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Enable 2.4GHz RC systems to work with 2.4GHz video systems

This post shows how to modify most 2.4GHz RC systems so that they won’t interfere anymore with 2.4GHz video transmission systems.

Introduction

Ever since the 90s I used my good old Graupner MC15 35MHz system. This worked always well, also with my first quadcopter “Mikrokopter”. But when I changed to my Naze32 system the RC started to become unreliable. This has led to at least three crashes which was quite annoying. Even with lots of ferrites and other filtering elements I could not get the system to work reliable anymore.

So it was time to look for alternatives. But… my video transmission based on wifibroadcast used 2.4Ghz technique. It is well known that 2.4GHz RC systems interfere with WIFI and and analog transmission.

One idea to overcome this issue was to trim the frequency of the TL-WN722N card below 2.4GHz. Here are the required changes to the kernel described. However, this approach has several disadvantages. First, the WIFI cards most likely do not have any calibration data if you leave the official channels. I tested my cards in my microwave oven and noticed a much higher packet loss rate. In fact is was so high that a wifibroadcast video transmission would not be possible. The second disadvantage is that this is illegal to use outside of a shielded cage. In summary: Not an option.

The second possibility would be to buy a 2.4GHz RC system and switch to 5GHz wifi equipment for wifibroadcast. The problem here is that I did not want to buy new wifi sticks. I developed everything for the TL-WN722N and I really like them. In the past I had to test a bunch of other cards until I found the TL-WN722N… Plus since they use 2.4GHz they offer a more robust transmission in case of obstacles. So again, not an option for me.

Mh, and now? There are DIY FrSky modules available. Refer to this nice project. This could have been used with an altered frequency scheme so that not the whole 2.4GHz band is used. But… RC is the most critical part of a quadcopter. And I am not really in the mood to do experiments on this part. It’s quite some work to get the RX and TX modules to run and then I don’t know anything about the reliability of the code or the range and quality of the TX modules… The answer is the same: Not an option.

But I liked the idea of using a 2.4GHz RC system that does not use the full spectrum. I looked around and found no systems that lets the user configure the spectrum. But what if we could modify a bought system in that respect? This is what this post is about.

How most 2.4GHz RC system are working

Most of the RC systems (Hott, Frsky, Hitec, …) use the TI CC2500 chip for the RF communication. The chip is used in a hopping scheme so that a narrow-band channel changes its frequency over the entire 2.4GHz spectrum at a high frequency (100 times per second in case of Hott). This is the trick why these RCs are so robust. Even if there are disturbances on some channels the chances are high that still packets on other channels come through.

The CC2500 is connected to a microcontroller via SPI and transmits or receives data under control of the microcontroller. This is the case for the transmitter as well as for the receiver.
If you take a look at the CC2500s data sheet then you’ll find that this chip has a channel register. This is what’s used to implement the frequency hopping. Both microcontrollers on TX and RX write the same channel sequence into this register so they hop “together” through the spectrum.
So all there is to do is to change the sequence they use and limit its values.

Ways to modify the channel register data

The most obvious way would be to change the software on the microcontroller in the RX and TX so that only a portion of channels is used. Unfortunately, firmware updates (in case of Hott) are encrypted. So one would need to find the encryption algorithm, possibly the key and need to reverse engineer the firmware. Things get even worse because you would have to do this twice, both for RX and TX. And again for every new firmware version. And again for other models. And again for other manufacturers. What a nightmare.

Another approach would be to alter the hardware. One would need to build a device that monitors the SPI communication between the microcontroller and CC2500 and modify the writes to the channel register. While this is a bit less elegant that a software-only solution it provides one major advantage: It works for all CC2500 based systems. Regardless of the firmware version, model, manufacturer… it always works. That’s why I’ve chosen to go down that road.

SPI injector

As said above, the SPI injector should watch for SPI writes to the channel register and overwrite its value. I’ve choosen to always set the MSB of the channel value to zero. This way the spectrum is always in the lower half of 2.4GHz.

