4
\$\begingroup\$

I am trying to create a circuit which can perform wireless communication over short ranges (less than 10 metres) at a very high data rate (approx 10 Mbps) and low power (in the range of 10-20 mW). There have been IEEE papers in which people have implemented this using UWB. They have used about 20-25 mW of power to transmit to about 4-5 metres at 10-15 Mbps.

As a result, I decided to do this using UWB technology. UWB technology employs using a large bandwidth even into the licensed regions to get a good bandwidth. FCC has put in an upper threshold of -41.3 dBm/MHz to limit its range and thereby the interference with other licensed products. Because of the low output spectral density, it is possible to have a good data rate consuming small power for small ranges.

UWB requires modulation techniques to be implemented. They are various choices available like PPM and BPSK. BPSK has been found to be more suitable compared to PPM because the power spectrum of PPM has peaks and hence severely limits the usage (recollect the FCC threshold of -41.3dBm/MHz). As a result, I landed up on BPSK (Binary PSK) or to be more precise DBPSK (Differential BPSK). Differential BPSK does not require coherent clocks on transmitter-receiver and hence preferred.

Instead of going into the design of the entire modulation/demodulation circuit, I want to use commercial ICs/modules. Designing has many challenges:

  1. Design of antennas and modulation/demodulation circuit is very difficult at such high frequencies.
  2. The wavelength at such high frequencies is proportional to the circuit dimensions and hence creating an entire new range of problems.
  3. Time is critical in my case. I do not have the time or resources to do circuit simulations and design a good circuit. Designing a circuit may be a follow-up of my work.

So, if you have any ideas about various commercial ICs or modules available for the following, it would be really great

  1. Transmitter: Modulator (+ Antenna) for Differential BPSK. There are power constraints and should be able to do communication at high frequencies.
  2. Receiver: Antenna + RF detector followed by Demodulator.

Just to summarise my requirements for the transmitter:

  1. Weight: 100 grams or less
  2. Data rate: 10 Mbps
  3. Power: Less than 50 mW (Smaller the better)
  4. Range: Less than 10 metres

Receiver has no such constraints. The transmitter is portable and battery operated and hence there are many constraints.

Please do let me know if you need more information. Thanks in advance.

Edit: Complete restructuring of the question

\$\endgroup\$
7
  • 1
    \$\begingroup\$ Why modulate on a carrier at all? You haven't said anything about requiring wireless operation. Both ethernet and USB can do what you want out of the box. It seems silly to invent your own modulation scheme. \$\endgroup\$ – Olin Lathrop Dec 12 '11 at 13:01
  • \$\begingroup\$ I am sorry I havent mentioned it explicitly but I am looking for a wireless scheme. \$\endgroup\$ – Neel Mehta Dec 12 '11 at 14:32
  • \$\begingroup\$ Do you realize you can't get reliable communication without bi-directional communication? If one unit is only a transmitter, then you have to be prepared for a certain error rate. \$\endgroup\$ – Olin Lathrop Dec 12 '11 at 19:50
  • \$\begingroup\$ @NeelMehta Can you give us some links to these IEEE papers that you talk of? \$\endgroup\$ – Kellenjb Dec 12 '11 at 21:44
  • \$\begingroup\$ I am a little rusty on the theory side of things... but doesn't -41.3 dBm/MHz mean that your EIRP has to be less then -41.3 dBm per MHz used? So to get up to 20 mW output you would need to spread it out over 270 GHz? \$\endgroup\$ – Kellenjb Dec 13 '11 at 2:59
2
\$\begingroup\$

Your question seems to be contradictory -- you seem to be saying you want to "create a circuit" without actually "designing a circuit".

I'm going to interpret that as saying you want to "build a complete system, including designing a high-level protocol and laying out a few circuit boards and soldering integrated circuits to the boards and plugging sub-assembly modules into those boards", but you'd rather not "design and fab a full-custom ASIC from scratch" or "design something from dozens of discrete transistors instead of a chip" or "design and do EM simulations and construct a full-custom antenna system and get FCC approval".

I've heard that, at least for low data rates, that UWB can be produced using simple circuits using off-the-shelf chips that were common long before anyone ever heard of UWB.

Alas, I don't know any specific chips that you could use for the data rate you want, much less if there exist off-the-shelf modules using those chips, but I hear that such chips exist. Let me give you some links that might lead to those chips.

My understanding is that there is currently only one UWB standard -- WiMedia’s Multiband OFDM, standardized as ECMA-368 and ECMA-369. My understanding is that "Certified wireless USB" and a potential future version of "Bluetooth" and a potential future version of "Zigbee" are higher-level layers on top of WiMedia's UWB standard.

My understanding is that there are several chip manufacturers producing chips that comply with this standard. a b

I hear that several other chip manufacturers are producing non-ECMA-compliant chips, including Pulse~LINK, DecaWave, IMEC, WiLinx, Wisair. Presumably those chip use some other proposed standard or proprietary UWB techniques.

If you can't find an off-the-shelf module, and you find yourself looking for individual chips, I suspect that many of the chips developed for HomePlug might be usable as part of a UWB system.

