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How Cellular
Phone Technologies Compare |
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Encoding and
Multiplexing |
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| Overview |
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| With thousands of cellular phone
calls going on at any given time within a city, it certainly
would not work for everyone to talk on the came channel at once
(as in CB and short-wave radios). Therefore, several different
techniques were developed by cell phone manufacturers to split
up the available bandwidth into many channels each capable of
supporting one conversation. The following sections will discuss
each technology and how it works. |
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| Analog vs.
Digital |
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| While the distinction between analog
and digital encoding is probably obvious to most readers, a
short discussion is included for those who are not. Essentially,
analog broadcasts audio as a series of continuously changing,
voltage levels representing the amplitude of the voice conversation.
When sent on the cell phone network using the standard frequency
modulation (meaning voltage levels translate into frequency
shifts) into channels separated by 30 kHz, we find that the
amplitude can be effectively transmitted at 15 kHz due to Nyquist
limitations. |
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| Instead of sending data as various
voltage levels, a digital signal quantizes the voltage levels
into a number of bins (typically 28 or 256 representing an 8-bit
encoding). These bins are encoded as a binary number and sent
as a series of ones and zeros. This allows for digital compression
in the encoding stage enabling voice to be sent at as little
as 8000 bits per second. |
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| FDMA |
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| FDMA stands for "frequency division
multiple access" and, though it could be used for digital systems,
is exclusively used on all analog cellular systems. Essentially,
FDMA splits the allocated spectrum into many channels. In current
analog cell systems, each channel is 30 kHz. When a FDMA cell
phone establishes a call, it reserves the frequency channel
for the entire duration of the call. The voice data is modulated
into this channel’s frequency band (using frequency modulation)
and sent over the airwaves. At the receiver, the information
is recovered using a band-pass filter. The phone uses a common
digital control channel to acquire channels. |
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| FDMA systems are the least efficient
cellular system since each analog channel can only be used by
one user at a time. Not only are these channels larger than
necessary given modern digital voice compression, but they are
also wasted whenever there is silence during the cell phone
conversation. Analog signals are also especially susceptible
to noise – and there is no way to filter it out. Given the nature
of the signal, analog cell phones must use higher power (between
1 and 3 watts) to get acceptable call quality. Given these shortcomings,
it is easy to see why FDMA is being replaced by newer digital
techniques. |
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| TDMA |
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| TDMA stands for "time division multiple
access." TDMA builds on FDMA by dividing conversations by frequency
and time. Since digital compression allows voice to be sent
at well under 10 kilobits per second (equivalent to 10 kHz),
TDMA fits three digital conversations into a FDMA channel (which
is 30 kHz). By sampling a person’s voice for, say 30 milliseconds,
then transmitting it in 10 milliseconds; the system is able
to offer 3 timeslots per channel in a round-robin fashion. This
technique allows compatibility with FDMA while enabling digital
services and easily boosting system capacity by three times.
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| While TDMA is a good digital system,
it is still somewhat inefficient since it has no flexibility
for varying digital data rates (high quality voice, low quality
voice, pager traffic) and has no accommodations for silence
in a telephone conversation. In other words, once a call is
initiated, the channel/timeslot pair belongs to the phone for
the duration of the call. TDMA also requires strict signaling
and timeslot synchronization. A digital control channel provides
synchronization functionality as well as adding voice mail and
message notification. Due to the digital signal, TDMA phones
need only broadcast at 600 miliwatts. |
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| CDMA |
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| CDMA stands for "code division multiple
access" and is both the most interesting and the hardest to
implement multiplexing method. CDMA has been likened to a party:
When everyone talks at once, no one can be understood, however,
if everyone speaks a different language, then they can be understood.
