Carrier frequency and Data Rates

[rakeshm55]

Hi,
 I have a basic query
In Digital communication is it possible to have data rates at lower carrier frequency (with sufficient band width)
eg a 2Mbps data rates at 433Mhz 5Mhz Bandwidth
Is there any communication standards which can achieve similar data rates ??
I have observed such data rates implemented for High Carrier frequencies(2.5Ghz)
If true Why is it this so?? Other than Band width what role does carrier frequency play

Is it something to do with RF related challenges??

[ataradov]

Atmospheric RF attenuation (absorption) is frequency dependent, so there is a number of "good" bands. But primarily lower frequency bands (<10 GHz) are defined by regulations based on prior allocation and demand by the military.

You can achieve any data rate and any carrier frequency, provided you have enough power. Ideally, of course.

[rakeshm55]

Can you suggest some communication standard technology that support such speed with less than 512Mhz carrier freq and Mbps data rates??

[ataradov]

I have no idea if there are actual implementations that would support that range. It is crowded with garbage, so having a reliable communication would be hard. And frequency is low enough that having long range communication will need a lot of power.

Here is a frequency allocation chart for the US: https://www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf The area you are looking for is pretty crowded, there are simply no channels wide enough. You can get into that 420-450 MHz range, but you will probably need a licence to transmit something more powerful than a key fob commands.

Why do you need this?

[BrianHG]

Where will you be doing this?
What kind of distance do you require?
Will you be operating completely within a RF shielded environment?
Do you just need to pass data in RF form down coax?

Depending on the answers to the above questions, answers will differ completely, however, sending 2 megabit via RF with a 5Mhz bandwidth isn't difficult.  Doing this with reliability increases difficulty.  Full duplex makes things more difficult.

It's that 420-450Mhz range where you have an opening, BUT, not with an allowable 5MHz bandwidth, which might leave you with only custom implementations.

For example, for unidirectional with a 4-6MHz bandwidth, there are cheap data modem chips using between 4QAM-256QAM used for digital broadcast over CATV & wireless CATV delivering you up to 25 megabits depending on the QAM modulation & price of components typically used for DTV and internet over cable modems.  Though not a typical over the air standard, they are designed to be used with a tuner front end operating between 54MHz through the upper GHz range.

For 1mbps, here is an all in 1 chip solution: https://www.silabs.com/documents/public/data-sheets/Si4464-63-61-60.pdf
Since they operate in the 1.5Mhz bandwidth, maybe you can use 2 in parallel and space you channels accordingly.

Or, you can go with these, but, NONE advertise baud rate throughput since they only feed around 64kbaud with the maximum allowable 50Khz bandwidth.
http://www.wirelessdata.com.au/sd4-400mhz-long-range-ip-serial-scada-radio.html
http://www.esteem.com/esteem-195c-450-470-mhz-serial/
https://www.tessco.com/products/displayItems.do?groupId=4206250&subgroupId=4206926&productFamilyId=2029959
http://www.gegridsolutions.com/Communications/catalog/SDSeries.htm

[IanMacdonald]

Analog modems achieve about 48kbps throughput with a 1200Hz carrier. This relies on trellis encoding.

For years this wasn't attempted because there was a textbook theory which stated that no comms channel could pass data at more then twice the carrier frequency. It was wrong.

[radiogeek381]

Analog modems achieve about 48kbps throughput with a 1200Hz carrier. This relies on trellis encoding.

For years this wasn't attempted because there was a textbook theory which stated that no comms channel could pass data at more then twice the carrier frequency. It was wrong.

Hmmm...   Claude Shannon published his work on information theory and channel capacity in 1948.  I'm not sure what textbooks after that period could have made the claim that datarate was limited by carrier frequency or channel bandwidth. 

One should note that the paper was published in the Bell System Technical Journal and so widely read and discussed that Shannon became something of a popular culture B-list celebrity.  There were articles written about the article, aimed at bringing much of the content within reach  of more readers.

It is more likely that the limiting factor was the density and cost of the logic and A/D D/A converters at each end of the channel.  To achieve high data rates on a voice grade channel requires a fair amount of resolution in time and amplitude.  Both took a lot of power, space, and money before the advent of moderately high levels of solid-state integration. The need for some kind of forward error correction on high data rate channels makes the computational requirement even greater.

There were high speed digital links where the bitrate exceeded the channel width, and I'm sure there will be posters who can name a few. But before VLSI these were substantial chunks o' stuff and only justified where the need was compelling or potential profits outweighed the cost.

So, after 1948, any textbook that suggested a channel limit based solely on bandwidth was junk and not likely to have been respected or even read by people who built modems.  Certainly the people at Codex, Racal, BTL, NASA, and a host of other places were well aware of the Shannon limit in the 50's and 60's.

