Skills needed for incorporating RF into my designs - Page 1
Forum section: electronics / projects

author: rx8pilot
author_rank: Super Contributor
timestamp: February 04, 2015, 08:13:19 pm
content: I have been hoping to add some digital RF links into some of my designs, but not sure what I am getting into. The early goals are to add Zigbee, Bluetooth, and Wifi. I am also interested in exploring longer range options of maybe a couple of miles at about 115KBd or so. Not sure what frequencies and regulations I will face for that dream.I have googled around the internet that seems to be littered with various pre-made modules for the short range standards based devices like 802.11x. For a commercial product, I am not sure how much I need to know about RF systems to successfully use these pre-made modules. From my growing experiences with SMPS design, I learned that I needed to learn a LOT about SMPS design to be successful. I tried using pre-made power bricks but they were always a compromise and expensive. Ultimately I have been rewarded by learning to roll my own SMPS, but the learning curve is steep and expensive. I have not seen anything on long(ish) range data link designs. The end solutions need to be as small as possible and battery powered so pre-made black boxes are not likely going to help.What skills should I focus on? What test equipment will I likely need? Is building a transceiver that can do a few miles at 115k bi-directional a matter for someone with decades of knowledge and a $million lab? At this point it all looks like magic and mystery to me. I have tried to read articles and watch videos. It is a whole new world for sure.I have a strong foundation in low speed analog electronics design, PCB layout, high-speed digital (3gbps). Not exactly genius levels, but I am a quick study in general. Keep in mind, this is not a hobby goal, but a commercial goal. I don't think I have the time to make a complete career change, just wanting to add a skill in a similar way that I have done with SMPS designs. I will never call myself an SMPS designer, but I know enough to get what I need.Hoping that some with RF experience can point me in the right direction. Classes, books, links, etc would be greatly appreciated.

author: calzap
author_rank: Frequent Contributor
timestamp: February 04, 2015, 08:40:46 pm
content: You should look into what the amateur radio community can offer. Many amateurs are experienced RF engineers, and some participate in this forum. For example, one member a local club to which I belong is an RF engineer with AT&T and has been for years. You'll not find a more helpful group. Long range digital transmissions have been part of amateur radio for a long time. The ARRL website is one starting point, but there are a lot of amateur radio websites. For the range that you mentioned, you'll need to know something about FCC regulations and perhaps similar regs for other countries.Mike in California

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 12:25:40 am
content: Thank you. That path may also offer up a social life....digging for some HAM sites.......

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 06, 2015, 12:37:47 am
content: Sparkfun have a number of tutorials on using their various wireless products.https://www.sparkfun.com/search/results?term=wirelessAnd it is a pretty good bet there there are many on YouTube, also.Its just a matter of slogging through all the dreck to find the good stuff.As calzap said, many of the amateur radio people ("hams") still "roll their own" RF circuits, and there is lots of theory as well as practical stuff available in ham circles.You didn't mention the nature of the data transmission, but perhaps APRS might interest you...http://www.aprs.org/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2015, 02:35:14 am
content: These days, you need/want very little RF knowledge at all. Everything that's MCU-ready is a pre-cooked module (at worst, add antenna from the datasheet-given layout) and offers way better performance (from radio frequency SNR and bandwidth to bitrate, to built-in PHY and protocol stack) than anything you'd want to spend five years developing.A basic RF signal path, that looks something like,TX: VCO with FM, RF amp, converter, power outputRX: preamp, VCO + mixer, IF filter, AGC/limiter, detectorcan be done in a couple discrete ICs or under 20 transistors, and you put maybe just basic keyed digital signaling into it (UART with lowpass to minimize sidebands), and add whatever stack to that you want (error detection / correction, compression, flow control, etc.). But man, that's a lot of work, even if you know what you're doing in RF; and doing it at 2.45GHz or whatever (a mobile-friendly sort of frequency), harder still.If you don't mind the adventure, shortwave isn't so hard to deal with. But you need big antennas (the top of the SW band is reachable by vehicle-mountable hardware, but walkie-talkies or smaller are unlikely to succeed). There's also a lot of well-entrenched services to potentially crap on, like marine, mobile, airborne and emergency bands. Any one band is quite thin (e.g., the 13.56MHz ISM band is only +/-7kHz), so don't count on getting much more bitrate* than that, either.*More complicated modulation methods (almost always QAM with constellations of 4, 8 or 16 nodes, in rectangular or polar coordinates) allow higher bitrates for the same baud, but depend on higher SNR. This is why wi-fi negotiates a bitrate: it's selecting modulation rate and method. Gigabit Ethernet takes advantage of the good SNR over copper, and throws a huge number of levels into it to get that bitrate up -- if it can negotiate such a connection rate.Also might be worth checking out the history of dial-up modems. You can probably use one to handle real over-the-air modulation modes, by setting it up with the right AT commands to receive a modulation without doing the usual dial/negotiate cycle.Tim