CDMA systems have no channels, but instead encodes each call
as a coded sequence across the entire frequency spectrum. Each
conversation is modulated, in the digital domain, with a unique
code (called a pseudo-noise code) that makes it distinguishable
from the other calls in the frequency spectrum. Using a correlation
calculation and the code the call was encoded with, the digital
audio signal can be extracted from the other signals being broadcast
by other phones on the network. From the perspective of one
call, upon extracting the signal, everything else appears to
be low-level noise. As long as there is sufficient separation
between the codes (said to be mutually orthogonal), the noise
level will be low enough to recover the digital signal. Each
signal is not, in fact, spread across the whole spectrum (12.5
MHz for traditional cellular or 60 MHz in PCS cellular), but
is spread across 1.25 MHz "pass-bands." |
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| CDMA systems are the latest technology
on the market and are already eclipsing TDMA in terms of cost
and call quality. Since CDMA offers far greater capacity and
variable data rates depending on the audio activity, many more
users can be fit into a given frequency spectrum and higher
audio quality can be provide. The current CDMA systems boast
at least three times the capacity of TDMA and GSM systems. The
fact that CDMA shares frequencies with neighboring cell towers
allows for easier installation of extra capacity, since extra
capacity can be achieved by simply adding extra cell sites and
shrinking power levels of nearby sites. CDMA technology also
allows lower cell phone power levels (200 miliwatts) since the
modulation techniques expect to deal with noise and are well
suited to weaker signals. The downside to CDMA is the complexity
of deciphering and extracting the received signals, especially
if there are multiple signal paths (reflections) between the
phone and the cell tower (called multipath interference). As
a result, CDMA phones are twice as expensive as TDMA phones
and CDMA cell site equipment is 3-4 times the price of TDMA
equivalents. |
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| GSM |
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| GSM stands for "Global System for
Mobile Communications." GSM is mostly a European system and
is largely unused in the US. GSM is interesting in that it uses
a modified and far more efficient version of TDMA. GSM keeps
the idea of timeslots and frequency channels, but corrects several
major shortcomings. Since the GSM timeslots are smaller than
TDMA, they hold less data but allow for data rates starting
at 300 bits per second. Thus, a call can use as many timeslots
as necessary up to a limit of 13 kilobits per second. When a
call is inactive (silence) or may be compressed more, fewer
timeslots are used. To facilitate filling in gaps left by unused
timeslots, calls do "frequency hopping" in GSM. This means that
calls will jump between channels and timeslots to maximize the
system’s usage. A control channel is used to communicate the
frequency hopping and other information between the cell tower
and the phone. To compare with the other systems, it should
be noted that GSM requires 1 Watt of output power from the phone.
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| Call Handoff |
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| It is apparent that cells must somehow
overlap, and when a user travels between cells, one cell must
hand the call off to the other cell. The cells must also not
interfere with each other. This is accomplished by giving each
cell a slightly different chunk of the frequency spectrum (note
that CDMA does not do this) and by measuring power levels. When
the power level of the user begins to fade, the cell tower determines
which cell is the closest cell. Upon finding this information,
the current cell tower sends an over-the-air message to the
new cell tower and to the cell phone. At this point, the new
cell tower picks up the call and the old one drops the call
as the cell phone switches frequencies. This type of handoff
is called a "hard handoff" since the audio feed is lost for
between 10 milliseconds and 100 milliseconds while the new tower
picks up the signal. Often these "hard" handoffs fail when the
new tower tries to pick the call up, leading to frequent dropped
calls. |
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| In most systems, each cell tower
typically receives a 1.8 MHz frequency spectrum. In normal cellular
systems that have a 12.5 MHz spectrum (not the high-band PCS
systems that have more bandwidth), this allows for 7 cells before
cells have to reuse frequencies. Generally, there are 1-2 cells
and 10-20 miles separating cells using the same frequency in
order to minimize interference. |
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| Security |
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| One of the largest problems in wireless
communication is security. There are two worries: Other people
listening into phone calls and other people illegally billing
time to a user’s account (called "phone cloning"). |
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| Unfortunately, analog phones transmit
in plain FM, and provide no security. For instance, a few years
ago, Newt Gingrich had a cell phone conversation taped by someone
using a simple police scanner, which is designed to receive
police activity on the CB frequencies. Since analog phones have
such weak security, the architects of digital technology designed
digital phones with much more robust security. |
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| Digital phones employ encryption
to secure the phone and the conversation. Encryption is used
in TDMA and CDMA to make sure that it is almost impossible to
"latch" onto a conversation. The encryption works by picking
a key that is used in an equation that compresses the audio.
The encrypted key is sent to the cell tower so the cell tower
knows how to decode the conversation. Therefore, even if the
person with the scanner finds the channel and time slice you
are using, they would need to find the encryption code to make
sense of the signal. It is also important to mention that CDMA
also uses its modulation code to provide increased security,
resulting in over four billion possible encryption codes. Cell
phones also must be protected from cloning. By encrypting the
cell phone number and related information when sending the information
to the switch, cloning is prevented. |
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| Source: |
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| How Cellular Phone Technologies Compare
Best of the Net Very good and concise explanation of how cell
phone networks work and what are the differences between CDMA,
TDMA, FDMA and GSM. |
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