Patent Publication Number: US-10326630-B2

Title: Extracting sub-bands from signals in a frequency domain

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/255,592 filed Sep. 2, 2016 and entitled “EXTRACTING SUB-BANDS FROM SIGNALS IN A FREQUENCY DOMAIN” (currently pending), which is a continuation application of U.S. patent application Ser. No. 14/255,739 filed Apr. 17, 2014 and entitled “EXTRACTING SUB-BANDS FROM SIGNALS IN A FREQUENCY DOMAIN” (issued as U.S. Pat. No. 9,438,318 on Sep. 6, 2016), which claims the benefit of U.S. Provisional Application Ser. No. 61/812,820 filed Apr. 17, 2013 and titled “Dividing Signals into Sub-Bands via Frequency Transforms,” both of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to telecommunications systems and more particularly (although not necessarily exclusively) to extracting sub-bands of interest from signals in a frequency domain. 
     BACKGROUND 
     A distributed antenna system (“DAS”) can provide a signal transport network for communicating signals between one or more base stations and mobile communication devices or other terminal devices. The DAS may include master units and remote antenna units. The master units may be connected to the base stations. The master units receive downlink signals from the base stations and distribute downlink signals in analog or digital format to multiple remote antenna units. The remote antenna units transmit downlink signals to mobile communication devices or other terminal devices within coverage areas serviced by the remote antenna units. In the uplink direction, the remote antenna units receive uplink signals from terminal devices in the serviced coverage areas. The remote antenna units may combine uplink signals and transmit the combined uplink signals to master units. The master units may transmit uplink signals to the serving base stations. 
     A signal transport network provided by a DAS may be implemented using analog systems or digital systems. A digital system can include one or more devices for digitizing analog downlink signals received from a base station. A digital representation of the analog waveform is used to transmit the downlink signal via the DAS. 
     A master unit may route an entire downlink signal to remote antenna units of the DAS. Routing an entire downlink signal to remote antenna units can involve unnecessarily routing frequency bands in which no voice data or other data is transmitted. 
     It is desirable to distinguish sub-bands of signals communicated via a DAS that include voice or other data from sub-bands in which no voice data or other data is transmitted. 
     SUMMARY 
     Certain aspects and features of the present invention are directed to distributed antenna systems that can extract sub-bands of interest from signals in a frequency domain. 
     In one aspect, a method for extracting sub-bands of interest from signals in a frequency domain for transmission via a distributed antenna system is provided. The method involves generating a transformed downlink signal by performing a frequency transform on a downlink signal. The transformed downlink signal represents the downlink signal in a frequency domain. The method also involves determining that at least one sub-band of the transformed downlink signal includes data to be transmitted via the distributed antenna system. The method involves extracting the sub-band from the transformed downlink signal for transmission via the distributed antenna system. 
     In another aspect, a unit for extracting sub-bands of interest from signals in a frequency domain for transmission via a distributed antenna system is provided. The unit can include a first interface section, a processor, and a second interface section. The first interface section can receive a downlink signal from a base station. The processor can generate a transformed downlink signal by performing a frequency transform on a downlink signal. The transformed downlink signal represents the downlink signal in a frequency domain. The processor can also determine that at least one sub-band of the transformed downlink signal includes data to be transmitted via the distributed antenna system. The processor can also extract the sub-band from the transformed downlink signal. The second interface section can provide the extracted sub-band to at least one remote antenna unit of the distributed antenna system. 
     In another aspect, a distributed antenna system is provided. The distributed antenna system includes a unit and at least one remote antenna unit. The unit can receive a downlink signal and generate a transformed downlink signal by performing a frequency transform on the downlink signal. The transformed downlink signal represents the downlink signal in a frequency domain. The unit can also determine that at least one sub-band of the transformed downlink signal includes data to be transmitted via the distributed antenna system. The unit can also extract the at least one sub-band from the transformed downlink signal. The unit can also provide the extracted sub-band to the remote antenna unit. The remote antenna unit can generate a wireless RF signal based on the extracted sub-band and transmit the wireless RF signal to a terminal device. 
     These illustrative aspects and features are mentioned not to limit or define the disclosure, but to provide examples to aid understanding of the concepts disclosed in this application. Other aspects, advantages, and features of the present disclosure will become apparent after review of the entire application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram depicting an example of a distributed antenna system in which sub-bands of interest can be extracted from signals in a frequency domain according to one aspect of the present disclosure. 
         FIG. 2  is a flow chart depicting an example of a process for extracting sub-bands of interest from a downlink signal in the frequency domain according to one aspect of the present disclosure. 
         FIG. 3  is a block diagram depicting an example of a unit of a distributed antenna system that can sub-divide downlink signals into sub-bands in the frequency domain according to one aspect of the present disclosure. 
         FIG. 4  is a block diagram depicting an alternative example of a unit of a distributed antenna system that can sub-divide downlink signals into sub-bands in the frequency domain according to one aspect of the present disclosure. 
