sjl@world.std.com (Scott J Loftesness) (12/13/90)
Would be interested in hearing a report from anyone who might have attended the IEEE/Globecom conference last week and listened to the discussion on Spread Spectrum given by the folks from Qualcomm.
dalyb@godzilla.UUCP (Brian Daly) (12/18/90)
In article <1990Dec13.122314.6448@world.std.com>, sjl@world.std.com (Scott J Loftesness) writes: > Would be interested in hearing a report from anyone who might have > attended the IEEE/Globecom conference last week and listened to the > discussion on Spread Spectrum given by the folks from Qualcomm. I attended GLOBECOM, but did not have the opportunity to participate in the workshop on CDMA, which was organized by QUALCOMM. (I assume that this is what you were referring to). I did attend the presentation on a paper which was co-authored by Mr. Padovani at QUALCOMM, along with Space Engineering of Italy and the ESA. This paper was an analysis of a CDMA-based land mobile satellite system for Europe. The results of their analysis are: "CDMA is the way to go for the provision of satellite land mobile services"; CDMA is feasible and dependable.(The paper compared CDMA with FDMA). -- Brian K. Daly WB7OML @ AG Communication Systems, Phoenix, Arizona UUCP: {...!ames!ncar!noao!asuvax | uunet!zardoz!hrc | att}!gtephx!dalyb Phone: (602) 582-7644 FAX: (602) 582-7111 ~
chik@eecg.toronto.edu (Raymond Chik) (12/20/90)
In article <4eabf5aa.1423f@godzilla.UUCP> dalyb@godzilla.UUCP (Brian Daly) writes: >In article <1990Dec13.122314.6448@world.std.com>, sjl@world.std.com (Scott J Loftesness) writes: > ........... >of their analysis are: "CDMA is the way to go for the provision of satellite >land mobile services"; CDMA is feasible and dependable.(The paper compared >CDMA with FDMA). >-- Could someone enlighten the rest with a little more detailed description of CDMA and FDMA ? ************************************************************************** * May the force be with you!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!* * Raymond Y. V. Chik |_ \ _| * * VLSI Research Grp. || -------- || * * Dept. of Elec. Eng. _||o-+ | -+-+- +-o||_ * * U. of Toronto | | | +-+ | | * * |_ | | _|_ | _| * *Internet: chik@eecg.toronto.edu ||--+ | | | | +--|| * * chik@vrg.toronto.edu _|| / | | | ||_ * * 8-) >-( |-< %-> | | | * **************************************************************************
lei@motcid.UUCP (Peter P. Lei) (12/20/90)
>> Would be interested in hearing a report from anyone who might have >> attended the IEEE/Globecom conference last week and listened to the >> discussion on Spread Spectrum given by the folks from Qualcomm. >I attended GLOBECOM, but did not have the opportunity to participate in >the workshop on CDMA, which was organized by QUALCOMM. (I assume that this >is what you were referring to). >I did attend the presentation on a paper which was co-authored by Mr. Padovani >at QUALCOMM, along with Space Engineering of Italy and the ESA. This paper was >an analysis of a CDMA-based land mobile satellite system for Europe. The result>of their analysis are: "CDMA is the way to go for the provision of satellite >land mobile services"; CDMA is feasible and dependable.(The paper compared >CDMA with FDMA). Anyone know if IEEE is publishing the proceedings from the conference and how to get a copy of them? ------------------------------------------------------------------------------- Peter Lei UUCP: ...uunet!motcid!lei Motorola, Inc. Internet: lei@motcid.uu.net Cellular Infrastructure Group Phone: (708) 632-4426 1501 W Shure Dr, IL-27/GSM Arlington Hts, Illinois 60004 Standard disclaimers apply =)
wang@motcid.UUCP (Jerry Wang) (12/21/90)
chik@eecg.toronto.edu (Raymond Chik) writes: >In article <4eabf5aa.1423f@godzilla.UUCP> dalyb@godzilla.UUCP (Brian Daly) writes: > Could someone enlighten the rest with a little more detailed >description of CDMA and FDMA ? Here is my limited knowledge: I. FDMA - Frequency Division Multiple Access This is how the current analog cellular system works. The spectrum is divided into multiple 30 kHz (25 kHz in UK TACS system) channels. Each cellular subscriber is assigned to one such channel during a cellular phone call without interfering with each other. II. CDMA - Code Division Multiple Access A. Spectrum Spreading CDMA is a digital multiple access communication system based on Direct Sequence Spread Spectrum (DS-SS). The digital information to be transmitted (speech, data or control) is multiplied by a pseudo random sequence prior to modulation. As a result the symbol rate entering to the channel is increased in proportion to the length of the sequence. This is why it's called 'spread' spectrum. The key advantage of spread spectrum is 'processing gain' over white noise, as evidenced by Shannon's channel capacity theory (Shannon said, you can get any BER performance you want given a S/N, as long as you have the 'bandwidth' to do it. So the bandwidth increase contributes to 'processing gain directly). Another advantage is 'anti-jamming' capability