The present invention relates to radio communication systems, and more particularly to operating two radio systems that are located in close proximity to one another and that operate in the same radio spectrum.
In the past few decades, progress in radio and Very Large Scale Integrated circuit (VLSI) technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as mobile radio communication devices, can now be produced having acceptable cost, size and power consumption. After the worldwide success of mobile telephony in licensed bands, capacity limitations and huge license fees have spurred an interest in radio applications operating in the unlicensed bands. For the past few years, systems such as Wireless Local Area Networks (WLAN) operating in accordance with the IEEE 802.11 standards (commercialized under the name “WiFi™”) and Wireless Personal Area Networks operating in accordance with the Bluetooth® standards (IEEE 802.15 standards) have been increasingly deployed in the unlicensed 2.4 GHz Industrial, Scientific, Medical (ISM) frequency band.
There is a general coexistence problem with radios that operate both in the same area and in the same radio spectrum. If Bluetooth® radios and WLAN radios are operating in close proximity, say within a few meters to a few tens of meters of one another, mutual interference gives rise to degradation of the radio link quality. The following discussion explains why this is so.
The Bluetooth® radio unit is designed to perform frequency hopping over a set of 79 hop carriers that have been defined at a 1 MHz spacing in a frequency band centered at 2.4 GHz. At any given moment, the Bluetooth® radio covers only about 1 MHz of bandwidth. In contrast, WLAN IEEE 802.11b for example uses a static carrier that can be dynamically selected out of 11 carriers, each occupying about 22 MHz of bandwidth. These 11 carries together occupy the same frequency band as is used by the Bluetooth® radio. Consequently, when a Bluetooth® radio and a WLAN 802.11b radio operate in the same area, there is a 22/79 probability that, at any instant of time, the Bluetooth® channel will overlap with the WLAN channel resulting in mutual interference.
Several solutions to this problem exist. One of these, Adaptive Frequency Hopping (AFH), has recently been released by the Bluetooth® Special Interest Group in a draft specification. Using this technique, the Bluetooth® radio can select a number of carrier frequencies that will be skipped during frequency hopping, thereby making them unused for radio communications. An example of an AFH scheme has been described in U.S. patent application Ser. No. 09/418,562 filed on Oct. 15, 1999 by J. C. Haartsen and published as WO0129984. However, with the increased deployment of Bluetooth® connectivity and WLAN IEEE 802.11 (“WLAN 802.11”) networks, coexistence has gone to the next level: co-location. By co-location is meant the placement of two radios at very close proximity to one another, for example about 10 cm or less, although this measurement should be construed loosely. Optimally, the two radios are implemented on the same platform and use a common antenna, such as the dual radio embodiment illustrated in FIG. 1. Devices that employ both Bluetooth® wireless technology and WLAN radios include laptop and desktop computers as well as lighter devices such as Personal Digital Assistants (PDAs). In the future, mobile telephones may also incorporate both types of technologies. Concurrent operation of these two types of radios poses a problem because they interfere with each other's transmissions and receptions. In those cases, AFH will not help: due to the small attenuation between the transmitter of one radio and the receiver of the other radio, the interfering signal is so much stronger that it drowns out the received information signal.
One possible method of combating the interference is applying active cancellation of the interfering signal as described in U.S. Pat. No. 6,539,204, which issued to Marsh et al. on Mar. 25, 2003. However, a WLAN transmitter may typically operate at high power levels, such as at +20 dBm, whereas a Bluetooth® receiver will typically be trying to receive incoming Bluetooth® signals at −85 dBm from a remote Bluetooth® unit. When these two units are co-located, the power differences are in the range of 30 to 50 dB, which is impossible to compensate for in active cancellation circuitry.