I’ve chosen a CPLD to do the work. The reason is that it would be quite tough to get the timing right with just a microcontroller. The SPI bus runs at 4MHz so a microcontroller that just runs two or three times faster might not be able to change just a single bit at the right time.

This schematic explains nicely how the CPLD integrated into the system:

schematic

Here you can see that the CPLD listens on the SPI lines. However, the original connection of the MOSI signal has been cutted and is feeded through the CPLD. Whenever the channel register is written, the CPLD zeros the MOSI signal at the right time.

This is also visible in the waveforms:
waveform

The red arrow shows the position where the CPLD has overwritten one bit (DIN is original data, DOUT is modified data).

I modified a Graupner GR-12 receiver. You can see the connections on the PIC microcontroller here:

gr12

Here is a picture of the modified receiver:

IMG_20150827_175508

If you want to apply this hack to a different receiver, you just need to follow the traces from the CC2500. Of course, you need to patch both the receiver and the transmitter (so that both agree on the channels they choose).

Checking if things work well

I needed a way to check if my changes did work and affected the spectrum. Luckily my TX (Graupner MZ-12) provides a view on which channels it received data.
Here you see the default (unmodified system) output where all channels are more or less available:

IMG_20150826_193129

When I integrated the SPI injector only on the receiver side, half of the channels (the upper channels) should be deactivated. The reason for this that now the TX and RX disagree on the upper half of the channels. TX still uses the original hopping scheme while RX uses the modified scheme. The TX spectrum agrees :)

IMG_20150826_192901

After both devices have been modified the spectrum looks ok again (because now both devices agree on the hopping scheme):

IMG_20150828_174925

Notice the symmetry? This is because the upper half of the channels is now remapped by the CPLD to the lower half. So it is to be expected that the spectrum diagram tells more or less the same for both halves.

How to rebuild

First, you need to clone the repository:

hg clone https://bitbucket.org/befi/cc2500_rc_freq_mod

In the repository you find the sources as well as a testbench for simulation, an UCF file for defining the pinout and prebuilt images.

Second, you need to buy two Xilinx XC9536XL CPLDs. These costs only around 2€ pp.

As a next step you need to flash a XC9536XL CPLD from Xilinx with the prebuilt files. There are many ways to do so. You could use the official Xilinx JTAG cable, a parallel port adapter, a bus pirate and many more. You find lots of information in this Hackaday howto.

The last step is to find out the traces on your TX and RX, cut the MOSI trace and wire up the CPLD. The pinout is described in the spi_mod.ucf file.

Conclusion

In this post I presented a cheap way to modify the spectrum of a RC system. One advantage is that only the channel scheme is modified. Since all other aspects of the RC system are untouched, you still have the good range and high reliability of the original system.
Most importantly, this modification works on all RC systems that use the CC2500 chip, irregardless of brand, model or firmware version.

Forward error correction for wifibroadcast

This post introduces a new feature to wifibroadcast: Forward error correction. This helps lowering the data rate and/or increasing the reliability of the transmission.

Introduction

Early after I published wifibroadcast there were suggestions on improving the forward error correction (FEC) mechanism. While I agreed on these suggestions, my primary goal was to first create something that is usable to see if the idea behind wifibroadcast is worth following. After some months of testing and dozens of other people having also great experiences with wifibroadcast I was confident enough to start to “improve” things.

Initially I implemented a simple retransmission. Each packet got sent several times, increasing the chance that at least one packet got through. This worked sufficiently well but had some issues:

  • Bandwidth: The only parameter to increase the reliability was the retransmission rate. And this rate basically multiplied the net bandwidth. This is quite obvious: If you transmit each packet three times, you need three times the bandwidth.
  • Relieability: Although it seems unlikely that all N retransmissions of the same packet are lost that phenomenon existed. The explanation is quite simple: If you roll a dice often enough, the likelihood of some strange combinations happening needs to be considered. For example, if you roll a million times, you could expect to see the sequence “1”-“2”-“3”-“4” nearly a thousandth times. The same is true for wifibroadcast. We are sending a lot of packets so unlikely things can easily happen.

