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Thread: Ideal frequency for cell phone communication

  1. #1 Ideal frequency for cell phone communication 
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    I read today an article about the FCC to be auctioning radio spectrum in the 600MHz range and it got me thinking. If spectrum allocation was not an issue and we could build a network for cell phones without restriction on frequency what would be the ideal frequency to use. I remember from my high school days and my basic understanding is that lower frequencies propagate matter better than higher ones and thus would work better at penetrating buildings? Correct me if I'm wrong! It seems wireless carriers want the lower frequencies a lot more than the GHz frequencies for that reason? Not sure and hence my question.

    Thanks in advance

    Regards

    Ryan


    Last edited by Myristate; May 16th, 2014 at 03:36 PM. Reason: Typo
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    Forum Radioactive Isotope MagiMaster's Avatar
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    Lower frequencies require larger antenna though. Imagine rabbit ears on your cell phone.


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  4. #3  
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    Quote Originally Posted by Myristate View Post
    I read today an article about the FCC to be auctioning radio spectrum in the 600MHz range and it got me thinking. If spectrum allocation was not an issue and we could build a network for cell phones without restriction on frequency what would be the ideal frequency to use. I remember from my high school days and my basic understanding is that lower frequencies propagate matter better than higher ones and thus would work better at penetrating buildings? Correct me if I'm wrong! It seems wireless carriers want the lower frequencies a lot more than the GHz frequencies for that reason? Not sure and hence my question.

    Thanks in advance

    Regards

    Ryan
    The word "ideal" is too loose to be of actual value. But here are the factors to consider:

    Lower frequencies suffer less atmospheric attenuation (generally speaking), but efficient radiation/reception requires longer antennas. An antenna cannot be too short -- relative to a wavelength -- if it is to be reasonably efficient. As a rough rule of thumb, radiating structures should have critical dimensions no smaller than about 10% of a wavelength, and preferably a couple of times greater than that. A 600MHz carrier frequency has a half-meter wavelength, so antennas need to be at least 5-10cm in length. That's somewhat on the longish side of what consumers would find acceptable in their modern cellphones.

    Propagation gets progressively worse (again, generally speaking) as frequency increases. Beyond about 5-10GHz or so, km-scale communications becomes unattractive.

    So, there is a decade-wide "sweet spot" between about 500MHz and 5GHz that corresponds to the ideal frequency range for handheld terrestrial wireless communications over kilometer distances.
    Last edited by tk421; May 16th, 2014 at 04:57 PM.
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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by Myristate View Post
    I read today an article about the FCC to be auctioning radio spectrum in the 600MHz range and it got me thinking. If spectrum allocation was not an issue and we could build a network for cell phones without restriction on frequency what would be the ideal frequency to use. I remember from my high school days and my basic understanding is that lower frequencies propagate matter better than higher ones and thus would work better at penetrating buildings? Correct me if I'm wrong! It seems wireless carriers want the lower frequencies a lot more than the GHz frequencies for that reason? Not sure and hence my question.

    Thanks in advance

    Regards

    Ryan
    The word "ideal" is too loose to be of actual value. But here are the factors to consider:

    Lower frequencies suffer less atmospheric attenuation (generally speaking), but efficient radiation/reception requires longer antennas. An antenna cannot be too short -- relative to a wavelength -- if it is to be reasonably efficient. As a rough rule of thumb, radiating structures should have critical dimensions no smaller than about 10% of a wavelength, and preferably a couple of times greater than that. A 600MHz carrier frequency has a half-meter wavelength, so antennas need to be at least 5-10cm in length. That's somewhat on the longish side of what consumers would find acceptable in their modern cellphones.

    Propagation gets progressively worse (again, generally speaking) as frequency increases. Beyond about 5-10GHz or so, km-scale communications becomes unattractive.

    So, there is a decade-wide "sweet spot" between about 500MHz and 5GHz that corresponds to the ideal frequency range for handheld terrestrial wireless communications over kilometer distances.
    The antenna issue had not even occurred to me, Thanks for the education! Did not think the sweet spot would be such a vast range but it is what it is.
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    I think it is a trade off for the telecommunications companies. Lower frequencies will have longer wavelengths and will follow the curvature of the Earth more truly and will be able to be received at longer distances, but yes the antennas have to be larger. Lower frequencies can also be susceptible to more reflections I think. There are other issues too as, if the information on the carrier wave is done by frequency modulation, then the FCC has to be sure that in the range of frequencies needed to carry both the carrier frequencies and the information frequencies, there are no others broadcasts because there will be mutual interference.

    Also here are today's real world cellphone frequency bands. Frequency Bands for Mobile Phones
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    Quote Originally Posted by Mayflow View Post
    I think it is a trade off for the telecommunications companies. Lower frequencies will have longer wavelengths and will follow the curvature of the Earth more truly and will be able to be received at longer distances, but yes the antennas have to be larger.
    Surface wave (groundwave) propagation is pretty much negligible by the time you get to cellular frequencies. A typical upper frequency is in the low numbers of MHz. At cellular frequencies, it's essentially all straight-line propagation, to first order.

    ETA: And since this thread is about cellular technology, it's important to note that the range is limited by design, so following the curvature of the earth would not be a goal at all. Indeed, it would be something to be avoided at all cost.

    Lower frequencies can also be susceptible to more reflections I think.
    Nope -- higher frequencies are. It has to do with the size of the objects encountered, relative to the wavelength. If the objects are small compared to a wavelength, diffraction tends to dominate. If the objects are large compared to a wavelength, reflection can (but does not have to) dominate. Radar depends critically on the truth of that observation. You cannot use low frequencies if you want to make a good radar.

    There are other issues too as, if the information on the carrier wave is done by frequency modulation, then the FCC has to be sure that in the range of frequencies needed to carry both the carrier frequencies and the information frequencies, there are no others broadcasts because there will be mutual interference.
    Cellular telephony relies on much more than simple assignment of a mutually non-interfering set of frequencies. Time-division techniques assign unique slots of time to each user, and code-division techniques convolve transmissions with (nearly) orthogonal codes so that signal processing can pick out an individual signal from among many others that share the same spectrum.

    Also here are today's real world cellphone frequency bands. Frequency Bands for Mobile Phones
    Note that they are all in the frequency range identified earlier as the sweet spot for km-scale wireless.
    Last edited by tk421; May 16th, 2014 at 11:51 PM.
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    If spectrum allocation were not an answer, the quick answer is "all of them". You'd want your phone to go up to wifi and above frequencies when you're close to a tower for huge data transfer rates, but you'd want those long waves for deep in the woods.

    Comcast has an interesting new idea, which is where your home wifi can be shared with other subscribers if they are within range.
    Comcast customer surprised to learn new router is also public hotspot | Ars Technica
    That's the kind of thing I'd expect to see moving forward though, phones that can access a shared network of every wifi hotspot, with traditional cell tech kicking in for greater distances, and even possible satellite integration down the road for really texts from the middle of nowhere, with no cell towers. So its a multi-frequency approach.
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