New protocol brings a 3X increase in bandwidth to Bluetooth designs without impacting upper-layer (above HCI) protocols. Here's a look inside at how the EDR protocol achieves this feat.

By David McCall, Cambridge Silicon Radio (CSR)
Courtesy of Wireless Net DesignLine Dec 21, 2004

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There's no doubt that Bluetooth technology has started to gain hold in the communication sector. The technology, which was once bashed for lack of adoption, is now shipping in 10s of millions in applications like cellular phones, wireless headsets, and automobiles.

But, like all wireless protocols, Bluetooth also needs to evolve to meet the changing demands of consumers. In particular, there is a clear call in the consumer electronics space to support the wireless transfer of higher-bandwidth/higher-quality audio signals, a request that will stretch the throughput capabilities delivered by existing 1-Mbit/s Bluetooth designs.

To address the need for next-generation audio and other short-range, high-bandwidth wireless applications, the Bluetooth Special Interest Group has developed an Enhanced Data Rate (EDR) protocol, which pushes the bandwidth delivered over Bluetooth connections up to a peak rate of 3 Mbit/s. In this article, we'll look at the reasons why the bandwidth of Bluetooth connections needed to be increased. We'll then take a detailed look at the modulation changes made in version 2.0 of the Bluetooth spec to support EDR operation.

Why is EDR Needed?
The EDR version of Bluetooth was ratified in November of 2004 and although it appears to be a mere addendum to the latest version 2.0 of the specification, its effect is far reaching.

When the Bluetooth specification was initially drawn up back in the late 90s, great care was taken to ensure that the specification allowed flexibility and adaptability in addition to ensuring that that this wireless communication medium remained robust and secure. Since its inception, Bluetooth has seen a number of adaptations that have led to major improvements in performance and interoperability as well as co-existence with other wireless standards such as 802.11. EDR Bluetooth is the latest such adaptation.

EDR operation is defined under the Bluetooth 2.0 specification. Specifically, the Bluetooth 2.0 spec defines modulation changes and additional packet types that allow designers to deliver a peak rate of 3 Mbit/s (2.1 Mbit/s real throughput) over a Bluetooth connection. In comparison, the Bluetooth 1.2 spec defined a maximum data rate of 1 Mbit/s (723 kbit/s real throughput) over a Bluetooth connection.

For any communications technology, faster is almost always better, and Bluetooth is no exception. However, the reasoning behind EDR, and the selection of the 3-Mbit/s data rate in particular, goes beyond a simple desire for more speed.

At present, there is no single Bluetooth application that demands more than the current 1-Mbit data rate. Even a high-quality stereo audio stream (using the mandatory SBC codec) tops out at 345 kbit/s.

Although future individual applications are expected to maintain data rate demands at this level, it seems almost certain that as Bluetooth grows in popularity, users will increasingly run multiple Bluetooth links at the same time. This is particularly true for PCs, where it is easy to imagine using a Bluetooth mouse and keyboard and listening to streaming stereo audio over a pair of Bluetooth headphones, all at the same time. EDR gives Bluetooth the extra capacity it needs to maintain all these links at data rates that users find acceptable.

Taking the example of stereo audio plus a mouse and keyboard and standard rate Bluetooth: A mouse and keyboard can each typically take 11% of the maximum theoretically available bandwidth and a medium quality stereo audio stream, using the default SBC codec, takes 35%. This leaves a margin of 43%, which would be plenty for a wired network.

However, for a wireless network the theoretical maximum is rarely reached due to interference and non-ideal packet scheduling. In practice the 43% margin is just enough to maintain acceptable performance in most situations — assuming the packet scheduling is just right. High levels of interference will, however, cause glitches in the audio or sluggish mouse and keyboard response.

The situation changes if the user wants to listen to a high-quality audio stream, which requires 345 kbit/s of data rather than the 237 kbit/s for medium quality. The stereo audio stream now takes up 53% of the available bandwidth, which, when combined with the 22% required for a mouse and keyboard, leaves a 25% margin. This simply isn't enough to allow for the retransmissions necessary to maintain acceptable performance under even the lightest interference.

Switching to EDR solves the problem. The mouse and keyboard still take 11% of the maximum available bandwidth, but a high-quality A/V audio stream now only takes up 18%. This leaves a margin of 60%, which is easily enough to maintain acceptable performance under even heavy interference and still leave room for additional applications, like printing a file or synchronising data.

Reducing Power; Providing Compatibility
In addition to supporting higher-quality audio, EDR is important because it helps lower power consumption in a Bluetooth design. The amount of power drawn by a Bluetooth radio depends on the length of time it is active. Since EDR allows data to be transmitted 3 times faster, the radio only needs to be active for a third of the time and draws a third of the power as a result.

Backwards compatibility with v1.2 was also a high priority during the development of EDR. The final specification is 100% backwards compatible and allows networks containing a mixture of EDR and standard rate devices. The new modulation schemes are also very compatible with standard rate, so a single transmit and receive chain can do both. This means that designing a product using EDR is no more complicated than designing a Bluetooth 1.2 product.

How EDR Works
So, how does EDR work? Essentially, the EDR protocol defines additional packet types that employ new modulation schemes for payload data.

All Bluetooth data is transmitted as part of a packet. Standard rate packets are made up of four sections:

1. Access Code — The receiving device uses this to recognise incoming transmissions.
2. Header — Describes the packet type and length.
3. Payload — The actual data.
4. Inter-Packet Guard Band — Radio retunes to the next frequency.

In the Bluetooth 1.2 specification, all three transmit sections use Gaussian frequency-shift keying (GFSK) to modulate the over-air RF signal. In GFSK, the carrier frequency deviates by +/-160 kHz to indicate a one or a zero thus encoding one bit per symbol. The symbol rate is 1 MSymbol/s, leading to a peak data rate of 1 Mbit/s. But, when designers account for access codes, headers, and guard bands, Bluetooth systems actually deliver a maximum payload data rate of 723 kbit/s.

In the Bluetooth 2.0 specification, EDR packets still use GFSK modulation for the access code and header. However, EDR uses one of two different modulation schemes for the payload: one mandatory and one optional (more below). The change of modulation scheme also requires the insertion of a small guard band and synchronization sequence between the header and the payload (Figure 1).

Figure 1: Diagram showing an EDR packet split into access code, header, sync sequence, and payload.

The 2-Mbit Improvement
The mandatory 2X (2-Mbit) data rate uses π/4 differential quaternary phase-shift keying (π/4-DQPSK). As the name implies, this modulation scheme varies the phase of the carrier rather than the frequency. Quaternary refers to the fact that there are four possible phase positions for each symbol, allowing two bits of data to be encoded per symbol. The symbol rate remains the same; hence the 2X data rate increase.