FEC method

Some commentators suggested to take a look at udpcast, a tool that is able to transfer data over a lossy unidirectional link. And well, that is exactly what wifibroadcast does! The FEC code there was well written and easy to adopt for my use case. In addition, the code is fast enough to run on a Raspberry PI A+. In the code Vandermonde matrices are used to perform the FEC.
The idea behind the code is very simple. Your data has to be divided into packets and a fixed number of them are grouped together to form a block. Lets assume we have a block size of 8 packets. The FEC code now allows you to calculate an arbitrary number of FEC packets (that have the same size as the DATA packets). Lets assume we use 4 FEC packets. If now a DATA packet gets lost, it can be replaced by any of the 4 FEC packets. So we might be able to repair up to 4 DATA packets, regardless which ones of the 8 are affected. This leads to two important advantages:

  • Bandwidth: Since we used in the example above 4 FEC packets per 8 DATA packets, the bandwidth requirements are +50%. This could not have been archived with retransmission. There the “smallest” bandwidth with redundancy would be +100%
  • Reliability: Since any DATA packet can be repair with any FEC packet, the chances of corruptions drop significantly. It is no more important which packets are corrupted, only how many.

Usage

The new FEC feature has been merged into the default branch of wifibroadcast (together with the new fifo interface introduced here)
To update (both on tx and rx!):

hg pull
hg update
make clean && make

The basic usage of wifibroadcast has not changed. Only some parameters have now a different meaning:

  • -b This parameter describes the number of DATA packets per block. So this parameter has more or less the same meaning as with the retransmission version
  • -r This parameter has changed from number of retransmissions to number of FEC packets. Each block will be appended with -r FEC packets.
  • -f Packet size. This parameter is identical to the retransmission version in that it defines the size of a packet. The only difference is that packets now cannot be smaller than -f.
  • It is also important to note that both tx and rx need to agree on -b, -r and -f. Otherwise the transmission does not work anymore.

    How to set the parameters

    The values of the parameters depend on your application. There are two things to consider: The coding rate and the block length. The coding rate is defined by the ratio between DATA and FEC packets. The block length is simply given by the -b parameter.

    • Changing the coding rate while keeping block size constant: Increasing the number of FEC packets increases the reliability of the link at the expense of higher bandwidth. A FEC ratio of 50% is roughly(!) comparable (in terms of reliability) to a retransmission rate of 2. If your block size is 8 DATA packets then this would result in 4 FEC packets.
    • Changing the block lengths while keeping the coding rate constant: An increased block length can increase the reliability of a link without increasing the bandwidth. However, this increases the latency since it needs to be waited until a full block of data has been received before the transmission can begin. Why does this increase reliability? Very often, blocks are only mildly corrupted so that not all FEC packets will be needed. However, these unused FEC packets are useless for the following block. By increasing the block size you are also increasing the “range” where the FECs can be applied.

    Since with this new FEC feature the bandwidth requirements of wifibroadcast can be lowered quite significantly, new WIFI modulations are now usable. I’ve added a new patched firmware under patches/AR9271/firmware/htc_9271.fw.mcs1 that if being copied to /lib/firmware/htc_9271.fw (on tx) enables the MCS1 modulation. This gives you 13mbit/s air data rate of which roughly net 10mbit/s are usable. So this modulation would be perfect to use if you have a h264 data rate of 6mbit/s with a 8/4 coding rate (resulting in a 9mbit/s bandwidth requirement).

    The lower modulation rate should give you a better range if you are flying at high distance or with massive occlusions.

Telemetry OSD for wifibroadcast

This post shows how to add an overlay with telemetry information onto a video stream

Since the last flyaway of my quad having a telemetry link that transmits GPS coordinates was high on my wish-list. Wifibroadcast was prepared long time ago to transfer a second data stream in parallel to the video data by defining ports. But until now I did not make use of it. You can see a video of the OSD in action here:

This post describes how to set up the OSD display. Additionally, two changes required to wifibroadcast are described before the actual OSD stuff.

