20 dBm

2.4GHz

RF220SU/SU-EU TX Up to 3 miles (4.8km) at 250 Kbps using a 5.5dBi antenna

* Since it is only 20 dBm, you can increase gain on antenna to at least 9 dBi?

Datasheet: http://www.electronics-chips.com/c/part/synapse-wireless/RF220SU-EU#Pdp-datasheet

Datasheets and firmware for the RF220 series: https://developer.synapse-wireless.com/modules/rf220/downloads.html

* LR-WPANs https://en.wikipedia.org/wiki/IEEE_802.15.4

21 dBm

Details/datasheet: http://www.sparklan.com/p2-products-detail.php?PKey=1338-NipRzMlbr1zsg_tkbOrTX4Deewu385mp1cEyA&WUBA-171GN

Chipset: Atheros AR9271 using https://wireless.wiki.kernel.org/en/users/Drivers/Atheros (Linux)

Pinouts:

1 WPS function ; Active “High” 

2 VCC (+5V)

3 USB_D- 

4 USB_D+
5 GND

Datasheet: http://www.skylabmodule.com/wp-content/uploads/SkyLab_SKW77_V1.03_datasheet.pdf

Chip:  MediaTek MT7620A https://wikidevi.com/wiki/MediaTek_MT7620 which uses https://openwrt.org/

Can we use WiFi to transmit video long-distances? What power will the receiver get for various distances/antenna gains?

How about we use WiFi to transmit video and telemetry via WLAN from the robot station to the base station? Can we use WiFi over long-distances?

First I thought about using a high-power WiFi module on the RF plugin. So I found this one to start, although there are plenty of others:  SkylabHigh power module SKW77. This has transmission power of up to +27dBm (so ~500mW)[1] and frequency range is 2.400GHz—2.4835GHz. IEEE 802.11b/g/n 2×2 300Mbps. IPEX connector (Hirose U.FL?) for antenna.

The 5 dBi antenna is the Antenna +4-5dBI 2.4GHz/5.1GHz/5.8GHz 90Deg SMA Rev 

* Remember that dBm is logarithmic scale!

Then what kind of antenna should be used? Here we have two 5dBi antenna. We can calculate the power received by the base station by entering the following into the Friis Equation[2]:

• Transmit Power (Pt) : 27 dBm
• Transmit Antenna Gain in dBi (Gt): 5 dBi
• Receiver Antenna Gain in dBi (Gr): 5 dBi
• Wavelength: 0.12491352 (for 2.4GHz)

If we have:

Antenna Separation in metres (R) = 1000, that gives -63 dBm at receiver 

If we have:

Antenna Separation in metres (R)= 500, that gives -57 dBm at receiver

Ok, so -63 dBm and -57 dBm are both great for transmitting video!

What happens if we increase the antenna separations (R)?

Antenna Separation in metres (R) = 1500, gives -66.5 dBm at receiver

That's still ok for transmitting video! 

Antenna Separation in metres (R) = 2000, gives -69 dBm at receiver 

That's not so good for video now. Since we wanted better that -67 dBm

What next?

This is just great! But it is all based on ideal conditions! No interference, no reflections, etc. It would certainly be great for robots in a desert, or going along the beach (e.g. trash picking robots!). I.e. in situations when we have LOS. But it wouldn't be so great for robots in a forest! And it would be no good at all for robots in a city! But of course, we would use 4G/5G for those robots.

Well, we can't just increase the transmission power, because there are regulations on this! And we do have to worry about our batteries too! These are the EIRP limits (https://en.wikipedia.org/wiki/Effective_radiated_power) . I only looked at this briefly, but it seems FCC (USA) rule for FCC 2.4 GHz BAND (POINT-TO-MULTIPOINT) is 36 dBm EIRP limit. 