         FIG. 5  is a block diagram depicting an example of a remote antenna unit of a distributed antenna system that can sub-divide uplink signals into sub-bands in the frequency domain. 
     
    
    
     DETAILED DESCRIPTION 
     Systems and methods are disclosed for dividing signals communicated via a distributed antenna system (“DAS”) into equally spaced sub-bands using frequency transforms of the signals. In some aspects, a master unit or other unit of a DAS can transform a digital downlink signal from the time domain to the frequency domain. The unit of the DAS can identify multiple, equally sized sub-bands from the digital downlink signal in the frequency domain. Dividing signals communicated via the DAS into equally spaced sub-bands can thus reduce the amount of bandwidth used to route the signals to different units of the DAS. For example, data for frequency domain representations of individual sub-bands of interest can be processed and routed to remote antenna units in the DAS. Data for frequency domain representations of other sub-bands can be discarded or otherwise omitted from routing. Processing and routing individual sub-bands of interest can obviate the need to route the entire downlink signal via the DAS. 
     In some aspects, a unit of the DAS can include an input section, a processor, and an output section. The unit can receive downlink signals via the input section and output downlink signals via the output section to a remote antenna unit or other unit in the DAS. The processor of the unit can generate a transformed downlink signal by performing a frequency transform on the downlink signal or by configuring one or more signal processing devices to perform the frequency transform on the downlink signal. Performing a frequency transform on the downlink signal can include transforming the downlink signal from a time domain to a frequency domain. Non-limiting examples of a frequency transform can include a fast Fourier transform (“FFT”), a discrete Fourier transform, and a discrete cosine transform. The processor can determine that at least one sub-band of the transformed downlink signal includes voice or other data to be transmitted via the DAS. The processor can extract, identify, or otherwise selected the sub-bands from the transformed downlink signal that include the voice or other data to be transmitted. The output section can route the sub-bands having the data to one or more remote antenna units or other units of the DAS. 
     In a non-limiting example, a master unit or other unit of a DAS can receive signals from base stations and convert the downlink signals into digital downlink signals. The master unit can decompose or otherwise sub-divide the digital downlink signals into multiple, equally sized sub-bands. The master unit can extract or otherwise select sub-bands of interest from the equally sized sub-bands. Sub-bands of interest can include sub-bands in which voice data or other data is included. The master unit can provide sub-bands of interest to one or more remote antenna units. A remote antenna unit receiving the sub-bands of interest can convert the sub-band into a composite signal for transmission to mobile communication devices in a coverage area serviced by the remote antenna unit. In some aspects, a master unit can route different sub-bands extracted from a transformed downlink signal to different sets of remote antenna units. 
     Dividing signals communicated via a DAS into sub-bands can allow for separation of individual sub-bands of interest from composite signals communicated via the DAS. For example, in a time domain, a composite signal can be filtered to extract individual channels of interest, remove frequencies other than the channels of interest, or both. Transforming signals into a frequency domain representation of the signals can allow for removing the individual frequency components or otherwise manipulating the signals in a more computationally efficient manner as compared to filtering signals in the time domain. Also, each sub-band of a signal that has been divided can include digital signals sampled at the same sampling rate. Dividing signals that have been transformed into the frequency domain can provide greater computationally efficiency than extracting channels of interest using the signals in the time domain. 
     In some aspects, the signal power for one or more sub-bands or groups of sub-bands extracted from a transformed downlink signal can be modified to increase or decrease the level of the signal in that sub-band. For example, the signal level of each sub-band or a group of sub-bands can be compared to a threshold value. The signal level of a sub-band can be scaled up or down based on whether the signal level is above or below the threshold. 
     Detailed descriptions of certain examples are discussed below. These illustrative examples are given to introduce the reader to the general subject matter discussed here and are not intended to limit the scope of the disclosed concepts. The following sections describe various additional aspects and examples with reference to the drawings in which like numerals indicate like elements, and directional descriptions are used to describe the illustrative examples but, like the illustrative examples, should not be used to limit the present disclosure. 
       FIG. 1  is a block diagram depicting an example of a DAS  100  in which sub-bands of interest can be extracted from signals in a frequency domain. The DAS  100  can communicate signals between one or more base stations  101   a ,  101   b  and mobile communication devices via master units and remote antenna units of the DAS  100 . The DAS  100  can include a unit  102  and multiple remote antenna units  112   a - c.    
     The unit can be a master unit or other suitable unit that can communicate with one or more base stations  101   a ,  101   b . The unit  102  can receive downlink signals from the base stations  101   a ,  101   b  and transmit uplink signals to the base stations  101   a ,  101   b . Any suitable communication link can be used for communication between the base stations  101   a ,  101   b  and a unit  102 , such as (but not limited to) a wired connection or a wireless connection. A wired connection can include, for example, a connection via a copper cable, an optical fiber, or another suitable communication medium. A wireless connection can include, for example, a wireless RF communication link. The unit  102  can combine downlink signals received from base stations  101   a ,  101   b . The unit  102  can transmit the combined downlink signals to one or more of the remote antenna units  112   a - c.    