Setup

My quad uses a Naze32 flight controller that is able to provide FrSky telemetry data over a serial link. The serial port of the Naze32 is connected to a USB2SERIAL converter that is plugged into the tx Raspberry. The telemetry data is then transmitted using wifibroadcast over a second port in parallel to the video. On the receiving Raspberry the telemetry data is received, parsed and drawn onto the screen.

Wifibroadcast minimum packet length

Telemetry data is very different to video data in that it consists of very small packets. For example, most of the FrSky packets are only 5 bytes long. Using the default settings of tx this would lead to a single packet being sent for each small telemetry data unit. This of cause would create a high overhead since the ratio of payload to required header data is very low. To avoid this issue I added a command line parameter -m that can be used to define a minimum payload length. If given, tx waits until it has captured the minimum number of bytes and only then sends a packet.

Several tx instances in parallel

As said above wifibroadcast had the “port” feature for quite some time. The idea was to start several instances of tx, each with a different -p parameter. I used this method for my first telemetry experiments but noticed something odd: Whenever I started a second tx instance in parallel to the video tx process, the video tx was influenced. This was even the case when the second tx did not send any data at all. Quite strange. The result of this was that the video transmission was less fluid and stuttering a bit. Quite a high price for just a bit of telemetry data…

I basically had two options: Trace the cause of this issue in the kernel or adapt tx so that a single instance is able to send several data streams. I went for the latter one because I think it required less effort.
The route I went for was that tx got a new parameter -s that describes the number of parallel streams that shall be transmitted. If this parameter is omitted then tx behaves just like before, transmitting a single stream that is read from standard input. If however two or more streams are requested, tx changes the way it works. It then creates named FIFOs under /tmp. For example, if -s 2 is given, tx creates two named FIFOs called /tmp/fifo0 and /tmp/fifo1. Everything that gets written into fifo0 will be transported over the first port and everything written into fifo1 gets transported over the second port. Quite simple. The actual port number can be influenced by the -p parameter. It serves as an offset. Assuming that -p 100 is given, then fifo0 sends on port 100 and fifo1 on port 101.

The next section will show a usage example of this new mode that should make things clearer.

This feature is still not in the master branch since I had not yet the time to test it well. But after my first successful flight I will merge it into the master.

Putting things together

These commands assume that you have already installed wifibroadcast (as described here).

Transmitting side

First, change to the “tx_fifo_input” branch in wifibroadcast:

cd
cd wifibroadcast
hg pull
hg update tx_fifo_input
make

Next, start a tx instance in the background with a retransmission block size of 4, a minimum packet size of 64 bytes and two parallel streams:

sudo ./tx -b 4 -m 64 -s 2&

All that is needed is to connect the data sources to the named FIFOs:

sudo su
stty -F /dev/ttyUSB0 -imaxbel -opost -isig -icanon -echo -echoe -ixoff -ixon 9600 #set up serial port

raspivid -ih -t 0 -w 1280 -h 720 -fps 30 -b 4000000 -n -g 60 -pf high -o - > /tmp/fifo0 & #video
cat /dev/ttyUSB0 > /tmp/fifo1 & #telemetry

Now video and telemetry data is transmitted in parallel.

Receiving side

The video reception can be left unchanged (refer to here) and should work out of the box.

To get the OSD working, we first need to check out and build my OSD project:

cd
hg clone https://bitbucket.org/befi/frsky_omx_osd
cd frsky_omx_osd
make

And finally you need to pipe the telemetry data into the OSD viewer (note the -p 1 for the telemetry port):

cd
cd wifibroadcast
sudo ./rx -p 1 -b 4 wlan0 | /home/pi/frsky_omx_osd/frsky_omx_osd /home/pi/frsky_omx_osd

Of cause, you can also save the telemetry data using the “tee” command.

In case you just want to test the OSD without the whole wifibroadcast stuff there is a FrSky telemetry log included in the repository:

./frsky_omx_osd . < testlog.frsky

Conclusion

The OSD works well for me. But be warned: Things are still a bit hacky around here. I wouldn’t guarantee that my FrSky protocol interpretations are always right. No wonder seeing how that protocol is designed. It is a perfect example of what happens if companies outsource their coding tasks to mental institutions ;)
It should be easy though to replace the frsky.c parser with a different one (also for a different protocol). Volunteers are welcome :)

Diversity for wifibroadcast

This post describes a new feature of wifibroadcast: Software diversity.


