WO2000072466A1

WO2000072466A1 - Wide band noise reduction system

Info

Publication number: WO2000072466A1
Application number: PCT/US2000/013887
Authority: WO
Inventors: Joseph Silvia; Stephen J. Graveline; Glenn T. Flebotte
Original assignee: Transcept, Inc.
Priority date: 1999-05-20
Filing date: 2000-05-19
Publication date: 2000-11-30
Also published as: AU5148200A

Abstract

In a wireless microcell distribution system, low loss saw filters within a cable microcell integrator eliminate wide band thermal and man-made noise introduced by reverse path signaling from the cable microcell integrator, which noise deleteriously affects CATV signals. The utilization of the low loss saw filters within the cable microcell integrator completely eliminates the need for both cascaded amplifiers that increase wide band noise and the fixed frequency external filter which was previously required. Additionally, with the elimination of the fixed frequency external filter at each cable microcell integrator, the system can be tuned to different channels or frequencies by merely tuning the local oscillators in the cable microcell integrator, thereby to facilitate rapid reverse path channel allocation.

Description

TITLE OF INVENTION WIDE BAND NOISE REDUCTION SYSTEM

FIELD OF INVENTION This invention relates to wireless microcell distributor systems, more particularly to a system for reducing wide band noise.

BACKGROUND OF INVENTION In typical wireless microcell distributor systems, a so-called cable microcell integrator or module is connected to a cable for communicating between a base station and wireless handsets. In the past, these cable microcell integrators have been a dominant noise source when connected to the cable. The result of the connection of a cable microcell integrator to the cable has been so severe in some instances that it has raised the noise floor to such an extent that the signal-to-noise ratios required for sufficient video quality cannot be maintained. In an effort to eliminate the noise component due to that associated with cable microcell integrators, in the past, a fixed frequency duplexing band pass filter was interposed between the cable microcell integrator and the splitter coupled to the cable. It was the purpose of this filter to provide a 12 MHz band pass for transmitting the reverse control and telephony signals while reducing wide band noise components in the forward communication channel to a level which does not interfere with the video, color or audio signals.

The problem with the interposition of such a filter is not only the expense, installation and weight of the unit, but also its insertion between the cable microcell integrator and the splitter results in the requirement for more power, additional cabling and additional splicing. More importantly, however, the external band pass filters are frequency specific, such that it is only with the replacement of these filters with other fixed frequency filters at each microcell that the cable plant could be reconfigured for different frequency allocations. Typically for the reverse path, in certain locales the band used for reverse path signaling is set between 5 and 17 MHz as a default. However, if the pay-per-view channel is set at 10 MHz, the frequency allocation for the reverse path signaling must be changed to another band, for instance between 20 to 32 MHz.

Generally, it is a matter of local preference as to what bands are allocated for reverse path signaling, with the considerations not only being where to place the pay-per-view channels, but also the cable modem channels and test signals. Thus, in the past, with a fixed frequency external filter it was necessary to change-out the filter at a cable microcell integrator based on the particular channel allocations. This as will be appreciated is an expensive process and one which precludes ready tunability of the cable microcell integrator to accommodate such channel allocations. Additionally, in the past, the cable microcell integrator was provided with saw filters, the purpose of which was to reject noise and interfering signals so as to be able to clean up the telephony signals transmitted back to the base station. The purpose of the aforementioned saw filters was thus to isolate the particular carrier by rejecting interfering signals.

The problem with the saw filters initially utilized in the system was that they were typically high loss filters on the order of 20-25 dB. The high loss of these filters necessitated amplification, which when cascaded raised the wide band noise floor, thus significantly degrading the CATV signals.

By way of further background, in a wireless microcell distributor system, a base station is connected at one end to the telephone network switch and at the other end to the cable on which the cable microcell integrators are hung. Each of the cable microcell integrators is provided with transmit and receive antennas and transceivers which permit communication with the handsets. As such, each of the cable microcell integrators constitutes a microcell with the microcells being interconnected by the cable. This type of system eliminates costly towers and siting by providing a number of microcells spaced out along the same cable that already carries CATV signals. Thus, in the past, existing cable television networks have been adapted to carry telephony signals between a telephone network and a remote transceiver site, the coverage of which defines cells or sectors.