due to the spectrum spreading effect. The multiple access capability comes from the auto-corelation and cross-corelation properties of the set of pseudo-random sequences used for spreading. B. Multiple Access Ideally each sequence resembles a noise pattern. Auto-correlation of noise produces a 'delta function' which has value of infinity at t=0 and zero for the rest of t. Cross-correlation of two independent noise patterns produces over all time. Assuming that continuos '1' is spreaded by two independent pseudo sequences (let's call that sequence A and sequence B) and transmitted to the channel by two transmitters, and a receiver uses sequence A to corelate the combined signals: transmitter with sequence A will produce a peak (the pseudo 'delta function') for every transmitted '1' at the receiver and transmitter with sequence B will produce no response at all at the receiver. The receiver's capability to selectively pick up the desired transmission by using different sequence (Code) is the main principle for CDMA. C. CDMA issues - a few example Not all the codes (pseudo random sequences) chosen have ideal auto-corelation and cross-corelation properties. Multiple transmissions over different codes decreases the jaming margin (processing gain plus input S/N, desired output S/N). Strong (near) transmitter further eats up jamming margin for a weaker transmitter (far). CDMA has a lot of advatntages over the existing FDMA and TDMA (Time Division Multiple Access, being adapted in Europe) for cellular application, but it also has it's own issues to be resolved. These are beyond the scope of this discussion. Qualcomm is headed by people who write 'bibles' in the field of communication and information theory, e.g. Viterbi, Jacobs, etc. However I have not seen the Qualcomm proposal in the public domain to verify it's claims. (And I don't believe 'odinary people' like myself can challenge them anyway). Jerry Wang - Motorola
karn@envy.bellcore.com (Phil R. Karn) (12/21/90)
In article <4528@manta5.UUCP>, wang@motcid.UUCP (Jerry Wang) writes: > length of the sequence. This is why it's called 'spread' spectrum. The > key advantage of spread spectrum is 'processing gain' over white noise, as > evidenced by Shannon's channel capacity theory (Shannon said, > you can get any BER performance you want given a S/N, as long > as you have the 'bandwidth' to do it. So the bandwidth increase > contributes to 'processing gain directly). Another advantage > is 'anti-jamming' capability due to the spectrum spreading effect. I don't think this is quite true. By itself, spread spectrum does not give you any power advantage over non-spread signals in the presence of gaussian ("white" or thermal) noise. That is, if it takes 10 watts to send 10 kb/s over a given path without spread spectrum, spreading will not change this figure. What *will* change this figure is forward error correction (FEC) coding, which spends bandwidth to save power. But, unlike spread spectrum, this excess bandwidth cannot be shared with other stations. The so-called "processing gain" in spread spectrum is simply an artifact of the "effective" (information) bandwidth of the signal being much smaller than its actual (occupied) bandwidth. If you have a signal that ordinarily fits in 10 KHz and you spread it to 1 MHz (a ratio of 100:1 or 20 dB) then your receiver will still perform as though its noise bandwidth were 10 KHz even though its front end bandwidth is 1 MHz. So the "processing gain" in this case is a wash. Where processing gain *does* buy you something is in the presence of narrowband interference in your signal passband ("narrowband" == much narrower than the spreading bandwidth). An interfering narrowband signal will be *spread* by the despreading process at the receiver to the full spreading bandwith of the system, so for our example its effect on the wanted signal will be reduced by the 20-dB processing gain of the system. So "processing gain" would be better described as spread spectrum's "unwanted signal rejection ratio", analogous to the "adjacent channel rejection" figures given for conventional narrowband receivers. Since the latter figures are often 60 dB or more, you can see that spread systems are actually at a disadvantage when the signals are not all closely matched in level. This is the well-known "near-far" problem, and solving it requires careful transmitter power control. So why use spread spectrum at all? There are several key advantages: 1. Jam resistance. This is primarily of military interest, but it can also help in civilian environments where the "jamming" may be more accidental than intentional. It may also eliminate the need for the bureaucratic overhead of frequency coordination. (I see the Part 15 SS rules at 902-928 MHz as a "regulatory experiment" to see if spread spectrum can indeed work this way). 2. Multipath resistance. Spread spectrum receivers can track just one of several reflected signals, rejecting the others as interference. This is very attractive in a mobile environment, especially in cities. 3. Suppression of narrowband interference. This is especially useful in making use of "garbage" spectrum that would not otherwise be useful for communications, e.g., the ISM bands. Phil