     The remote antenna units can provide signal coverage in respective coverage zones  114   a - c . Providing signal coverage in the coverage zones  114   a - c  can include transmitting downlink signals received from the unit  102  to mobile communication devices or other terminal devices in the coverage zones  114   a - c . Providing signal coverage in the coverage zones  114   a - c  can also include receiving uplink signals from the mobile communication devices or other terminal devices in the coverage zones  114   a - c . The remote antenna units  112   a - c  can transmit the uplink signals to the unit  102 . The unit  102  can combine uplink signals received from remote antenna units for transmission to the base stations  101   a ,  101   b.    
     Although  FIG. 1  depicts a direct connection between the unit  102  and the remote antenna units  112   a - c , other implementations are possible. In some aspects, the unit  102  can be connected to the remote antenna units  112   a - c  via one or more extension units or other intermediate devices. 
     The unit  102  can include a processor  104 , an interface section  106 , a signal processing section  108 , and an interface section  110 . The interface section  106  can include one or more physical layer (“PHY”) devices for communicating with base stations  101   a ,  101   b . For example, the interface section  106  can include an external repeater, an internal RF transceiver included on an interface card, or other suitable RF interface device to communicate with the base stations  101   a ,  101   b . The interface section  110  can include one or more PHY devices for communicating with remote antenna units or other units of a DAS  100 . The signal processing section  108  can include one or more modules for conditioning, filtering, combining, or otherwise processing signals received via an interface section  106  and communicated to other devices in the DAS  100  via an interface section  110 . 
     The processor  104  can include any processing device or group of processing devices configured to execute one or more algorithms for identifying sub-bands of interest. The processor  104  can configure the signal processing section  108  to sub-divide signals into equally spaced sub-bands. The processor  104  can configure the signal processing section  108  to extract or otherwise select sub-bands of interest from the equally spaced sub-bands. The processor  104  can include any device suitable for executing program instructions stored in a non-transitory computer-readable medium or other memory device to control operation of the unit  102 . Examples of processor  104  include a microprocessor, an application-specific integrated circuit (“ASIC”), a field-programmable gate array (“FPGA”), or other suitable processor. 
     The unit  102  can extract or otherwise identify sub-bands of interest from a downlink signal in the frequency domain.  FIG. 2  is a flow chart depicting an example of a process for extracting sub-bands of interest from a downlink signal in the frequency domain. 
     In block  210 , the unit  102  generates a transformed downlink signal by performing a frequency transform on a downlink signal received from a base station. One or both of the processor  104  and the signal processing section  108  can execute computationally efficient algorithms for generating the transformed downlink signal and for determining the equally spaced sub-bands into which downlink signals can be divided. 
     In a non-limiting example, an FFT can be applied to a downlink signal to convert the downlink signal from the time domain to the frequency domain. One or more FFT modules or modules in the signal processing section  108  of the unit can generate an FFT for a frequency spectrum used by the DAS  100 . Each bin of the FFT can correspond to a sub-band of the downlink signal. Each bin can include information about the downlink signal within the bandwidth of the bin, such as a magnitude and a phase for the sub-band in the bin. A length of the FFT being used can determine the bandwidth for each sub-band. For example, the processor  104  can obtain a 1,024-point FFT of a signal received by the unit. The sampling rate of the digital signal can be divided into the 1,024 FFT bins. The processor  104  can identify sub-bands of interest within the frequency spectrum from the FFT of the frequency spectrum. The unit  102  can use the processor  104  to extract the identified sub-bands for processing and routing. 
     In block  220 , the processor  104  determines that at least one sub-band of the transformed downlink signal includes data to be transmitted via the DAS  100 . The processor  104  can identify that the transformed downlink signal includes sub-bands of interest. For example, the processor  104  can determine that one or more bins of an FFT or other frequency transform include data having a magnitude exceeding a threshold magnitude. 
     In some aspects, the processor  104  can analyze the downlink signal in the time domain to determine if a signal of interest is present. The processor  104  can calculate or otherwise determine which FFT bins include the signal of interest. 
     In additional or alternative aspects, the processor  104  can identify sub-bands of interest by examining the downlink signal in both the time and frequency domain. The processor  104  can detect signal characteristics consistent with mobile wireless signals. For example, the processor  104  can detect a particular modulation format and thereby identify signals of interest. 
     In additional or alternative aspects, the processor  104  can be configured by a user to extract specific spectral segments. The processor  104  can determine the FFT bins corresponding to the configured spectral segments. 
