Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
1997-09-17
2003-07-08
Luther, William (Department: 2664)
Pulse or digital communications
Spread spectrum
Frequency hopping
C375S137000
Reexamination Certificate
active
06590928
ABSTRACT:
BACKGROUND
The present invention relates to uncoordinated wireless multi-user systems, and more particularly to self-organized connectivity in an uncoordinated wireless multi-user system.
Radio Local Area Networks (LAN) typically cover an area of technology where the computer industry and the wireless communications industry merge. Conventional computer networking has relied on wired LANs, typically packet-switched and targeted for data transfer. By contrast, wireless networking, and in particular cellular networking, has relied on wide area networks, typically circuit-switched and targeted for voice transfer. Most efforts in the design of radio LANs have reused the principles that are used in wired LANs. This, however, is a questionable procedure because the environments of the wired medium and of the wireless medium differ in important ways. Moreover, multimedia communications require additional features due to the special traffic characteristics posed by data, voice and video. Finally, the residential environment has its own requirements which can be decisive for the design of the system.
Almost one hundred percent of the computer networks today use a wired infrastructure. The wired medium can range from a simple twisted pair to an optical fiber. Due to its shielded and controllable environment, the wired medium is characterized by low interference levels and stable propagation conditions. Consequently, the wired medium has potential for high to very high data rates. Because of the latter, all participants in wired LANs typically share this single medium. The medium constitutes a single channel which is used by only a single one of a number of different users at any given time. Time-division multiplexing (TDM) is used to allow different users to access the channel at different times.
The protocols for accessing wired media have been standardized by the IEEE in its 802 series. Typically, multiple access reservation techniques like carrier-sensing (e.g., Ethernet, 802.3 Carrier-Sense Multiple Access/Collision Detect (CSMA/CD) or tokens (e.g., 802.4 token buses, or 802.5 token rings) are used to gain access to the medium. These protocols can be used in a distributed sense in that the user occupying the channel reserves the medium by its present transmission or by its token. In these schemes, every user can hear all traffic. That is, in a single LAN, all of the users share not only the channel, but all of the information carried on that channel as well. When the number of participants grows, the LAN can be divided into smaller LANs or segments, which channels operate independently. LANs can be interconnected via bridges or routers which form interfaces between the different local networks. These configurations result in more complex networks. For example, reference is made to D. Bertsekas and R. Callager,
Data Networks,
2nd Edition, Prentice-Hall, London, 1992. For the discussion of the residential LANs, it suffices to consider the single LAN. The LAN typically provides a connectionless packet-switched service. Each packet has a destination address (and usually a source address as well) so that each user can determine whether the packet that passes by is intended for him or not.
It will be understood that the net throughput per user in a single LAN is determined by the peak data rate on the channel and by the number of users that share this channel. Even if the peak data rate is very high due to the wide band-width of the wireline medium, the effective user throughput can be low if the channel has to be shared among many users.
Since the type of communication that takes place over current wired LANs is asynchronous and connectionless, it is ill-suited for supporting delay-critical services like voice. Voice services demand synchronous or isochronous connections, which require priority techniques in the Medium Access Control (MAC) protocols in order to give voice users precedence over non-voice users. Different studies in existing data networks have shown that this is not a trivial task.
During the last several years, standards bodies in the United States and in Europe have worked on wireless LANs (WLANs). In the United States, this has resulted in the IEEE 802.11 standard (Draft standard IEEE 802.11, P802.11/D1, December 1994), whereas in Europe this has resulted in the ETSI HIPERLAN standard (ETSI, RES10/96/etr, “Radio Equipment and Systems (RES); High Performance Radio Local Area Networks (HIPERLANs), July 1996).
Looking first at the IEEE 802.11 standard, as the name indicates, it is an extension of the 802 LAN standard. The wireless connection is either a radio link or an infrared link. The radio medium is the Industrial, Scientific, Medical (ISM) band at 2.4 GHz. However, for a single radioLAN, only a 1-2 Mb/s channel is available at any given time. This relatively narrow channel has to be shared among all participants of the radio network. Both a configuration based on a wired infrastructure and a configuration based on an ad-hoc structure have been defined. With a wired infrastructure, the radio system merely provides a wireless extension between the wired LAN and the user terminal. Fixed access points interface between the wireline domain and wireless domain. In an ad hoc network, wireless units create their own wireless network. No wired backbone is involved at all. It is the ad hoc nature provided with wireless communications that gives the WLANs an important advantage over wired LANs in certain applications.
To avoid interference with other networks or other applications in the 2.4 GHz ISM band, either direct-sequence spreading or slow frequency hopping is used. Access to the channel is accomplished by a special form of Carrier-Sense Multiple Access/Collision Avoidance (CSMA/CA) that provides a connectionless service. In an architecture based on a wired infrastructure, the fixed part takes the role of a central controller which schedules all traffic. In an ad hoc architecture, the distributed CSMA/CA protocol provides the multiple access to the channel.
All in all, the IEEE 802.11 standard is very similar to that of the wired Ethernet, but wherein the wire has been replaced by a 1 Mb/s radio channel. It will be understood that the effective user throughput decreases quickly when the number of participants increases. In addition, since the spreading factor for Direct Sequence Spread Spectrum (DSSS) is only 11 and the hop rate for Frequency Hopping Spread Spectrum (FHSS) is only on the order of 10 to 20 hops/s, little immunity is provided against interference in the ISM band. Although different networks can theoretically coexist in the same area (different networks either use different DSSS carrier frequencies of which seven are defined, or use different FHSS hop sequences), thereby increasing the aggregate throughput. In fact, in A. Kamerman, “Spread-Spectrum Techniques Drive WLAN Performance,”
Microwaves
&
RF,
September 1996, pp. 109-114, it was claimed that the aggregate throughput, defined as the average throughput per user times the number of co-located users (not necessarily participating in the same network), can never exceed 4-6 Mb/s with either technology. For co-locating different networks under the IEEE 802.11 standard it is preferred that the networks be based on a wired infrastructure: a limited number of co-located fixed access points can create their own network. A certain amount of coordination via the wired network is then possible. However, for networks based on an ad hoc structure, this is much more difficult under IEEE 802.11 because the MAC protocol does not lend itself to this creation. Instead, units that come in range of an ad hoc network will join an existing network and not create their own network.
HIPERLAN has followed a similar path as IEEE 802.11. The system operates in the 5.2 GHz band (not available in the United States). The standard is still under development and consists of a family of sub-standards, HIPERLAN 1 to 4. The most basic part, HIPERLAN 1 (ETSI, ETS 300652, “Radio Equipment and Systems (RES); High Performance Radio Loc
Burns Doane Swecker & Mathis L.L.P.
Luther William
Telefonaktiebolaget LM Ericsson (publ)
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