Bluetooth Core 5.x

Bluetooth 5 and expanded LE capabilities

The introduction of Bluetooth 4.0 and its Low Energy (LE) variant enabled a new generation of small, battery-powered devices. As the Internet of Things (IoT) landscape matured, the limitations of early Bluetooth LE standards became apparent. The "personal area network" model did not cover connected homes, smart cities, and industrial sensor networks.

Three practical limitations drove the next phase of development. First, range could limit use beyond a room or building. A 10-meter reference value is fine for connecting a headset to a phone in your pocket, but it is insufficient for a smart lock on a garden gate needing to talk to a hub inside the house, or for environmental sensors spread across a farm. Second, the data rate, while power-efficient, was slow for certain tasks. Over-the-air (OTA) firmware updates for an entire fleet of devices could take an impractically long time at 1 Mbps. Third, the advertising capability, while foundational to LE, was limited. The small 31-byte payload of advertising packets was too restrictive for more sophisticated "beacon" applications or for broadcasting richer sensor data without establishing a formal connection.

In response to these needs, the Bluetooth Special Interest Group (SIG) adopted the Bluetooth 5 specification on December 6, 2016. It expanded Bluetooth Low Energy capabilities through three features often summarized as "2x the speed, 4x the range, 8x the data". This shorthand refers to specific LE feature comparisons and does not guarantee the same parameters for every device. The enhancements extended Bluetooth LE applications from personal connectivity to whole-home automation and industrial-scale asset tracking.

The First Pillar: 2x the Speed (LE 2M PHY)

One of the main changes in Bluetooth 5 was the doubling of the nominal PHY rate for Low Energy connections. This was achieved through the introduction of a new optional physical layer called LE 2M PHY.

How Does It Work? The GFSK Enhancement

Doubling the PHY rate without a large increase in power consumption required a change in transmission parameters. Bluetooth 5 retained the same modulation scheme, which remains Gaussian Frequency Shift Keying (GFSK). The LE 1M PHY uses a symbol rate of 1 Msym/s, while the optional LE 2M PHY uses 2 Msym/s. The symbol-rate change produces the nominal 2 Mbps PHY rate; frequency-deviation increase alone would not produce that rate.

• The standard LE 1M PHY uses a symbol rate of 1 Msym/s. Each symbol carries one bit before protocol overhead and retransmissions are considered.
• The optional LE 2M PHY uses a symbol rate of 2 Msym/s and a nominal PHY rate of $2 \text{ Mbps}$. Application data rate is lower because of headers, inter-frame spacing, acknowledgments, and link conditions.

Bluetooth SIG reference values put the approximate maximum application data rate at about 800 kbps for LE 1M and 1,400 kbps for LE 2M. These are reference application rates separate from radio PHY rates, and actual throughput depends on packet size, retransmissions, scheduling, and implementation.

The Trade-off: Speed vs. Range

The higher LE 2M PHY rate comes with a lower link budget than LE 1M. The receiver may require a stronger signal to decode data reliably. SIG materials describe about 80 percent of the LE 1M range for a typical comparison, while the result depends on power, sensitivity, antennas, and the environment.

Practical Applications of LE 2M PHY

The 2 Mbps mode is intended for applications where large amounts of data need to be transferred quickly and devices are relatively close. Key use cases include:

• Faster Over-the-Air (OTA) Firmware Updates: For a company managing thousands of deployed IoT sensors, updating their firmware using LE 1M could take a long time. In an ideal case, doubling the PHY rate approximately halves the radio portion of the transfer, while the total time depends on overhead, retransmissions, and device flash memory.
• Rich Data Collection:Medical devices, such as an electrocardiogram (ECG) monitor, or high-performance sports sensors can collect large volumes of data. The 2M PHY can offload this data to a smartphone or gateway faster when both devices support it, which can shorten the sensor's active period.
• Enhanced Wearables: Smartwatches and advanced fitness trackers can synchronize larger data files, such as workout logs, music playlists, or even low-resolution images, with a companion app more rapidly.

The Second Pillar: 4x the Range (LE Coded PHY)

An important feature of Bluetooth 5 was the ability to extend the communication range of LE devices. This was achieved by introducing the LE Coded PHY. The feature can extend coverage in a home or outdoors. Link budget, antennas, power, and obstacles determine the result, so a fixed 10-meter value does not describe this feature.

How It Works: Coding and redundancy

Increasing the range does not require increasing transmitter power. The LE Coded PHY makes the signal more resilient to noise and uses coding theory to add redundancy to the data being sent.

• Forward Error Correction (FEC): The core principle is Forward Error Correction. Before transmission, each bit of user data is passed through a coder that outputs a pattern of multiple bits. This adds redundancy.
• Two Coding Schemes (S=2 and S=8): The LE Coded PHY offers two levels of coding.
  • S=2 Coding: Each input data bit is mapped to a 2-bit symbol pattern. This mode doubles the on-air time and improves resistance to noise.
  • S=8 Coding: For maximum range, each input data bit is mapped to an 8-bit symbol pattern. This greatly increases the redundancy.
• Error Correction at the Receiver: The receiver knows the valid coding patterns. Even if some of the transmitted bits are corrupted by noise over the long distance, the receiver can often use the redundant information in the pattern to reconstruct the original bit correctly.

