When designing a short-range wireless system, you look for one that will give you the maximum range (distance between transmitter and receiver) and reliability that fits the application. Often, the technology choices do not always give you the range you need. So before you go nailing down a technology or standard, it is best to do some upfront calculations. First, look at the available technologies and determine their maximum reliable ranges. The table linked below is an estimate given by the various standards organizations and alliances. It will at least get you into the ballpark for an initial selection. (see table)
Next you can get a good estimate of the range by going back to the basic formula for received power:
Pr = Pt(Gt)(Gr)λ2/16π2d2
Pr is the received power, Pt is the transmitter output power, Gt is the gain of the transmitting antenna, Gr is the gain of the receiving antenna, λ is the wavelength in meters, and d is the range or distance. Playing around with all the variables and solving for d will let you determine just how much range you can get... theoretically. The formula assumes an open unobstructed, but not necessarily line of sight, path between transmit and receive antennas. If you have lots of obstructions like walls, other buildings, trees, or other stuff, your range is going to be much less than the theoretical. If you have worked with radio before, you know that testing is a must in the real world.
In the formula above, the antenna gains are power ratios, not dB. If the gains you have are referenced to a dipole as many specs give, these values must be multiplied by 1.64 to get them into a isotropic gain reference value for this formula. The distance d is given in meters.
Now here are the top 10 ways to extend range. Some are related to the formula above, but some are not:
1. Use a lower frequency
Look at the aforementioned formula and note the range is proportional to the square of the wavelength (λ). And, of course the λ = 300/f where f is the frequency in MHz. So decreasing the frequency of operation is a quick way to get more range. Lots of wireless standards operate in the 2.4 GHz band, but that does restrict range. If you really need to get maximum range, drop down to the 900 MHz band or even the 433 MHz band.
2. Increase transmit power
This is usually restricted by the FCC in whatever standard you use. Just use the max you are allowed. Sometimes the chip companies make chips that run at less power than allowed, so you may have to add an external power amplifier (PA) later. Wi-Fi chips and modules are like this. And Bluetooth has three power level options.
3. Increase receiver sensitivity
Receiver sensitivity is a measure of the gain the receiver has. It is usually expressed in dBm. Typical IC receivers have sensitivities in the -80 to -115 dBm range with the higher dBm value being the most sensitive. Choose a chip or module with the greatest sensitivity if maximum range is one of your key design goals.
4. Use an LNA
Most IC receivers already contain some kind of low noise amplifier front-end. But you can boost receiver sensitivity by adding an external LNA between the antenna and the IC input. Just be darned sure that this LNA has the lowest possible noise figure or you may be hurting yourself more than you are helping if the noise kills the signal.
5. Reduce transmission line losses
If your system uses a coax transmission line between the antenna and transceiver, be sure to use the lowest loss line you can find. Most wireless systems have short transmission lines, but at the normal UHF and microwave frequencies, coax has enormous losses. You have no idea until you take a look at this. Even a few feet of line can drop transmitted and receiver power by many dB. Shorten the line as much as possible or use a better, lower loss line. Or if you can, put the transmitter and/or receiver at the antenna.
6. Use directional antennas
A directional antenna focuses the radiated power in one direction. Such antennas are said to have gain. It is a way to get a power boost without actually increasing the transmitter power, and it increases the level of the received signal. The formula above clarifies just how important this is. Just be careful not to violate FCC rules, as some regulations do not allow gain antennas. Check the FCC CFR 47 Part 15 for applicable rules.
7. Increase antenna height
In most applications you will be using UHF and microwave frequencies, so line of sight (LOS) rules apply. If your receive antenna cannot "see" the transmit antenna, good luck in making a reliable link. The way to overcome this problem is to increase antenna height. Range automatically increases with height according to the formula:
D = √(2ht) + √(2hr)
D is the range or distance in miles, ht is the transmitter antenna height in feet, and hr is the receiver antenna height in feet.
Putting the antennas on a tower or tall building will get you in the clear for best reception, but remember; if you use a tower, your transmission line length may increase decreasing the power at the antenna thereby offsetting some of the benefit of height. There are cases where height is more important than line loss just because the signal will get through if it is LOS. You can always make up the loss with a LNA or PA, or you can put the transceiver on the tower with the antenna. Let the transmission line be twisted pair carrying the data.
8. Consider the modulation schemes
When designing a digital radio system, you will be looking at things like carrier to noise ratio (C/N) and bit error rate (BER). The modulation scheme used by your system will determine the relationship between the two. This is no place for a college course on the subject, but what it boils down to is that some modulation methods give a better BER with a lower C/N. FSK is better than ASK/OOK. BPSK is better than FSK, and so on. If you are really dealing with a critical link, be sure to check into this. You may even want to consider direct sequence spread spectrum (DSSS), which spreads the bandwidth of the signal to provide process gain that will add to the range. Cutting data rate can also usually lead to lower BER and longer range.
9. Minimize obstructions
When you evaluate the applications, try to eliminate as many obstructions as possible. Walls kill range more than anything. They won't stop a signal, but they will attenuate the daylights out of it. If walls have to be involved, try to minimize the number implicated. If you can, put the transceiver outside the wall. If outdoor obstructions are present, locate antennas appropriately to avoid them. Put the antennas on towers or find some way around the obstruction. Relocate the whole application. Take a hard look at this, as obstructions will make or break the application faster than anything—and this goes for indoor operation as well.
10. Use a repeater or mesh network
A repeater is just a transceiver operating on a different frequency that serves as a radio relay station. Many VHF and UHF radio systems use repeaters where they are mounted on a high building or a tower on a hill. The transmitter sends the signal to the repeater, which receives the signal and retransmits it on a different frequency. It works great. The big problem is that the system becomes more complex and more expensive. And, repeaters are often not available for or allowed in some wireless services because of the extra frequencies needed.
A good alternative is to use a mesh network. A mesh uses lots of transceiver nodes that talk to one another over a short distance. If the nodes are spread out, a wide range can be covered. To send data, one transceiver sends the signal to one of its closest neighbors. This neighbor node serves as a repeater/router and transmits the data to the next node and so on. With lots of nodes, a truly extended range can be achieved. ZigBee is a natural choice because mesh is inherent. Other wireless technologies can be adapted to mesh but it requires extensive proprietary design. Wi-Fi, the popular WLAN 802.11 technology, is now regularly used in mesh applications you can buy. An IEEE mesh standard (802.11s) is in the works. A real benefit of mesh is that it not only extends the range but also increases reliability since if one node goes down, the signal can usually find an alternate path through the mesh.