Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
2000-12-22
2004-05-04
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S296000
Reexamination Certificate
active
06731705
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of wireless communications systems, and in particular, to a method and system for interference mitigation in adaptive array systems.
BACKGROUND OF THE INVENTION
One advance in increasing the capacity of communication systems has been in the area of resource sharing or multiple access. Examples of multiple access techniques include code division multiple access (CDMA), frequency division multiple access (FDMA), and time division multiple access (TDMA). For example, in a TDMA system, each remote user terminals communicates with a hub communication device (e.g., a base station) in a frequency channel shared with other remote user terminals, but in its own (i.e., non-overlapping) time slot. As such, in a TDMA system, multiple remote user terminals may communicate with the hub communication device within the same frequency channel, but within non-overlapping time slots. (The term “channel” as used herein refers to any one or a combination of conventional communication channels, such as frequency, time, code channels).
Unfortunately, communications systems, especially those employing multiple access techniques, may suffer from inter-channel interference (inter-channel interference is also sometimes referred to as adjacent channel interference; however, the term inter-channel interference is used herein to emphasize that interference may occur between channels that are not necessarily adjacent, but may nonetheless affect each other). For example, in an FDMA cellular communication systems, when a base station transmits a downlink signal to a first receiver (which may be a cellular telephone handset or other remote user terminal) on a primary frequency channel, a second receiver that is tuned to receive in a non-primary frequency channel, which channel may be adjacent to or relatively near the frequency band of the primary frequency channel, may nonetheless experience inter-channel interference due to transmitter, receiver, and/or channel characteristics or limitations that cause energy from the primary downlink signal to be detected as interference on one or more non-primary channels. Similarly, in a TDMA system, receivers operating in adjacent time slots may experience inter-channel interference. Nonetheless, this is currently employed in some systems, such as GSM system.
Inter-channel (and/or co-channel) interference experienced by receivers, such as remote user terminals, that are not the intended recipient of the “primary” transmission of a base station or other communication device may be caused by one or a combination of factors attributed to the limitation(s) of the receiver(s), the characteristics of the channel and/or environment, and/or by generation of “ghost” signals by the transmitter (e.g., by the base station). For example, factors that are attributed to limitations of a receiver, such as a remote user terminal, and which factors may cause inter-channel interference to occur include, but are not limited to, relatively limited dynamic range in the receive path of the remote user terminal, phase noise in the remote user terminal's oscillator, relatively poor analog and/or digital filtering or channel selectivity of the remote user terminal. On the other hand, factors attributed to a transmitter, such as a base station, may also cause inter-channel (and/or co-channel interference) that may be experienced by one or more receivers. For instance, a transmitter may generate unwanted “ghost signals” to appear on “primary” or “non-primary” channels when the transmitter transmits a downlink signal on the primary channel.
Unfortunately, techniques for alleviating inter-channel interference by improving the remote user terminal's selectivity—i.e., its ability to discard unwanted signals in nearby frequency, time, and/or code channels—generally entail additional cost or power consumption. On the other hand, relatively limited selectivity of a remote user terminal's receiver may cause a number of undesirable effects in a communications system. In fact, if adjacent channels are occupied by signals of sufficient power, the resultant interference to the remote user's receiver may render the remote user terminal relatively unreliable or even inoperable.
One technique to reduce or eliminate inter-channel inteference is to leave unoccupied (i.e., unused) adjacent channels and/or other relatively nearby channels that may be susceptible to (or cause) inter-channel interference. For example, if a remote user terminal in communication with a base station is using a given channel, the base station may be programmed not to assign adjacent or other relatively nearby channels to other remote user terminals whose relatively limited channel selectivity may render such adjacent or nearby channels susceptible to inter-channel interference. However, by leaving some otherwise usable channels unused, this solution leads to a relatively significant loss in spectral efficiency. In systems where there may be a relatively large number of remote user terminals, such a loss in spectral efficiency may render this solution impractical.
Another prior technique for reducing inter-channel interference involves dynamic channel allocation. One example of dynamic channel allocation is employed in the Personal Handyphone System (PHS), a cellular network architecture currently implemented in a number of geographical areas, including, for example, in portions of Japan. PHS remote user terminals (also known as PHS handsets) are capable of transmitting control messages to a PHS base station. When a PHS handset detects a deteriorated signal quality (e.g., due to inter-channel interference), the PHS handset informs the PHS base station, via a control message, that a new channel is needed, and such new channel may be allocated by the PHS base station to the PHS handset during a communication session (e.g., during a voice or data “call”).
However, before a PHS handset accepts a newly assigned channel, the handset measures the interference on the newly assigned channel to determine whether it is significant relative to a threshold. When the PHS handset performs the measurement of interference on the newly assigned channel, the handset uses the same receiving apparatus that is used during normal traffic of voice or data exchange with the PHS base station. As such, even during the measurement phase for a newly assigned channel, the PHS handset may experience interference from signals on adjacent or nearby channels. If the level of such interference is too high, for example, as compared with a threshold, the PHS handset may again request a new channel from the base station.
Eventually, if network load—namely, the number of users (e.g., PHS handsets) or other signal sources or receivers—does not exceed a threshold, the PHS handsets and base stations in the PHS network may find a pattern of time slots and frequencies that facilitate communication with a tolerable amount of inter-channel interference. If, on the other hand, no suitable channel can be found by a PHS handset in a number of attempts or within a predefined time-period, a call may be dropped—i.e., communication may involuntarily be terminated between the PHS handset and the base station. Furthermore, even if communication is not terminated, voice quality or data integrity is typically significantly reduced when a PHS handset switches between channels.
Adaptive arrays (also known as “smart antennas”), which employ antenna arrays along with signal processing hardware and/or software, also have been utilized to decrease interference and improve performance in wireless communications. Antenna arrays typically include a number of antennas that are spatially separated and coupled to one or more digital signal processors and/or general purpose processors. Adaptive antenna arrays, or simply, adaptive arrays, periodically analyze the signals received from each of the antennas in an array to distinguish between desired signals (e.g., from a desired remote user terminal, such as cellular telephone
Kasapi Athanasios A.
Trott Mitchell D.
ArrayComm, Inc.
Blakely & Sokoloff, Taylor & Zafman
Chin Stephen
Kim Kevin
LandOfFree
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