Update 06/02/1025: The new diversity code from 06/01 worked fine on my desktop pc but showed some rare problems on a Raspberry A+. The TX_RESTART was triggered sometimes by out of order packets (with a block distance of 10 blocks!). Due to that I increased the threshold for the TX_RESTART trigger. The Downside is that the empirical tuning of the -d parameter does not work anymore. So I’ll have to write a tool that determines the -d value automatically…

Update 06/01/2015: I’ve added a new parameter to wifibroadcast for diversity. With -d you can define the number of retransmission blocks that will be kept as a window buffer. The parameter defaults to one (which minimizes latency). However, if you use diversity with wifi adapters of different types, they might deliver their data at different times. The -d parameter should be set so that the window buffer spans over the time difference between the two adapters.
The empirical approach of finding the right setting is simple: If -d is too low, you should see often the message “TX RESTART”. Increase the -d parameter until this message disappears and you have found the correct setting.

An active commenter on this blog, malkauns, has started development on software support for diversity. The general idea is: You use more than one WIFI receiver and combine the received data. Due to multipath and other effects the probability that you receive a single packet correctly increases with each added receiver. Together with malkauns I integrated this feature into the default branch of wifibroadcast.

Usage

The usage is extremely simple: Just call wifibroadcast with more WIFI adapters:

./rx wlan0 wlan1

Please note that currently you need two identical adapters. Different types can introduce a delay between the received data streams that currently cannot be processed by wifibroadcast. In this case the systems works but you gain little advantage from diversity.

First measurements

My first experiments showed that the reception quality increases drastically. I tested the feature in a noisy environment with lots of surrounding networks. In every case the packet drop rate was at least one order of magnitude lower when using two adapters. In 90% of the cases it was even better by two orders of magnitude.

Conclusion and outlook

The results are very promising. I think in the future I’ll use diversity on all of my flights. Since the TL-WN722N is very cheap, this feature should be useful to many adopters. There are still some open issues with diversity that need to be solved. The support of different adapter types in parallel is one thing that is planned. Also, there is possibly some potential for smarter FEC in this case. Currently the diversity only works on packet basis. It may be a good idea to dive into their content. We’ll see :)

Getting used to FPV

This post presents just another FPV video transmitted using wifibroadcast

After my last crash I rebuilt my quad and tried it out today. And I am getting more and more used to the FPV feeling. I am still a total noob compared to others but there is progress :)
Since I was a bit afraid to crash again I went for maximum safety: triple retransmissions, 5MBPS, 30fps with a keyframe every 15 frames (thus 2Hz). Due to that and a dirty lens the image quality is not optimal. Again, I had the receiver in my pocket and my body occasionally blocked the line of sight. Therefore, there are some disturbed video frames. I really need something to put the antenna on!

Here is the video (recorded at the ground station):

Relieable 3+km HD FPV solution

This post presents my first experiences using the wifibroadcast video transmission.

To be honest, this post is a bit contradictory.
The title uses the word “reliable” and later in this post I’ll describe my first three FPV flights where the last one ended in a really bad crash.
Analyzing the cause showed that it was actually my fault, not the transmissions one.
But before I come to the bad part let’s start with the good news which is a range test.

Extended range test

The last range test was a bit limited in terms of range since the area I was in was limited to 500m line of sight (LOS).
This time I had a car available and drove to a small hill where I’ve placed the video sender.
You can see a picture of the valley down the hill here:

IMG_20150504_122200

I used the ALFA AWUS36NHA with the omni antenna that came with it as a transmitter.
The H264 data rate was 7mbps with a double retransmission.
On the rx side I used the TP-LINK TL-WN722N with my home-made double biquad antenna:

DSCF1110

The results were really astonishing.
I drove down the valley all the way to the end right before I lost line of sight.
At this point (at 3km distance to the transmitter) I had rock-solid video image.
Since the double biquad antenna is a directional antenna its sensitivity decreases when the transmitter is not in the center of the receive coil.
And again I was surprised that I could move the antenna +-30° both horizontally and vertically until I lost too many packets.
This is really good news: In case your quad stays in a radius of 3km you are safe to fly withing a 60° sector.
This also indicates that the maximum distance is way beyond 3km.
My guess would be that at least 5km should be possible.