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 05:22:39 am
content: Using the pre-made modules will almost certainly be a necessity for the well established short range applications like BT and WiFi. One question is this: Is it typical to design in one of these modules with an app-note antenna and just trust the specs and performance? Or, is it typical to have the skills and equipment necessary to analyze the actual performance?The quest for the long range solution logically would follow some smaller projects to get my feet wet with the commodity IP available in modules. I have seen some modules that are essentially RF modems that can do 19.2k at significant ranges. Too slow for what I am after, but its a start. Again, not sure how knowledgeable I will need to be to incorporate those into a design anyway. It seems that I will need to have a substantial investment in test gear in addition to the time spent learning about this specialty area. Then again, maybe not. Maybe it's like a digital designer putting someones SMPS in the corner of their box and trusting that it will just work as advertised.

author: georges80
author_rank: Frequent Contributor
timestamp: February 06, 2015, 05:52:48 am
content: BT and wifi is easy enough with an off the shelf module. Most can be had with a small pcb antenna onboard. Treat the BT or wifi module as a uart and it's all just 'software'. Plenty to choose from on ebay if you are just playing.For long range I've had very good success with various modules from Digi (use to be maxstream). I use their 900MHz HP modules which are available with different data rates - basically the slower module you use, the longer distance you can hit (the lower data rate with the same bandwidth gives you better sensitivity). Several miles line of sight is quite feasible with decent antennas on them. The interface again is essentially a uart implementation, so again the work is just 'software'.Sticking with a module and the built-in antenna or using a recommended/approved antenna (especially the Digi modules) means you can hide under their FCC testing etc and you don't have to do anything extra on the radio front. Meet Digi's antenna requirements and distance requirements from a human body and you're also good on that front.cheers,george.

author: Howardlong
author_rank: Super Contributor
timestamp: February 06, 2015, 06:09:52 am
content: I think you have summed it up very well. I do plenty of RF, but for sone things using a pre-baked module makes a lot of sense.Regarding range, you probably realise this, but there's a direct tradeoff between bandwidth and range. Narrowband is the way to achieve greater range. However you can also improve range by choosing a better modulation an coding scheme. Generally a PSK scheme is significantly better than OOK or AFSK, but needs a more complex receiver, although these days that can be done in software. PSK also typically needs a fairly linear transmitter too, so efficiency at the transmitter reduces.Adding error correction schemes also help in improving the perceived sensitivity, but this is a whole subject in itself. Typically they will add some latency, by spreading the information temporally they avoid burst noise.Also for narrowband, when you start hitting 2.4GHz, you need much more accurate reference oscillators, so more expense.Yes, particularly if you roll your own, for commercial stuff you're going to need some test equipment, not just for development and testing, but also for at least a modicum of pre-compliance testing, probably up to at least the third harmonic of your carrier, which is typically the biggest spike after the carrier itself.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 06:11:46 am
content: Digi has some interesting options that are very small and low power. FYI, this effort is aimed toward a commercial product that will have to go through the usual suspects of certifications. I am highly motivated to make wise choices early on to prevent a certification nightmare at the 11th hour.http://www.digi.com/products/wireless-wired-embedded-solutions/zigbee-rf-modules/point-multipoint-rfmodules/xbee-pro-900hpSo, do you think that if put something like this in my product, I could get by without turning myself into an RF engineer? The interface and software part is well within my skill set - but the RF link is the mystery. I feel like I should understand enough to know what is happening under the hood even if I could not possibly design it myself. For example, if the module is crippled because something else in the system is creating some sort of RF problem, I would want to be able to troubleshoot those kinds of things. Essentially, it would be nice to understand and be able to verify the performance of the system.I can hardly believe how inexpensive these little modules are. At least that part is very exciting.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 06:20:47 am
content: Quote from: Howardlong on February 06, 2015, 06:09:52 amIt seems that I would be years away from rolling my own solution. Since you have done RF, could you point me to a resource that can help me build an overview of the technologies and techniques used to encode data on RF? At least at this very early stage, I am hoping better understand the scope of what I am after. It may sound silly, but I don't even know what questions to ask. Some study of the history up to some of the common and current techniques will help.I feel like someone that knows nothing about cars in a repair shop trying to describe a funny sound under the hood.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 06, 2015, 07:39:19 am
content: What volumes are you thinking about?What price point are your products?How large/small are your products? Just L W H roughly.How much bandwidth do you TRULY need?What kind of data are you sending?Are you willing to fuss with licensed spectrum or is license free a critical requirement.Is this for military/govt use or civil sector only?Do you need encryption? Not so you want it, do you NEED it?Do you want all these RF options you've listed in one product, or one at a time?What's your power budget like? What size or Wh batteries are we talking about?What countries do you want to operate or sell into?I would STRONGLY suggest going with an existing, pre-certified radio. There's a lot of companies out there that will OEM you a solution based on a twist of their existing products. If I know more about what you're trying to accomplish, I can probably point you to a source.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 06, 2015, 07:57:13 am
content: Oh and do you need simplex, duplex, half duplex?Is this just raw data or are you trying to something like MODBUS over the air?