     In additional or alternative aspects, the processor  104  can communicate with the base station  101   a ,  101   b  to determine which channels are used by the base stations  101   a ,  101   b . The processor  104  can identify the FFT bins that include the signals of interest based on which channels are used by the base stations  101   a ,  101   b . For example, the base stations  101   a ,  101   b  may transmit control signals that are addressed to the unit  102  and that identify the channels being used. In some aspects, the control signals can be transmitted via the same communication links as downlink signals that are to be communicated to terminal devices. In other aspects, the control signals can be transmitted via a dedicated communication link that is different from communication links used to transmit downlink signals to the DAS  100 . The unit  102  can include an interface card or other device used to communicate with one or more of the base stations  101   a ,  101   b  via the dedicated communication link. 
     In block  230 , the unit  102  extracts the at least one sub-band from the transformed downlink signal. In some aspects, the processor  104  can extract or otherwise select the sub-bands of interest by selecting FFT bins corresponding to the sub-bands of interest. Each bin can represent a given amount of frequency spectrum. The unit  102  can transmit bins corresponding to the sub-bands of interest to one or more other remote antenna units  112   a - c . In a non-limiting example, a portion of the downlink frequency band may lack any voice data or other data to be transmitted. The unit  102  can remove sub-bands from the downlink signal that correspond to the portion of the frequency band without voice or data. Removing the sub-bands can reduce the occupied bandwidth on the transport media between the unit  102  and the remote antenna units  112   a - c.    
     In some aspects, selecting the bins of interest can cause one or more discontinuities in the frequency domain that may result in distortion in the time domain. Distortion can be minimized by filtering the sub-bands of interest from the frequency spectrum used by the DAS  100 . For example, the unit  102  can select bins corresponding to the sub-bands of interest by applying a windowing function to an FFT of the frequency spectrum to minimize distortion in the time domain. 
     Extracting sub-bands of interest can differ from filtering a signal in the time domain. Filtering signals may involve decimating the signal to reduce the sampling rate of the original signal in order to reduce the amount of bandwidth required to transport a digital version of the signal. A receiving device may interpolate or otherwise up-sample the received signal. In the time domain, each sub-band may be down-converted via a mixer and decimated in a low-pass filtering operation. To combine multiple sub-bands at an up-converter may involve tracking the phases of the down-converter and the up-converter. Tracking the phases of the down-converter and the up-converter can involve identifying the delays between the down-conversion process for de-composing or otherwise dividing the sub-band and the up-conversion process for reconstructing the signal at a receiving device. Subdividing signal in the frequency domain can obviate the need to track the phases of down-conversion and up-conversion processes. A phase relationship from bin to bin can be identified by the process of transforming the signal into the frequency domain. 
     In additional or alternative aspects, a filter can be applied to a signal in the time domain to sub-divide the signal into the sub-bands prior to converting the signal to the frequency domain. For example, the unit  102  can include a filter bank or a set of discrete filters in the signal processing section  108  prior to an FFT module or other frequency transform module. The filter bank or a set of discrete filters can be used to sub-divide a downlink signal (either in analog or digital format) into multiple downlink signal components in the time domain. Each of the signal components can correspond to one of the sub-bands. The downlink signal components outputted by the filter bank or other set of filters can be transformed into the frequency domain using the FFT module or other frequency transform module. The bins of the frequency domain representation that correspond to sub-bands of interest can be selected by the unit  102  and communicated via the DAS  100 . 
     In additional or alternative aspects, using frequency transforms to divide downlink signals into sub-bands can allow the unit  102  to modify a signal level in some portions of the downlink frequency band relative to signal levels in other portions of the downlink frequency band. For example, the unit  102  can modify a signal level for a portion of the downlink frequency band by multiplying sub-bands within the portion of the downlink frequency band by an appropriate scaling factor. Multiplying sub-bands within the portion of the downlink frequency band by the appropriate scaling factor can allow for simplified equalization of signal levels from different signal sources. 
     In additional or alternative aspects, dividing downlink signals into sub-bands can allow for flexible routing and combining of signals from multiple base stations  101   a ,  101   b  to different groups of remote antenna units. Dividing downlink signals into smaller sub-bands allows for flexible routing of signals by identifying channels of interest or sub-bands of interest and focusing signal processing to those channels of interest or sub-bands of interest. 
     In one non-limiting example, the unit  102  can determine that one or more sub-bands of a digital downlink lack data to be transmitted via the DAS  100 . For instance, the processor  104  of the unit  102  can identify one or more bins of the frequency domain representation of the downlink signal (e.g., an FFT, a discrete Fourier transform, a discrete cosine transform, etc.) that have signal level values that are less than a desired threshold. A signal level can include, for example, a signal power, a voltage, a magnitude, a variance, or any other signal parameter that is suitable for determining whether a signal is present. The unit  102  can discard data from the identified bins or otherwise modify the frequency domain representation of the downlink signal to exclude the sub-bands associated with the identified bins. The unit  102  can reduce the sampling rate of the modified digital downlink signal based on the sub-bands without data being excluded from the digital downlink signal. 
