Multiplex communications – Communication over free space – Using trunking
Reexamination Certificate
2000-05-08
2004-02-03
Ton, Dang (Department: 2666)
Multiplex communications
Communication over free space
Using trunking
C370S433000, C370S437000, C375S132000, C455S434000, C455S513000
Reexamination Certificate
active
06687239
ABSTRACT:
BACKGROUND OF THE INVENTION
Devices incorporating wireless communications techniques are becoming increasingly prevalent in modem society. An inevitable result of this trend is that frequency spectrums are becoming more crowded and prone to interference. At the same time, consumers are becoming increasingly concerned about the privacy and security of communications, Consequently, systems engineers designing a variety of wireless communications systems, including cellular and cordless telephones, are increasingly turning to digital spread spectrum signaling methods to achieve better voice quality, greater security, and more efficient bandwidth utilization than can be achieved with conventional signaling methods, such as amplitude or frequency modulation without bandwidth spreading.
One popular spread spectrum signaling technique is frequency-hopping spread spectrum (“FHSS”). A FHSS transceiver operates by rapidly changing its tuned carrier frequency in a known pattem, called the hop sequence. By using different hop sequences, multiple users can communicate simultaneously over differing communications channels all within a common frequency bandwidth. FHSS offers better voice quality than other solutions in noisy environments because a short segment of voice data transmitted on a “bad” channel is simply muted. When the number of bad channels in the hop sequence is relatively low, the resultant degradation in voice quality is not noticeable to the user.
Another aspect of FHSS systems which is particularly advantageous is the ability to circumvent interference at a particular frequency by dynamically changing the channels in the hop sequence, substituting a new frequency channel for a detected/identified “bad” channel. Numerous methods of monitoring channel performance and determining when a channel should be removed from the hop sequence are known in the art. However, the specific implementation of typical FHSS protocols compromises the effectiveness of many prior art dynamic channel allocation techniques in extremely noisy environments.
For example, many FHSS systems transmit traffic information in predetermined data packet structures consisting mostly of primary traffic data, and sometimes containing some lesser amount of secondary control data. For example, in the context of a wireless telephone, a typical data packet transmitted/exchanged during the course of a conversation may be comprised primarily of voice data, with a small allocation for any control information that may be necessary, such as link control commands. Because of the minimal bandwidth allocated to control data, a complete command to substitute a poor frequency channel in the hop sequence must generally be broken up and transmitted in the control data field of multiple packets. As a result, channel substitution transmissions are much more sensitive to errors than any given primary traffic transmission, because an error in any one of the multiple packets in which the control command is transmitted will result in a failed substitution. Furthermore, transmission of a channel substitution command takes appreciably more time than transmission of a single traffic packet. While these factors are inconsequential when the communications link is clean, they can seriously degrade the efficacy of dynamic channel allocation techniques in the presence of significant interference. Indeed, as the packet error probability goes up in a system that divides a channel substitution command into five transmitted packets, the probability of a failed channel substitution increases at five times the rate of the probability of a packet error. Consequently, dynamic channel allocation performance breaks down in extremely noisy environments.
Although channels could be allocated more quickly and with less chance of error by transmitting the substitution command in a data packet comprised entirely of command data, such a dedicated data packet structure necessarily requires an undesirable interruption in the throughput of the voice data. Such an interruption results in the user's voice being temporarily “blanked” during transmission of the command data packet.
Another problem faced by many prior art dynamic channel allocation techniques is ensuring that a receiving party properly receives a channel substitution command sent by a transmitting party. Without confirmation that a channel substitution was received, the transmitting party may begin communicating on a channel that is not tuned by the receiver. Thus, all communications on that channel in the hop sequence would cease, and muting of the voice signal may result. Furthermore, because channel substitutions typically occur only when communicating in a noisy environment, the risk of a transmitted channel substitution command being received improperly is significant.
One approach to ensuring receipt of channel substitution commands is the introduction of handshakes between communicating parties to confirm that a channel substitution request was properly received before the request is carried out. However, such request-reply command pairs further increase the throughput required to enact each substitution. In particularly noisy systems, it may be desirable to exchange multiple handshake signals. As a result, either the rate at which noisy channels can be replaced is reduced, or the voice data throughput is further sacrificed; either effect results in increased muting of the voice channel.
It is therefore an object of this invention to provide a method of applying known dynamic channel allocation techniques in a way that the continuity of primary traffic is preserved under conditions of minor interference, and yet a communications link suffering heavy interference can be maintained and rapidly improved.
SUMMARY OF THE INVENTION
This invention consists of a method of dynamic channel allocation in a frequency hopping communications system. Specifically, this method provides improved efficiency and reliability of channel replacement over prior art techniques.
The invention involves a wireless communications link between two or more transceivers. While the invention will be explained in the context of a link between two wireless transceivers, the methods could be applied by one of ordinary skill in the art to systems involving any number of transceivers that are time or frequency division multiplexed on a common link.
In one embodiment of the invention, a first and second transceiver engage in a frequency hopping wireless communication link. During the course of the communications, a transceiver begins evaluating the performance of each channel in the hop sequence, starting at a fixed point in the sequence. The transceiver proceeds sequentially through the hop sequence, until a channel fails to satisfy a predetermined quality criterion. A channel which fails to satisfy the quality criterion is considered bad.
Upon locating a bad channel, the transceiver selects a replacement channel from a set of available channels. The bad channel is then replaced in the hop sequence by the replacement channel. dr
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a schematic block diagram of one embodiment of the invention, whereby a wireless telephone can dynamically optimize the communications link performance by allocating channels and data packet content.
FIG. 2
is a diagram of a VMUX packet structure.
FIG. 3
is a diagram of a DMUX packet structure.
FIG. 4
is a state diagram of one aspect of the invention providing a level of dynamic control over data packet content, according to one possible embodiment.
REFERENCES:
patent: 5103461 (1992-04-01), Tymes
patent: 5541954 (1996-07-01), Emi
patent: 5737359 (1998-04-01), Koivu
patent: 6115407 (2000-09-01), Gendel et al.
patent: 6252910 (2001-06-01), West et al.
patent: 2 261 141 (1993-05-01), None
patent: 0111197 (2001-12-01), None
Mehra Inder Pal
Shaw Pittman LLP
Ton Dang
VTech Telecommunications, LTD
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