on to talk about cells and sectors and the kind of
antennas needed: "These days most cells are divided into
sectors. Typically three but you might see just two or rarely
six. Six sectored sites have been touted as a Great Thing by
manufacturers such as Hughes and Motorola who want to sell you
more equipment. In practice six sectors sites have been more
trouble than they're worth. So, typically, you have three antenna
per sector or 'face'. You'll have one antenna for the voice transmit
channel, one antenna for the set up or control channel, and two
antennas to receive. Or you may duplex one of the transmits onto
a receive. By sectorising you gain better control of interference
issues. That is, you're transmitting in one direction instead
of broadcasting all around, like with an omnidirectional antenna,
so you can tighten up your frequency re-use"
"This is a large point of confusion with, I think, most
RF or radio frequency engineers, so you'll see it written about
incorrectly. While at AirTouch, I had the good fortune to work
for a few months with a consultant who was retired from Bell
Labs. He was one of the engineers who worked on cellular in the
60s and 70s. We had a few discussions on this at AirTouch, and
many of the engineers still didn't get it. And, of course, I
had access to Dr. Lee frequently during my years there. It doesn't
get much more authoritative than the guys who developed the stuff!"
Jim Harless, a regular contributor, recently checked in regarding
six sector cells. He agrees with Mark about the early days, that
six sector cells in AMPS did not work out. He notes that "At
Metawave (link now dead) I've been actively involved in converting
some busy CDMA cells to 6-sector using our smart antenna platform.
Although our technology is vendor specific, you can't use it
with all equipment, it actually works quite well, regardless
of the added number of pilots and increase in soft
handoffs. In short, six sector simply allows carriers to
populate the cell with more channel elements. Also, they are
looking for improved cell performance, which we have been able
to provide. By the way, I think the reason early CDMA papers
had inflated capacity numbers were because they had six sector
cells in mind."
Mark says "I don't recall any discussion of anything
like that. But Qualcomm knew next to nothing about a commercial
mobile radio environment. They had been strictly military contractors.
So they had a lot to learn, and I think they made some bad assumptions
early on. I think they just underestimated the noise levels that
would exist in the real world. I do know for sure that the 'other
carrier jammer' problem caught them completely by surprise. That's
what we encountered when mobiles would drive next to a competitors
site and get knocked off the air. They had to re-design the phone.
Now, what about those hexagon shaped cell sites?
Mark van der Hoek says the answer has to do with frequency planning and vehicle traffic. "After much experimenting and calculating, the Bell team came up with the solution that the honeybee has known about all along -- the hex system. Using 3 sectored sites, major roads could be served by one dominant sector, and a frequency re-use pattern of 7 could be applied that would allow the most efficient re-use of the available channels."
A cell cluster. Note how neatly seven hexagon shaped cells fit together. Try that with a triangle. Clusters of four and twelve are also possible but frequency re-use patterns based on seven are most common.
Mark continues, "Cellular pioneers knew most sites would be in cities using a road system based on a grid. Site arrangement must allow efficient frequency planning. If sites with the same channels are located too closely together, there will be interference. So what configuration of antennas will best serve those city streeets?"
"If we use 4 sectors, with a box shape for cells, we either have all of the antennas pointing along most of the streets, or we have them offset from the streets. Having the borders of the sites or sectors pointing along the streets will cause too many handoffs between cells and sectors -- the signal will vary continously and the mobile will 'ping-pong' from one sector to another. This puts too much load on the system and increases the probablity of dropped calls. The streets need to be served by ONE dominant sector."
Do you understand that? Imagine the dots below are a road. If you have two sectors facing the same way, even if they are some distance apart, you'll have the problems Mark just discussed. You need them to be offset.
<-------Cell Site A ---------> <------Cell Site B-------> .............................................................................
"For a more complete discussion of the mathematics behind the hex grid, with an excellent treatment of frequency planning, I refer you to any number of Dr. Bill Lee's books."
Basic Theory and Operation
Cell phone theory is simple. Executing that theory is extremely complicated. Each cell site has a base station with a computerized 800 or 1900 megahertz transceiver and an antenna. This radio equipment provides coverage for an area that's usually two to ten miles in radius. Even smaller cell sites cover tunnels, subways and specific roadways. The area size depends on, among other things, topography, population, and traffic.
When you turn on your phone the mobile switch determines
what cell will carry the call and assigns a vacant radio channel
within that cell to take the conversation. It selects the cell
to serve you by measuring signal strength, matching your mobile
to the cell that has picked up the strongest signal. Managing
handoffs or handovers, that is, moving from cell to cell, is
handled in a similar manner. The base station serving your call
sends a hand-off request to the mobile switch after your signal
drops below a handover threshold. The cell site makes several
scans to confirm this and then switches your call to the next
cell. You may drive fifty miles, use 8 different cells and never
once realize that your call has been transferred. At least, that
is the goal. Let's look at some details of this amazing technology,
starting with cellular's place in the radio spectrum and how
The FCC allocates frequency space in the United States for
commercial and amateur radio services. Some of these assignments
may be coordinated with the International Telecommunications
Union but many are not. Much debate and discussion over many
years placed cellular frequencies in the 800 megahertz band.
By comparison, PCS or Personal Communication Services technology,
still cellular radio, operates in the 1900 MHz band. The FCC
also issues the necessary operating licenses to the different
Although the Bell System had trialed
cellular in early 1978 in Chicago, and worldwide deployment
of AMPS began shortly thereafter, American commercial cellular
development began in earnest only after AT&T's breakup in
1984. The United States government decided to license two carriers
in each geographical area. One license went automatically to
the local telephone companies, in telecom parlance, the local
exchange carriers or LECs. The other went to an individual, a
company or a group of investors who met a long list of requirements
and who properly petitioned the FCC. And, perhaps most importantly,
who won the cellular lottery. Since there were so many qualified
applicants, operating licenses were ultimately granted by the
luck of a draw, not by a spectrum auction as they are today.
The local telephone companies were called the wireline
carriers. The others were the non-wireline carriers.
Each company in each area took half the spectrum available. What's
called the "A Band" and the "B Band." The
nonwireline carriers usually got the A Band and the wireline
carriers got the B band. There's no real advantage to having
either one. It's important to remember, though, that depending
on the technology used, one carrier might provide more connections
than a competitor does with the same amount of spectrum.
[See A Band, B Band]
Learn more about cellular switches
Mobiles transmit on certain frequencies,
cellular base stations transmit on others. A and B refer to the
carrier each frequency assignment has. A channel is made up of
two frequencies, one to transmit on and one to receive.
Next page -->
[A Band, B Band] Actually,
the strange arrangement of the expanded channel assignments put
more stringent filtering requirements on the A band carrier,
but it's on the level of annoying rather than crippling. Minor
point. (back to text)
Pages in This Article (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)
Next page -->