Re: [Discuss-gnuradio] typical bandwidth?

[Dave Emery]

On Wed, Mar 20, 2002 at 08:31:45PM -0700, Stephen Nichols wrote:
> 
> I'm still looking for a good (read affordable) front end & A/D board
> combination. My question is what is a good/typical bandwidth to digitize? I
> know it depends on the type of signal you want to deal with but say I don't
> consider video; what is a good IF bandwidth that would cover the most
> "typical" signals? Or is "typical" too hard to define ? Or does anyone know
> of a good reference that has a list of the bandwidts of various signal types

        It all boils down to what specific signals you are interested in.

        Simple as that seems, it also is pretty fundemental - specific
signals on the air in specific places use particular transmission
standards (air interfaces), many for historical reasons, some for sound
current engineering design criteria.  And these different transmission
standards involve different bandwidths.

        For what it is worth most communications signals in the VHF/UHF
spectrum (30 mhz to 1 ghz)  used to be narrow band fm (nbfm). 
Originally most of this was 15 khz deviation (about 30-50 khz
bandwidth), but many years ago the FCC refarmed most everything down to
5 khz deviation (about 15-20 khz occupied bandwidth).   That has been
the standard until rather recently when the FCC again has started
refarming down to 2.5 to 3 khz deviation and 12.5 khz or less occupied
bandwidth.  And their eventual goal is to make most SCPC communication
signals 6.25 khz bandwidth (which implies a digital mode of some sort or
ACSSB).  But so far there are no voice systems on the air with 6.25 khz
occupied bandwidth except for a few lonely ACSSB  analog SSB signals in
the 220 mhz band.

        An important historical exception to the nbfm rule has been
aircraft (air traffic control) communications, both military and
civilian.  These signals are double sideband AM, which implies they
occupy a 6 khz bandwidth for communications grade voice audio. 
Traditionally aircraft signals have been on 25 khz spaced channels, but
in Europe and soon in the US some will be on 8.33 khz spaced channels. 
And in the US some of these signals will be converted to a 25 khz wide
TDMA digital format.

        In recent years a considerable number of digital data and
message (such as pagers and Mobitex and MDTs) systems have been
implemented in the VHF/UHF spectrum - most of these have used the
traditional nbfm voice channelizing (written into FCC rules) and most of
these signals occupy either 15-20 khz bandwidths or 12.5 or less
(narrowband) khz bandwidths.  FSK or GMSK modulation is almost the
universal standard here with both 4 level FSK filtered FSK and GMSK more
and more common.   There are also narrower band (6.25 khz or less) FSK
data signals around in the VHF/UHF spectrum on so called splinter
channels.   Data rates up to 19.2kb are acheived with 9600 being very
common.

        And increasingly in the last two or 3 years, public safety and
some commercial users have been converting over to APCO-25 digital voice
transmission which currently uses C4FM modulation (filtered 4 level FSK
basicly) in a 12.5 khz bandwidth.   This is going to become more and
more common and popular in the USA, and eventually a 6.25 khz wide
version of the signal is supposed to be introduced but hasn't yet.
APCO-25 voice can be secure (encrypted) or clear, but there is also a
good bit of federal and some public safety traffic that uses DES or
Fascinator/Indictor encrypted FSK voice in standard 25 khz channels
using around 4.5 khz deviation and 12 khz bit rate.  This stuff has been
around since the mid 80s and most federal law enforcement agencies use
it now for most of their radio traffic (more and more of their actual
operational traffic is on cellphones or NEXTEL radios however).

        Of course you presumably are aware that the original USA AMPS
analog cellphone service used nbfm in 30 khz wide channels with around
10 to 12 khz deviations.  A little used varient (NAMPS) used 10 khz wide
channels instead.    TDMA cellphones use the same 30 khz wide channels
as AMPS but using pi/4-QPSK (but TDMA is dying and being phased out and
replaced with GSM or CDMA).  GSM uses 300 (approximately) khz wide GMSK
signals (probably need a 400 to 500 khz bandwidth here to process them
with good BER). Current CDMA systems (and some of the 3 G stuff) uses
1.25 mhz wide signals. Both CDMA and TDMA signals are vector modulated
(basicly phase shift keyed) and complex - but you probably knew that.  
NEXTEL (IDEN) phones use 25 khz wide channels and fill them pretty much
end to end with digital energy - modulation is a form of multiple QAM
modulated subcarriers.

        Cordless telephones started out as nbfm with 15 -20 khz
bandwidth, but some of the more modern "secure" digital types use 900
mhz signals including some direct sequence spread spectrum signals that
are as wide 500 khz or even a mhz or so.  Many of the digital kind are
in the 50-250 khz wide range.
 
        Of course it need not be mentioned that broadcast FM stations
use 75 khz deviation with lots of baseband ultrasonic energy (the stereo
difference signal) and occupy a nominal 200 khz, but in order to recover
the audio with really good fidelity one needs about 300-400 or more khx
bandwidth in order to pass the upper order bessel sidebands.

        And as you alluded, NTSC AM VSB TV signals occupy a nominal 6
mhz bandwidth with the video being VSB AM with the 1.25 mhz of the lower
sideband transmitted with increasing roll off and 4.0 to 4.2 mhz of the
upper sideband transmitted with fairly sharp rolloff above that.  TV
sound has a nominal bandwidth of 250 khz (50 khz deviation as I
remember) with lots of ultrasonic subcarrier energy (the stereo and SAP
channels).  The TV sound FM carrier is 4.5 mhz above the video.

