Dive Into the BLE PHY Layer—The Basics of Bluetooth LE PHY Radio

In this article, we will explore the various Bluetooth LE (BLE) PHY radio modes. However, before diving too far, let's start with a basic discussion on the physical or PHY layer and what is involved within this protocol layer.

 

Basics Behind the Bluetooth LE PHY Layer

The PHY layer is a key part of wireless communication protocols like Bluetooth LE (Figure 1).

 

Figure 1. An example protocol stack showing the PHY layer's placement. Image used courtesy of Microchip

 

When it comes to Bluetooth devices, this layer is all about how LE radio transmitters and receivers send and receive digital data using radio signals. In general, the PHY layer sets up important rules and properties to ensure devices can communicate effectively. Some of the main aspects it covers include:

• Frequency band
• Gaussian frequency shift keying (GFSK) modulation scheme
• Transmission speeds
• Power
• Receiver sensitivity
• Time division

With those aspects in mind, let's take a look at the first one: the frequency band.

 

The BLE Frequency Band

The Bluetooth LE communication occurs within the globally unlicensed 2.4 GHz ISM (industrial, scientific, and medical) band. This band has been divided into 40 channels,  each 2 MHz wide (Figure 2).

 

Figure 2. The spectrum for Bluetooth LR communication. Image used courtesy of Microchip

 

Additionally, 37 of these channels are dedicated to data transmission and secondary advertising, while the remaining three serve as primary advertising channels used to discover and initiate connections between devices.

 

BLE Modulation Scheme—GFSK

All LE radio transmitters and receivers use the GFSK modulation method to send and receive data through radio signals. Generally, GFSK encodes data by changing the carrier radio signal frequency depending on the transmitted digital information. The carrier is a continuous radio signal with a specific frequency, which serves as a basis for transmitting the information. 

When a digital '1' needs to be sent, the frequency of the carrier is increased, and when a digital '0' needs to be sent, the frequency is decreased. These changes in frequency are called frequency deviations. In Bluetooth LE, this frequency shift is usually around ±185 kHz.

Now, let's talk about the Gaussian aspect of GFSK. When we say "Gaussian," we're referring to the Gaussian filter, which is used to shape and smoothen the changes in frequency during the encoding process. Without Gaussian filtering, the frequency shifts would be abrupt, making the signal harder to transmit and decode accurately. The Gaussian filter smooths out these sudden frequency transitions. The smoother frequency transitions reduce signal distortion, make communication more reliable, and help save energy—all crucial factors when it comes to low-power wireless communication between devices.

 

Bluetooth LE Transmission Speeds, Power, and Receiver Sensitivity

In the physical layer of Bluetooth LE, transmission speeds are measured in symbols per second, not bits per second. This is because the physical layer focuses on the actual radio signals being sent and received, not the digital bits they represent. To clarify, a symbol refers to the smallest unit of an analog signal, while a bit represents the smallest unit of digital information.

The PHY layer also describes the allowed output power level of the LE radio transmitter, which is between 0.01 and 100 mW (or -20 and +20 dBm). However, regulatory bodies in different regions may have their own requirements that could supersede the Bluetooth SIG's specifications. As a result, implementers must ensure that their devices comply with the local regulations applicable to the areas where they plan to use or sell their products.

Two additional example characteristics defined by the PHY layer are receiver sensitivity and bit error rate (BER). Receiver sensitivity measures the minimum signal strength a receiver can effectively decode, while BER refers to the ratio of erroneous bits received to the total number of bits transmitted. A lower BER indicates a more reliable and accurate communication link, as fewer errors occur during transmission. Manufacturers often specify receiver sensitivity as the minimum signal strength required to achieve a particular BER. Typically, the Bluetooth Core specification allows a maximum BER of 0.1% when transmitting data packets containing 37 octets or fewer.

To put it into perspective, an octet is equivalent to 8 bits. Thus, a packet containing 37 octal totals 37 x 8 = 296 bits. With a maximum BER of 0.1%, the allowable number of bit errors in such packets would be 0.1% of 296 bits. This translates to a maximum of one-bit error for every 3 to 4 packets of 37 octets.

 

BLE Time Division Duplex (TDD)

All Bluetooth LE radios are half-duplex devices, meaning they can either transmit or receive data but not both simultaneously. According to the PHY layer, the LE radios should use the time division duplex (TDD) scheme to mimic the behavior of a full-duplex system. In a TDD, the devices rapidly switch between transmitting and receiving modes within the same frequency band, using separate time slots for each mode. This technique allows ongoing bidirectional communication between devices while still operating as half-duplex devices. 