     In another non-limiting example, a unit  102  may receive downlink signals from three base stations that transmit downlink in a common frequency band. A first base station and a second base station may transmit downlink signals using the same frequencies with the frequency band. The unit  102  can route downlink signals from the first and second base stations to non-intersecting subsets of the remote antenna units  112   a - c . For example, the unit  102  can route downlink signals from the first base station to one or more remote antenna units  112   a  in the coverage zone  114   a  and can route downlink signals from the second base station to one or more remote antenna units  112   b  in the coverage zone  114   b . The third base station may transmit downlink signals using frequencies with the frequency band that are different from the frequencies used by the first and second base stations. For example, the third base station may operate in frequencies that do not overlap the frequencies used by the first and second base stations. Downlink signals from the first base station and the third base station can be combined and sent to a subset of the remote antenna units  112   a - c  for transmission. Downlink signals from the second base station and the third base station can be combined and sent to a different subset of the remote antenna units  112   a - c  for transmission. For example, the unit  102  can route downlink signals from the first and third base stations to one or more remote antenna units  112   a  in the coverage zone  114   a  and can route downlink signals from the second and third base stations to one or more remote antenna units  112   b  in the coverage zone  114   b.    
     Downlink signals from base stations  101   a ,  101   b  can each be sampled at a rate that allows the downlink signals from the base stations  101   a ,  101   b  to be represented in a combined signal sum. For instance, in the example provided above, a unit  102  may receive downlink signals from three base stations that transmit downlink in a common frequency band. Downlink signals from the first base station and the second base station may be converted to a digital signal with a sampling rate of X samples/second. Downlink signals from the third base station may be converted to a digital signal with a sampling rate of Y samples/second. Routing downlink signals from the three base stations to the remote antenna units  112   a - c  may involve sampling the downlink signals received from the base stations at a sampling rate of at least X+Y samples/second. Communication links between the unit  102  and the remote antenna units  112   a - c  may require a total bandwidth of 3×(X+Y) for routing the downlink signals from the three base stations to the remote antenna units  112   a - c.    
     The bandwidth requirements for the DAS  100  described in the example above can be reduced by identifying sub-bands of interest for the downlink signals received from the three base stations. For example, the unit  102  can combine a first set of downlink signals from the first base station and the third base station and can route the combined downlink signals to a first subset of the remote antenna units  112   a - c . The unit  102  can convert downlink signals from the first and third base stations into digital downlink signals using a sampling rate of 2×(X+Y) and can transmit the digital downlink signals to the first subset of remote antenna units  112   a  over one or more communication links having a bandwidth of 2×(X+Y). The unit  102  can also combine a second set of downlink signals from the second base station and the third base station and route the combined downlink signals to a second subset of the remote antenna units  112   a - c . The unit  102  can also convert the downlink signals received from the second and third base stations into digital downlink signals using a sampling rate of 2×(X+Y) and can transmit the digital downlink signals to the second subset of remote antenna units  112   a  over one or more communication links having a bandwidth of 2×(X+Y). Sub-dividing downlink signals into sub-bands can thus provide more efficient use of resources in the DAS  100 . 
       FIG. 3  is a block diagram depicting an example of a unit  102  that can sub-divide downlink signals into sub-bands in the frequency domain. The unit  102  can include the processor  104 , which is configured to communicate with signal processing devices in the signal processing section  108 . In a downlink direction, the unit  102  can include RF circuitry  302   a ,  302   b , analog-to-digital converters  304   a ,  304   b , FFT modules  306   a ,  306   b , a combiner  310 , framers  312   a ,  312   b , and PHY devices  314   a ,  314   b . In some aspects, one or more of the FFT modules  306   a ,  306   b , the combiner  310 , and the framers  312   a ,  312   b  can be implemented as software modules executed by the processor  104 . In additional or alternative aspects, one or more of the FFT modules  306   a ,  306   b , the combiner  310 , and the framers  312   a ,  312   b  can be implemented using dedicated signal processing circuitry, such as an FPGA. Although  FIG. 3  depicts two downlink paths for illustrative purposes, any number of downlink paths can be implemented in the unit  102 . 
     As depicted in  FIG. 3 , the interface section  106  can include the RF circuitry  302   a ,  302   b  that is configured for receiving downlink signals from the base stations  101   a ,  101   b . Non-limiting examples such RF circuitry include a wireless RF transceiver, an interface card for receiving RF signals over coaxial cable or another wired connection, etc. 