One thing we can do is to increase the antenna gain from 5 dBi to 9 dBi. Keeping the power at 500mW. That should be inside the regulations! And would give us power of around -67 dBm with antenna separation of 4000 metres! Example antenna being ANT-2G-OM-9-N-V (~£22)

Another thing we can do is increase the gain on an receiver antenna! We could go really high with the gain on this! For instance, TP-Link TL-ANT2424B 2.4GHz 24dBi Grid Parabolic Antenna (~£70) gives 24 dBi gain!! But wait, won't this break the EIRP limits? It would do, if it were to transmit! But does it have to transmit? Well it has to transmit as 802.11 specifies bidirectional, and we are going to have problems with security if we don't transmit too! BUT! There have been some exploits of the ESP32 to make it just a one-way data stream to the receiver! I don't want to go into them, because they could be used for bad purposes :-( . Maybe we could RX on the 24 dBi antenna (for video), and TX on a 9 dBi antenna to conform to 802.11 standard? Ok, that doesn't sound like the best idea! But it sure would give a long-range! And is worth investigating!

So with an antenna gain of 24 dBi on the receiver, and 9 dBi antenna gain  on transmitter, we could get power of  -66 dBm at the receiver with antenna separation of 20km! 

[1] Converting dBm to Watts

P_(W) = 1W ⋅ 10^(P(dBm) ^{/ 10)} / 1000 = 10^((P(dBm)^{- 30) / 10)}

[2] Friis Equation

https://en.wikipedia.org/wiki/Friis_transmission_equation

Following on from the 5.8Ghz log, I'm investigating some of the video transmitters used by drones.

MatekSys  VIDEO TRANSMITTER 5.8G VTX-HV W/ BFCMS CONTROL

• Input voltage: 7~27V(2~6S LiPo)
• 40 standard channels, A/B/E/F/R Band
• Output Power: 25, 200, 500mW switchable
• BEC Output: 5V/1A for Camera or FC

You can see details here http://www.mateksys.com/?portfolio=vtx-hv#tab-id-4

TBS UNIFY 5G8 PRO HV

RACEDAYQUADS MACH 2

These are all fairly low-cost (the VTX-HV is around £30), but they are not open-source, so I don't know the components used etc. But I'm sure we can have a look at them anyway! Even if they are not going to be used in the project!

POSSIBLE 5.8Ghz MODULES FOR USAGE IN PROJECT

Anyhow, just looking around, there are some 5.8Ghz TX modules that can be incorporated into our RF plugin for the robot station. We'll be making our own PCB with this and the 433Mhz module on it! We can't just use one of these video transmitters from drones! Because they are not open-source!

FX756T 5.8G 600mW 32CH 

So yes, a lot of the 5.8Ghz modules are lower power than this (600mW), so wouldn't be much good for our goal of 1km! This one seems promising!

So, I went to investigate how drones transmit video! They use FPV modules/systems (first-person video). Many use 5.8Ghz. So they might use 200 milliwatt, 5.8GHz module to transmit video 2.4km range. Or 800 milliwatt for 4.8km range. Note that drones broadcast an omnidirectional radiation pattern, using circularly polarized omnidirectional antennas on transmitter and receiver. We don't need that for the robots, since we are in just one direction from them. Maybe we could use cast surface grid antenna, or dish antenna, on the base stations and robot stations?

I've found a video intro to FPV, and antenna type, dbi/gain, power, diversity controllers, here! 

Remember, like I said, the robots can use a narrow beam vs. omnidirectional that drones need - so we can get much better range without same power costs. The idea of a diversity controller is quite interesting! We have this attached to the base station. It would have different antenna on it, and it will select the antenna getting the best signal. This is the one it would switch to the base station. So it could have a circular polarised antenna on it, and a dish antenna, and a patch antenna, and it will select which is getting the best signal! Drone users are making even 6-channel diversity controllers! Something I'll look into! The next step up for drones, is to use motorized tracking antenna that will point at the drone. These can give up to 115km range for video transmission e.g. http://www.uavfactory.com/product/86 !!

Can we use TCP over RF? Is there going to be an issue here about overloading the sender with NAK or NACK (negative-acknowledgement [ACK]) signals because we are getting many lost packets/packets out of order/etc.?

I found a white paper on this issue here: http://www.rtlogic.com/~/media/datasheets/whitepapers/rtl-mwp-079_adapting%20rfif%20over%20ip%20within%20range%20applications.pdf

Well, we are going to need line-of-sight (LOS) for this! And hope that birds won't fly through the laser!

