Table Of Contents
Table Of Contents

The Physical Layer

Overview

Wireless network interfaces contain a radio model component, which is responsible for modeling the physical layer (PHY). [1] The radio model describes the physical device that is capable of transmitting and receiving signals on the medium.

Conceptually, a radio model relies on several sub-models:

  • antenna model
  • transmitter model
  • receiver model
  • error model (as part of the receiver model)
  • energy consumption model

The antenna model is shared between the transmitter model and the receiver model. The separation of the transmitter model and the receiver model allows asymmetric configurations. The energy consumer model is optional, and it is only used when the simulation of energy consumption is necessary.

Generic Radio

In INET, radio models implement the IRadio module interface. A generic, often used implementation of IRadio is the Radio NED type. Radio is an active compound module, that is, it has an associated C++ class that encapsulates the computations.

Radio contains its antenna, transmitter, receiver and energy consumer models as submodules with parametric types:

antenna: <antennaType> like IAntenna;
transmitter: <transmitterType> like ITransmitter;
receiver: <receiverType> like IReceiver;
energyConsumer: <energyConsumerType> like IEnergyConsumer
    if energyConsumerType != "";

The following sections describe the parts of the radio model.

Components of a Radio

Antenna Models

The antenna model describes the effects of the physical device which converts electric signals into radio waves, and vice versa. This model captures the antenna characteristics that heavily affect the quality of the communication channel. For example, various antenna shapes, antenna size and geometry, antenna arrays, and antenna orientation causes different directional or frequency selectivity.

The antenna model provides a position and an orientation using a mobility model that defaults to the mobility of the node. The main purpose of this model is to compute the antenna gain based on the specific antenna characteristics and the direction of the signal. The signal direction is computed by the medium from the position and the orientation of the transmitter and the receiver. The following list provides some examples:

  • IsotropicAntenna: antenna gain is exactly 1 in any direction
  • ConstantGainAntenna: antenna gain is a constant determined by a parameter
  • DipoleAntenna: antenna gain depends on the direction according to the dipole antenna characteristics
  • InterpolatingAntenna: antenna gain is computed by linear interpolation according to a table indexed by the direction angles

Transmitter Models

The transmitter model describes the physical process which converts packets into electric signals. In other words, this model converts an L2 frame into a signal that is transmitted on the medium. The conversion process and the representation of the signal depends on the level of detail and the physical characteristics of the implemented protocol.

There are two main levels of detail (or modeling depths):

  • In the flat model, the transmitter model skips the symbol domain and the sample domain representations, and it directly creates the analog domain representation. The bit domain representation is reduced to the bit length of the packet, and the actual bits are ignored.
  • In the layered model, the conversion process involves various processing steps such as packet serialization, forward error correction encoding, scrambling, interleaving, and modulation. This transmitter model requires significantly more computation, but it produces accurate bit domain, symbol domain, and sample domain representations.

Some of the transmitter types available in INET:

Receiver Models

The receiver model describes the physical process which converts electric signals into packets. In other words, this model converts a reception, along with an interference computed by the medium model, into a MAC packet and a reception indication.

For a packet to be received successfully, reception must be possible (based on reception power, bandwidth, modulation scheme and other characteristics), it must be attempted (i.e. the receiver must synchronize itself on the preamble and start receiving), and it must be successful (as determined by the error model and the simulated part of the signal decoding).

In the flat model, the receiver model skips the sample domain, the symbol domain, and the bit domain representations, and it directly creates the packet domain representation by copying the packet from the transmission. It uses the error model to decide whether the reception is successful.

In the layered model, the conversion process involves various processing steps such as demodulation, descrambling, deinterleaving, forward error correction decoding, and deserialization. This reception model requires much more computation than the flat model, but it produces accurate sample domain, symbol domain, and bit domain representations.

Some of the receiver types available in INET:

Error Models

Determining reception errors is a crucial part of the reception process. There are often several different statistical error models in the literature even for a particular physical layer. In order to support this diversity, the error model is a separate replaceable component of the receiver.

The error model describes how the signal to noise ratio affects the amount of errors at the receiver. The main purpose of this model is to determine whether the received packet has errors or not. It also computes various physical layer indications for higher layers such as packet error rate, bit error rate, and symbol error rate. For the layered reception model it needs to compute the erroneous bits, symbols, or samples depending on the lowest simulated physical domain where the real decoding starts. The error model is optional (if omitted, all receptions are considered successful.)

The following list provides some examples:

Power Consumption Models

A substantial part of the energy consumption of communication devices comes from transmitting and receiving signals. The energy consumer model describes how the radio consumes energy depending on its activity. This model is optional (if omitted, energy consumption is ignored.)