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 08:08:25 am
content: Quote from: LabSpokane on February 06, 2015, 07:39:19 am500-1000/year (small qty)Quote from: LabSpokane on February 06, 2015, 07:39:19 amA few $k is typical. Professional applications.Quote from: LabSpokane on February 06, 2015, 07:39:19 amAs small as I can possibly make them. Users have to carry these products all day.Quote from: LabSpokane on February 06, 2015, 07:39:19 amThat is a good question. For the core functionality, I can get away with modest bandwidth for low rate telemetry. The goal would be to design in a path to far higher bandwidth options to the extent possible. I understand that at some point I would need to change approach to make a leap in bandwidth. If all the stars aligned, I would be pushing uncompressed HD video - but that is not the starting point. To get my feet wet, I would be happy with 115k+ with a practical and reliable indoor range of 1000ft.Quote from: LabSpokane on February 06, 2015, 07:39:19 amTelemetry data, audio, video. I can format the data any way that will best suit the RF link. Telemetry can be anything and is not as timing sensitive as audio and video.Quote from: LabSpokane on February 06, 2015, 07:39:19 amI do not anticipate any need to cryptoQuote from: LabSpokane on February 06, 2015, 07:39:19 amInitially, one at a time. Over the next year or so, I am hoping to go from a simple BT link that will be used as a short range status monitor. next would be WiFi to communicate the same data over a network. Followed by the long range RF link where there is no hardwire network to leverage.Pushing audio and HD video over RF is a separate dream that will only make sense after I get these other more basic solutions.Quote from: LabSpokane on February 06, 2015, 07:39:19 amVariable from batteries to AC mains. The batteries can range from 100W/hrs to 500W/hrs 12v nominal so there is significant power available and the run-time only needs to be a few hours per battery. Obviously the lower the power the better, but not really stressed for power like running off of a few AA batteries.Quote from: LabSpokane on February 06, 2015, 07:39:19 amStarting in the US, but we need to work toward a worldwide options. That is probably not all that simple, but it is at least the goal.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 08:14:51 am
content: Quote from: LabSpokane on February 06, 2015, 07:57:13 amFull duplex. multiple streams of RS232, 422 where a 422 command may create a 232 response going the opposite direction.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 08:17:24 am
content: Quote from: LabSpokane on February 06, 2015, 07:39:19 amAbsolutely! I have no dreams of re-inventing the wheel. Thank you for the time, I hope my answers are enough. If not I will continue to refine them.