These characteristics mentioned above are just a few taken from the Bluetooth specification. For a comprehensive understanding, refer to the complete Bluetooth specification document.

 

Bluetooth LE PHY Symbol Rate Modes—1M and 2M

Bluetooth LE technology offers several PHY radio modes, each with its distinct advantages and limitations. Let's take a closer look at each of these modes.

 

1M—Overview and Advantages

The 1M PHY mode is the standard Bluetooth LE radio mode that has been around since the inception of Bluetooth LE (v4.0). It operates at a symbol rate of 1 Megasymbols per second (Msym/s), meaning each payload bit takes just 1 µs to transmit.

Every Bluetooth LE device must support the 1M PHY mode, making it the only mode fully backward compatible with BLE devices that don't support BLE 5. It is also considered the baseline for comparing other PHY modes. It provides a good balance of power consumption and range, making it suitable for most everyday applications. The 1M PHY mode is always the default starting point when connecting two Bluetooth LE devices. From there, if both devices support additional modes, the peers can request a switch to a more advanced mode to meet specific requirements. 

Below are some of the advantages of this mode.

 

Advantages of the 1M Mode:

• Low power consumption
• Compatible with all Bluetooth LE devices

 

2M—Overview, Use Cases, and Advantages

Introduced with Bluetooth 5.0, the 2M PHY mode offers twice the data rate of the 1M PHY mode, operating at 2 Msym/s. This means each payload bit takes just 0.5 µs to transmit. The 2M mode allows for quicker data transfer, which may reduce power consumption during active radio communication, particularly for longer data transmissions in which the overhead associated with negotiating for the switch to this mode becomes a negligible portion of the overall communication. 

Below we'll outline some general use cases and this mode's advantages.

 

Ideal Use Cases for 2M Mode:

• High-throughput applications such as firmware updates
• Large data volume applications such as buffered sensors

 

Advantages of the 2M Mode:

• Faster data transfer
• Shorter communication duration, reducing power consumption
• Improved spectral efficiency

 

BLE 5.0 Foward Error Correcting PHY Modes: Coded S2 and Coded S8

The coded PHY modes, introduced in Bluetooth 5.0, were developed to extend the range and robustness of Bluetooth LE communication. Coded PHY modes use forward error correction (FEC) to enhance link reliability in noisy environments. This technique allows the receiver to detect and correct errors in the received data without needing to request retransmission. FEC algorithms add redundant bits, known as "parity bits," to the original data before transmission. The receiver can then use these parity bits to identify and correct errors up to a certain limit. 

Compared to the 1M PHY, the coded PHY improves BLE’s reliability at the cost of less throughput and increased power consumption. Two variants of the coded PHY modes are coded S2 and coded S8. The main difference between S2 and S8 lies in the coding scheme used.

Below we'll give a brief breakdown of each coding scheme.

 

Coded S2

In coded S2, the data payload is coded with two symbols, meaning two symbols are transmitted for every bit of payload data. In other words, the data rate is halved compared to the 1M PHY mode. This coding scheme provides an extended range of approximately two times the range of the 1M PHY mode while sacrificing some data throughput.

• 2x data rate reduction (compared to 1M PHY)
• Increased range by a factor of 2
• Uses 2-symbol coding—thus being called S2
• Lower throughput compared to 1M and 2M PHY

 

Coded S8

Coded S8 takes FEC to another level. It uses an eight-symbol coding scheme, meaning eight symbols represent each bit of payload data during transmission. This coding results in a data rate reduction by a factor of eight compared to the 1M PHY mode. The advantage of the S8 coding is an even more extended range, approximately four times the range of the 1M PHY mode (Figure 3), at the cost of significantly lower data throughput.

 

Figure 3. An overview of the S2 and S8 coding. Image used courtesy of Bluetooth

 

• 8x data rate reduction (compared to 1M PHY)
• Increased range by a factor of 4
• Uses an even more robust coding and error correction scheme than the coded S2 PHY
• Uses 8-symbol coding —thus being called S8
• Lowest throughput among the PHY modes

 

Ideal Use Cases of Coded PHY Modes:

• Long-range applications
• Industrial automation
• Smart home

 

Advantages of Coded PHY Modes:

• Increased communication range
• Improved link reliability in noisy environments
• Robustness against interference

 

Comparing the PHY Symbol Rated and Forward Error Correcting Modes

Compounding on the above conversation, Table 1 shows a breakdown and comparison of the different modes.

 

Table 1. Comparison between 1M, 2M, Coded S2, and Coded S8. Data used courtesy of Bluetooth

 

Researchers Test The Performance Claims of the PHY Modes

Researchers at the Graz University of Technology conducted a study to evaluate the actual performance of BLE 5 PHY modes since their release in June 2016. 