Anyway, I'm just gathering some useful research on this at the moment:

10m/500Mbps WDM visible light communication systems https://www.osapublishing.org/oe/abstract.cfm?uri=oe-20-9-9919

Introduction

[31/03/18]

I've decided the RF plugin should use 433Mhz for serial communication: in order to transmit control commands from base station to robot, and transmit telemetry (GPS, temperature, battery voltage, etc.) from robot to the base station. This is fairly simple, so I won't list the modules.

The RF plugin should use ~2.4Ghz for video transmission from robot to base station. Unfortunately, there is a lot of stuff using 2.4Ghz so there will be interference depending on area. 

I was thinking to try the Synapse RF266PC1 first. This comes with Synapse SNAP network operating system [pdf], an embedded Python virtual machine . So you don't need an MCU on the module. Here's the SNAP SNAPcore firmware primer [pdf]. BUT this has been discontinued! So, instead, I will try the Synapse RF220SU-EU instead. This seems even better! With 4.8km LOS range giving 250Kbps. 

I doubt we are going to get much of range in urban environment at 500Kbps non-LOS! But let's not be put-off, because we would likely have 4G there instead! The RF comms is going to be used in environments that don't have 3G/4G/5G! So probably not many buildings in these places, which means we might even have LOS. And nothing else using 2.4Ghz either!

2.4Ghz Modules

Here are some of the 2.4Ghz modules I've found as candidates for this!

Synapse RF220SU-EU

4.8km range [this is actually LOS outdoors at 250Kbps]

2.4Ghz band operation

250Kbps, 500Kbps, 1Mbps, 2Mbps data rates

Transmit power: +20 dBm

150mA TX at +20 dBm 

22mA RX

U.FL Connector / rp-sma connector for antenna

Datasheet: http://www.synapse-wireless.com/upl/downloads/resources/supporting-documents/product-brief-rf220uf1-and-rf220su-b874d350.pdf

Nordic Semiconductors nRF24L01+ Single Chip 2.4GHz Transceiver

100m range 

2.4GHz ISM band operation

 250kbps, 1Mbps and 2Mbps on air data
rates

11.3mA TX at 0dBm output power

13.5mA RX at 2Mbps air data rate

Datasheet: https://www.sparkfun.com/datasheets/Wireless/Nordic/nRF24L01P_Product_Specification_1_0.pdf

[there are supposedly lots of fakes for this chip!]

Synapse RF266PC1 - 2.4GHz (chip antenna)

1200m range (at 250kbps)

so I assume we can get to 2Mbps at shorter ranges

2.4GHz ISM band operation

250kbps, 1Mbps and 2Mbps on air data
rates

Transmit Current (Typ@3.3V) 130mA [TX]
Idle/Receive Current (Typ@3.3V) 25mA [RX]

Datasheet: https://cdn.sparkfun.com/datasheets/Wireless/General/Synapse-RF266PC1-Engine-Data-Sheet.pdf

[possibly discontinued - see the latest modules here http://www.synapse-wireless.com/upl/downloads/library/rf-modules-comparison-chart.pdf ]

TANGO-24PA-PCBANT -  RADIO TRANSCEIVER, SPI, 2MBPS

1km range 

2.4Ghz ISM band operation

250Kbps / 1Mbps / 2Mbps  data rates

Transmit  100mW (+20dBm) Maximum Transmit Power

Datasheet: http://www.farnell.com/datasheets/2267055.pdf

MRF24WB0MB/RM -  RF Transceiver Radio Module, 802.11B, U.Fl Connection, 2.483 GHz

[IEEE 802.11 compliant WiFi transceiver module]

400m range

2.483GHz operation

1Mbps/2Mbps data rates

Designed for use with PIC18, PIC24, dsPIC33, and PIC32 MCU's with downloadable TCP/IP stack

Datasheet: http://www.farnell.com/datasheets/1738902.pdf

 As you can see, we won't be able to transmit HD video over long-distances (1km+) using 2.4GHz. There is nothing to stop us implementing repeater stations however! Exploration robots could deposit their own repeaters as they progress I suppose! All of these modules are low-cost (<£30). 

Design of comms plugin cellular / software library