Bluetooth 5: Go Faster. Go Further.

Introduction

According to a paper by Goldman Sachs, in the 1990s there were approximately 1 billion devices connected to the internet. In the 2000s, the age of the smartphone, this figure rose to 2 billion. ABI Research now forecasts that by 2021 there will be 48 billion devices connected to the internet, in what we're likely to term the age of the IoT. Of those 48 billion devices, 30% are forecasted to include Bluetooth technology.

Bluetooth Low Energy (LE) has been actively evolved to make it a key enabler of the Internet of Things (IoT), focusing on the edge tier of IoT systems. Bluetooth 5 brings some major advances to the technology and makes it ideal for an even broader range of IoT scenarios.

This paper will present and explore the key advances in Bluetooth 5.

Key Statistic: By 2021, 48 billion devices are projected to be connected to the internet, with 30% of these expected to utilize Bluetooth technology.

A Choice of Three PHYs

Bluetooth utilizes a full protocol stack, with the bottom layer referred to as the Physical Layer (PHY). Bluetooth 5 introduces two new PHY variants to the specification used in Bluetooth 4, resulting in three PHYs in total: LE 1M, LE 2M, and LE Coded.

The PHYsical Layer

The PHY is the bottom layer of the Bluetooth protocol stack. Each PHY variant has unique characteristics and was designed with specific aims.

LE 1M

LE 1M is the PHY used in Bluetooth 4. It employs Gaussian Frequency Shift Keying and has a symbol rate of 1 megasymbol per second (Ms/s). Its support is mandatory in Bluetooth 5.

LE 2M - Double The Speed

The new LE 2M PHY enables the physical layer to operate at 2 Ms/s, allowing for higher data rates compared to LE 1M and Bluetooth 4.

Factors Behind the Introduction of LE 2M

While many Bluetooth LE use cases involve transmitting small amounts of data occasionally, there is a growing demand for low-power wireless communication technologies that support higher data rates. This is driven by advancements in areas like sports and fitness devices, which increasingly measure multiple human body dimensions with greater accuracy, and medical devices. The evolution of the ECG from a single-lead to a 12-lead device exemplifies this trend, generating significantly more data.

Furthermore, devices acting as buffered sensors, such as those used in Lifestyle Analysis, collect data over extended periods before transferring it. Transmitting data in a reduced amount of air time also improves spectral efficiency.

Technical Aspects of LE 2M

The LE 2M PHY doubles the symbol rate of the LE 1M PHY. It continues to use 2-level Gaussian Frequency Shift Keying (GFSK). To mitigate inter-symbol interference at higher symbol rates, the LE 2M PHY uses a frequency deviation of at least 370 kHz, compared to the LE 1M PHY's minimum of 185 kHz.

4x Range

Range and Bluetooth 4

Bluetooth LE offers a longer range than commonly believed. Informal testing with a standard smartphone and a Bluetooth LE MCU demonstrated successful receipt of notifications at over 350 meters in a sub-optimal environment. Commercial Bluetooth modules also advertise ranges of up to 500 meters.

Why Increase the Range of Bluetooth?

While Bluetooth 4 provides a healthy range for low-power wireless communications, increased range is advantageous for various use cases, particularly in the smart home sector.

The LE Coded PHY

The LE Coded PHY approximately quadruples the range compared to Bluetooth 4 without increasing transmission power.

Increasing Bluetooth's range without increasing transmitter power involves achieving the same maximum permitted Bit Error Rate (BER) at a greater distance, which corresponds to a lower Signal-to-Noise Ratio (SNR).

Dealing with Errors

Communication systems address errors through Error Detection and Error Correction. Bluetooth uses a Cyclic Redundancy Check (CRC) for error detection, appending a 24-bit CRC value to each packet. The receiver recalculates the CRC and compares it to the appended value. If they differ, an error is flagged.

When errors are detected, systems can either abandon the communication or request retransmission. Bluetooth handles this by not acknowledging a failed CRC check, prompting the transmitter to resend the data.

Error Correction

Bluetooth LE version 4 performs error detection only. Bluetooth 5's LE Coded PHY utilizes Forward Error Correction (FEC) to correct errors by adding redundant bits to packets. This allows data to be decoded correctly at a lower SNR and greater distance.

The FEC process in Bluetooth LE involves two stages: FEC encoding and a Pattern Mapper. The Pattern Mapper converts bits from the FEC encoder into symbols, with the number of output symbols depending on the coding scheme (S=2 or S=8). The S=8 scheme, for instance, produces 4 output bits for each input bit.