In these systems, the base transceiver acts as the interface between the telephone network and the wireless telephones. To carry the wireless telephony signals over a broad band distribution network, a predetermined bandwidth on the network is typically allocated for this purpose. To most efficiently use a given bandwidth to carry wireless telephony signals between wireless telephones and the telephone network, code, frequency or time division multiplexing is utilized to support code division multiple access, time division multiple access, and frequency division multiple access. This requires appropriate base station equipment that acts as the interface with the telephone network and the wireless telephone system.

The base station is typically coupled to a head end interface converter, the purpose of which in the forward direction is to provide the required number of telephony signals and both control and reference signals to a coupler which couples these signals to a cable-to-fiber transducer. The cable-to-fiber transducer injects the requisite signals into the cable, with a splitter being provided at a point on the cable to split out CATV signals and the telephony signals. The splitter provides the telephony signals as well as the CATV signals to a microcell hung on the cable, with the microcell having three antennas, one a transmitting antenna and the others a primary and diversity antenna which serve as the receiving antennas. It will be appreciated that the communication between the head end interface converter and each of the microcells is bi-directional. In the forward direction, control over power and gain of each of the microcells is provided through the forward signal to each of the modules. Likewise, channel assignment for the telephony signals and the CATV signals is provided by forward control signals. Moreover, fault enable and disable control signals are provided in the forward direction to each of the modules. Additionally, transmit enable and disable signals are also provided to the microcells, along with a status monitoring and fault reporting signal. Moreover, software updates for each of the microcell can be provided by the forward control signal. In the reverse direction, signals from the module are transmitted to the head end interface converter which include responses to all of the queries contained in the forward control signals. These reverse control signals are provided back to the head end interface converter at frequencies in the reverse CATV band. It is the reduction of the wide band noise generated by the cable microcell integrator as it provides these reverse path signals to which the subject invention is directed.

SUMMARY OF THE INVENTION In order to reduce the wide band noise caused by the cascaded amplification required by the prior high loss saw filters in the cable microcell integrators, and in order to eliminate the external band pass filters themselves, in the subject invention carrier isolation is performed by low loss saw filters, with the loss associated with these filters being between 5 to 10 dB in one embodiment. The low loss characteristic of these filters eliminates the necessity for amplification and therefore the aforementioned cascading effect.

Moreover, since external filters are eliminated, the utilization of the low loss saw filters in the cable microcell integrator permits the system to be tunable through control of the local oscillators within the cable microcell integrator. Thus, the subject system permits setting the reverse path channel allocations at will without having to physically go to each of the cable microcell integrators and provide a different filter. The use of the low loss saw filters reduces the noise floor in one embodiment by 12 dB, an amount which virtually eliminates the cable microcell integrator as a dominant source of noise in the system. The reason that the wide band noise is reduced in this manner is that the amplification required by the prior high loss saw filters is eliminated. It will be appreciated that by the use of the low loss saw filters the cascade is altered which results in an improvement in the noise figure. This in and of itself reduces wide band noise.

In one embodiment, for the primary and diversity paths, series-connected saw filters are interposed between the two down conversion stages in the cable microcell integrator used between the associated antennas and the cable plant. In one embodiment, series-connected saw filters are utilized for improved rejection, with each saw filter providing out-of-band rejection of signals to the J-STD-019 for CDMA service. The two cascaded saw filters are used in order to knock down an adjacent higher power incoming signal to a point in which the desired signal is not desensed causing a dropped call. Note that in one embodiment the two saw filters have identical transfer characteristics. In summary, in a wireless microcell distribution system, low loss saw filters within a cable microcell integrator eliminate wide band thermal and man-made noise introduced by reverse path signaling from the cable microcell integrator, which noise deleteriously affects CATV signals. The utilization of the low loss saw filters within the cable microcell integrator completely eliminates the need for both cascaded amplifiers that increase wide band noise and the fixed frequency external filter which was previously required.

