Telecommunications – Transmitter and receiver at separate stations – Having diverse art device
Reexamination Certificate
2000-01-08
2004-07-20
Banks-Harold, Marsha D. (Department: 2686)
Telecommunications
Transmitter and receiver at separate stations
Having diverse art device
C455S008000, C455S009000, C361S035000, C361S066000, C700S292000, C343S784000
Reexamination Certificate
active
06766143
ABSTRACT:
BACKGROUND OF THE INVENTION
Prior Art: Wireless Chip Sets
Note that the term “wireless” is generally used hereinbelow in place of the term “radio” as being more descriptive of the related technology.
Use of wireless frequencies and communications methods which do not require individual device licensing is preferred. The following is taken from Reference 1 hereinbelow; Intersil Application Note AN9804.1 by Jim Zyren and Al Petrick, dated June 1998:
Spread Spectrum Radios
The term “spread spectrum” simply means that the energy radiated by the transmitter is spread out over a wider amount of the RF spectrum than would otherwise be used. By spreading out the energy, it is far less likely that two users sharing the same spectrum will interfere with each other. This is an important consideration in an unlicensed band, which is why the regulatory authorities imposed spread spectrum requirements on radios which transmit over −1 dBm (about 0.75 mW).
The frequency allocation by countries is found in Reference 2 hereinbelow; Intersil Application Note AN9829 by Jim Zyren and Al Petrick:
U.S.
2.4000-2.4835 GHz
Europe
2.4000-2.4835 GHz
Japan
2.4710-2.4970 GHz
France
2.4465-2.4835 GHz
Spain
2.4450-2.4750 GHz
Again from Reference 1:
In the U.S., license-free bands are collectively designated as Industry, Science, and Medicine (ISM) bands. Operation in these bands with approved devices does not require an FCC license. By waving licensing requirements, these bands have been made generally accessible to virtually everyone. This is mainly why ISM bands are so important for commercial and consumer applications.
As mentioned above, radios employing spread spectrum methods are allowed to radiate up to 1.0W (30 dBm) of RF energy, as compared to less than 1 mW for non-spread radios. There are two common types of spread spectrum systems. The easiest to understand is Frequency Hopped Spread Spectrum (FHSS). In this method, the carrier frequency hops from channel to channel in some pre-arranged sequence. The receiver is programmed to hop in sequence with the transmitter. If one channel is jammed, the data is simply retransmitted when the transmitter hops to a clear channel. The major drawback to FHSS is limited data rate. In the 2.4 GHz band, FCC regulations require that the maximum occupied bandwidth for any single channel is 1 MHz. This effectively limits the data rate through this type of system to about 1 Mbps.
By contrast, Direct-Sequence-Spread-Spectrum (DSSS) systems in the ISM bands provide much higher data rates. DSSS systems do not jump from frequency to frequency. Instead, the transmitter actually spreads the energy out over a wide portion of the RF spectrum. This is accomplished by combining the data stream with a much higher rate Pseudo Random Numerical (PRN) sequence via an exclusive “or” (XOR) function. The result is a digital stream at the same rate as the PRN. When the RF carrier is modulated by the higher speed digital stream, the result is a spreading of the RF energy . . . . At the receiver, the pseudo random code is used to “de-spread” the received data . . . . It is during this process that the matched filter rejects unwanted interference because it is uncorrelated with the PRN. By careful selection of the PRN sequence, the matched filter provides an additional benefit. It can reject multipath signals which are delayed relative to the main signal . . . by 44 ns (or more).
At this time of writing Intersil has introduced their PRISM II-11 Mbps (Mega bits per second) chip set using DSSS technology supporting IEEE Standard 802.11 2.4 GHz Wireless Local Area Networks (WLANs). Moreover this chip set also supports peer-to-peer ad hoc networks.
The following partial specifications are published by the Intersil website:
Variable data rates: 11, 5.5, 2, 1 Mbps
Frequency band: 2.4 GHz ISM Band
Dual modes: 1—WLAN, and
2—Independent Basic Service Sets (IBSSs) consisting of two or more stations (IEDs) which have recognized each other and have established communications. Within an IBSS, stations communicate directly with each other on a peer-to-peer level. Selectively, groups of IBSSs then are combined through a distribution system to an Access Port (AP) to communicate with wired LANs.
Again the following is selectively quoted from Intersil tutorial AN9829 dated February 1999 by Jim Zyren and Al Petrick, reference 2 below, on IEEE standard 802.11 which the PRISM II chip set supports:
DSSS is the same technology used in GPS satellite navigation systems and in CDMA (Code Division Multiple Access) cellular telephones . . . the data stream is combined via an XOR function with a high-speed PRN sequence using an 11 chip Barker Code. The term “chip” (note: not to be confused with a chip set of components) is used instead of “bit” to denote the fact that the Barker Code does not carry any binary information by and of itself. The result is an 11 Mbps digital stream which is then modulated onto a carrier frequency using Differential Binary Phase Shift Keying (DBPSK).
WLAN radios are half duplex and cannot receive while transmitting. Therefore a collision cannot be detected by a radio while transmission is in progress. The basic access method for 802.11 is the Distributed Coordination Function (DCF) which uses Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). Stations (STAs/IEDs) sense the medium to determine if it is idle. If not each STA waits until transmission stops, and then enters into a random back-off procedure. This reduces the probability of data clashes. Packet reception in DCF requires acknowledgement (ACK) . . . The period between completion of packet transmission and start of the ACK frame is one Short Inter Frame Space (SIFS).
Fast acknowledgement is one of the salient features of the 801.11 standard, because it requires ACKs to be handled at the Multiple Access Communications (MAC) sub-layer. Transmissions other than ACKs must wait at least a DCF inter-frame space (DIFS) before transmitting data . . . STAs wishing to transmit must wait an integer number of Slot Times depending on an internal timer setting (0 to 7 on first attempt). Upon expiration of a DIFS, the timer begins to decrement and on reaching zero the STA begins transmission. If a collision is detected the window is increased to 15 Slot Times and doubled after each unsuccessful attempt up to a maximum value of 256 Slot Times.
This method relies on the ability of each STA to sense all others . . . . If not the probability of collision is greatly increased. This is known as the Hidden Node. In order to combat this problem, a second carrier sense mechanism, Virtual Carrier Sense is described in the standard. Virtual Carrier Sense is implemented by reserving the medium for a specified period of time for an impending transmission.
The standard, supported by PRISM II, provides for security by two methods: authentication and encryption. It further supports synchronization of STA clocks by periodic transmissions of beacons containing time stamp information. It further supports power management by defining awake and doze modes of operation.
In addition . . . a waveform supporting 20 to 30 Mbps is under development. A task force is drafting a standard based on Orthogonal Frequency Division Multiplexing for implementation of these higher data rates.
A SXT810 chip by Level One, as given by References 4 and 5 hereinbelow, appears to have the advantage of being a single chip however, being designed for use in cordless telephones, it has the disadvantage of a limitation to approximately a 40 KHz data rate and non-compliance with IEEE standard 810.11.
The chip sets described hereinabove are typical of those at the time of writing. Additional chip sets and single chip devices are expected to become available. It is to be understood that the invention contained herein is not limited to the chip sets described herein.
Prior Art: The Electric Power Industry
Electric power distribution substations supply approximately 80% of the power in the U. S. A. directly over distribution lines and cables feeding many homes and businesses. The re
Aubel Led J.
Banks-Harold Marsha D.
Perez-Gutierrez Rafael
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