author: coppice
author_rank: Super Contributor
timestamp: February 06, 2015, 08:31:30 am
content: Quote from: rx8pilot on February 06, 2015, 08:08:25 amIf you know little about RF, and lack a range of RF tools, use pre-approved modules for 1000 a year.If you are an RF expert, and have access to a full range of RF tools, use pre-approved modules for 1000 a year.You'll never recover the approvals cost if you design for yourself, because you just won't be making the same design for very long. Every couple of years Bluetooth, and WiFi move on to a new version, and ZigBee probably won't be around much longer.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 06, 2015, 08:50:34 am
content: Quote from: coppice on February 06, 2015, 08:31:30 amAbsolutely. Unless this is some very high-end (read: expensive) gear AND you have all the expertise and resources to "roll your own" it just doesn't make sense for such a small volume. Especially when there is such an extraordinarily wide range of OEM modules available these days. AND particularly if you will ultimately need international approvals.

author: LukeW
author_rank: Frequent Contributor
timestamp: February 06, 2015, 12:36:20 pm
content: If you need FCC Part 15 (or similar) for your product, using a pre-made module that is pre-certified can make iteasierto engineer your overall product for approval, especially if the pre-made module incorporates all the intentional radiation (transceiver) components of the design. However it doesn't obviate the need for testing and approval of theoverallproduct.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2015, 02:07:43 pm
content: Quote from: rx8pilot on February 06, 2015, 08:14:51 amEhh... you're obviously not doing RS-anything because the signal levels are RF. RS is a signaling standard, not a coding standard.Guessing you mean UART sequences?Tim

author: LabSpokane
author_rank: Super Contributor
timestamp: February 06, 2015, 03:35:22 pm
content: I think I understand where RX8 is going with this. He basically wants an RF conduit for his serial ASCII data. Sometime in the future, he would like to send high-bandwidth streaming data through the same environment.Is that a fair synopsis?

