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With a wideband signal, the different propagation paths of a wireless radio signal can be resolved at higher accuracy than with signals at a lower bandwidth.
Because destructive cancellation (look up Fresnel Clearance) is wavelength specific. If you have a signal that is being canceled by a multipath reflection, moving the frequency by a percent or so will create a path where the cancelation is not as deep.
Taking it to a case where a mobile station (MS, i.e. cell phone) communicate with a base station (BTS), and the uplink (from MS to BTS) reaches the BTS in several paths, then you say that in order to solve this problem, the MS would need to decrease/increase (?) its carrier frequency? (in order to increase/decrease the wavelength, respectrively).
However, I don't think that it's possible for the MS to change its carrier frequency; is it?
Taking it to a case where a mobile station (MS, i.e. cell phone) communicate with a base station (BTS), and the uplink (from MS to BTS) reaches the BTS in several paths, then you say that in order to solve this problem, the MS would need to decrease/increase (?) its carrier frequency? (in order to increase/decrease the wavelength, respectrively).
However, I don't think that it's possible for the MS to change its carrier frequency; is it?
This will only work using "frequency-hopping" or "spread-spectrum" type of radio services. On VHF fixed channel radios, moving the transmitter or receiver a couple of meters usually is enough. On the 2mtr amateur band, I frequently talk to a buddy who lives about 25km away with a mountain between us. If we experience severe multipath distortion on say 146.520MHz, just switching to say 147.540 MHz will clear up the signal. On other days, it might be just the reverse. Another curious thing: during such a contact, if an airliner flys by, the multipath can get better/worse...
High quality FM car radios and "pro" wireless mics use "diversity" to eliminate multipath distortion. Two receiving antennas spread apart less than 1m and two tuners with a detector selecting the best one. It works very well.
This will only work using "frequency-hopping" or "spread-spectrum" type of radio services. On VHF fixed channel radios, moving the transmitter or receiver a couple of meters usually is enough. On the 2mtr amateur band, I frequently talk to a buddy who lives about 25km away with a mountain between us. If we experience severe multipath distortion on say 146.520MHz, just switching to say 147.540 MHz will clear up the signal. On other days, it might be just the reverse. Another curious thing: during such a contact, if an airliner flys by, the multipath can get better/worse...
High quality FM car radios and "pro" wireless mics use "diversity" to eliminate multipath distortion. Two receiving antennas spread apart less than 1m and two tuners with a detector selecting the best one. It works very well.
When I was in the Army back in 1955, the AN/GRC26 vans used diversity.
Two receivers and 2 antenna's. USAREUR Org Charts - AN/GRC-26
The transmitter that we used at that time was a BC-610
There are usually two factors involved with multipath, sometimes three. There is almost always a time delay involved in multipath due to the relected path having a greater distance traveled from transmitter. This is the 'ghosts' in old analog TV picture.
There is the RF carrier phase cancellation based on spacial position. At cellular frequencies (1-2 GHz) the position of these RF cancelling nulls are inches apart in spacing.
The third potential factor is Doppler shift if either the transmitter, reflecting object, or the receiver is moving. This causes the reflection to have a slightly different frequency which sets up another nodel nulling pattern which is dynamic in time based on difference in frequency.
Modulation envelope delay, positional nulling, and Doppler nulling are all lumped together under the name of multipath reception.
Digital cellular has forward error correction which can fix some of the received bit errors. Wideband signals generally are less effected by the narrow frequency bandwidth dropouts of multipath.
Wider bandwidth CDMA also has has benefit of time correlation. Time shifted reflections will not be correlated by primary signal demodulator. Most cellphones have multiple 'rakes' in their demodulator. Rakes are independent correlators and the phone actually looks for reflections and correlates them independently, realign the results in time, and sums together the 'ghost' components to build a stronger recovered signal.
CDMA also has 'soft handover'. This means that your handset transmitted signal received by multiple cell towers are combined together. GSM is 'knife switched'.
In the BS (Before Sat) era we used MUF/FOT charts for HF frequency selection for long range data circuits. I often had use frequency and space diversity (stations separated by hundreds of miles) to maintain a long distance radio connection.
We would sometimes use several channels to transmit the same signal then combine them at the receiving station using signal re-generators and delay modules.
Movement also causes the channel characteristics to constantly change, which makes the job of the channel estimator / compensator / decorrelator a lot harder. The faster you go, the more transmission power you need for a given quality/bandwidth.
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