Stepping through the evolution technologies for enhanced Bluetooth enabled devices

This is an abridged version of a paper that will be presented at the IEEE International Conference in Consumer Electronics, which opens in Los Angeles.

Personal Area Networks (PANs) enable consumer electronic devices to communicate directly with one another using wireless technology. The most famous of these standards is Bluetooth. This technology operates in the unlicensed 2.4GHz Industrial, Scientific and Medical (ISM) band. Each radio packet is sent out on a different frequency with the terminal cycling through a total of 79 1MHz hop channels (or 23 1MHz hop channels in Japan, France and Spain). One of the drawbacks with the current standard is a restricted bit-rate of 1Mbit/second. The data rate is limited by the choice of modulation scheme and the available bandwidth per channel. Although 1 Mbit/sec may seem adequate for low bit rate applications, such as data modems, cordless telephones and low bit rate videophones, it is insufficient to support high bit rate VCR/TV quality digital video (2-12 Mbit/sec).

One method to increase the data rate of a digital communication system is to employ a higher order modulation technique. however this normally reduces range). Modulation is the process by which voice, music and data signals are added to radio waves produced by a transmitter. This process can be performed by varying the amplitude, frequency or the phase of the radio waves depending on the signal stream. The changes in amplitude, frequency or phase translate to different types of digital modulation. The only other way to increase the data rate is to assign more radio bandwidth.

Previous research has shown that higher data rates can be achieved by employing coherent M-state phase shift keying (M-PSK) and quadrature amplitude modulation (QAM) schemes in Bluetooth devices instead of the current Gaussian frequency shift keying (GFSK) scheme. Data throughput and coverage analysis were carried out for a typical home environment using a state of the art indoor space-time propagation model. Software simulated results showed that M-PSK and QAM schemes offer wider coverage and reliable data rates that facilitate a greater range of consumer electronic applications compared to the current GFSK system.

M-PSK schemes in particular have the added advantage that the transmitted symbols can be decoded at the receiver by measuring the relative change in phase (differential detection as normally termed by engineers). Therefore, this reduces the potential cost of the receiver when compared to QAM schemes — where relative changes in both amplitude and phase are required. The underlying challenge for the engineer of course lies in deploying M-PSK schemes for Bluetooth and increasing the bandwidth by a factor of 5-10 times to achieve a reasonable trade off between bandwidth efficiency, radio coverage, power requirements and data rates.

Although the study summarized above enhances data throughput performance, fundamentally the performance may still be limited due to the interference present in the 2.4GHz band. The main sources of interference in the 2.4GHz band are microwave ovens, other wireless local area network (WLAN) devices such as those based on the IEEE 802.11b and 802.11g standards as well as Bluetooth devices themselves. In order to achieve reliable performance, particularly for time bounded applications such as video and audio streaming, these interference issues need to be addressed and where possible, suitable interference mitigation techniques must be deployed at the Bluetooth device.

The Bluetooth communication structure is based on an ad-hoc network architecture. Ad- hoc connectivity is purely based on close range peer communications. No wired infrastructure is required to support connectivity between portable electronic devices and it does not rely on distinctive base stations. A group of Bluetooth units sharing the same channel is known as a piconet. Each piconet contains a master and up to seven active slaves. All Bluetooth units within a piconet hop using the same hop pattern defined by the Bluetooth device address and the Bluetooth clock of the master. Since each piconet contains a master with a unique address and a different clock, the hop pattern varies from one piconet to another. In the current Bluetooth system, single and multi-slot packets have been defined for data transmissions. Each single time slot packet is transmitted on a different hop frequency whereas a single hop frequency is used for the entire span of a multi time slot packet.

Initial investigations focussed on co-located piconets containing enhanced data rate Bluetooth devices. The fundamental issue with Bluetooth piconets operating within the same environment is that they are not time synchronised to each other and thus, a radio packet collision will occur in both time and frequency. As a result, data transmissions on unwanted piconets can interfere with the data transmissions on a wanted piconet. Consequently, the requirement to retransmit packets will increase, thus reducing the overall data throughput.

Performance issues

The investigation carried out demonstrated that the frequency collision statistics are linearly dependent on the available bandwidth. Frequency collisions between piconets will obviously depend also on the proximity of piconets within the environment and the transmit power levels (1mW for ranges up to 10m and 100mW for ranges up to 100m). Although the statistics seem high for the 79-hop system in Bluetooth, in the presence of low number of interferers (other Bluetooth piconets), the performance degradation is not catastrophic especially for shorter data packets that employ error protection. However, in the presence of high number of interferers, the performance degradation is significant.

The performance of enhanced Bluetooth in the 23-hop system is certainly unfavourable for time-bounded applications in the presence of high number of interferers unless a suitable mechanism was deployed using an access point that could time synchronize co-located Bluetooth piconets. The results suggest that enhanced data rate Bluetooth enabled devices are only capable of handling mutual interference provided the number of interferers are low.

An alternative solution for mitigating mutual interference between Bluetooth devices was proposed. In this study the advantages of exploiting antenna diversity was investigated. Antenna diversity is a technique used to increase the capacity of a wireless channel. A receiving station obtains multiple observations of the same signal sent by a transmitting station. The redundancy built into the multiple observations can be used to recover the transmitted signal with a higher degree of accuracy at the receiver, hence reducing the requirement of retransmissions.

Plan details a typical office environment (18.5 m x 13.8 m) containing various WLAN devices. The main sources of interference in the 2.4-GHz band are microwave ovens, other WLAN devices like those based on the IEEE 802.11b and 802.11g standards, and Bluetooth devices. Source: University of Bristol

In the study carried out, antenna diversity was exploited using a technique called space time block codes (STBC) with maximum likelihood (ML) decoding. A simple 2 transmit and 2 receive antenna architecture was deployed at the Bluetooth devices in the wanted piconet. The test environment used for this simulation was a typical open plan office environment containing several unwanted Bluetooth piconets (interferers) that were modelled without any antenna diversity. Results showed that the reliability of both time-bounded and non-time bounded Bluetooth enabled devices in the presence of interferers were enhanced by using STBC coupled with suitable interference cancellation techniques. The improvement was significant particularly when a high number of interferers were present in the environment.

Current research focuses on addressing cross interference between Bluetooth and enhanced data rate Bluetooth and a plethora of other unlicensed systems that coexist in the 2.4GHz ISM band. Two systems in particular are being considered; IEEE 802.11b and IEEE 802.11g.

In 2000, 802.11b became the standard wireless Ethernet networking technology for both business and home use. The avalanche of 802.11b devices in the market today primarily in the form of adapters come in two major form factors; PC cards for laptops and USB for desktops. In addition, there are PCI adapters that allow users to plug a PC card into a PCI slot. This system provides raw data throughputs up to 11Mb/s of which, 4-6Mbit/sec is the net data throughput — the actual content of information on files that are being transmitted.

More promisingly the 802.11g standard runs, like 802.11a, at 54 Mbit/sec but with full backward compatibility with 802.11b. The 802.11g specification uses a new method of encoding bits onto radio waves in such a way that will squeeze up to 54 Mbps of raw data on narrowband 312.5 kHz channels covering approximately 16.25MHz. The net throughput of this standard is between 20-30Mbit/sec.