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 05:52:04 pm
content: Quote from: T3sl4co1l on February 06, 2015, 02:07:43 pmI understand, the hope was to communicate the signals I am starting with and ultimately need to end with. The original question was do I need Full Duplex operation, and the answer is yes. Is it universal to use a standard UART to feed an digital RF link module the same way you would if it was wired?Quote from: LabSpokane on February 06, 2015, 03:35:22 pmThat is fair to say.The big question I am trying to gather answers for is this......When using a pre-designed RF module what level of knowledge and skills is appropriate to incorporate and verify the performance of the device? Making educated choices about the various technologies, frequencies, encoding schemes, power budgets, antenna options, interference, etc. These are just some of the topics that I am guessing I need to know. I don't really know what questions to ask. This study is to ultimately build a commercial family of products that is reliable and capable. My fear is that if I simply drop in a module based on the data sheet, I would be in for a rude awakening. Maybe this is an irrational fear or maybe it is sound.I had a few engineers on payroll (not anymore) that would only use data sheet example circuits because they did not understand the "data" part of the data sheet well enough. When the circuits did not work, there was no way for them to troubleshoot because they did not have any understanding of the underlying principles. If I go down this path, I need to have some solid confidence that I will end up with a marketable product. The RF component is only a sub-section of the whole system, but it could bring the whole project to its knees if it has problems at the end. Poor decisions early in the process could be a major financial smack in the end.My knowledge of how data is carried over RF nearly zero which seems dangerous. How much power do I need? I have no idea. What kind of antenna do I need? I have no idea. Without becoming an RF engineer, I should at least be able to have an intelligent conversation with one, right? "Managers Knowledge" so to speak.I hope that paints a better picture of where I am starting and where I am hoping to go.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 06, 2015, 06:08:52 pm
content: The actual process of encoding/modulating your data onto the RF carrier (and conversely the method of demodulating/decoding the data at the receiving end) are handled internally by the store-bought RF modules. They typically have quite good data sheet and application notes published that should meet all your information and integration questions.The bigger question (at least IMHO) is selecting the proper band, power, etc. for the particular product you are designing. For example what is the working distance? Is this supposed to inter-operate with existing RF services (cell-phone service, wireless data service, WiFi, Bluetooth, etc. etc.) Or is your product providing both ends of the communication path? What kind of equipment/service is it? For example medical devices have special bands/modes not available to other kinds of applications, etc etc.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 06, 2015, 06:37:33 pm
content: RX8,I was going to write you a long winded answer, but the short answer is talk to Freewave. They are in Boulder and pretty much the leader in OEM 900 MHz. You'll need 2.4GHz for the video, but the data link should be 900 for robustness. If you have 1000 unit/yr quantities you actually have a good position to negotiate from for pricing. The Digi modules certainly are interesting, but I'm not sure about the history or support. From what I've read, the Digi modules need more self-support and may not be as "plush" as other alternatives. It's tough to argue about the price point, though. It is very attractive.The Freewave will not be a $3 or $10 module. Think north of $100 each. However, virtually all other aspects besides shoving data in and reading data out will be taken care of for you. It will pay large dividends in NRE and support.Your RF link budget will be easy, because it will be what it will be. Jamming a 3' Yagi on your device is not much of an option. You're limited to +36dBm in the US. The radio will be +30dBm (1 watt), giving you a little overhead for cable loss and a 3 to 6 dBi antenna. You can calculate the free space path loss and fresnel zones and all that, but you really are going to be limited by metal, rocks, and dirt between TX and RX. The other limiting factor will be the noise floor where you're operating. 902-928 MHz is getting pretty heavy use by gas/oil and utilities, so if you're near their infrastructure, you might have some range issues. You should not have too many jamming issues, because all the radios are frequency hopping spread spectrum, so they largely play well together.I don't know what kind of infrastructure you're trying to transmit through, so it's hard to tell what your output power should be. What you will want is at least 20 dB of fade margin for a solid link. What that means is that whatever dB signal you're getting at your receiver, you want 20 dB above its minimum sensitivity - or a better metric - is 20 dB above what you deem the minimum is for a reliable data link. The radio will retry sending data on error, basically lowering the throughput. It will be up to you or your application to decide on how much waiting / lost data is acceptable. All this testing is largely empirical. Indoor radio is a very inexact science.Freewave has a different frequency module that's the same form factor for outside of the US.For prototyping such an application, you have tons of choices. Dataradio, GE/MDS, Freewave, etc. The MDS iNet and iNet II are nice little radios and readily available on eBay. You'll just need to make sure they are reset to factory defaults so that you're not locked out of the radio when you get them. I see iNets for $40 to $300 ea on a regular basis.For service and support, Calamp/Dataradio is the leader in my opinion. If you can get a set of old Integra H or the new Phantoms cheap, do it. Very easy to use and very reliable. They are far too expensive to use as an OEM radio in my opinion. You might call Calamp and see if they offer an OEM solution.========================================================If you can manage serial data in your app, you can use an OEM radio module. They really do take the black magic out of the equation.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 06:41:34 pm
content: For BT and WiFi, obviously needs to interface with anyone.For the longer range, I will handle both ends so it does not need to inter operate with any other equipment or a particular standard.Minimum reliable working distance indoors would need to be around 1000 feet to be useful. Considering the enormous amounts of variables, I would probably design for a substantial margin above that. It is entirely possible that this could be operated in a location that does not support wireless RF very well, but I will design for places like office buildings, homes as examples of the most challenging. I would assume that when operated in an open desert plain, the range will probably be better.More range is better especially if the end user can throttle the power output based on available power. For outdoor LOS, a couple of miles (reliably) would be great.This is for control and monitoring of portable equipment that would normally be operated over copper of fiber cables. The goal is to operate this equipment where cables cannot be used or are impractical - vehicle or pedestrian traffic for example. It is also possible that the equipment may be in a position where safety is a concern, so the operator needs to be at a safe distance with losing the time needed to lay cable. The system itself has nothing to do with providing safety or life critical operations. It is essentially a suite of sensors (we do not make the sensors, just the platform that controls the sensors).The fundamental controls are relatively slow - existing as various serial standards and open-drain type status signals. I am assuming that I would be responsible for muxing the data in whatever way the chosen module wants it. Not sure what to expect the data to look like, but I suppose that is relatively trivial. A lot of the data can be parsed as well, reducing the bandwidth needed. For example, some parameters are constantly transmitting at full speed even if they have not changed, that is a simple opportunity for data reduction. It makes sense to get an understanding of the trade-offs with range vs. data rate vs. cost to decide what and how the data will be prepared for the link. Maybe some manufactures offer compression as part of the module?