The team, led by Michael Spörk, examined the performance of 2M PHY, which promises to double the throughput and coded PHYs—also known as Bluetooth long range—which aim to increase communication reliability.

The experiments were conducted in a vacant university lab using the nRF52840 DK device from Nordic Semiconductor. The setup consisted of a BLE client and peripheral for all four PHY modes to measure power consumption, throughput, and reliability in different configurations.

As anticipated, the 2M PHY mode yielded the lowest average power consumption due to its rapid data rate. However, the coded S8 PHY exhibited the highest power consumption, primarily because of the overhead from its coding scheme. Compared to the 1M PHY mode, the 2M PHY consumed about 8% less power, while the coded S2 and S8 PHYs consumed around 61% and 70% more power, respectively. A breakdown of these results is shown in Figure 4.

 

Figure 4. Average power consumption of a BLE slave with varying PDU lengths and PHY modes, using a 125 ms fixed connection interval. Image used courtesy of Spörk et al.

 

In terms of throughput, the 2M PHY delivered the highest performance, achieving between 178% and 212% of the 1M PHY mode's throughput, effectively doubling its capacity. Conversely, the coded S8 PHY demonstrated the lowest throughput.

The study also assessed communication reliability by measuring the packet reception rate (PRR) for different link qualities. The 2M PHY had the lowest PRR, while the coded S2 and S8 PHYs significantly increased reliability for poor link qualities due to their coding schemes.

Finally, the researchers evaluated the robustness of the four PHY modes under Wi-Fi interference. As expected, the coded S8 PHY mode offered the highest PRR and reliability. Under Wi-Fi interference, the coded S2 and S8 PHYs maintained almost 100% PRR, while the 2M PHY managed only a 54% PRR for a Wi-Fi transmission power of 5 mW (Figure 5).

 

Figure 5. Packet reception rate of different PHY modes at different attenuation levels. Image used courtesy of Spörk et al.

 

These researchers also found something interesting about link quality. The optimal choice of PHY mode depends on the quality of the connection. When the connection is stable and interference is low, the 2M PHY mode is the best choice for maximizing data throughput and energy efficiency. Its higher data throughput allows faster and more power-conserving communication than other PHY modes.

However, when the connection quality is poor, packets are frequently corrupted. In that case, the coded S8 PHY mode becomes more suitable, as it can recover most corrupted packets without retransmission and is thus more energy-efficient. Interestingly, there is a small transition area—between -10 dBm and -15 dBm attenuation—where the 1M PHY mode slightly outperforms the other PHY modes in terms of power consumption.

 

Choosing the Right Bluetooth LE PHY Mode

When selecting the optimal Bluetooth LE PHY mode for your application, consider the following factors:

• Data rate requirements: Choose a PHY mode that meets your data throughput needs. 2M provides the highest throughput, while coded S8 provides the lowest.
• Range requirements: If long-range communication is crucial, the coded PHY modes might be more suitable.
• Reliability requirements: Determine the level of reliability needed for your application, and consider the PHY modes that enhance reliability.
• Power consumption: Consider the power constraints of your device and choose a PHY mode that offers the right balance of power efficiency. 2M PHY mode is the most energy-efficient, while the coded S8 PHY mode has the highest power consumption.
• Link quality: Assess the typical link quality of your application's environment and select a PHY mode that performs best under those conditions.
• Compatibility: Ensure the selected PHY mode is compatible with the devices you intend to communicate with. Typically, 1M is mandatory, while 2M and coded PHY modes are optional.

Overall, choosing the right Bluetooth LE PHY mode often involves data rate, range, and power consumption trade-offs. By understanding the specific requirements of your application, you can find the optimal balance to achieve maximum performance. In the end, you need to decide what's more important for your particular application: making data transfer twice as fast or greatly improving the connection's reliability. If your application demands high data throughput, you may opt for the 2M PHY mode despite its higher power consumption and reduced range. On the other hand, if long-range communication is a more pressing need, then coded PHY modes might be more suitable despite their lower data rates.

One last thing to consider is link quality. The link quality affects your choice of PHY because it can directly impact the effective data throughput and power consumption. A PHY mode that works well under good link quality conditions might perform differently than expected in low-quality environments. 

All in all, Bluetooth LE offers various PHY modes, making it suitable for broader applications. Knowing the pros and cons of each PHY mode helps you choose the best option, balancing speed, distance, and battery usage for your specific needs. As Bluetooth LE technology keeps improving, we can look forward to even better performance and efficiency in future wireless devices.