I also tested a dipole antenna at the receiver.
This gave a maximum distance of 900m with the advantage of a 360° coverage.

The end of my last FPV flight

As I have mentioned before my last flight ended quite brutally.
And I have learned a lot from it.
Two main problems have caused that crash: Strong wind (30km/h) and blocked line of sight.

I was flying at a distance of around 200m against the wind.
When I turned around the quad was moving with the wind and suddenly lost height which blocked the LOS between the antennas.
The stream was quickly so disrupted that I could not guess its current position and height.
While I was trying to move the quad back to my position (I did not know that the LOS was blocked due to the height) the strong wind pushed the quad away horizontally.
When I (blindly) increased altitude the quad was already pushed away so far that the LOS was now blocked horizontally due to trees close to me.
I still received data but that was mostly unusable.
Maybe each 5 seconds a good frame blinked quickly and was then disrupted again.
When I lost contact completely I was forced to “land”.
The problem now was: Where is it? I knew that the battery was almost dead so the RC activated beeper would soon be quiet.
I ran around like a mad dog listening for the beeping.
After one hour I gave up and “accepted my loss”.
Back home I converted the received video stream of the crash flight to single images using gstreamer.
This way I was able to recover a more or less intact image 10s before the crash:

img1217

This image allowed me to localize the approximate position and finally I have found my quad sitting in a field (luckily the green antenna stood out of the long grass).

The impact took place at a distance of 700m to the rx station.
This means that the wind pushed the quad in 30s around 500m away from me…

Aftermath and conclusions

Although the crash was quite hard all the electronics and motors stayed intact.
Most likely this is a result of the sandwich construction that protects the electronics between two plates.
However, all of the 8mm carbon fiber rods that hold the motors broke quite brutally.
Under normal circumstances these rods are really durable.
So the crash seemed to be quite hard.

My conclusion is that a blocked line of sight using 2.4GHz transmission is deadly.
In the future I’ll take care that my rx antenna has a clear field of view of at least 180°.
Also my first reaction in case of a lost video connection will be to gain altitude.
This incident also motivates me to buy a GPS receiver that transmits the position of the quad and also provides an automatic “return to home” function.

PS: A little cheer up

For not letting this post end with bad feelings I show you the video of my first FPV-only flight.
I shot it using the ALFA AWUS36NHA in the air with a dipole antenna and a TP-LINK TL-WN722N also with a dipole on the ground. The video is decoded and displayed using a raspberry pi as shown here.

The occasional errors in the video are because I had the receiver in my pocket. Due to that my body blocked quite often the line of sight between the two antennas. The images are wobbly because it was quite windy that day (around 20km/h). I know, compared to other FPV videos this is sooo boring. But hey, it was my first flight ;)

Picture of my ground station and sample video of a weak link

In this post I’ll explain my ground station that receives and displays video data using wifibroadcast

My ground station

Let’s start with a picture:

DSCF1118

Here you see my ground station. To the left you see my FPV glasses which contain a 7″ HDMI display and a fresnel lens. The enclosure is made out of foam and at the back a strap from ski-goggles is attached. Right next to the “helmet” you see a 3-cell battery that powers the system. On the right you find the Raspberry Pi A+ that has been integrated into a USB hub enclosure (there was enough room to keep the hub :). The Raspberry is powered by a cheap 5V DC/DC converter.

Sample video of a weak link

Following is a video where I have intentionally removed the antenna from the receiving wifi dongle. This shows some of the artifacts you could experience under bad signal conditions. I know, it looks really bad but remember: A lot of bad images and some good images is better than no images at all. And to get bad images like that with an antenna attached you really need a high distance or a solid block of the line of sight.

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