The following list provides some examples:

  • StateBasedEpEnergyConsumer: power consumption is determined by the radio state (a combination of radio mode, transmitter state and receiver state), and specified in parameters like receiverIdlePowerConsumption and receiverReceivingDataPowerConsumption, in watts.
  • StateBasedCcEnergyConsumer: similar to the previous one, but consumption is given in ampères.

Layered Radio Models

In layered radio models, the transmitter and receiver models are split to several stages to allow more fine-grained modeling.

For transmission, processing steps such as packet serialization, forward error correction (FEC) encoding, scrambling, interleaving, and modulation are explicitly modeled. Reception involves the inverse operations: demodulation, descrambling, deinterleaving, FEC decoding, and deserialization.

In layered radio models, these processing steps are encapsulated in four stages, represented as four submodules in both the transmitter and receiver model:

  1. Encoding and Decoding describe how the packet domain signal representation is converted into the bit domain, and vice versa.
  2. Modulation and Demodulation describe how the bit domain signal representation is converted into the symbol domain, and vice versa.
  3. Pulse Shaping and Pulse Filtering describe how the symbol domain signal representation is converted into the sample domain, and vice versa.
  4. Digital Analog and Analog Digital Conversion describe how the sample domain signal representation is converted into the analog domain, and vice versa.

In layered radio transmitters and receivers such as ApskLayeredTransmitter and ApskLayeredReceiver, these submodules have parametric types to make them replaceable. This provides immense freedom for experimentation.

Notable Radio Models

The Radio module has several specialized versions derived from it, where certain submodule types and parameters are set to fixed values. This section describes some of the frequently used ones.

The radio can be replaced in wireless network interfaces by setting the radioType parameter, like in the following ini file fragment.

**.wlan[*].radioType = "UnitDiskRadio"

However, be aware that not all MAC protocols can be used with all radio models, and that some radio models require a matching transmission medium module.

UnitDiskRadio

UnitDiskRadio provides a very simple but fast and predictable physical layer model. It is the implementation (with some extensions) of the Unit Disk Graph model, which is widely used for the study of wireless ad-hoc networks. UnitDiskRadio is applicable if network nodes need to have a finite communication range, but physical effects of signal propagation are to be ignored.

UnitDiskRadio allows three radii to be given as parameters, instead of the usual one: communication range, interference range, and detection range. One can also turn off interference modeling (meaning that signals colliding at a receiver will all be received correctly), which is sometimes a useful abstraction.

UnitDiskRadio needs to be used together with a special physical medium model, UnitDiskRadioMedium.

The following ini file fragment shows an example configuration.

*.radioMediumType = "UnitDiskRadioMedium"
*.host[*].wlan[*].radioType = "UnitDiskRadio"
*.host[*].wlan[*].radio.transmitter.bitrate = 2Mbps
*.host[*].wlan[*].radio.transmitter.preambleDuration = 0s
*.host[*].wlan[*].radio.transmitter.headerLength = 100b
*.host[*].wlan[*].radio.transmitter.communicationRange = 100m
*.host[*].wlan[*].radio.transmitter.interferenceRange = 0m
*.host[*].wlan[*].radio.transmitter.detectionRange = 0m
*.host[*].wlan[*].radio.receiver.ignoreInterference = true

As a side note, if modeling full connectivity and ignoring interference is required, then ShortcutInterface provides an even simpler and faster alternative.

APSK Radio

APSK radio models provide a hypothetical radio that simulates one of the well-known ASP, PSK and QAM modulations. (APSK stands for Amplitude and Phase-Shift Keying.)

APSK radio has scalar/dimensional, and flat/layered variants. The flat variants, ApskScalarRadio and ApskDimensionalRadio model frame transmissons in the selected modulation scheme but without utilizing other techniques such as forward error correction (FEC), interleaving, spreading, etc. These radios require matching medium models, ApskScalarRadioMedium and ApskDimensionalRadioMedium.

The layered versions, ApskLayeredScalarRadio and ApskLayeredDimensionalRadio can not only model the processing steps missing from their simpler counterparts, they also feature configurable level of detail: the transmitter and receiver modules have levelOfDetail parameters that control which domains are actually simulated. These radio models must be used in conjuction with ApskLayeredScalarRadioMedium and ApskLayeredDimensionalRadioMedium, respectively.

[1]Wired network interfaces could similarly contain an explicit PHY model. The reason they do not is that wired links normally have very low error rates and simple observable behavior, and there is usually not much to be gained from modeling the physical layer in detail.