Base station
As you drive
along the highway, you may notice cellular towers or cellular base stations
appearing every few miles. A base station is the interface between wireless
phones and traditional wired phones. It’s what allows you to use your cell phone
to call your home phone. The base station, a wireless system, uses
microwave radio communication.
It is composed of several antennas mounted on a tower and a building with
electronics in it at the base. When you make a call on your cell
phone, the cell phone and base station
communicate back and forth by radio, and the radio
waves they use are in the microwave
region of the electromagnetic spectrum.
The base
station antenna is mounted on tall towers because from this high point it is
easier to stay in communication with cell phone users, who are often near the
ground. The actual antenna elements of a base station are usually less than ten
centimeters (about 4 inches), but may be grouped into clusters or “arrays” with
heights of about a meter (about 3 feet). They need to be mounted on a tower to
overcome obstacles, such as trees, hills, or tall buildings, that stand between
the base station and your cell phone. You might see several racks of antennas
at different heights along the tower. Each rack actually belongs to a different
cellular or PCS service provider. Because the tower structure is so expensive,
it is often economical for service providers to rent space on a tower that is
owned by a competitor, or a third party. Most cells are divided into three
sectors, and so each antenna rack will have a triangular shape. Each face of
the rack will usually have three antennas installed.
Of the three
antennas, two are for receiving and one is for transmitting. Two are used on
the receive side so that the base station can compare signals and select the
best antenna for each user within the cell. This is known as 'diversity'
reception to more equally manage the power differences between the cellular
base station transmitter and the small battery powered cellphone transmitter.
The cellular tower transmitting antenna is usually placed between the two receiving
antennas. Even though the receive frequency is different from the transmit
frequency, the antennas are separated by several meters because of the large
difference in power levels.
The physical
size of the antennas is generally related to the frequency of their operation,
eg, the 450Mhz Nordic Mobile System circa 1990 used antennas around 4m tall,
wheres say the 1900MHz GSM systems use much smaller antennas to achieve similar
gain and antenna radiation patterns. There are two types of cellular antennas
used, Omnidirectional and Sector antennas. Omnidirectional antennas are
generally only used in low traffic volume areas, or for very small or indoor
'cells'.
Under the
tower is usually a small building that houses the electronics. To transmit
calls, the base station requires a powerful transmitting amplifier to generate
strong signals. This “power amplifier” is linked to the transmitting antenna by
a length of coaxial cable. Connected by cable to the receiving antennas are
low-noise amplifiers that can detect the weak incoming signals and separate it
from any background noise that is present.
A bank of
electronic circuits called the transceiver rack connects to the low noise
and power amplifiers, and converts their radio signals into digital signals,
and vice versa. The transceiver is connected to the electronic switching
device that routes calls between the base station and the main telephone
system.
The electronic
switching function is known as the "Mobile Switching Centre" (MSC)
for GSM, AMPTS, NMT and CDMA systems, and for 3G/4G mobile systems it is known
as the "Media Gateway" (MGW)/ "Gateway MSC Server"
(GMSC).
How
Attenuators Can Improve Resolution
If the pre-amp is based on IC op-amps, there are quite likely to be other sonic
benefits in addition to the improved signal/noise performance. Though it isn’t
widely acknowledged, all op-amps have a push-pull class B or class A/B output
stage which will inevitably suffer from some residual crossover distortion.
