This invention relates to communication systems. In particular, it relates to cordless, cellular, Wireless Local Area Network (WLAN), or Wireless Personal Area Network (WPAN) systems controlling low power terminals.
In the last decades, progress in radio and semiconductor very large scale integration (VLSI) technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as mobile radios, can now be produced having acceptable costs, sizes, and power consumptions.
Although wireless technology is today focused mainly on voice communications (e.g., handheld radios), this field is rapidly expanding to provide greater information flow to and from other types of mobile, or nomadic, devices and fixed devices. Progress in technology and rising production volumes provide inexpensive radio equipment, which can be easily integrated into many devices. This reduces the number of cables currently used. For instance, radio communication can eliminate or reduce the number of cables used to connect master devices with their respective peripherals. The aforementioned radio communications can require an unlicensed spectral band with sufficient capacity to allow for high data rate transmissions. A suitable band is the ISM (Industrial, Scientific and Medical) band at 2.45 GHz (gigahertz), which is globally available. This ISM band provides 83.5 MHz (megahertz) of radio spectrum.
To allow different radio networks to share the same radio medium without coordination, signal spreading is usually applied. The U.S. Federal Communications Commission currently requires radio equipment operating in the 2.4 GHz band to apply some form of spreading when the transmit power exceeds about 0 dBm. Spreading can either be at the symbol level by applying the direct-sequence (DS) spread spectrum technique or at the channel level by applying the frequency hopping (FH) spread spectrum technique. The latter is attractive for the radio applications mentioned above since it more readily enables cost-effective radios.
A communication system called BLUETOOTH® has been developed to provide pervasive connectivity especially between portable devices like mobile phones, laptop computers, personal digital assistants (PDAs), and other nomadic devices. A BLUETOOTH® system uses frequency hopping to enable the construction of low-power, low-cost radios with small footprints. The system supports both data and voice, and the latter is optimized by applying fast frequency hopping with a nominal rate of 800 hops/second through the entire 2.4 GHz ISM band in combination with a robust voice coding. The air interface uses time slots having nominal lengths of 625 microseconds (μs), which corresponds to the dwell time of the FH. During a time slot, a single packet can be sent.
BLUETOOTH® wireless terminals can create so-called “ad hoc” piconets, which include a “master” device and one or more “slave” devices connected via the FH piconet channel. The FH sequence used for the piconet channel is completely determined by the address or identity of the device acting as the master. The system clock of the master device determines the phase in the hopping sequence. In a BLUETOOTH® system, each device has a free-running system clock. The slave devices add time offsets to their clocks such that they become aligned with the clock of the master device. By using the master address to select the proper hopping sequence and using a time offset to align to the master clock, the slave devices keep in hop synchrony to the master device, which is to say that the master and slave devices remain in contact by hopping synchronously to the carrier frequencies.
For more details of BLUETOOTH® communication systems, the reader is referred to the literature, which includes J. C. Haartsen, “The Bluetooth radio system”, IEEE Personal Communications Magazine, vol. 7, no.1, pp. 28–36 (February 2000).
In many wireless applications, traffic demand in the slave devices is low. For example, normal use of a headset wirelessly connection to a telephone may not exceed 20–40 minutes per day. Another low-traffic example is a keyboard or a mouse or a game controller wirelessly connected to a personal computer (PC). Accumulated over a day, there may be no traffic for a few hours. Yet another low-traffic example is a monitoring system of sensors distributed in a house or factory. Once in a while a sensor reading must be reported, but the duty cycle of such a monitoring system is low.
In these examples, slave units are in a sleep mode most of the time in order to save power. Many such wireless terminals receive their power supply, not from the power mains, but from batteries, which imposes stringent requirements on the power consumption of the terminals. Simple, low-cost, low-power terminals can be built when a star network is considered. FIG. 1 depicts a conventional wireless communication system 100 arranged as a star network. A central controller 110 provides one or more communication channels on which remote terminals 120, 130, 140, 150, 160 and controller 110 can communicate. The intelligence needed to operate the system 100 can be concentrated in the controller 110, so the remote terminals can be simpler, “dumb” devices. On the other hand, the terminals and controller can all be similar, as in many BLUETOOTH® networks. While no traffic is offered to the channel, the terminals can operate in low-power mode in order to extend the battery life of the terminals.
Terminal operation in low-power mode can be facilitated by a beacon signal broadcast by the central controller 110. The terminals remain time-synchronized to the beacon. When the beacon is about to be transmitted, the terminals wake up, receive the beacon signal, process it appropriately, viz., the terminals “read” the beacon. Depending on information read from the beacon, further actions are taken by the terminals. FIG. 2 is a timing diagram of a conventional beacon signal provided by a central controller and activity periods of two remote terminals. A beacon signal 210 is broadcast by the central controller 110 at a regular interval, and the terminals, represented by Terminal A and Terminal B in FIG. 2, wake up during the beacon transmission and read, or scan it, and sleep otherwise. In the case depicted in FIG. 2, data links to the terminals are not established, so they operate in an idle or standby mode.
Communication systems using such beacon signals are cordless telephone systems such as the Digital Enhanced Cordless Telephone (DECT) system and cellular telephone systems such as those specified by the GSM and IS-95 telecommunication standards. In such systems, however, only a single type of terminal is attached to the network, i.e., a voice terminal. Latency requirements do not differ between terminals, and the same latency characteristics are expected in the uplink (terminal-to-controller path) and the downlink (controller-to-terminal path). In addition, the number of terminals sharing a single beacon is not extremely large, and so the probability that a terminal's channel access will be blocked due to contention with other terminals' simultaneous channel access attempts can be kept manageably low.
U.S. Pat. No. 6,028,853 to Haartsen discloses a method and arrangement for synchronizing peripheral units in an ad hoc wireless network. The synchronization is achieved by two series of beacon signals staggered in time. The beacon series have the same repetition rate.
U.S. Pat. No. 6,351,468 B1 to LaRowe, Jr., et al. discloses a WPAN having a hub and several peripheral units. Synchronization of the network is achieved by transmission of a beacon signal from the hub.
U.S. Patent Application Publication No. 2002/0019215 A1 by Romans discloses a power management method and apparatus for use in a WLAN. A control point transmits beacon signals at regular intervals and peripheral units switch to their active modes to receive at least some of the beacon signals.
International Patent Publication No. WO 02/23818 A2 by Jamieson et al. and its counterpart U.S. Patent Application Publication No. 2002/0034959 A1 disclose a method and a system for transferring data in a master/slave radio network. The slaves are grouped into categories, and data-pending indications that relate to entire categories are included in a beacon signal from the master that is monitored by all slaves. Channel access is based on carrier sense multiple access (CSMA), which requires a mechanism for channel contention resolution.