author: LabSpokane
author_rank: Super Contributor
timestamp: February 06, 2015, 06:48:48 pm
content: If you need to aggregate different statuses and actions, you could consider Modbus. That might introduce a lot of re-engineering of existing hardware, or engineering new protocol adapters.If it was my own hardware and was a relatively standard set of parameters, I'd just build a standard message on one end with fixed fields or delimiters, or XML tags, and parse it out on the other end. Not pretty or proud, but simple. Toss in a checksum of your choosing at the end and you're done.=============================================================================Compression is usually defacto/default on a good radio. And your data lines might run at 115Kb right now, but I would be surprised if you really had a solid 115K stream of data. Most industrial applications don't. The 115Kb is there for latency, not throughput. If you are able, it would be worth tossing in a data tap on your 232/422/485 lines and seeing just how much data you have going back and forth over time. It would also be good to take a uC with two hardware UARTS and build yourself a variable-delay data buffer just so you can see what effect latency has on your application. There are usually adjustments that need to be made.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 06, 2015, 08:13:39 pm
content: As for the throughput of the link, the plan is to pack it as full as possible. The amount of useful data is substantially bigger than 115k and from the beginning, we need to create a priority scheme to determine how to manage the available kbps. There are some signals/command/data that can be delayed 500ms and no on would care. There are others that are more timing critical. Also, we control the data truncation from the sources to balance the data payload with resolution of a sensor.We can carefully manage the data by priority and vary the payload to fit the available link quality. Also, we have the option of recording a significant amount of data to buffer short term link failures.I took a look at the Freewave website and reached out to them. The quick breeze through the site looks promising. Looking forward to digging through various integration resources they have. As a matter of self-interest, I suspect they will be happy to offer some guidance. Thank you for that tip!

author: LabSpokane
author_rank: Super Contributor
timestamp: February 06, 2015, 08:39:35 pm
content: If you truly have 115kb solid, try prototyping with a 512k or better radio that will let you dumb down its transfer rate. The radio will have a certain amount of overhead for error correction etc.Another good feature that folks like Calamp have is online diagnostics (RSSI, bad packets, temp, etc) via a second serial channel. I'm guessing Freewave offers that as well. If you're going to stuff the pipe as full as you're planning, you'll need that info during normal operations.

author: Axl
author_rank: Newbie
timestamp: February 22, 2015, 03:24:33 am
content: Hi, have you considered a GPRS/GSM module on an MCU board?

author: hans
author_rank: Super Contributor
timestamp: February 22, 2015, 10:21:50 am
content: Quote from: rx8pilot on February 06, 2015, 08:13:39 pmI would strongly recommended partnering up with an OEM manufacturer of digital radio modems.Here are some more to check out I know off:-AR West-Satel-Pacific Crest-MicroHardNot all of them make the digital modems you're probably looking for. I suspect you need 900MHz frequency hopping in the US, or 430MHz modems if you can afford to get a license for all those modems (PITA).Nor will all of them make really fast modems (115200+ Baud for example), because the marketed long-range is achieved with noise-free environment and a very very sensitive modem + good antenna's.In practice, always account formuchless range. Some manufacturers claim 40 miles - probably out at sea with excellent line-of-sight and directional antenna's.Some of them make fast mesh modems though, which may be of interest if your products happen to be clustered around an installation. You probably do need to ask the manufacturer about the actual throughput vs baudrate, because those are far from equal.My personal experience is that you get 4-5km range with 1Watt 400MHz modems 2 meters of ground level, outside urban landscape (no buildings, not directly under trees). When you can transmit like 30 metres up high, you may double that range on average.Not sure about 900MHz, probably much less than 400MHz because the signal propagation is not as good.What even more hurts this is that the FCC is pushing for more dense traffic in the air. They banned 25kHz/9600Baud operation a little while ago I believe; so modem manufacturers will be focussing on higher baud rates in smaller channels. This leads to the use of more convoluted encoding schemes, which typically worsen the sensitivity.Rolling your own long-distance solution is really though I think. Although you can probably get them to work from the application note for a chip from like SiLabs or CML microcircuits (actually these chips are in most modems I just listed), the design verification and debugging is key and makes their trade though. I've seen quite a lot of issues/trouble with various operation conditions (humidity/temperature level/temperature transients) and interferences (high signal power, EMI issues from other products nearby, etc.), so I don't think you can also expect a completely bump-free integration path.