Additionally, with the elimination of the fixed frequency external filter at each cable microcell integrator, the system can be tuned to different channels or frequencies by merely tuning the local oscillators in the cable microcell integrator, thereby to facilitate rapid reverse path channel allocation.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the Subject Invention will be better understood taken in connection with the Detailed Description in conjunction with the Drawings of which:

Figure 1 is a block and schematic diagram of a wireless microcell distributor system, illustrating the utilization of an external fixed frequency duplexing filter to eliminate noise generated by the high loss saw filters in the associated cable microcell integrator;

Figure 2 is a graph showing the relationship between the video, color and audio signals to the noise floor, also indicating the increased noise floor due to the utilization of high loss saw filters in the cable microcell integrator and the reduced cable microcell integrator noise component due to the utilization of the fixed frequency duplexing filter of Figure 1;

Figure 3 is a diagrammatic representation of the duplexing filter between the CATV plant and a cable microcell integrator; Figure 4 is a schematic and block diagram of the external filter of Figure 3, illustrating a forward path and a reverse path, each requiring a differently configured band pass filter; and

Figure 5 is a block diagram of the subject cable microcell integrator, illustrating the utilization of cascaded low noise saw filters between the two down conversion sections in the cable microcell integrator.

DETAILED DESCRIPTION Referring now to Figure 1, in a typical Cable TV plant a base station 10 is connected to a head end interface converter 12 which is in turn connected to a coupler 14 at which the CATV signal 16 may be injected. The coupler is connected to a cable-to-fiber transceiver 18 which is coupled to a cable 20, which is in turn coupled to a filter-to-cable transceiver 24. This fiber-to-cable transceiver is coupled to a splitter 26 in turn coupled through an external filter 28 to a cable microcell integrator 30. On the transmit side the cable microcell integrator has a transmit antenna 32. On the receive side the cable microcell integrator has a primary antenna 34 and a diversity antenna 36, with the antennas providing for the transmission and receipt of signals to and from a wireless transceiver 38.

The signals of concern in the subject application are those along the reverse paths which are used to transmit information from the cable microcell integrator back to the head end interface converter. As illustrated by waveform 40, the reverse path signals include the primary and diversity signals 40 and 42, respectively, along with the reverse control signals as illustrated at 44. These signals lay in a band, referred to herein as the reverse channel band, typically between 5 MHz and 52 MHz.

As mentioned hereinbefore, the dominant source of wide band noise in the cable plant was due to the high loss saw filters used in the cable microcell integrators. The reason, as mentioned before, is that the high loss saw filters utilized in the cable microcell integrators required cascaded amplification which raised the noise level as illustrated by a waveform 50 from the position noted by dotted line 52 to the position noted at 54.

As illustrated in Figure 2, for video, color and audio components of the CATV signal, respectively, 56, 58 and 60, when the cable microcell integrators were coupled to the cable plant, the required 50 dB signal-to-noise ratio between the peak of the video signal and the noise floor could not be maintained. The result was that noise would creep into the video signal, thereby degrading it. As illustrated in Figure 2, the peak of the video signal in some cases was no more than 38 dB above noise floor 54, which was the result of connecting the cable microcell integrator to the cable.

It was to lower noise floor 54 that filter 28 was devised. Referring now to Figure 3, duplex filter 28 was interposed between the CATV cable and cable microcell integrator 30 so as to be able to filter out noise in the reverse path, here illustrated at 62. It is important, however, that the forward path illustrated at 64 not be affected by the filtering system utilized to filter out wide band noise in the reverse path.

For this purpose and referring now to Figure 4, forward control and reference signals passed through a suitable band pass filter 68 in the direction illustrated by arrow 70. Reverse path signals were passed through a specially configured band pass filter 72 for transmission back to the head end interface converter. Band pass filter 72 was given a pass band characteristic such as illustrated by waveform 74 in Figure 1 in which the filters in one embodiment had a 12 MHz pass band. The filter in one embodiment passed frequencies from 5 to 17 MHz.

