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RF vs PHY | What is the difference between RF and PHY layers

This page compares RF vs PHY and mentions the functional difference between RF and PHY layers. The comparison between RF and PHY have been derived based on their functions, interfaces, applications and other parameters.

Introduction: OSI model has been developed to establish communication between open systems. OSI model has seven layers. Each of these layers has well defined functions and they support the layer above and below of itself. Both RF (Radio Frequency) and PHY (Physical) layers are mapped to layer-1 of the OSI stack. RF layer is present when medium is wireless. PHY is known as baseband layer where as RF is known as wireless layer based on operations performed. The figure-1 depicts OSI stack with position of PHY and RF layers one above the other.

RF and PHY in OSI Stack
Figure-1 : RF and PHY in OSI stack

Layer-1 deals with transmission of information in the form suitable for wired/wireless medium such that they can be retrieved at the receiver end. It takes care of error recovery at the receiver caused by channel (wired medium or wireless medium). Refer article on basics of OSI and TCPIP layers >>.

The figure-1 depicts OSI stack of two hosts. The figure shows peer to peer communication between two hosts and unit of data exchanged at different layers. All the layers consist of transmitter (Tx) and receiver (Rx) parts inside both the hosts. RF layer exists only in wireless nodes or hosts.

Example #1: The communication between AP (Access Point or router) and Station (STA or client) follows the same communication mechanism in WLAN (Wireless LAN).
Example #2: Similarly communication between Base Station and Mobile subscriber follows the same protocol mechanism as per standard specifications.

What is RF Layer

RF is the short form of radio frequency which is used for transmission of EM waves into air using antenna. RF signals are characterized by its frequency (i.e. number of cycles per second measured as hertz). One hertz equals 1 cycle per second.

➨Frequency of RF Signal = c/λ
Where,
c is the speed of light, equals 3 x 108 m/sec
λ is signal wavelength (i.e. distance between 2 consecutive peaks)
• Lower is the frequency, longer is the wavelength. Hence lower frequencies travel longer distance compare to higher frequencies.

Different RF frequencies are used as per standards such as WLAN, Zigbee, GSM, LoRa, Sigfox, 4G LTE, 5G etc. The figure-2 below depicts RF and Baseband (i.e. PHY) parts of a wireless node or device.

RF Frequencies are measured in units of Hz, KHz, MHz, GHz, THz etc.
➨Period of EM wave = 1/Frequency
1 Hz of frequency equals 1 second of period, 1 KHz equals 1 ms and so on.

Following parameters are very useful and used often with RF signal.
• Frequency: Number of cycles per second, measured in Hertz as explained above
• Wavelength: Distance between 2 consecutive peaks, measured in meters/centimeters as explained above
• Power: Strength of RF signal, measured in dB, dBm, dBW etc. Higher transmit power of RF signal helps it to travel more distance from its originating source. RF passive devices such as attenuators, RF Mixers, Isolators cause RF signal to get attenuated. RF amplifiers cause RF signal to get amplified. These gains and losses are measured in unit of decibel.
• Bandwidth: Width of RF spectrum, measured in Hz/KHz/MHz/GHz. It helps to know how much of information an RF signal can carry.

RF Part
Figure-2 : RF modules

The baseband data such as voice or data or video are passed through the modulator and given as input to RF transmitter. After necessary frequency translation by RF upconverter, the RF upconverted signal is amplified by PA (Power Amplifier) before transmission into the air by RF antenna. At the receiver reverse operations take place such as LNA (Low Noise Amplification), RF down conversion, demodulation etc. to retrieve baseband information. The upconversion and down conversion is carried out using appropriate RF mixers, suitable LOs or RF synthesizers, Band Pass Filters (BPFs), amplifier MMICs, attenuator pads etc. Refer articles on C band RF Frequency Converter , mmwave RF Up Converter and Down converter and RF Transceiver design and development for more information.

Up Down conversion using RF mixer
Figure-3 : RF Up conversion and Down conversion process

Following are the functions of RF layer.
➨At the transmitter, it modulates baseband data using RF carrier as per system using RF Up converter. This modulated RF signal is amplified using PA (Power Amplifier) before transmission into air (by Antenna) or other medium as per system design.
➨At the receiver, the received RF modulated signal is amplified using LNA (Low Noise Amplifier) first. This amplified signal is down converted using RF down converter and fed to the baseband receiver.

What is PHY Layer

PHY stands for Physical, the keyword often used for Physical layer of OSI stack layer-1. It interfaces with MAC/upper layers at one end and with wired or wireless medium (i.e. RF) at the other end. Physical medium can be copper wire, twisted pair, fiber optic cable or space/vaccum.

The amplitude, frequency and phase of RF signal are used to map or demap PHY data during transmission and reception respectively. Refer AM/FM/PM, ASK/FSK/PSK, BPSK, QPSK, QAM for more information.

