With Bluetooth expected to grow rapidly, the problems of the technology's coexistence with different classes of wireless systems have become significant. Popular wireless LAN standards such as IEEE 802.11b and HomeRF are being worked on the interference testbench, and systems that cannot live together could see their support severely undermined.
Each of those three major wireless standards operates in the 2.4-GHz band, which is available virtually worldwide on an unlicensed basis. Naturally, when different wireless transmission schemes seek to share the same frequency band there will be interference-the only question is the magnitude of that interference. Since the three technologies are complementary and all are poised for widespread deployment, the probability of interference is expected to increase as different wireless standards are lined up within overlapping coverage areas.
Bluetooth is a point-to-point protocol intended to wirelessly connect laptops, handheld computers, cellular phones, digital cameras and printers. It operates over short distances of up to 30 feet, eliminating the need for data cables and infrared. Now being discussed is an extension of this range to 300 feet by increasing the transmit power from 1 or 10 milliwatts, as the standard currently allows, to 100 mW.
Bluetooth radios transmit short packets using frequency-hopping spread spectrum (FHSS) at 1,600 hops per second, a much higher rate than that used by HomeRF and other radios. Faster hopping has the advantage of spending less time on a frequency channel that may be experiencing interference, which is generally seen as good. On the other hand, it is very throughput inefficient, both in terms of wasted time during the hop and in the lack of time to transmit long packets without exceeding the hop interval.
Regarding interference with other systems, the hop rate can be viewed from two perspectives. For a target system carrying delay-sensitive traffic and therefore concerned about latency, faster hopping interferers are preferred because they leave a given channel sooner. But for those mainly concerned about the throughput of relatively long Ethernet mapped packets, slower hop rates are better because the probability that a given target packet will collide with the interference source is greatly reduced.
In contrast to Bluetooth, HomeRF is a wireless LAN protocol that can connect many devices simultaneously, with a range of more than 150 feet. Although the two protocols are optimized quite differently and thus are designed for different applications, their RF hardware has significant details in common. Though the 802.11b standard was originally developed for corporate networks, HomeRF was designed from the start to meet the needs of consumers in home-networking applications: affordability, robustness and ease of use. An important differentiator from 802.11b is the ability of HomeRF's Shared Wireless Access Protocol (Swap) to transport isochronous data, enabling true computer-telephony integration. Swap is essentially a hybrid combination of time-division multiple access (TDMA) for isochronous services and carrier-sense multiple access for asynchronous data transmission. The isochronous services support up to eight simultaneous voice connections based on the European Digital Enhanced Cordless Telephony voice standard.
As in Bluetooth, HomeRF-compliant products use the FHSS transmission scheme to combat interference. If the transmission on any frequency channel is corrupted by interference, the FHSS system simply hops to the next frequency and retransmits. Successive hops usually span a wide enough portion of the spectrum to exceed the contiguous bandwidth of the interferer. Hence, if interference interrupts the transmission on a particular hop, there is only a slight chance that it will affect the next channel in the hopping sequence. Users may see a decrease in throughput each time the device encounters a blocked frequency, but since it is frequency agile, the network will continue to operate.
For delay-sensitive voice applications, HomeRF includes a unique frequency-diversity retry mechanism that provides increased immunity. A Swap frame incorporates up to two contention-free TDMA periods, with the TDMA period at the end of the frame reserved for the initial isochronous transmission. Voice packets that are blocked because of interference are automatically resent at the beginning of the next frame, less than 10 ms later, but separated by a hop. In this way, frequency diversity is achieved with minimum latency between the transmission of the original voice packet and any retransmissions that may be required.
The 802.11b wireless Ethernet standard is aimed primarily at large offices and business campus environments. In contrast to Bluetooth and HomeRF, 802.11b is based on direct-sequence spread spectrum (DSSS) technology. DSSS systems transmit on a statically allocated carrier frequency and spread the signal by mapping each symbol into a redundant bit pattern called the chip code. A high ratio of chips per bit increases the signal's resistance to interference. If relatively weak interference mangles some of the chips, it is still possible to decipher the signal and extract the data. Not surprisingly, proponents of DSSS point to its superior throughput and its ability to reject multipath interference. But due to its static nature a DSSS system has no mechanism for avoiding interference, so in more severe interference situations a DSSS system can be completely inoperable. An FHSS system can hop around the signal, experiencing only limited throughput degradation.
Of course, the performance of a system in the presence of interference depends on many other components, such as proximity, transmit power and signal-to-noise ratio.