The only viable method for preventing the co-location interference is applying time division multiplexing (TDM) so that only one radio operates at a time. This results in perfect isolation between the radios. TDM has been investigated for co-located radios before. See, for example, the white paper presented by Mobilian Corporation at WinHEC 2001 entitled “Wi-Fi™ (802.11b) and Bluetooth™: An Examination of Coexistence Approaches”, Apr. 11, 2001. In the described approach, a packet arbitration method is provided that operates at the Medium Access Control (MAC) level. The mechanism called MEHTA (which stands for “Mac Enhanced Temporal Algorithm”) takes into account the activity and duration of the activity of the two radios. A block diagram of a system implementing this method, also known as Packet Traffic Arbitration or “PTA”, is illustrated in FIG. 2. As shown, a WLAN device 201 and a Bluetooth® device 203 are co-located with one another. The WLAN device 201 includes an IEEE 802.11 MAC 205 that communicates with an IEEE 802.11 PLCP+PHY layer control block 207. The Bluetooth® device 203 similarly includes an IEEE 802.15.1 LM+LC block 209 that communicates with an IEEE 802.15.1 baseband controller 211. A PTA controller 213 is provided that determines which of the WLAN and Bluetooth® devices 201, 203 will be permitted to transmit at any given moment. To accomplish this, the PTA controller 213 includes a WLAN (802.11b) control portion 215 and a Bluetooth® (802.15.1) control portion 217 which each receive present status information from each of the WLAN and Bluetooth® devices 201, 203. This present status information indicates the activity and expected time duration of the activity of each of the two radios. When the WLAN device 201 wishes to transmit, it communicates a transmission request 219 to the WLAN control portion 215 and waits for the WLAN control portion 215 to reply with a transmission confirmation 221 before proceeding with the transmission. Similarly, when the Bluetooth® device 203 wishes to transmit, it communicates a transmission request 223 to the Bluetooth® control portion 217 and waits for the Bluetooth® control portion 217 to reply with a transmission confirmation 225 before proceeding with the transmission. Each of the WLAN and Bluetooth® control portions 215, 217 makes its determination whether to permit the requested transmission based upon the totality of status information provided to it.
Although PTA takes into account the real-time conditions at the radio interface, it is a suboptimal solution to the problem of co-located radio devices because it cannot satisfactorily anticipate the needs of priority services, such as voice communication. Rather, it only considers the instantaneous conditions in the considered radios. Consequently, a Bluetooth® priority packet will have to interrupt ongoing WLAN traffic and will result in disturbance of the WLAN link.
An alternative TDM-based method is the Alternating Wireless Medium Access (AWMA) technique. As illustrated in the timing diagram depicted in FIG. 3, the AWMA technique divides time into segments during which the Bluetooth® radio and the WLAN radio are alternately active. However, this setup requires a rather static allocation of the bandwidth between the WLAN and Bluetooth® radios, and can only slowly adapt to changing traffic conditions. Real-time or priority services, such as voice service in accordance with the Bluetooth® standards, cannot be supported. Another drawback is that the WLAN 802.11 specification would have to be modified in order to add a field in the WLAN beacon specific to the AWMA mechanism. In addition, synchronization between the WLAN and the Bluetooth® link is required, which is only feasible when the Bluetooth® unit co-located on the platform acts as a master. The latter is a severe limitation because the co-located Bluetooth® radio may just as likely be allocated the slave role. AWMA as well as PTA are described in a presentation held by Intersil entitled Tim Godfrey, “802.11 and Bluetooth Coexistence Techniques”, presented to Bluetooth Developers Conference Dec. 11, 2002. In this presentation, another technique was proposed called Blue802™. In this technique, the power save mode in the 802.11 standard is used. When the Bluetooth® radio needs bandwidth, the Bluetooth® terminal informs the WLAN Access Point (AP) that it will enter the sleep mode. Again, this is not a suitable solution if the Bluetooth® terminal needs to support priority services, such as voice communication, because the 802.11 system cannot be put to sleep for every Bluetooth® voice packet.
It is therefore desirable to provide a mechanism that allows two incompatible transceivers, such as a Bluetooth® radio and a WLAN IEEE 802.11 radio, to coexist in close proximity (e.g., on the same platform, possibly using the same antenna). It is also desirable to provide such a mechanism in which undisturbed real-time services, such as voice communication, are supported on one transceiver's link (e.g., the Bluetooth® link), while keeping high efficiency in best-effort services being carried on the other transceiver's link (e.g., the WLAN link). It is yet further desirable for any such mechanism to not require changes in the specifications for either transceiver (e.g., the specifications of the WLAN and Bluetooth® transceivers).