\$\endgroup\$
2
\$\begingroup\$

Doing your own RF communication at 10 Mbit/s is not trivial and will require regulatory approval, as does any intentional radiator. Since you are here asking about the basics, I'll assume a full blown licensed or approved RF development is out of your scope. It would take significant time and money in any case. I therefore see two options:

  1. Use something off the shelf. WiFi seems like the obvious choice. You can buy already licensed modules, and it can support the data rate.

  2. Don't use RF. You said "short" range, so maybe some kind of light modulation is a possibility. Light emitting devices can be modulated at these speeds, but detection circuits at that bandwidth will not be trivial. Then there is the issue of converting the data sliced output to a meaningful and verified bit stream. At 10 Mbit/s, that will need to be done in a FPGA. I don't see a microcontroller having enough compute power to do manchester decoding, for example, at those speeds.

\$\endgroup\$
10
  • 1
    \$\begingroup\$ I would think 10MBit/sec should be possible in unlicensed bandwidth. There are off-the-shelf devices to send video wirelessly in 900Mhz and 2.4GHz. I would think that one could design a device to convert a 1.25MByte/second data stream to a video stream with enough forward error correction to provide a usable data rate (e.g. even if one restricts oneself to video format data, leaving the horizontal and vertical front/back porches blank, sending 320 pixels per line, 240 lines/field, at 8 gray levels each would be a raw data rate of 13.8 mbits/sec. \$\endgroup\$ – supercat Dec 12 '11 at 16:01
  • \$\begingroup\$ I don't want to suggest that as an optimal format for sending wireless data, but the bandwidth would seem to be there. \$\endgroup\$ – supercat Dec 12 '11 at 16:01
  • \$\begingroup\$ It is possible to transmit at low energy levels using a very huge bandwidth. This is where the UWB comes into picture. The FCC has put in a power spectral density emission limit of -41.3 dBm/MHz. The wiki article on UWB briefly explains the same. Since, I am looking at low power and portable system, WiFi doesnt seem to be a good possibility. Hence I am looking at UWB. Since it is not very popular, there are not commercial modules available. Hence, I am trying to make a circuit using various ICs. \$\endgroup\$ – Neel Mehta Dec 12 '11 at 16:21
  • 3
    \$\begingroup\$ @Olin, You say the "low power unlicensed bands" don't have the bandwidth required to transmit 10 Mbit/second, but then go on to say that they can use WiFi. As far as I know, 2.4 GHz falls into what I would consider a low power unlicensed band. You do have to get FCC approval to manufacture/sell the device, but this isn't a license to use the band, rather an approval that you are following policies, but you would have to do this for any frequency band anyways... Can you clarify what you mean? \$\endgroup\$ – Kellenjb Dec 12 '11 at 16:25
  • 2
    \$\begingroup\$ @Neel: This UWB thing you keep talking about sounds like some spread spectrum approach. How much power do you have available at either end to support this 10 Mbit/s data channel? \$\endgroup\$ – Olin Lathrop Dec 12 '11 at 17:14
2
\$\begingroup\$

Realistically, your choices are to pick a highly integrated chipset (or more likely module), or else acquire a large FPGA and some data converters or chips implementing the various functional blocks and lease a spectrum analyzer and several generators and begin a 6-24 month odyssey of learning.

Which is more appropriate really depends on if your goal is to solve some current application problem, or to seek future work in the low-level design of such modules.

\$\endgroup\$
3
  • \$\begingroup\$ Do you have a highly integrated chipset to recommend? \$\endgroup\$ – Kellenjb Dec 12 '11 at 18:45
  • \$\begingroup\$ @Kellenjb - I thought we didn't do shopping questions. Wifi as a variety of schemes and spectral allocations has already been mentioned and certainly merits real consideration. There are many vendors offering solutions for that and other technologies, the choice amongst them depending on numerous factors that haven't been included in the question. \$\endgroup\$ – Chris Stratton Dec 12 '11 at 18:56
  • \$\begingroup\$ @ChrisStratton: I have tried to provide more details stating everything explicitly. Please see if you can provide me with pointers to chipsets (or modules). \$\endgroup\$ – Neel Mehta Dec 12 '11 at 19:42
0
\$\begingroup\$

Your statements "very high data rate (approx 10 Mbps)", "I decided to do this using UWB technology.", and "I do not want to design a circuit at such high frequencies" appear to conflict.

I think you need to either lower your expectations of data rate, or expect to tackle some of the challenges of RF design.

\$\endgroup\$
2
  • \$\begingroup\$ What I meant by "I do not want to design a circuit" was that I do not intend to make a circuit using basic components. I want to use commercial off the shelf components for this. i.e. I want to use commercial ICs for modulation, etc instead of designing the circuits for the same. I hope this clears the conflict. \$\endgroup\$ – Neel Mehta Dec 12 '11 at 14:38
  • 3
    \$\begingroup\$ This looks like a comment to me. \$\endgroup\$ – Kellenjb Dec 12 '11 at 14:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.