Following are the functions of PHY layer. It provides services to layers above and below of itself i.e. Network/upper layers and wired medium or wireless (i.e. RF) medium.
• It makes higher layer data to be suitable for transmission over the desired medium. It adds header and trailer as per transmission medium used.
• It introduces redundant data as per FEC technique (CC/CTC) at the transmitter to provide error correction at the receiver.
• It converts bits to symbols as per modulation technique used. This increases bandwidth efficiency of the system. For example,
In QPSK, 2 bits are mapped to 1 symbol
In 16QAM, 4 bits are mapped to 1 symbol and so on.
• It uses modules such as scrambler and interleaver to meet specific requirements of wireless channel.
• PHY receiver takes care of time synchronization, frequency synchronization and channel equalization.
Refer data flow through typical wireless physical layer.


Physical layer (PHY) modules in wimax device
Figure-4 : PHY (i.e. Baseband) Transmitter Modules (as per IEEE 802.16-2004 WiMAX standard)


The figure-4 depicts typical PHY layer modules. The MAC layer output (i.e. "Data In") is given as input to PHY Tx (Transmitter) chain. The "Data Out" refers to PHY Tx output which is fed to RF Transmitter (i.e. Upconverter) through DAC. Different PHY layers will have different FEC configurations and modulation formats.

RF PHY and MAC interface diagram
Figure-5 : Interfacing RF and PHY

As shown in the figure-5, PHY layer is implemented on FPGA/Processor and upper layers such as MAC/IP are implemented on Processor. Memory is used between PHY and MAC for data exchange. RF and PHY layers are interfaced using DAC in the transmit chain and using ADC in the receive chain as shown. Refer interfacing between RF PHY and MAC >> layers.

Difference between RF and PHY

Following table mentions difference between RF and PHY layers with respect to various parameters.

Parameters RF Layer PHY Layer
Full Name Radio Frequency layer, It is also known by RF Transceiver. Physical layer, It is also known by modem (Modulator Demodulator) or baseband layer.
Function Converts information (in analog or digital form) into Electro-Magnetic (EM) waves suitably to be transmitted over the air medium and vice versa. Different frequency channels are used for transmission and reception of EM waves as per standards such as WLAN, Bluetooth, Zigbee, GSM, 4G LTE, 5G etc. RF transmitter converts bits to EM waves where as RF Receiver converts EM waves to bits. Converts MAC/Upper layer data bits in the suitable form to be transmitted over wired/wireless medium. It performs number of operations on MAC data such as randomization, encoding, interleaving, data mapping and so on at the PHY transmitter. It performs reverse operations such as data demapping, de-interleaving, decoding, de-randomization at the PHY receiver.
Interfaces RF layer interfaces with PHY layer at one end through ADC/DAC and with RF antenna at the other end as shown. PHY layer interfaces with RF layer at one end through ADC/DAC and with MAC layer at the other end as shown.
Frequency Radio Frequency uses 1 to 3 GHz in EM spectrum. Microwave uses 3 to 30 GHz and mmwave uses 30 to 300 GHz. PHY or baseband data uses very low frequencies, which are modulated using higher RF carrier frequencies before transmission as per standards/country wide regulatory allocations. Audio signal occupies 20 Hz to 20 KHz. Frequency of digital signal ranges from 0 Hz to sampling frequency (fs) of ADC (Analog to Digital Converter).
Medium of propagation EM waves propagate in vacuum, space, solid state medias such as circuit board metals or coaxial cables, waveguides etc. PHY or baseband data signals propagate through wired mediums such as PCB circuit traces, ethernet or using RS232/RS485 etc.
Modules It consists of basic modules such as RF Transceiver (RF Transmitter and Receiver), PA (Power Amplifier) and LNA (Low Noise Amplifier). It consists of basic modules such as scrambler or randomizer, interleaver, FEC encoder (Convolutional encoder, CTC encoder), Interleaver, data mapper, IFFT etc. in the PHY transmitter and reverse modules in the PHY receiver. It addition PHY receiver consists of time, frequency and channel equalization modules to compensate for time offset, frequency offset and channel impairments.
Measurements RF Frequency and Power are very basic measurements of RF signal. The advanced RF measurements include spurious, harmonics, phase noise, frequency stability, 1dB/TOI measurements etc. Refer RF Measurements tutorial >>. Time period, Pulse amplitude and eye diagram are very basic measurements of PHY layer signal. The advanced PHY measurements include I/Q diagram, EVM, spectral flatness, PAPR etc. Refer Physical layer measurements >>.
Examples MAX2830 from Maxim (Now Part of Analog Devices) which supports 2.4 GHz to 2.5 GHz is an RF Transceiver suitable for WLAN (802.11g/b) standard. This SoC houses PA, LNA, Rx/Tx antenna switch, LO, PLL and so on. RF Transceiver SoCs are available for different standards (WLAN, WiMAX, Zigbee, Bluetooth, LoRa, Sigfox, GSM, 4G LTE, 5G etc.) from various vendors. Refer RF Transceiver vendors and 5G NR (New radio) RF Transceiver manufacturers for more information. WF200 from Silicon Labs is a WiFi Transceiver IC. It houses 802.11b/g/n PHY layer, MAC layer and RF layer. Refer WiFi SoC manufacturers, WiMAX SoC manufacturers , Zigbee SoC Manufacturers , 5G NR Modem chipsers for more information.

RF vs Baseband

➨Refer RF vs Baseband >> for more information.



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