        ATSC TV (covered elsewhere here by those working on decoding it)
is 8 level VSB with a suppressed carrier (but also with a pilot).  It
occupies more or less an entire 6 mhz channel end to end.  So its
bandwidth is a very little less than 6 mhz.

        And in addition to these signals in the VHF/UHF range, there are
are a variety of oddball signals of various sorts in that frequency
range depending on what you are near and how sensitive your receiver
front end is and what sort of antenna you are using.  These signals
range from very narrowband (ham weak signal work on 2 M and 70 cm) to
quite broadband (automobile key entry and alarm transmitters) to very
broadband (some low frequency military radars, some secure spread
spectrum communications systems.  It all depends on what signal is of
interest...

        Above a ghz or so (which of course is above the tuning range of
the cable box tuners that have been employed with the prototype GNUradio
stuff), there are signals with all sorts of bandwidths.  Many of the
current generation wireless networking device signals are quite wide (1
to as much as 10 mhz) with frequency hopping common.  Some microwave
radar signals are hundreds of mhz wide (and very complex). FM video
links (still quite common in the microwave range) are usually 10 mhz
wide (4 mhz deviation) 18 mhz wide (6 mhz or so deviation) or 27 mhz
wide (10 mhz deviation). Old fashioned fixed point to point microwave
links range from 200 khz wide to 10 to 20 mhz wide depending on channel
capacity and modulation (most are digital now with QAM and QSPK being
common, but there are a few FM-FDM-SSB links left).

        But on the other hand, not everything above a ghz is wideband,
there are lots of narrow band signals used on various satellite uplink
and downlink bands ranging from signals less than 3-4 khz wide (on
INMARSAT for example) to 25 or 50 khz wide to a mhz or couple of  mhz.

        And going in the other direction, on the HF spectrum (2-30 mhz)
very few signals are wider than 6 khz and many are as narrow as 200 hz
or less.   Most voice signals are SSB with 3 khz nominal bandwidth
(often a little less). Shortwave broadcasters are DSB or occasionally
SSB AM with a nominal 6 khz or perhaps 10 khz bandwidth and 5 khz
spacing.   Much ham traffic is SSB, CW (500 hz or less bandwidth), or
various RTTY and digital modes (ranging from 100 hz wide to 2.4 khz). In
general most HF users use receivers and transmitters in SSB mode which
generally limits the bandwidth of the signal to 3 khz or less given the
design of the gear employed, and much of the military/aviation
government portion of the HF spectrum is largely allocated in 3 khz
spaced channels which increasingly are used for digital data
transmissions of various sorts using modern modulations at speeds of up
to 4800 baud. There are a few exotic military wideband signals on HF
(and the military is planning to use more) with 10-200 khz or more
bandwidth, and quite a few OTH radars with 5-50 khz wide high power
signals, so not absolutely everything on HF is narrowband, but most
intelligable communications signals are.

        Below HF, there is the standard AM broadcast band with signals
nominally 20 khz wide and various LF beacons (narrow band) and VLF 
military communications (very narrowband for the most part - 200 hz or
less).

        A quick summary:

        NBFM voice  -  15-20 khz (wide) 12.5 khz (narrow)
        APCO-25 voice  12.5 khz - 6.25 khz in future
        FM broadcast - 200 khz nominal 300 khz hifi
        TV             6 mhz video 5.4 mhz, sound 250 khz
        AM broadcast   20 khz (current FCC, used to be 10 khz)
        ACSSB voice    4.5 khz
        pagers/MDTs    usually 12.5 khz or 20 khz, can be narrower
        aircraft       6 khz
        AMPS cellphone 25-30 khz
        TDMA cellphone 30 khz 
        CDMA cellphone 1.25 mhz
        GSM  cellphone 300 khz
        NEXTEL         25 khz
        cordless phone 15 khz to more than 1 mhz depending on type
        SSB voice      3 khz
        CW (morse code)200 hz
        RTTY/digital HF 200hz - 2.4 khz

        And yes, I simplify a good bit here and approximate some numbers
rather than being really fussy about the exact width of certain signals.

        But again, the basic point of most listening efforts and
essentially all interoperable transmitting efforts is going to be aimed
at some very specific signal with (usually) well specified and well
analyzed signal characteristics - so in reality one has to first ask
what the exact target is and then choose equipment parameters
accordingly.

        But as one can see, most traditional HF/VHF/UHF signals can be
processed with system bandwidths in the order of 30-50 khz.  Handling
modern digital wireless signals increasingly involves significantly
wider bandwidths and faster and more complex processing, with multi mhz
system bandwidths and lots and lots of processing being required for
CDMA and especially 3G CDMA stuff.  Broadcast FM and TV reception of
course requires wider bandwidths too, but aside from broadcast signals
and modern wideband digital wireless signals (cell and PCS) there aren't
many wideband signals in the HF/VHF/UHF to 1.3 ghz band  that require
much more than 30-50 khz system bandwidths.

> 
> 
> stephen
> 
> 
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-- 
        Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass. 
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