Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
2000-12-15
2004-06-22
Bayard, Emmanuel (Department: 2631)
Pulse or digital communications
Spread spectrum
Frequency hopping
Reexamination Certificate
active
06754250
ABSTRACT:
BACKGROUND
This invention relates to frequency hopping (FH) radio systems. In particular, it relates to multiple, uncoordinated FH radios that try to form a wireless network. The invention describes how links between several FH radios can be established and maintained.
Radio Local Area Networks (radio LANs or RLANs) 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 designed for data transfer. By contrast, wireless networking (particularly cellular networking) has relied on wide area networks, typically circuit-switched and designed for voice transfer. Most efforts in the design of radio LANs have followed the design principles that are used in wired LANs. However, the best wireless network design may not be obtained using the wired LAN design principles because the environments are different in the wired medium and the wireless medium. Moreover, multimedia communications require additional features due to the special data traffic requirements of data, voice and video. Also, the residential environment has unique requirements that can impact the design of the system.
Almost all 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 has low interference levels and stable propagation conditions. Consequently, the wired medium has potential for high to very high data transmission rates. Within the wired infrastructure, 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. To expand user access, time-division multiplexing (TDM) is used to allow different users to access the same channel at different times. The protocols for accessing wired media have been standardized by the IEEE in its 802 series of standards. 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 are used in a distributed sense such that the user occupying the channel reserves the medium by its present transmission or by its token. Using these protocols, every user can hear all data traffic that is transferred on the LAN. In a single LAN, all of the users share not only the channel, but also can access all of the information carried on that channel as well. As the number of users grows, the LAN can be divided into smaller LANs or segments that have independently operating channels. The individual LANs can be interconnected via bridges or routers that form interfaces between the different local networks. These configurations result in more complex networks (see, for example, D. Bertsekas and R. Callager, “Data Networks”, 2nd Edition, Prentice-Hall, London, 1992). In discussing residential LANs it is sufficient here to consider a single LAN.
Each 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 or not each packet transferred on the LAN is intended for him. 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 (as is expected in the wired 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 wired LANs is typically asynchronous and connectionless, it is ill suited to support delay critical services like voice. Voice services demand synchronous or isochronous connections, which require priority techniques in the Medium Access Control (MAC) protocols. The priority techniques give voice users priority over non-voice users. Different studies evaluating existing data networks have shown that providing voice and other time sensitive data over existing data networks is a difficult task.
During the last several years, standards bodies in the United States and in Europe have worked on wireless LANs (WLANs). However, the United States and Europe have adopted different standards. In the United States, the WLAN standard is the IEEE 802.11 standard (see, Draft Standard IEEE 802.11, P802. 11/D1, Dec. 1994), whereas in Europe, the ETSI HIPERLAN standard has been developed for WLANs (see, ETSI, RES10/96/etr, “Radio Equipment and Systems (RES); High Performance Radio Local Area Networks (HIPERLANs)”, July 1996).
The IEEE 802.11 standard, as the name indicates, is an extension of the 802 LAN standard. The wireless connection is either a radio link or an infrared link. The radio link can be established using the Industrial, Scientific, Medical (ISM) band at 2.4 GHz. The standard provides for only a 1-2 Mb/s channel for each single WLAN at any given time. This relatively narrow bandwidth channel has to be shared among all users of the radio network. The standard defines both a configuration based on a wired infrastructure and a configuration based on an ad-hoc structure. 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. In general, the IEEE 802.11 standard is very similar to that of the wired Ethernet, except the wire has been replaced by a 1 Mb/s radio channel.
The effective user throughput decreases quickly as the number of participants increases. In addition, little immunity is provided against interference in the ISM band because the spreading factor for Direct Sequence Spread Spectrum (DSSS) is only 11 and the spreading factor for Frequency Hopping Spread Spectrum (FHSS) is only on the order of 79 channels. Different networks can theoretically coexist in the same area thereby increasing the aggregate throughput. The different networks either use different DSSS carrier frequencies of which seven are defined, or use different FHSS hop sequences. However, the aggregate throughput, defined as the average throughput per user times the number of collocated users (not necessarily participating in the same network), can never exceed 4-6 Mb/s with either technology (see, A. Kamerman, “Spread-Spectrum Techniques Drive WLAN Performance,” Microwaves & RF, Sept. 1996, pp. 109-114).
For collocating different networks under the IEEE 802.11 standard, it is preferred that the networks be based on a wired infrastructure. If a limited number of collocated fixed access points can create their own network, then a certain amount of coordination via the wired network is then possible. However, it is much more difficult to create ad-hoc networks under IEEE 802.11, because the MAC protocol does not lend itself to an ad-hoc structure. Instead of forming ad-hoc networks, units that come in range of an existing network will join the existing network and not create their own network.
HIPERLAN has followed a simila
Bayard Emmanuel
Telefonaktiebolaget LM Ericsson (publ)
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