     The signal processing section  108  can include the analog-to-digital converters  304   a ,  304   b , the FFT modules  306   a ,  306   b , and the combiner  310 . The analog-to-digital converters  304   a ,  304   b  of the unit  102  can convert analog downlink signals received by the unit  102  to digital downlink signals. The FFT modules  306   a ,  306   b  can transform the digital downlink signals into the frequency domain. The processor  104  of the unit  102  can extract or otherwise select sub-bands of interest from the downlink signals in the frequency domain. The processor  104  can be communicatively coupled to some or all of the signal processing devices of the unit  102  via any suitable structure for transporting electrical signals between devices or components within the unit  104 . For example, the processor  104  can communicate with the signal processing devices of the unit  102  via, for example, a printed circuit board (not depicted) or other conductive components that can be used to communicate electrical signals within the unit  102 . In some aspects, the processor  104  can provide a control signal to the combiner  310  to ignore data in the frequency domain received from one or more of the FFT modules  306   a ,  306   b  other than the data in the frequency domain corresponding to the sub-bands of interest from the FFT module. In additional or alternative aspects, the processor  104  can configure one or more of the FFT modules  306   a ,  306   b  to only send bins corresponding to the sub-bands of interest to the combiner  310 . In additional or alternative aspects, the processor  104  can configure one or more of the combiner  310  or one or more of the framers  312   a ,  312   b  to only send bins corresponding to the sub-bands of interest to other devices or modules in the downlink path. The combiner  310  can combine the extracted sub-bands of interest from the downlink paths of the unit  102  into serialized downlink data for transmission to remote antenna units. Although  FIG. 3  depicts FFT modules  306   a ,  306   b  for performing frequency transforms, any suitable frequency transform device or module can be used in the downlink direction. 
     The interface section  110  can include the framers  312   a ,  312   b  and the PHY devices  314   a ,  314   b . The framers  312   a ,  312   b  can packetize serialized downlink data received from the combiner  310  for transmission to one or more of the remote antenna units  112   a - c  as a packetized data stream. The PHY devices  314   a ,  314   b  can transmit the packetized downlink data streams to the remote antenna units  112   a - c.    
     In an uplink direction, the unit  102  can include the PHY devices  314   a ,  314   b , deframers  316   a ,  316   b , bin aligners  318   a - d , gain adjustment modules  320   a - d , bin summers  322   a ,  322   b , gain adjustment modules  324   a ,  324   b , inverse FFT modules  326   a ,  326   b , digital-to-analog converters  328   a ,  328   b , and RF circuitry  330   a ,  330   b . In some aspects, one or more of the deframers  316   a ,  316   b , the bin aligners  318   a - d , the gain adjustment modules  320   a - d , the bin summers  322   a ,  322   b , the gain adjustment modules  324   a ,  324   b , and the inverse FFT modules  326   a ,  326   b  can be implemented as software modules executed by the processor  104 . In additional or alternative aspects, one or more of the deframers  316   a ,  316   b , the bin aligners  318   a - d , the gain adjustment modules  320   a - d , the bin summers  322   a ,  322   b , the gain adjustment modules  324   a ,  324   b , and the inverse FFT modules  326   a ,  326   b  can be implemented using dedicated signal processing circuitry, such as an FPGA. Although  FIG. 3  depicts two uplink paths for illustrative purposes, any number of uplink paths can be implemented in the unit  102 . 
     In an uplink direction, the interface section  110  for communicating with the remote antenna units  112   a - c  can include the PHY devices  314   a ,  314   b  and the deframers  316   a ,  316   b . The PHY devices  314   a ,  314   b  can receive packetized uplink data streams from one or more of the remote antenna units  112   a - c . The de-framers  316   a ,  316   b  can extract uplink data from packetized uplink data streams received from remote antenna units  112   a - c.    
     The signal processing section  108  can also include the bin aligners  318   a - d , the gain adjustment modules  320   a - d , the bin summers  322   a ,  322   b , the gain adjustment modules  324   a ,  324   b , the inverse FFT modules  326   a ,  326   b , and the digital-to-analog converters  328   a ,  328   b . Each of the bin aligners  318   a - d  can align frequency bins from FFTs or other frequency transforms of uplink signals. For example, FFT data for the same uplink signals that are received by the unit  102  from different remote antenna units  112   a - c  may be shifted in time with respect to one another. Each of the bin aligners  318   a - d  can be used to account for the FFT data being shifted in time by ensuring that the same frequency bins of FFT data from different remote antenna units  112   a - c  are added together in bin summers  322   a ,  322   b . Although  FIG. 3  depicts two bin aligners  318   a ,  318   b  for providing uplink signals to the bin summer  322   a  and two bin aligners  318   c ,  318   d  for providing uplink signals to the bin summer  322   b , any number of bin aligners can be used to align bins that are to be summed at a bin summer. Each of the bin aligners  318   a - d  can provide an uplink signal to a respective one of the gain adjustment modules  320   a - d . Each of the gain adjustment modules  320   a - d  can adjust the gain of the uplink signal outputted from a respective one of the bin aligners  318   a - d.    
