Physical Layer (PHY)¶

Introduction¶

The physical layer (PHY) is the lowest layer of the Bluetooth low energy protocol stack. It configures the physical parameters of the radio transmission and reception. It determines how a bit (and its value) are represented over the air. By switching the PHY, the physical properties of the RF signal is changed.

The supported PHYs include LE 1M, 2M, and Coded PHY

LE 2M PHY¶

The CC33x1 device supports transferring data over the mandatory symbol rate of 1 megasymbol per second (Msym/s) where 1 symbol represents 1 bit. This results in a bit rate of 1 megabit per second (Mb/s), which is referred to as LE 1M PHY.

The stack also supports an optional symbol rate of 2 Msym/s, with a bit rate of 2 Mb/s, which is referred to as LE 2M PHY.

LE Coded PHY¶

LE Coded PHY allows the ability to have the signal range increase up to nearly quadruple the range of LE 1M PHY. This is achieved by coding the signal, so the Tx power stays the same. This means that the power consumption per time stays the same. On the other hand, this coding entails a lower data throughput.

LE Coded S2 and S8¶

The LE Coded PHY can be operated with two data rates:

S2 : In LE Coded S=2 mode, each bit is represented by two symbols. Thus, the data rate is 500kbps. In this mode the range is roughly doubled compared to the LE 1M PHY.
S8: In LE Coded S=8, each bit is represented by eight symbols. This gives a data rate of 125kbps. In this mode the range is roughly quadrupled compared to the LE 1M PHY.

Every packet sent on LE Coded PHY contains a coding indicator (CI), which indicates the coding of the packet. Thus, when a packet is being received on the LE Coded PHY, the receiver uses the coding indicator to determine the coding of the packet. The full packet format of packets sent on LE Coded PHY is found in the `Bluetooth Core Specifications Version 5.3`_, Vol 6, Part B, Chapter 2.2 PACKET FORMAT FOR THE LE CODED PHY.

Link Budget¶

The LE Coded PHY gives a higher sensitivity than the LE 1M PHY. The receiver sensitivity is defined by how weak signals the receiver can receive compared to the level of interfering RF power received simultaneously. If no interference is present, the sensitivity level is determined by the thermal background noise and will correspond to the sensitivity specified in the datasheet.

In an operating RF link, the transmitter will transmit at a specified RF power level, and a (usually very tiny) fraction of that RF power will be picked up by the receiving antenna and fed to the receiver. If that fraction is too small, the received power level will drop below the receiver sensitivity level and the link will fail.

The link budget is the ratio between transmitted power and the Rx sensitivity level. For convenience, the link budget (LB) is usually denoted in a logarithmic scale (dB). Output power and sensitivity are usually denoted in a logarithmic scale relative to 1mW (dBm). This means that:

$\textrm{LB}_{\textrm{dBm}} = \textrm{Tx Power} (\textrm{dBm}) - \textrm{Rx Sensitivity Level} (\textrm{dBm})$

There are two ways to improve the link budget:

Increase the output power

Improve (reduce) the receiver sensitivity level

Increasing the output power is fairly straight forward, but it comes at the cost of (sometimes significantly) increased power consumption. In addition, all regulatory jurisdictions have limits on RF emission levels and unwanted spurious emissions, both of which will increase if the transmit power is increased.

The other option, improving receiver sensitivity, is the path chosen by the Bluetooth SIG when Bluetooth 5 was adopted with the intention of offering four times the RF range. This was also chosen to provide the longest range Bluetooth Low Energy solutions at lowest power consumption. In free space, a doubling of the range requires a quadrupling of the link budget (or 6 dB increase). This means that four times the range requires 12 dB better sensitivity for the LE Coded PHY, when compared with the LE 1M PHY. The reference for LE 1M PHY is set to -93 dBm. (Note that -93 dBm was chosen as a reference point for a standard radio at the time the spec was written.) Since the reference for LE 1M PHY is -93 dBm, the LE Coded PHY needs a sensitivity of -105 dBm in order to achieve four times the range.

The link budget increase is achieved through a two-way approach:

The biggest improvement comes simply from the fact that the data rate is reduced to 1/8th, which means that every bit carries eight times more energy for any given power level. Theoretically, this allows the receiver to receive signals at 9 dB lower power levels and still accumulate the same energy per bit as before.
The last 3 dB can be achieved due to the coding employed. The -93 dBm comparison level (for 1Mpbs) assumed a standard differential demodulator, where each received symbol (1 symbol per bit) is determined to be “1” or “0” based on a comparison with the previous symbol. The Coded PHYs facilitate a semi-coherent receiver where eight symbols make up 1 bit, and correlators can search for these known symbol sequences. This is called Forward Error Correction (FEC).

It is worth noting that even though the Tx power is the same for LE Coded PHY as for LE 1M PHY, the data rate is lower, thus the device will spend a longer time to send the same amount of data. This will give a higher power consumption for sending the same amount of data on LE Coded PHY compared to LE 1M PHY.

You can read more about how the LE Coded PHY in this blog: How does Bluetooth® 5 increase the achievable range of a Bluetooth Low Energy connection?

PHY Comparison¶

A comparison of the Bluetooth Low Energy PHYs is given below:

Table 2 Comparison of the PHYs.¶
Parameter	LE 1M	LE 2M	LE Coded S=2	LE Coded S=8
Symbol Rate	1Msps	2Msps	1Msps	1Msps
Data Rate	1Mbps	2Mbps	500kbps	125kbps
Error Correction	None	None	FEC	FEC
Range Multiplier	1	~0.8	~2	~4

LE 2M PHY vs LE 1M PHY¶

The LE 2M PHY feature uses the same transmit power as the LE 1M PHY, the only change is in the modulation of data in the PHY. Using the LE 2M PHY, the energy consumption decreases due to higher data modulation at the same output power. The following table lists some of the differences between the two PHYs:

Table 3 Tradeoff between 1M and 2M PHYs¶
Parameter	Comparison
Power consumption	Energy consumption is reduced using the same transmit power.
Data Rate	LE 2M PHY is 2x faster to transmit data than LE 1M PHY.
Receive Sensitivity	The link budget will be lower relative to LE 1M PHY, due to the increased symbol rate.
Transmit Power	The output power is same for both PHYs.

The main advantage to use the LE 2M PHY is for high throughput applications to transfer data at a higher speed.

LE Coded PHY vs LE 1M PHY¶

The LE Coded PHY feature uses the same transmit power as the LE 1M PHY, the only change is in the modulation of data in the PHY. Using the LE Coded PHY, the energy consumption increases because the radio is in Tx longer. Thus the main application for LE Coded PHY should be applications that need a long range, but a low data rate. According to the Bluetooth spec, there are limitations on what packets can be sent on the LE Coded PHY.