This type of distortion affects low level signals much more than it does high
level signals. Negative feedback is used to correct this distortion, and
at full output voltage swing the specifications will be very good. However, the
full output voltage swing on op-amps is about 36 volts (peak-peak) and this is
much greater than is required in a pre-amp. Sure, op-amp specs are great
at 36 volts, but most power amps are driven to their maximum output with a
signal of just 750mV so it should be clear that the op-amps within a hi-fi
pre-amp can never run at anywhere near their optimum. Also consider that
750mV is the signal required for full power. When listening at more modest
volume levels the signal will be much less. Assume that a more modest
level means 20dB below maximum, then the pre-amp’s output signal will be just
75mV. Now consider that low level details within the programme material
are at an even lower level than this. Reverb times, for example, are
specified as the time taken for a reverberant sound to decay to 60dB below its
original value, so it’s reasonable to assume that signals 60dB below the
programme peaks are still audible. That means that signals 60dB below 75mV
are significant ie 0.075mV. When you now consider that op-amp
performance measurements are taken with a signal swing of 36 volts but what is
really important is what is happening below a tenth of a millivolt, it becomes
obvious that the low-level crossover distortion inherent in op-amps may be
audible as subtle masking of detail. The In-Line Attenuators will improve the
situation by allowing the op-amps to work with a higher signal level without
the music becoming too loud (due to the 10dB signal reduction at the inputs to
the power amp), so the crossover distortion will be effectively reduced by
swamping it with a larger signal swing.
What is Amplitude
Modulation
In order that a steady radio signal or "radio carrier"
can carry information it must be changed or modulated in one way so that the
information can be conveyed from one place to another. There are a number of
ways in which a carrier can be modulated to carry a signal - often an audio
signal and the most obvious way is to vary its amplitude.
Amplitude Modulation has been in use since the very earliest
days of radio technology. The first recorded instance of its use was in 1901 when
a signal was transmitted by a Canadian engineer named Reginald Fessenden. To
achieve this, he used a continuous spark transmission and placed a carbon
microphone in the antenna lead. The sound waves impacting on the microphone
varied its resistance and in turn this varied the intensity of the
transmission. Although very crude, signals were audible over a distance of a
few hundred metres. The quality of the audio was not good particularly as a
result of the continuous rasping sound caused by the spark used for the
transmission.
Later, continuous sine wave signals could be generated and the
audio quality was greatly improved. As a result, amplitude modulation, AM
became the standard for voice transmissions.
Amplitude modulation applications
Amplitude modulation is used in a variety of applications. Even
though it is not as widely used as it was in previous years in its basic format
it can nevertheless still be found.
- Broadcast transmissions: AM is still widely used for broadcasting
on the long, medium and short wave bands. It is simple to demodulate and
this means that radio receivers capable of demodulating amplitude
modulation are cheap and simple to manufacture. Nevertheless many people
are moving to high quality forms of transmission like frequency modulation,
FM or digital transmissions.
- Air band radio:
VHF transmissions for many airborne applications still use AM. . It is
used for ground to air radio communications as well as two way radio links
for ground staff as well.
- Single sideband:
Amplitude modulation in the form of single sideband is still used for HF
radio links. Using a lower bandwidth and providing more effective use of
the transmitted power this form of modulation is still used for many point
to point HF links.
Single Sideband, SSB
Modulation
Single sideband modulation is a form of
amplitude modulation. As the name implies, single sideband, SSB uses only one
sideband for a given audio path to provide the final signal.
Single sideband modulation, SSB, provides a
considerably more efficient form of communication when compared to ordinary
amplitude modulation. It is far more efficient in terms of the radio spectrum
used, and also the power used to transmit the signal.
In view of its advantages single sideband
modulation has been widely used for many years, providing effective
communications, as well as forms being used for some analogue television
signals, and some other applications.
Single sideband
modulation basics
Single sideband modulation can be viewed as an amplitude
modulation signal with elements removed or reduced. In order to see how single
sideband is created, it is necessary to use an amplitude modulated signal as
the starting point.
From this it can be seen that the signal has
two sidebands, each the mirror of the other, and the carrier. To improve the
efficient of the signal, both in terms of the power and spectrum usage, it is
possible to remove the carrier, or at least reduce it, and remove one sideband
- one is the mirror image of the other.
A single sideband signal therefore consists of
a single sideband, and often no carrier, although the various variants of
single sideband are detailed below.
It can be seen that either the upper sideband
or lower sideband can be used. There is no advantage between using either the
upper or lower sideband. The main criterion is to use the same sideband as used
by other users for the given frequency band and application. The upper sideband
is more commonly used for professional applications.
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