     Each of the bin summers  322   a ,  322   b  can sum or otherwise combine frequency bins from uplink signals in the frequency domain (e.g., OFDM uplink signals). Each of the bin summers  322   a ,  322   b  can add the same frequency bins of FFTs from multiple remote antenna units. For example, three FFTs may be obtained from three remote antenna units, where each FFT includes 1,024 bins. A bin summer can add the first bin from the three FFTs and can save the sum in the first bin of the FFT sum. The bin summer can add the second bin from the three FFTs and can save the sum in the second bin of the FFT sum. The bin summer can repeat the process until all bin values for each of the 1,024 bins of the FFT are added. 
     The inverse FFT modules  326   a ,  326   b  can transform FFTs of uplink signals into the time domain for transmission to the base stations  101   a ,  101   b . Although  FIG. 3  depicts inverse FFT modules  326   a ,  326   b  for transforming FFTs into the time domain, the unit  102  can include any suitable inverse frequency transform device for transforming frequency domain representations of signals into the time domain. The digital-to-analog converters  328   a ,  328   b  can convert digital uplink signals in the time domain to analog uplink signals for transmission to base stations  101   a ,  101   b  via the RF circuitry  330   a ,  330   b  included in the interface section  106 . 
       FIG. 4  is a block diagram depicting an alternative example of a unit  102 ′ that can sub-divide downlink signals into sub-bands in the frequency domain. The unit  102 ′ can include downlink filters  402   a ,  402   b  and decimators  404   a ,  404   b  in the downlink paths. The analog-to-digital converters  304   a ,  304   b  may sample the downlink signals received from the base stations  101   a ,  101   b  at a higher rate than is desirable for representing the signals that occupy the bandwidth of the input RF signal. The decimators  404   a ,  404   b  can be used to reduce the sampling rate of the digital downlink signal to a rate at or near the minimum used to represent the downlink signals (e.g., the Nyquist rate). Reducing the sampling rate of the digital downlink signal can conserve transmission resources, such as the available bandwidth of communication links between the unit  102  and the remote antenna units  112   a - c . A frequency domain decimation function can be provided to reduce the sampling rate. 
     Each of the downlink filters  402   a ,  402   b  can be a low-pass filter that can filter the transformed downlink signal outputted from a respective one of the FFT modules  306   a ,  306   b . Filtering the transformed downlink signals can prevent or reduce aliasing caused by the decimators  404   a ,  404   b . The downlink filters  402   a ,  402   b  can also attenuate signals in frequency bins from the FFT that are not of interest. Attenuating the signals that are not of interest using the downlink filters  402   a ,  402   b  can allow the decimators  404   a ,  404   b  to reduce aliasing to an acceptable level by removing the attenuated frequency bins from the FFT representations of the downlink signals. Each of the decimators  404   a ,  404   b  can reduce the sampling rate of downlink signals by removing the frequency bins that have been attenuated. 
     The unit  102  depicted in  FIG. 4  can also include interpolators  406   a ,  406   b  and uplink filters  408   a ,  408   b  in the uplink paths. Each of the interpolators  406   a ,  406   b  can increase the sampling rate of a respective uplink signal outputted from a respective one of the gain adjustment modules  324   a ,  324   b . Each of the uplink filters  408   a ,  408   b  can filter the signal outputted from a respective one of the interpolators  406   a ,  406   b  to prevent or reduce aliasing caused by the interpolators  406   a ,  406   b.    
     In additional or alternative aspects, frequency transforms of uplink signals can be used to reduce the bandwidth requirements for communicating the uplink signals via the DAS  100 . For example, the unit  102  can combine uplink signals received by multiple remote antenna units  112   a - c  into a combined uplink signal for transmission to one or more of the base stations  101   a ,  101   b . The unit  102  can add or otherwise combine individual sub-bands of uplink signals from different remote antenna units  112   a - c . The unit  102  can exclude other sub-bands of the uplink signals from the combined uplink signal. For example, a given sub-band or group of sub-bands from one remote antenna unit may be unexcited. An unexcited sub-band can include a sub-band in which only noise without any signal component is received at a remote antenna unit. Unexcited sub-bands may be excluded from a combination of sub-bands in the same frequency range received from other remote antenna units (e.g., a squelch operation). In some aspects, sub-bands of interest can be identified and extracted by a processor  104  of a unit  102  receiving uplink signals from remote antenna units. In other aspects, sub-bands of interest can be identified and extracted by a processor of a remote antenna unit. 
       FIG. 5  is a block diagram depicting an example of a remote antenna unit  112  that can sub-divide uplink signals into sub-bands in the frequency domain. The remote antenna unit  112  can include an interface section  502  and a signal processing section  503 . The interface section  502  can include a PHY device  504 , a deframer  506 , and a framer  522 . The signal processing section  503  can include an interpolator  508 , a low-pass filter  510 , an inverse FFT module  512 , a digital-to-analog converter  514 , a processor  515 , an analog-to-digital converter  516 , an FFT module  518 , a filter  520 , and a decimator  524 . 
