Most cell phones and wireless internet routers, modems, and access points that use spread spectrum usually broadcast and receive their data on FM-radio waves. Just out of curiosity, I ask, why not use AM?
Let's say a DSSS/FHSS type of spread-spectrum is transmitted and received using the AM radio waves in the UHF spectrum [i.e. spread info for transmission throughout the UHF band and receive AM radio waves throughout all UHF frequencies]. What would be the disadvantages of this?
Normally DSSS and FHSS are transmitted/received on FM radio waves. So I ask what would be the disadvantages of using AM instead of FM for this?
AM radio tends to be more vulnerable to unwanted magnetic disruptions than FM, however this only affects analog reception. Digital reception on AM should be unaffected even by the strongest-interfering analog RF magnetic signal. Right?
FM demodulation produces a handy 'capture' effect. If a weak and a strong signal are present together, then the recovered baseband signal from the weaker FM signal is greatly reduced. This is useful in rejecting interference from adjacent stations on the same frequency.
With AM, the demodulated baseband signals are present in amplitudes that are proportional to their RF amplitudes. In addition, the AM carriers will 'beat' together to produce an additional and unwanted tone-modulation of the received RF signal.
But if the signal is digital, won't it remain immune to EMI/RFI [analog disruption] even if received on AM? DSSS and FHSS and digital. So I would think that the analog magnetic interferences wouldn't affect it.
Also, doesn't FM have the disadvantages in that it hogs more bandwidth than AM?
Yeah but even if the signal being transmitted/received is digital, it's carrier wave is still analog. Right?
AFAIK, there is no such thing as a digital carrier wave. The carrier wave is always analog just like a cable link is also always analog. The signal transmitted through the analog medium maybe digital, though.
If a PCM signal [digital] is transmitted on an AM carrier wave [analog], the AM wave's peak-to-peak amplitude will vary according to the PCM signal in the following manner:
A positive amperage of the PCM signal will cause the AM carrier wave's peak-to-peak amplitude to increase while a negative amperage [i.e. going below the x-axis when graphed] will cause a decrease the AM carrier's peak-to-peak amplitude.
A increase in frequency of the PCM signal will cause the AM carrier's peak-to-peak amplitude to vary more rapidly while a decrease in the PCM signal's frequency will cause the AM wave's peak-to-peak amplitude to vary more slowly.
That's how I understand it. The PCM signal is digital but causes a measurable affect on the analog AM carrier wave. Upon reception, an AM-demodulator can retrieve this PCM signal and play it back. I could be wrong though.
I'm not sure how. All transmission is analog until the received signal is digitized. FM (and it's step-brother, FSK) offer better immunity to impulse noise than AM at the expense of spreading the transmitted power over the redundancy that exists in their spectra.
I'll ignore cellular modulation methods because you already beat that to death in sci.electronics.design. I'll assume that you've had a brain transplant and are now capable of understanding various answers to your marginal questions.
802.11 wireless modulation is pure FM for 1 and 2Mbits/sec data rates.
5.5 and 11 add AM to the FM modulation, thus yielding higher bits/Hz modulation density, which is a measure of spectral efficiency. Therefore, 802.11 wireless data does use some AM.
6Mbits/sec thru 54Mbits/sec use OFDM, which are 48 orthogonal (non-overlapping) QAM (quadrature amplitude modulation) carriers. Each carrier is modulated by both FM and AM. Each carries a part of the data stream which when reassemble, yields even higher bits/Hz modulation density and efficiency. Quadrature implies phase modulation (or FM if you prefer). The AM in QAM implies that it's also amplitude modulated.
The FCC would have you arrested and returned to wherever you escaped from. Modulation methods and technology have evolved over the years to satisfy many conflicting requirements. Pure AM is one of the oldest, and has the advantage of simplicity. Everything else about it is inferior to other modulation methods. FAA/FCC regulations protecting obsolete technology has insured that it's still in use in the aviation and broadcast business.
Pure AM has a number of disadvantages, depending on what you're trying to accomplish. The big one is that it's grossly inefficient in both the utilization of power and spectral efficiency. The carrier hogs at least half the available power. AM has its place, but there are better ways.
I'll skip all the intermediate modulation schemes and jump directly to the present. Microprocessors have become so cheap, that it's now economical to do DSP (digital signal processing) in every radio. Digital has the huge advantage of offering error correction, noise immunity, bandwidth compression, and high spectral efficiency. Schemes have been devised that will extract useful audio or data from signals that are well below the thermal noise floor. You can't do that with AM (or FM). If you attempted to do Wi-Fi using pure AM technology, the data thruput would be horrible and/or the error rate would be hideous.