author: Howardlong
author_rank: Super Contributor
timestamp: February 22, 2015, 06:35:15 pm
content: My finger-in-the-air says that 1000' indoors at a 115kbps sustained data rate isn't feasible in unlicensed spectrum I'm afraid.Even with an efficient modulation scheme and the best concatenated FEC coding at Shannon limit, the ERP allowed on unlicensed bands isn't going to cut it indoors unless you knock the walls down and have line of sight between each end.Mesh might work, but as well as adding lots of latency, there's a whole lot of extra complexity inserted into your path.Perhaps Wifi with extenders?By the way, I missed that you were looking for something on the subject. A good book for the engineer who's after a practical introductory text on modulation schemes and coding is "Wireless Digital Communications: Design and Theory" by Tom McDermott ISBN 0-9644707-2-1. It might say "Theory" in the text but it's not full of unapproachable numerous equations that many academic texts present. It is very practical as a primer, although it's quite old but still relevant. I recommend it to postgrads who I think have been taught too much theory and not enough practice when I do the odd stint at universities.One other thing about using unlicensed spectrum is that one should be very wary of becoming dependent on it. As there is no co-ordination, unlike licensed spectrum, you are potentially building a system on a sink hole. It's quite common to have unlicensed wireless systems just break because, for example, a crane operator turns up at a construction site a couple of blocks away. Spread spectrum can help, but within the unlicensed spectrum you are limited to what you can do.

author: rx8pilot
author_rank: Super Contributor
timestamp: February 25, 2015, 12:16:42 am
content: Thank you for the book tip.As I have continued to learn and speaking with various companies that manufacture OEM radios, a picture is starting to develop. I hear what you say about the cross-your-fingers nature of unlicensed bands, but it is a limitation I will be stuck with since the end users would prefer to deal with that limitation than the cost and complexity of using licensed bands. The system is mobile and will travel which is another big problem, especially when it needs to go to other countries. From what I can tell, 2.4Ghz is the only spectral freebie that covers the world. Of course there is an enormous amount of 2.4Ghz traffic out there.I do have the option to design the mechanics to make the radio a clip-on module that can be switched out by the user. it may be another can of worms, but they could choose 900Mhz, 867Mhz, or 2.4Ghz depending on location and performance desired.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 25, 2015, 03:45:26 am
content: Your analysis is correct. Meshed 2.4GHz is what you'll be stuck with. Watch out in the EU because they dumb down the EIRP pretty severely.

author: Howardlong
author_rank: Super Contributor
timestamp: February 25, 2015, 03:10:18 pm
content: Yes, regretfully the noise floor in the 2.4GHz band is so bad these days due to its popularity that making something reliable and consistent is next to impossible. You can have surveys done, but that would work out very expensive if it's designed to be a mobile system.915 and 869 are nice because there's high power available and the propagation is a little better than 2.4GHz. 869 in ITU region 1 offers 500mW over several 25kHz channels that can be aggregated. However as I remember it, you cannot just transmit all the time, there are listen before talk and/or duty cycle limitations too. ISTR 915 requires spread spectrum for use of 1W in ITU region 2, anyway when I last looked at the band a couple of years ago it would need a completely different modulation method to region 1's 869 allocation. There are also national limits applied despite the ITU region recommendations making it very hard as it seems you've discovered.Also remember that either way, you are also restricted on the antennas you can use legally. Technically there is good reason to use directional antennas for point to point links, which it sounds like what you are doing, this not only gives gain but also reduces the noise floor by somewhat nulling out other in band transmissions. Typically the EIRP restrict you to little more than a dipole however at full power. But remember that antennas are reciprocal, they have gain on transmit and receive, so you could limit the power to keep within EIRP restrictions but have the benefit of gain on the receive side as well as nulling out unwanted co-channel interference.One last thing, remember that if this is a bidirectional link there is going to be latency as I assume you are going to be operating half duplex on the same channel. For a mesh network, this latency will get worse as more nodes are placed in the path. You could operate full duplex by having, say, a low bandwidth command channel on a different ISM band, and allowing the high bandwidth channel to operate full time. Trying to operate full duplex in band brings a whole heap of other problems, mostly that you'll need some pretty spiffy RF filters and will probably have to operate fixed channels as a result.