Far Field Communication System
Most common far field wireless connectivity in consumer electronics are powered by Wi-FI and Bluetooth standards. In this article, we will go over principles of wireless communication, system design, and overview of Wi-Fi and Bluetooth.
The purpose of this article is teach the reader how to design an RF system by going over basic principles and walk through a typical wireless block diagram, and it's design choices.
Wireless communication has been around for more than a century and evolved from ranging from simple radio to satellite communication. However, the principle of communication is the same; using electromagnetic waves as a message carrier to achieve long distance communication.
A simple rf transceiver design is consisting of transmiter, receiver, antenna and baseband processor. In modern rf systems, more frequency bands (i.e 2.4 GHz and 5GHz WiFi bands) and different channels allocations require more thorough signal conditioning and planning via a combination of filters, switches, amplifiers, multiple RF paths and antenna in order to maintain quality connectivity for the user experience. However, principles of radio transmission and reception is remains the same.
What is far field?
It's a mode of RF transmission characterized by the in phase relationship between the E and M components of the EM field. This in-phase E to M propagation relation occurs at a far distance away from the transmiter; hence the propagation wave shape of the EM field at far distance is called far field.
What is the difference between WiFI and Bluetooth?
Wifi and Bluetooth are two different wireless standards with main differences in radio control function, signal modulation, channel access scheme, and lastly network firmware stack. However both WiFI and Bluetooth share 2.4 GHz ISM band, which makes two coexistence of two devices challenging.
See system coexistence for more information.
Band: 2.4 ISM GHz
Range: ~10m indoor
Frequency Range: 2402 MHz - 2480 MHz: 79 channels for classic and 40 Channels for LE
Channel Bandwidth: 1 MHz
Bluetooth Classic BDR : 1 Mb/s
Bluetooth Classic EDR: 3 Mb/s
Bluetooth LE ( introduced in Bluetooth 4.0): 1 Mb/s
Range: ~35m indoor
Channel Bandwidth: 20/40 MHz
Data Rate: 144.4 Mbps or 2 spatial streams
Frequency range: 2.4 -2.497 GHz: ch1 - ch14
Standards: 802.11 b/g/n (typ.)
Standard: 802.11 a/n/ac (typ.)
Channel Bandwidth: 20/40/80MHz
Data rate: up to 867 Mbps for 2 spatial streams
Frequency range: 5.15 - 5.35 GHz: Ch36 - Ch64; 5.47 - 5.725 GHz: Ch100 - Ch140; 5.725 - 5.85GHz: Ch149 - ch165
Following example is a 2 transmitters and 2 receivers (2T2R/ 2x2 MIMO) wireless system that is commonly found in today's tablets, OTT stick, and portable wireless devices such as smart speaker.
Wifi and BT Combo Chip Module
Integrated transceivers (i.e mixer, PA, LNA, etc.)
Integrated network/baseband processors
Integrated MAC controller
Peripheral,PMU, Clock, memory, etc.
Dual band Antenna
Dual band antenna supports two distinct frequency ranges. In this case, it's the frequency range for wifi 802.11 a/b/g/n/ac and bluetooth.
It provides directional isolation for receiver when transmitter is on in order to avoid damage to receiver due to high transmitter power.
it provides frequency isolation between two bands (i.e as 2.4 GHz and 5 GHz bands) and allows independent operation between two 2.5GHz and 5GHz transceivers while sharing one dual band antenna. Operating efficiency and cost and space reduction are achieved.
BPF (band Pass Filter)
band pass filter is used to prevent out of band noise.
UART for control
PCM for audio
SDIOV3.0 for control and data
The antenna should be placed is opposite directions two maximize polar pattern coverage.
Does not use power plane for RF ICs. Power plane does not provide enough isolation for RF power pins, which is needed to reduce noise leakage among RF components.
Use star connection for power for RF components.
RF component and paths requires ground clearance in the layers below to avoid signal attenuation. Follow RF routing guideline of from the vendor
Use via fensing for all RF traces ideally the spacing between each successful via is < 1/10 of the smaller RF wave length of signal of interest. This provides noise immunity to RF signal and most important creates a PCB wave guide for RF signals.
Design trade offs
Why choosing a wifi/bt combo chip module?
As a system designer with limited domain expertise in RF system, selecting WiFi and Bluetooth combo chip module simplifies design, RF testing, and cerfitication. There is a lot of module vendors who sells wifi/bt compliant module in a easy assembled system in package parts.
Are there an alternative to buying a module?
One can use the chip by itself and design in the necessary matching network, RF switches, and amplifiers to save cost and space. This is design approach generally referred to COB, chip on board; however one needs to be very stringent is RF components selection, layout, and testing.
Why using a dual band antenna?
Dual band antenna as the name suggests that this antenna supports frequency range on two bands, 2.4 GHz and 5.0 GHz respective; therefore, there is no need to use two separate antenna, one for each band, resulting in cost and space saving.
Why using SDIO 3.0 interface for wifi Host interface?
Wifi host interface is not limited to SDIO. Other common supported interface would be PCIE and USB. However SDIO is extensively used to interface with wifi and bluetooh chips.
How does Wifi and BT radio coexist on 2.4 GHz band without collision?
Traffic Collision avoidance is achieved via a signaling handshake system between wifi and bt radio to coordinate for transmit and receive. For instance, bt radio send a request to wifi radio for transmission, wifi radio grants that request and delays its transmission.
Where is the antenna for bluetooth radio?
Both wifi and bluetooth operates on the same frequency range on 2.4 GHz ISM band; therefore, it shares the same antenna with WiFi radio. Another cost and spacing saving in this design.
What are some popular wireless vendors?
Broadcom and Realtek are quiet common for cost friendly parts.
All wireless devices must undergone compliance test in regulator agencies such as FCC and CE. Example RF testing standards for CE are EN 300 893 for 5G and EN 300 328 for 2.4G.
Summary & Conclusion
We learned that far field radio transmission allows longer range communication compared to NFC. Wi-Fi and Bluetooth are the most popular standards. We see that most of wireless system can be done quiet easily by buying a certified module. All the designer needs to do is select the right antenna, following strict RF placement and routing guidelines, and lastly select the correct host interface to communication with the application processor.
As we can see, RF design is challenging but manageable if we pick the right parts.
To over the overall about Wi-Fi Physial Layer and testing, please download the poster from following link: TEktronic WIFI Poster
To learn about Bluetooth Physical Layer and testing, please download the poster from following link: TEktronic BT Poster