The Trade-off: Range vs. Speed

The range gain comes at a cost to the data rate. Because each data bit is represented by multiple symbols on the air, both the PHY rate and application throughput are reduced.

• With S=2 coding, the protocol rate drops from 1 Mbps to approximately $500 \text{ kbps}$, with an approximate application rate of 400 kbps.
• With S=8 coding, the rate drops further to $125 \text{ kbps}$, with an approximate application rate of 100 kbps. SIG describes this variant with an approximate reference range multiplier of 4 compared with the standard LE 1M PHY.

Practical Applications of LE Coded PHY

The long-range mode supports additional Bluetooth use cases in IoT deployments:

• Smart Home and Building Automation: Devices like smart door locks, window sensors, smoke detectors, and garden irrigation systems can communicate with a central hub over a greater distance when link budget and gateway placement are suitable.
• Industrial Asset Tracking: In a large warehouse or factory, Bluetooth tags attached to tools, pallets, or equipment can be tracked by placed gateways, depending on propagation.
• Agriculture: Environmental sensors monitoring soil moisture or temperature can be scattered across a field and report to a gateway when range and terrain allow it.
• Smart Cities: Applications like smart parking sensors or lighting controllers can cover a wider area, while the required number of gateways depends on the radio plan and environment.

The Third Pillar: 8x Advertising Capacity (Advertising Extensions)

A major enhancement in Bluetooth 5 was a new extended advertising mechanism. In Bluetooth 4.x, advertising was constrained to a small 31-byte payload broadcast on three primary channels. This limited the potential of connectionless applications. Bluetooth 5 introduced Advertising Extensions, which changed how devices broadcast information.

How Advertising Extensions Work: Offloading the Traffic

The new mechanism uses a two-step procedure to free up the primary advertising channels and send more data:

1. Primary Advertising on Primary Channels: The device still sends a very short advertisement packet on one or more of the three primary advertising channels (37, 38, 39). However, this packet no longer needs to contain the main data payload. Instead, it contains a simple pointer, instructing a scanning device on which data channel to listen to and at what time to find the rest of the information.
2. Secondary Advertising on Data Channels: The main, larger advertising payload is then transmitted on one of the 37 data channels at the time specified by the primary advertisement. This offloading of the main data packet from the advertising channels reduces congestion and the probability of collisions.

The Benefits of the New Approach

This advertising architecture increases the available host data field:

• Larger Payload: The extended advertising PDU can carry up to 255 octets. After the extended header, the host advertising data field is up to 254 bytes; headers and an AuxPtr where present consume part of the packet budget.
• Packet Chaining: Multiple advertising extension packets can be logically chained together. This allows a device to broadcast more than 254 bytes, up to about 1650 bytes of host data with fragmentation, without establishing a connection.
• Improved Coexistence:By moving bulky data transmissions to the 37 data channels, the 3 critical advertising channels remain less congested, which can help device discovery and connection initiation. This supports the "8x the data" metric as an advertising-capability comparison; it does not specify guaranteed throughput.

Practical Applications of Advertising Extensions

• Advanced Beacons: Retail stores can use beacons to broadcast a simple ID together with product information, promotional offers, or a URL that a smartphone can use to open a specific webpage.
• Connectionless Sensor Networks: An environmental sensor can now broadcast its temperature, humidity, and pressure readings directly in its advertising packets. A gateway can simply listen for these broadcasts without needing to establish a power-intensive connection with each sensor individually.
• Element of the LE Audio ecosystem: Advertising Extensions can carry information used to discover audio streams, but LE Audio and Auracast also require later Core features, isochronous channels, and the relevant profiles. A source such as a TV in an airport lounge can then broadcast to multiple receivers, with the number of headphones depending on the implementation.

Further Enhancements and Backward Compatibility

Beyond changes to speed, range, and advertising capacity, the Bluetooth Core 5 family includes releases 5.0 (December 6, 2016), 5.1 (January 21, 2019), 5.2 (December 31, 2019), 5.3 (July 13, 2021), and 5.4 (January 31, 2023). Core 5.0 added Channel Selection Algorithm #2 and the LE PHY and advertising features discussed here; 5.1 added Direction Finding; 5.2 added isochronous channels used by LE Audio; 5.3 added encryption key size control and Connection Subrating; 5.4 added Periodic Advertising with Responses. Direction Finding provides angle-of-arrival or angle-of-departure measurements but does not determine a position by itself. Positioning accuracy depends on the antenna array, reference-point geometry, propagation conditions, and the calculation algorithm.

The three pillars discussed here, 2M PHY, Coded PHY, and Advertising Extensions, apply to the Bluetooth Low Energy radio. Core 5.0 also includes the BR/EDR Slot Availability Mask, so it is too broad to say that the Classic radio was completely unchanged. For a connection-oriented feature, both endpoints must support the feature and its procedure. Broadcast advertising instead uses advertiser and observer roles, so an observer can receive a compatible advertisement without establishing a connection. A Bluetooth 5 smartphone can connect to an older Bluetooth 4.2 fitness tracker, but their common feature set will be limited. When compatible PHY support exists, the controller and host can negotiate 1M, 2M, or Coded according to link conditions and application preferences.