The generator polynomials for FEC Encoding are:

G₀(x) = 1 + x + x² + x³

G₁(x) = 1 + x² + x³

The choice of coding scheme (S=2 or S=8) affects the range and data rate. S=2 approximately doubles the range, while S=8 approximately quadruples it. However, the redundancy required for FEC impacts the number of symbols transmitted, reducing the overall data rate.

PHY Selection

Changing the Current PHY

The Host Controller Interface (HCI) supports a new command, the Change PHY Procedure, allowing the host to select the desired PHY. Applications can switch to 2Ms/s mode for higher data rates or to long-range mode as needed.

Comparing the Three PHYs

Advertising Extensions

Advertising in Bluetooth 4

Bluetooth 4 advertising packets are 37 octets long, with a 6-octet header and a payload of up to 31 octets. They are transmitted on dedicated channels 37, 38, and 39, out of 40 available 2MHz-wide radio channels (numbered 0-39). The same payload is typically transmitted on all three channels sequentially.

Bluetooth 5 Advertising Extensions in Summary

Bluetooth 5 introduces significant changes to advertising, including eight new PDUs for advertising, scanning, and connecting. These enhancements allow for larger data broadcasts in connectionless scenarios, deterministic advertising, and multiple distinct advertising data sets. Improvements in contention and duty cycle are also notable.

Bluetooth beacons are a key use case for advertising. Bluetooth 5 enables richer, multi-faceted contextual data to be broadcast by beacons, going beyond simple IDs or URLs. Examples include vending machines or refrigerators broadcasting location, temperature, stock level, and battery status.

Larger Packets and Advertising Channel Offload

Bluetooth 5 supports packets up to 255 octets by offloading the payload to secondary channels (0-36), which were previously used only for connection events. This allows larger packets in connectionless scenarios and offers other benefits.

Advertising Packet Chaining

For larger data broadcasts, packets can be chained together, with each packet containing a subset of the data and referencing the next packet via the AuxPtr header field on a different channel.

Advertising Sets

Bluetooth 5 introduces a standard mechanism for distinct advertising sets, each with an ID and unique advertising parameters. Advertising sets can utilize primary or secondary channels. Scheduling and transmission are managed by the Link Layer in the Controller, making it more power-efficient than host-driven methods.

Periodic Advertising

Bluetooth 5 enables periodic and deterministic advertising, allowing scanners to synchronize with the advertising device's schedule. This is more power-efficient and supports new uses like audio applications. Periodic advertising uses a SyncInfo field for timing information and a new GAP PDU called AUX_SYNC_IND.

Reduced Contention and Duty Cycle

Bluetooth 5 utilizes primary advertising channels (37, 38, 39) for small headers and secondary channels (0-36) for the main payload. This reduces contention on the primary channels. Unlike Bluetooth 4, which repeated the payload on channels 37, 38, and 39, Bluetooth 5 transmits the payload once on a secondary channel, reducing the overall data transmitted and duty cycle.

High Duty Cycle Non-Connectable Advertising

The minimum Advertising Interval for non-connectable advertising has been reduced from 100ms to 20ms, enabling faster recognition and response to advertising packets from devices like beacons.

Slot Availability Masks

Bluetooth 5 introduces Slot Availability Masks to improve coexistence with other radio technologies, such as LTE, operating in adjacent bands. This system allows Bluetooth to indicate the availability of its time slots and synchronize optimally with adjacent MWS bands, mitigating potential interference.

Improved Frequency Hopping

Bluetooth uses Adaptive Frequency Hopping during connections, an algorithm that determines radio channels for transmission and reception. This algorithm frequently changes the selected channel, enabling data transmission across a wide selection of channels and improving performance in busy radio environments.

Bluetooth 4's channel selection algorithm produced only 12 distinct sequences, using the same channel for all packets within a connection event, which is suboptimal for applications like audio. Bluetooth 5 introduces a new channel selection algorithm (#2) that uses pseudo-random hopping sequences with a vast number of distinct possibilities. Devices can indicate support for this algorithm, which utilizes a shared event counter to ensure peers select the same channel from the next available one in a pseudo-random sequence.

The Significance of Bluetooth 5

Bluetooth 5 represents a significant advancement in Bluetooth technology. It provides whole-home and building coverage with the long-range LE Coded PHY. The higher symbol rate of LE 2M enhances spectral efficiency and supports emerging use cases in sports, fitness, and medical equipment. Advertising extensions pave the way for next-generation beacons, advanced audio applications, and new industrial and smart city applications.

Bluetooth 5 is poised to have a substantial impact across many sectors, further solidifying its position as the preferred low-power wireless technology for the Internet of Things.

References

[1] Bluetooth SIG, Bluetooth 5 Core Specification. Available at: https://www.bluetooth.com/specifications/bluetooth-core-specification