DSSS are pure FM at 1 and 2Mbits/sec data rates. 5.5 and 11Mbits/sec are a mixture of AM and FM. OFDM is neither pure AM or FM. It's QAM modulation, a form of PSK (phase shift keying), with multiple sub-carriers to reduce the effects of frequency selective fading.
Lousy spectral efficiency, difficult linearity requirements, slow AGC, excessive bandwidth, high error rate for data, and it's not fashionable. I suppose there's some benifits to resurrecting stone age technology, so that we don't forget our past, but I wouldn't want to live like a cave man. That's essentially what you're suggesting by reverting to pure AM technology.
Magnetic? Wave a magnet around your cherished AM BCB (broadcast band) radio. Hear anything different? How are magnets causing disruptions?
No. Magnets don't affect radio waves much. There is Faraday rotation, which causes EM (electro-magic) waves to change polarization in the presence of a magnetic field. That has some effect on microwave signals and is used to good advantage in RF circulators and isolators. However, for RF below about 1GHz, it's a non-issue.
So what are you proposing this time? Cellular service at HF frequencies? Ultra high index of modulation AM schemes? Long range cellular where you would have only a few simultenous users on a given continent? Laws to limit acronyms to two letters?
That is one way. Usually you will also need some synchronizing information to know which bits are which. Sometimes it is done as a subcarrier, modulating a carrier synchronous to the bit rate, and then using that to amplitude modulate the real carrier.
You could also generate a pulse for 1 and no pulse for 0, though there are synchronizing problems with that. Next easiest is a pulse for 1, no pulse for zero, and a clock pulse half way in between. (The coding used on single density floppy disks.)
You need some kind of modulation for an AC coupled signal path.
Only if the power of the co-channel is very much less otherwise you get complete destruction of both to a point when they flip over and you hear the other channel. Power saving is the big thing with FM.
Ye cannae re-write the laws of physics captain - digital or analogue FM is the same. You do the Maths and see.
Digital comms still breaks down except it has error-correcting codes built in - so you don't see it! Ocassionally you may well find that your cell-phone losses connection too. It's all or nothing with digital.
Somehow this reminds me that there have been discussions on using digital techniques to demodulate standard AM and FM radio signals. Now that we have digital radio, what seems to be called HD radio (radio stations seem to advertise it pretty often).
Neither AM nor FM can cope with co-channel interference when the powers are roughly equal.
Power saving is the big thing with FM.
It all depends on what baseband Signal-to-Noise ratio is acceptable. Very high S/N applications (e.g. Hi-Fi music) require an AM RF-level that is high relative to the FM RF-level. For low S/N applications (e.g. voice communication) the positions are reversed.
Ah yes. After technology comes the politics. I'll resist jumping in with both feet.
Incidentaly, the HD in HD Radio is "hybrid digital" not "high definition" (or "heavy duty). Marketing at work.
It's an old axiom, that you don't get something new, without losing some of the old. In this case, you get hi-fi, low noise, better frequency response, 2 or more audio channels, data, realtime traffic updates, additional services, and possibly profitability for AM stations. What you lose is some legacy AM compatibility and increased adjacent channel garbage along with the usual added complexity. In my never humble opinion, it's a reasonable trade. While I got my start in radio listening to DX stations on my parents Grundig radio, I would personally be willing to lose some of that, in favor of technical progress and a radical improvement in performance and features.
Note that in hybrid mode, as commonly used by US AM BCB stations, the digital portion of the xmit power is only about 1% of the total xmitted station power, where most of the power is also in the adjacent channels. There isn't going to be much 2nd adjacent channel (also known as alternate channel) splatter from that low power level. It's kinda academic because few AM receivers have the IF selectivity necessary to reject the adjacent channels anyway.
However, the adjacent 9KHz channels will certainly be polluted. From a recent FCC report:
According to McLarnon, the hybrid IBOC AM system creates two new "stations" in the first adjacent channels, each with a total power of -16 dBc. He states that for a 50 kW station, each would therefore be 1250 watts and current allocation rules provide protection of +6 dB D/U for first adjacent channels. According to McLarnon, if a station currently at +6 dBD/U adds IBOC, it creates a new source of cochannel interference to first adjacent channels at +22dB D/U. He believes that this is significant since it is 4 dB more interference power than is permitted by the Commission?s allocation rules for co-channel stations. McLarnon further states that the majority of existing allocations were created when first adjacent protection was only 0 dB D/U, and this figure still applies to the Canada-US bilateral agreement on AM broadcasting.
I do have some not very nice things to say about the FCC endorsing a proprietary technology, from a single vendor (formerly Lucent), and other administrative oddities. Also, the FCC ignored European Eureka