It will be appreciated that filter 72 is a fixed frequency filter and had to be preset to the desired reverse channel. As mentioned hereinbefore, the channels range from 5 to 52 MHz, and it was only with difficulty that each of the filters at each of the cable microcell integrators could be switched out for different applications.

Referring now to Figure 5, in the subject invention the external filter for each cable microcell integrator is eliminated due to the use of low loss saw filters interposed in the primary and diversity paths as illustrated by dotted boxes 80 and 82 between down convert sections 84 and 86 in each of these paths.

As can be seen, signals received at primary receive antenna 34 and diversity receive antenna 36 are coupled through respective band pass filters 86 and 88 to respective mixers 90 and 92 in each of the paths.

Mixers 90 and 92 are provided with the output from a local oscillator 94, the frequency of which is controlled by a frequency control unit 96, with the output of mixers 90 and 92 being applied to band pass saw filters 80 and 82. The output of each of these banks of saw filters is provided to mixers 98 and 100, with these mixers provided with the output of a local oscillator 102 and 106, the frequencies of which are controlled by a frequency control unit 104. The output of mixers 98 and 100 are provided to a summation circuit 110, the output of which is provided through a further band pass filter 112 to the cable plant.

The types of saw filters which are cascaded in the stacked arrangement shown within dotted boxes 80 and 82 are low loss saw filters 120 which have identical pass band characteristics and which introduce no more than a 5 to 10 dB loss due to their insertion. Such filters are commercially available.

The result of so doing is to be able to provide sufficiently isolated telephony carriers while at the same time not introducing significant losses, thereby eliminating the need for cascaded amplification. With the elimination of the cascaded amplification, wide band noise is greatly reduced. Moreover, the costly heavy external filters are eliminated from the system.

Not only are the costly external filters completely eliminated from the subject system, tunability is brought back to the cable microcell integrators such that the channels for reverse channel signals can be freely set through frequency control units 96 and 104. The overall net affect of the utilization of the low loss saw filters is the lowering of the noise floor, providing tunability for the system and elimination of installation expense and weight of the external filters.

Having now described a few embodiments of the invention, and some modifications and variations thereto, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by the way of example only.

Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention as limited only by the appended claims and equivalents thereto.

Claims

WHAT IS CLAIMED IS:

1. In a wireless microcell distributor system, a method for reducing wide band thermal and man-made noise introduced by reverse path signaling from a microcell, comprising the steps of; providing a cable microcell integrator having primary and diversity antennas having outputs coupled through at least one stage of down conversion; and, interposing low noise saw filters after the at least one stage of down conversion, whereof reverse path noise is reduced through the use of said low loss saw filters due to the elimination of amplification occasioned by the use of high loss saw filters while at the same time eliminating the necessity for external fixed frequency filters at each microcell and the resulting limitation on changing channel assignments occasioned by the fixed frequency filters.

2. The method of Claim 1 wherein the loss associated with the saw filters is less than 10 dB.

3. A method of providing tunability to microcells in a wireless microcell distribution system while at the same time reducing wide band thermal and man-made noise, comprising; providing each microcell with low noise saw filters in the primary and diversity paths thereof.

4. A method for eliminating external filters at each microcell in a wireless microcell distribution system while at the same time reducing wide band thermal and man-made noise, comprising; providing each microcell with low loss saw filters in the primary and diversity paths thereof.

5. A cable microcell integrator for use in a wireless microcell distribution system, comprising; primary and diversity antennas to establish corresponding primary ad diversity paths; a pair of first down conversion stages coupled to respective primary and diversity antennas; at least one low loss saw filter coupled to the output of a down conversion stage; a pair of second down conversion stages coupled respectively to the outputs of the saw filters in said primary and secondary paths; and, means for coupling the outputs of said second down conversion stages to said distribution system.

6. The cable microcell integrator of Claim 5 wherein each of said saw filters has a loss characteristic of less than 10 dB.