Setting the Stage

Setting the Stage

By baseband, we mean the original information rate. For analog voice, baseband would be
used to refer to the 3 kHz of audio bandwidth. This would then be preprocessed. Preemphasis/
de-emphasis would be used to tailor the high-frequency response and reduce
high-frequency noise. Companding (compression/expansion) would be used to compress
the dynamic range of the signal. The signal would then be modulated onto an RF carrier
using amplitude or frequency modulation. Usually, an intermediate step between baseband
and RF would be used, known as the IF processing stage (intermediate frequency).
We still use IF processing today and will discuss its merits/demerits in a later section.

In a 2G handset, baseband refers to the information rate of the encoder (for example,
13 kbps) and related digital signaling bandwidth. The data is then channel coded�"that
is, additional bits are added to provide error protection�"and then the data is modulated
onto an RF carrier, usually with an IF processing stage. In a 3G handset, baseband
refers to the information rate of the vocoder, parallel image and video encoder rates,
other data inputs, and related channel coding.
First-generation handsets therefore have a baseband running at a few kilohertz, and
second-generation handsets a few tens of kilohertz.
Third-generation handsets have a user data rate that can vary between a few kilohertz
and, in the longer term, several megahertz. The user data is channel coded and
then spread using a variable spreading code to a constant baseband rate known as the
chip rate�"for example, 1.2288 Mcps (million chips per second; a clock rate of 1.2288
MHz) or 3.84 Mcps (a clock rate of 3.84 MHz). This baseband data, after spreading, has
to be modulated onto an RF carrier (producing a 1.25 or 5 MHz bandwidth), sometimes
via an IF. The RF will be running at 1900/2100 MHz.
Essentially, the higher the frequency, the more expensive it is to process a signal. The
more we can do at baseband, the lower the cost. This is not to downplay the importance
of the RF link. The way in which we use the RF bandwidth and RF power available
to us has a direct impact on end-to-end quality of service.
Ever since the early experiments of Hughes and Hertz in the 1880s, we have
searched for progressively more efficient means of moving information through free
space using electromagnetic propagation. By efficiency we mean the ability to send and
receive a relatively large amount of information across a relatively small amount of
radio bandwidth using a relatively small amount of RF power generated by a relatively
power-efficient amplifier in a relatively short period of time.
The spark transmitters used to send the first long-distance (trans-Atlantic) radio
transmissions in the early 1900s were effective but not efficient either in terms of their
use of bandwidth or the efficiency with which the RF power was produced and
applied. What was needed was an enabling technology.
Thermionic and triode valves introduced in the early 1900s made possible the application
of tuned circuits, the basis for channelized frequencies giving long-distance (and
relatively) low-power communication. Tuned circuits reduced the amount of RF power
needed in a transceiver and provided the technology needed for portable Morse code
transceivers in World War I.
Efficiency in RF communication requires three performance parameters:
Sensitivity. The ability to process a low-level signal in the presence of noise
and/or distortion
Selectivity. The ability to recover wanted signals in the presence of unwanted
signals
Stability. The ability to stay within defined parameters (for example, frequency
and power) under all operating conditions when transmitting and receiving
The higher the frequency, the harder it is to maintain these performance parameters.
For example, at higher frequencies it becomes progressively harder to deliver gain�"that
is, providing a large signal from a small signal�"without introducing noise. The gain
becomes more expensive in terms of the input power needed for a given output transmission
power. It becomes harder to deliver receive sensitivity, because of front-end noise, and to deliver receive selectivity, due to filter performance. On the other hand,
as we move to higher frequencies, we have access to more bandwidth..
For example, we have only 370 kHz of bandwidth available at long wave; we have
270 GHz available in the millimetric band (30 to 300 GHz). Also, as frequency
increases, range decreases. (Propagation loss increases with frequency). This is good
news and bad news. Agood VHF transceiver�"for example, at 150 MHz�"can transmit
to a base station 40 or 50 kilometers away, but this means that very little frequency
reuse is available. In a 900 MHz cellular network, frequencies can be used within (relatively)
close proximity. In a millimetric network, at 60 GHz, attenuation is 15 dB per
kilometer�"a very high level of frequency reuse is available.
Another benefit of moving to higher frequencies is that external or received noise
(space or galactic noise) reduces above 100 MHz. As you move to 1 GHz and above,
external noise more or less disappears as an influence on performance (in a noise
rather than interference limited environment) and receiver design�"particularly LNA
design�"becomes the dominant performance constraint.
An additional reason to move to higher frequencies is that smaller, more compact
resonant components�"for example, antennas, filters, and resonators�"can be used.
Remember, RF wavelength is a product of the speed of light (300,000,000 meters per
second) divided by frequency, as shown in Table 1.1.
During the 1920s, there was a rapid growth in broadcast transmission using long
wave and medium wave. The formation of the BBC in 1922 was early recognition of the
political and social importance of radio broadcasting. At the same time, radio amateurs
such as Gerald Marcuse were developing equipment for long-distance shortwave communication.
In 1932, George V addressed the British Empire on the shortwave world
service. In practice, there has always been substantial commonality in the processing
techniques used for radio and TV broadcasting and two-way and later cellular radio�"
a convergence that continues today.