Bluetooth technology is one solution for wireless personal area networks (WPANs). Two other technologies of interest, namely IrDA and HomeRF, are described here. The IEEE™ 802.15 standards work also addresses WPANs and the IEEE™ 802.11 working group addresses wireless LANs; this work is discussed separately in other articles.
We choose to discuss HomeRF and IrDA because they have interesting relationships with Bluetooth wireless technology. Strictly speaking, these technologies are not limited to the WPAN domain, although they can be considered part of it. Certainly there are other technologies that could be employed in the WPAN domain: wearable computing devices are becoming more popular and could be considered WPANs or elements of WPANs. IrDA is relevant for comparison to Bluetooth wireless communications because the two technologies share some similar usage models and protocols. HomeRF, like Bluetooth wireless technology, is a relatively short-range RF communications scheme that operates in the 2.4 gigahertz (GHz) industrial, scientific, and medical (ISM) band. These technologies are described next, with the objective of providing a context in which Bluetooth wireless communications as a WPAN technology can be better understood.
IrDA is an infrared wireless communication technology developed by the Infrared Data Association. Here we compare and contrast specific features of these technologies.
IrDA is a specific use of infrared light as a communications medium; Bluetooth technology is a specific use of radio waves as a communications medium. Like the Bluetooth special interest group (SIG), the IrDA specifies hardware and software protocols for wireless communication intended to promote interoperable applications.
Although both technologies are wireless, they use different parts of the electromagnetic spectrum with quite different signal propagation characteristics. Because infrared uses the nonvisible infrared light spectrum, IrDA communication is blocked by obstacles that block light (such as walls, doors, briefcases, and people). The signal wavelength used with Bluetooth communication (about 12.5 cm, at its associated frequency of 2.4GHz) is three orders of magnitude greater than that of IrDA. At this wavelength, radio frequency (RF) communications can penetrate many of these sorts of obstacles. Recent advances in infrared technology have enabled more diffuse transmission patterns, although much of the IrDA equipment in use today uses a relatively narrowly focused beam, which usually requires that the two devices engaged in IrDA communication be aligned with (pointed at) each other. RF transmission patterns radiate in some pattern (ideally, spherical) around the radio antenna, so any two devices within range can communicate with each other, whether or not they are "pointed at" each other (in fact, the second device might not be visible at all to the user of the first device, as it could be in another room behind doors and walls or even on another floor of a building, for example).
The initial IrDA data rate of 115 kilobits per second (Kbps) has now been enhanced to 1 megabit per second (Mbps), comparable to that of the first Bluetooth radios. Today, IrDA can achieve data rates of up to 4Mbps, with even higher rates already specified and beginning to be implemented. Bluetooth wireless communication occurs at a raw data rate of 1Mbps, with higher speeds being investigated.
The effective range for Bluetooth wireless communication is about 10 meters using the standard 0 dBm radio. With optional power amplification of up to 20 dBm, range on the order of 100 meters can be achieved. IrDA range is about 1 meter and, as noted already, generally requires a line of sight to establish a connection.
Both Bluetooth wireless technology and IrDA communication are optimized for low power consumption and low cost. Compared to other RF communication technologies, Bluetooth communication consumes very little power and is projected to have very low module costs in the foreseeable future. However, IrDA consumes significantly less power than Bluetooth technology, because far less power is required for infrared transceivers than for RF transceivers. IrDA hardware also already is less expensive than Bluetooth radio modules, owing largely to the maturity and wide deployment of IrDA.
This brief discussion indicates that IrDA technology compares favorably to Bluetooth technology in the areas of cost, power, and data rate. However, Bluetooth technology can add convenience and user mobility by relaxing the device alignment and line of sight requirements of IrDA communications. This illustrates an additional design parameter for WPANs: enhanced device usability enabled by the unconscious application of communications technology. RF technologies are well suited for this aspect of WPANs, and Bluetooth wireless technology further emphasizes device usability characteristics in the profiles. All of these considerations, both objective and subjective, may influence the choice of a WPAN technology for a particular application.
HomeRF is a wireless radio communications technology developed by the HomeRF working group, an industry consortium not unlike the Bluetooth SIG (in fact, several companies participate in both groups). Like Bluetooth wireless communications, HomeRF operates in the 2.4GHz ISM band, using radio waves for relatively short-range voice and data communication among various types of devices. The impetus for developing the HomeRF technology was to provide a solution for wireless in-home networks.
HomeRF and Bluetooth technologies have many parallels. Both were developed at roughly the same time, and both operate in the unlicensed 2.4GHz ISM band for RF communications. Each one enables both voice and data traffic; and each one is designed for relatively short-range, low-power operation. The first products using these technologies were available by 2001.
The two technologies do have differences, though. In addition to technical differences such as packet structure and protocol layers that are not detailed here, other characteristics distinguish them. HomeRF was designed from the outset for home networking environments, whereas Bluetooth technology is optimized for use with WPANs and mobile. HomeRF communication range is about 50 meters (designed to cover a typical home), which is greater than the 10-meter range of the typical Bluetooth radio. Initially, the data rates of the two technologies were similar at about 1Mbps; recently, HomeRF version 2.0 has specified data rates up to 10Mbps. With all other factors being equal, higher data rates and greater range require more power consumption, so HomeRF often could require more power, although both technologies include power-saving schemes in their respective specifications.
Like any technologies that operate in the same frequency spectrum, HomeRF and Bluetooth wireless communications can interfere with each other. Radio frequency interference is to be expected in the unlicensed 2.4GHz ISM band. One means to mitigate RF interference is the use of frequency-hopping spread-spectrum (FHSS), and both of these technologies use FHSS as a way to deal with undesired interference. Other techniques also can be used to minimize the problem of RF interference among these and other RF emitters in the ISM band.
Bluetooth wireless communications is one instance of a WPAN technology. Other technologies also can be used in WPANs, and we reviewed two of these, IrDA and HomeRF.
These and other mechanisms can be employed to implement WPANs, but each is optimized for specific usage, applications, or domains. Although in some respects, certain technologies might be viewed as competing in the WPAN space, we believe that at least the ones discussed here are complementary to each other. Each has specific advantages that make it particularly suitable for certain environments or applications, and we expect that multiple WPAN technologies will exist in the foreseeable future.