To recap, Shannon showed that the channel capacity was proportional to the bandwidth of the channel and its signal-to-noise ratio.

[David Hess]

In Digital communication is it possible to have data rates at lower carrier frequency (with sufficient band width)
eg a 2Mbps data rates at 433Mhz 5Mhz Bandwidth

Sure, just the occupied bandwidth and signal to noise ratio matter.

Is there any communication standards which can achieve similar data rates ??

The same modulation and bandwidth which produces the desired data rate at a higher carrier frequency can be used at a lower carrier frequency.  If you doubt this, just consider using a mixer and image filter to translate the higher frequency signal to a lower frequency and then doing the reverse at the receiver.

I have observed such data rates implemented for High Carrier frequencies(2.5Ghz)
If true Why is it this so?? Other than Band width what role does carrier frequency play

Is it something to do with RF related challenges??

At higher carrier frequencies, a given bandwidth is smaller in comparison with the entire band available allowing tuned circuits to be used.  At lower frequencies, the channel is much wider in proportion to the carrier frequency making design more difficult.

As a practical matter, it is easier to find a continuous 2MHz at 2.5GHz than at 433MHz and I am not aware of any 5 MHz allocations in the 70 centimeter band which would be legal to use for this kind of data transmission.  The closest I am aware of would be in the 33cm/900MHz ISM band which is sometimes used for WiFi.  There might be some "white space" allocations in the vacated UHF TV frequencies between 70cm and 33cm.

Atmospheric RF attenuation (absorption) is frequency dependent, so there is a number of "good" bands. But primarily lower frequency bands (<10 GHz) are defined by regulations based on prior allocation and demand by the military.

Atmospheric attenuation has a very small effect except during exceptional weather where rain or clouds are present.  Below the various microwave absorption peaks, almost all of the difference in path loss is simply because for the same antenna scaled for operating frequency, the higher frequency antenna has a smaller aperture.  If you use antennas with equal apertures like dishes, then the path loss is practically identical.  This is easy enough to show by considering what the inverse square law means.

[CharlieEcho]

OP might find this interesting:

An Architecture for High Data Rate Very Low Frequency Communication
http://publications.drdo.gov.in/ojs/index.php/dsj/article/view/3760/2157

It proposes the use of "Gaussian minimum shift keying (GMSK) modulation and low delay parity check (LDPC) channel coding" to achieve higher throughput for VLF ship-to-shore comms. Much lower frequencies, of course.

[BrianHG]

Silabs also now has reportedly 2000kbps, or 2mbps transceiver ICs.

[IanMacdonald]

"Hmmm...   Claude Shannon published his work on information theory and channel capacity in 1948.  I'm not sure what textbooks after that period could have made the claim that datarate was limited by carrier frequency or channel bandwidth. "

I'm not sure if Shannon wrote this or if it was a misinterpretation of Shannon's work, but it was certainly doing the rounds for many years. An example of how things get 'whitewashed' out of the science books. Not only is the mistake corrected but it's made to seem like it never existed!

[coppice]

"Hmmm...   Claude Shannon published his work on information theory and channel capacity in 1948.  I'm not sure what textbooks after that period could have made the claim that datarate was limited by carrier frequency or channel bandwidth. "

I'm not sure if Shannon wrote this or if it was a misinterpretation of Shannon's work, but it was certainly doing the rounds for many years. An example of how things get 'whitewashed' out of the science books. Not only is the mistake corrected but it's made to seem like it never existed!

You replied to something poorly worded, which might be the same kind of poor wording that has confused others over the years. Shannon showed that the potential data rate is limited by a combination of channel bandwidth and SNR for a memoryless AWGN channel. So, he did say that bandwidth is a limiting factor. He also said SNR is a limiting factor. For a constant SNR the bandwidth tells you the RELATIVE potential data rate (e.g. double the bandwidth for a constant SNR and you double the potential data rate). For a constant bandwidth the SNR also tells you the RELATIVE potential data rate (e.g. double the SNR for a constant bandwidth and you double the potential data rate). What neither tell you on their own is the ACTUAL potential data rate. You need to look at the interplay between the two to find that.

Anyone claiming carrier frequency has an impact on data rate (other than needing to be high enough to keep the lower band edge above zero) is completely off the mark, and always was.

[BrianHG]

Well, from personal experience, with a 5 Mhz bandwidth, even transmitting as low as 54Mhz, with a simple 16QAM modulation, I can achieve 20 megabits per second half duplex with components from the mid 90s with a DAC/ADC feeding a cheap analog TV modulator/tuner with the 4.5MHz audio filter trap removed.  Jumping to the mid 2000s, it would become relatively easy to double the throughput using 256QAM as large and fast enough FPGAs and direct digital synthesis RF DACs/ADCs began to enter the affordable range.