What makes radio frequency spectrum cover a larger distance for data transfer?
Bluetooth, WiFi and ZigBee all use the same band (2.4 to 2.5 GHz) but their coverage area is different. Why?
If it's from gain, then is it possible to replace a Bluetooth antenna with a high gain antenna and send to larger distance just like WiFi?
I think I need to learn a few fundamentals of radio waves. Please educate me. I asked professors and industry persons also and they didn't clarify the concept clearly.
To simplify things, wireless communication can happen if the received signal strength is higher than the receiver’s sensitivity.
The received signal strength is a function of:
Free space loss is a function of distance and frequency.
So with most things equal (TX power, wiring, antennas, frequencies…), the remaining variable is distance.
The receiver sensitivity is highly related to the data rate. It’s like when you speak ve-ry slow-ly whi-le ar-ti-cu-la-ting ca-re-fu-lly, it’s much easier to understand you even if you don’t speak loudly or there’s ambient noise than ifyouspeakveryquickly.
So you have on one side RX signal strength which depends on distance, and on the other sensitivity which depends (along other things) on data rate.
The standard BLE data rate is 1 Mbit/s. The standard Zigbee data rate is 250 kbit/s. This explains the difference in range.
Recent versions of BLE (5.0 or thereabouts) introduced new data rates, including the new “long range” PHY, which has a data rate of… 250 kbit/s.
This is also why LPWAN solutions like LoRa or Sigfox use even slower data rates. LoRa data rates are in the hundreds to thousands of bits/s, this is what explains the very long distances they can achieve with limited TX power.
Note that this is oversimplified. Modulation techniques, bandwidth (the size of the slice of spectrum that is used) and the quality of the hardware also have an influence.
That’s mostly where WiFi differs. WiFi uses a lot more bandwidth (in terms of spectrum use). The latest iterations of WiFi use up to 80 MHz (where Zigbee uses 2 MHz and BLE 1 MHz). They also use spatial diversity (MIMO) with multiple antennas. The modulations are also more complex, which uses more power, because WiFi devices are less constrained.
Note also that not all devices are created equal. In most regions there are strict EIRP limits (radiated power, taking into account all transmitter gains and losses including the antenna), and while some devices are exactly at the limit, others may transmit well below that. Likewise, not all devices have the same RX sensitivity even with all other parameters equal.
Finally, let’s look at antenna gain. An antenna always transmits the same amount of power the radio chip sends it, the difference is how this is “concentrated”. Antenna gain is defined by comparison with an “isotropic” antenna, which is a theoretical antenna which transmits the signal in all directions (both horizontally and vertically). Most common antennas are dipole antennas, which are omnidirectional, in the sense they send in all horizontal directions (with the antenna vertical), but a lot more in the horizontal plane and close to it than up or down. Such antennas have a gain of about 2.2 dBi. Higher gain antennas will “focus” more of the signal in a limited number of directions, either by reducing even more the vertical angle it transmits on, or in more extreme cases in a very narrow cone. Helpful for point to point links, not at all for covering a wide area.
Also, antenna gain is included in EIRP, which is in many regions what defines the allowed TX power, so increasing antenna gain requires you to drop radio TX power, and you don’t gain anything (on the TX side — the story is different on the RX side).