wb8foz@mthvax.cs.miami.edu (David Lesher) (12/22/90)
In <1990Dec20.231903@envy.bellcore.com> karn@envy.bellcore.com (Phil R. Karn) writes: >So why use spread spectrum at all? There are several key advantages: >1. Jam resistance. >2. Multipath resistance. >3. Suppression of narrowband interference. 4) Hiding the fact you are transmitting at all, and/or making it harder to figure out who is. In a noisy environment, how well can your rf-source seeking missile figure out who to blow up? -- A host is a host from coast to coast.....wb8foz@mthvax.cs.miami.edu & no one will talk to a host that's close............(305) 255-RTFM Unless the host (that isn't close)......................pob 570-335 is busy, hung or dead....................................33257-0335
john@qip.UUCP (John Moore) (12/24/90)
In article <1990Dec20.231903@envy.bellcore.com> karn@thumper.bellcore.com writes:
]Where processing gain *does* buy you something is in the presence of
]narrowband interference in your signal passband ("narrowband" == much
]narrower than the spreading bandwidth). An interfering narrowband
]signal will be *spread* by the despreading process at the receiver to
]the full spreading bandwith of the system, so for our example its
]effect on the wanted signal will be reduced by the 20-dB processing
]gain of the system.
]
]So "processing gain" would be better described as spread spectrum's
]"unwanted signal rejection ratio", analogous to the "adjacent channel
]rejection" figures given for conventional narrowband receivers. Since
]the latter figures are often 60 dB or more, you can see that spread
]systems are actually at a disadvantage when the signals are not all
]closely matched in level. This is the well-known "near-far" problem,
]and solving it requires careful transmitter power control.
Try this on for size. This stuff is subtle enough that I hope I
got it right:
Some of this depends on whether it is best to think of it in the
"time" domain or "frequency" domain. For example, if we achieved
our "spread spectrum" by relatively slow frequency hopping, we would
only have degradation when the signals coincided - which could be
very rare. Consider a system where the frequency is swept in a sawtooth
manner. If the the sawtooths of two stations are anti-synchronized
(synchronized out of phase), you would only experience interference
as a click when the other guy reset his ramp (and thus generated a
broadband signal). This would be a system best looked at in the
time domain (and perhaps it shouldn't really be called spread "spectrum").
On the other hand, if an interfering transmitter has essentially a white noise
modulation, I agree that the processing gain does nothing for you over
a narrow band system.
Specifically - assume a 10kHz information bandwidth spread over
1 MHz. Sure enough, the receiver sees a 20dB better SNR than
a receiver using the 1 MHz bandwidth for 1 MHz of information.
However, so would a 10kHz narrow band receiver. No processing
gain here!
Assume that there are multiple systems operating in the same
frequency band. If they are narrowband, sometimes there will be collisions
that completely disable the colliding systems. If those same systems
were spread spectrum, and the relative signals strengths were
right (VERY IMPORTANT!), they could co-operate. I think this is the
intent behind the 900 MHz part 15 devices. The Japanese, however,
have taken an alternative approach in their 900 MHz personal radio
service: they detect interference and simply shift frequencies.
Where you really gain bandwidth by using SS, I suspect, is in situations
like the satellite business described by the previous poster. In this
case, you have controlled signal strengths (everyone is at the
same distance from the satellite, and presumably transmitting the
same power). I believe you also have more or less random usage
patterns - bursty data, etc. In such a situation, you have an allocation
problem - how do you give the bandwidth to the guy that needs it. An
old solution uses some form of TDMA - such as CSMA or CSMA/CD. These
are not very efficient at high usages - they tend to start thrashing.
The SS solution in that case should be more elegant - increased interference
yields linear decreases in SNR, rather than collisions and retries.
Up to some level of usage, the system should be well behaved.
I suspect this is why the VSAT folks are pushing it. Their needs fit
this very specialized use.
For those in love with SS, let me offer a challenge:
Develop a VHF/UHF repeater technology for Spread Spectrum that
would actually yield better spectrum usage than currently existing
narrow band systems (such as trunked systems). Have fun!
--
John Moore HAM:NJ7E/CAP:T-Bird 381 {ames!ncar!noao!asuvax,mcdphx}!anasaz!john
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