Hi Ross 200kHz is enough in the mobile urban environment, but indoors up to 10 MHz (say at > 1000MHz) provides diversity gain. at 400 MHz, my feeling is 5MHz is probably a good number because of the sort of things and sizes and distances relative to a wavelength...
On 200 MHz DTV, I have observed in an suburban environment 2-3 MHz frequency ripple. My suggestion is read up on 3G , IS-95, AMPS, GSM, LTE, all these questions have been well studied and comprehensively answered. you just got to take into account different scale of fading patterns at our lower frequencies. MSK is not structy a phase modulation defined mode. MSK is a special form of FSK where the frequency shift is half the information rate. It can look like other phase encloded modulations, but it is a bit different. BPSK having the least number of transitions and the greatest transistition distance would be superior in SNR performance but is less spectrally efficient, and requires a linear radio in order to fully achieve the SNR advantages. MSK is is pretty good. At the end of the day, using CDMA, it does not really matter which modulation you choose- as CDMA is just one method of spreading the signal over a wider bandwidth-. It doesnt matter whether you spread MSK, BPSK, SSB, AM etc, it is a linear process- spread, despread. CDMA runs into its own sync problems with multipath . heavy multipath leads to suboptimal correlator alignment. LTE is the rocker.....it is a 'extreme' and very organised (blocks) form of FDMA. On 16/02/2015 10:03 PM, Ross Whenmouth wrote: > Hi Glen, > >> Yes.. you can improve the demod to cope with multipath, but for a flat >> fading channel like 2m or 70cm, it does not matter how good your demod >> is if you have no signal when you stop at the traffic lights..... that >> is why the system needs some sort of diversity. It doesnt matter what >> type, space, frequency, site just some type. > If one was to use DSSS/CDMA to achieve frequency diversity (say on 70cm > where there is a bit more bandwidth), how much RF bandwidth would be > required to greatly reduce multipath fading effects to the receiver? > (and with a linear RX you could also do clever things like use a "rake" > demodulator). > > Also, instead of using BPSK modulation for DSSS/CDMA, if one was to use > MSK modulation instead (+/- 90 degree phase shift instead of +/- 180 > degree), would there be a reason why a non-linear transmit chain could > not be used? (as far as the TX is concerned it's just high speed MSK). > > ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=190641631&iu=/4140/ostg.clktrk _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
