Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
1999-10-15
2001-05-08
Nguyen, Lee (Department: 2683)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S450000, C455S509000, C370S337000, C370S347000, C370S362000, C375S132000, C375S135000, C375S136000, C375S138000
Reexamination Certificate
active
06230026
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to wireless communication networks (e.g. cellular and personal communication systems) and is particularly directed to an architecture used to support frequency hopping associated therewith.
BACKGROUND OF THE INVENTION
The basestations used by the providers of current day multiple channel wireless communication services, such as cellular mobile telephone (CMT) and personal communication systems (PCS), typically designate signal processing equipment for each single receiver channel. This is probably a result of the fact that each basestation is configured to provide communication capability for only a limited predetermined number of channels in the overall frequency spectrum that is available to the service provider.
A typical basestation may thus contain several racks of equipment which house multiple sets of receiver and transmitter signal processing components that service a prescribed subset of the available channels. For example, in an IS-136 TDMA cellular system, a typical basestation may service a pre-selected number of RF channels, such as 12, simultaneously supporting a total number of 36 mobile units, of the total number, such as 416, of the RF channels available to the service provider.
Wireless service providers would prefer, however, to employ equipment that would be more flexible, both in terms of where it can be located, as well as in the extent of the available bandwidth coverage provided by a particular transceiver site. This is particularly true where relatively large, secure, and protective structures for multiple racks of equipment are not necessarily available or cost effective. Additionally, service providers desire equipment that can accommodate subscriber growth with features making more efficient use of the available RF spectrum, such as for PCS applications.
One way to resolve this difficulty is to implement a basestation transceiver using a high speed analog-to-digital (A/D) converter and equipment which makes use of efficient digital filtering algorithms such as the Fast Fourier Transform (FFT) to separate the incoming signal energy into multiple baseband channels. On the transmit side, this implementation includes an inverse FFT processing combiner which outputs a combined signal representative of the contents of the baseband signal provided to it.
U.S. Pat. No. 5,940,384 assigned to the same assignee as the present invention and hereby incorporated by reference describes a method of flexibly allocating modulators and demodulators (in the form of digital signal processors or DSPs) to ones of these baseband channels as additional resources are needed, for example, during times of high message traffic. By making the basestation's implementation of call processing resources modular, the basestation can initially be configured to support a limited number of channels. Then, as the demand for services grows, additional channels can be supported by the addition of additional DSPs. The DSPs allow a change or expansion in the type of service, for example, into one of several air interface standards such as code division multiple access (CDMA) as well as time division multiple access (TDMA).
To ensure non-interfering coverage among dispersed basestations, each basestation uses only a subset of the available RF channels, so that mutual interference among any of the channels of the network is reduced. To further reduce interference, frequency hopping can be used. Frequency hopping can significantly reduce the average interference on a given RF channel compared to statically tuned channels. With reduced interference, higher frequency reuse is possible allowing more efficient use of the available RF spectrum, thus enabling higher capacity within the network.
SUMMARY OF THE INVENTION
The present invention provides a method of performing frequency hopping with a wideband transceiver configured in a modular manner, by periodically changing the mapping between DSPs and their associated channels to comply with a frequency hopping schedule. The ability to freely allocate DSPs among channels, used to provide flexibility in handling changing traffic loads and the like, as will be described, is thus harnessed to provide frequency hopping modifications.
Specifically, frequency hopping reduces co-channel interference for each call by switching the frequency on which a particular traffic channel is transmitted. Thus, frequency hopping reduces the probability of RF interference events between co-channels. This allows improved call quality with more overall system capacity via higher frequency reuse. To further clarify, frequency hopping varies the frequency used at predetermined intervals and uses different hop sequences that are non-correlated between basestations using a subset of the same frequencies. Accordingly, the present invention supports frequency hopping utilizing real-time switching RF carriers over a time division multiplexed (TDM) bus between RF transceiver resources and digital signal processing resources.
The RF transceiver resource converts received RF carriers to a baseband signal. Likewise, it converts signals to be transmitted from a baseband representation to the proper RF frequency. An implementation of this invention utilizes dual port (DP) random access memories (RAM) that are used to map a baseband signal to a logical digital signal processor (DSP) resource that produces that signal. A microprocessor possessing the frequency hop sequence of the baseband signal is used to update the DP memory as the RF carrier's frequency changes. The RF carrier is mapped from the RF frequency that the carrier is using in real-time to the same DSP resource for processing.
To support frequency hopping, as provided in GSM for example, the architecture of the present invention also allows determination of changes in the frequency and the ability to effect those changes in synchronization with a mobile unit. To effect these changes, the invention divides the DP-RAM into two sections. One section contains the current frequency set of all RF channels, and the other section contains the next frequency set in the hop sequence. This ping/pong structure of the DP-RAM allows a digital signal processor to swap the sections of the DP-RAM at predetermined intervals that are used to map RF channels to the TDM bus. The same DSP has access to the DP-RAM and fills the inactive section of the DP-RAM with the frequency information for the next frequency hop. Therefore, all swapping or ping/pongs of the DP-RAM occur at the frequency hop (FHOP) rate. Furthermore, the ping/pongs of the DP-RAM invoked by a DSP must be synchronized with other DSPs performing baseband processing of the RF channels as the FHOP rate is determined by the framing structure of the GSM TDMA RF (radio frequency) signal. One scheme used to achieve this synchronization is to align the GSM framing time of the DSP controlling the DP-RAM with baseband processing DSPs using a common TDMA frame signal generated by digital clocking logic and driven by an accurate timing source such as a global positioning system (GPS) receiver. All DSPs monitor this TDMA framing signal in the frequency hop mode, and via this architecture, frequency hopping is supported for the broadband basestation.
In accordance with the present invention then, the co-channel interference associated with signal processing architectures currently employed by multichannel wireless communication service providers is reduced by this new and improved broadband architecture and methodology that supports frequency hopping.
REFERENCES:
patent: 5430713 (1995-07-01), Gupta et al.
patent: 5592480 (1997-01-01), Carney et al.
patent: 5940384 (1999-08-01), Carney et al.
Schwaller John F.
Smith Jeffrey Wilson
Williams Terry L.
AirNet Communications Corporation
Nguyen Lee
Senterfitt Akerman
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