     In a downlink path, the PHY device  504  can receive packetized downlink data streams from the unit  102 . The deframer  506  can extract downlink data from the packetized downlink data streams. The interpolator  508  can increase the sampling rate of a digital downlink signal received from the deframer  506 . The filter  510  can be a low-pass filter or other filter that is suitable for preventing or reducing aliasing caused by the interpolator  508 . The inverse FFT module  512  or other suitable inverse frequency transform device can transform digital downlink signals from the frequency domain into the time domain. The digital-to-analog converter  514  can convert the digital downlink signals to analog downlink signals for transmission to mobile communication devices or other terminal devices via suitable RF circuitry. 
     In an uplink path, the analog-to-digital converter  516  can convert analog uplink signals to digital uplink signals. The analog uplink signals can be received using suitable RF circuitry of the remote antenna unit  112 . An FFT module  518  or other frequency transform device can transform the digital uplink signals into the frequency domain. The processor  515  can extract or otherwise select sub-bands of interest from the uplink signal in the frequency domain. The processor  515  can include any device suitable for executing program instructions stored in a non-transitory computer-readable medium or other memory device to control operation of the remote antenna unit  112 . Examples of processor  515  include a microprocessor, an ASIC, an FPGA, or other suitable processor. 
     The filter  520  can be a low-pass filter or other filter that is suitable for preventing or reducing aliasing caused by the decimator  524 . The decimator  524  can decrease the sampling rate of the digital uplink signal for transmission via the DAS  100 . The framer  522  can packetize uplink data received from mobile communication devices for transmission to the unit  102 . The PHY device  504  can transmit packetized uplink data streams to the unit  102 . 
     In some aspects, one or more of the elements in the signal processing section  503  can be implemented as software modules executed by the processor  104 . In additional or alternative aspects, one or more of the elements in the signal processing section  503  can be implemented using dedicated signal processing circuitry, such as an FPGA. Although  FIG. 5  depicts the downlink path as including an interpolator  508  and a low-pass filter  510 , other implementations are possible. In some aspects, the interpolator  508  and the low-pass filter  510  can be omitted. Although  FIG. 5  depicts the uplink path as including the  520  and the decimator  524 , other implementations are possible. In some aspects, the filter  520  and the decimator  524  can be omitted. 
     In some aspects, a DAS  100  can be used to communicate orthogonal frequency-division multiplexing (“OFDM”) signals. Some telecommunication technologies, such as OFDM-based signals transmitted via long-term evolution (“LTE”) systems, use FFTs and inverse FFTs for processing signals that are communicated with mobile communication devices or other terminal devices. These types of signals can be sub-divided into sub-bands using an FFT. Each FFT can be aligned with a complete OFDM symbol. 
     For example, LTE signals can include multiple resource blocks. Each resource block may have a size of one FFT bin. Each FFT bin can be encoded via quadrature amplitude modulation (“QAM”) to convey information. A sub-banding unit of a DAS  100  can decompose an LTE signal into FFT bins. Unused resource blocks may not be transmitted from the master unit to the remote antenna unit. A guard time (called the cyclic prefix) used between LTE symbols may not convey information. The guard time or cyclic prefix may not be transmitted from the master unit to the remote antenna unit. Excluding un-used resource blocks or guard times from signals communicated via a DAS  100  can reduce the bandwidth used to communicate the signals. 
     OFDM signals can be communicated in an FFT format. If a known OFDM-based signal uses an FFT format, the unit  102  can synchronize the start and stop of FFT frames to correspond with the start and stop of the OFDM-based symbols to be transported. Synchronizing the start and stop of FFT frames to correspond with the start and stop of the OFDM-based symbols can include analyzing the OFDM signal with a measurement module implemented in the processor  104  of the unit  102 . The measurement module can determine the timing of the received signal. The timing of the signal can be used to control when to start and stop FFT frames. 
     In additional or alternative aspects, the unit  102  of the DAS  100  can equalize a frequency response throughout the DAS  100 . Some analog circuitry for devices in a DAS  100  can have a frequency response that is not as constant (or “flat”) as desirable. A desirable frequency response can have a magnitude and group delay that is constant across the frequency spectrum used by the DAS  100 . Imperfections in analog circuitry can introduce ripples and other imperfections across the frequency spectrum. The unit  102  can execute one or more operations in the frequency domain to apply the inverse of the frequency response of the analog circuitry. Applying the inverse of the frequency response can include applying different complex weightings to different bins corresponding to frequencies of interest to compensate for variance of magnitude and/or group delay across the frequency spectrum used by the DAS  100 . Applying the inverse of the frequency response can equalize the frequency response throughout the DAS  100 . 
     In some aspects, a magnitude (i.e., weight) associated with each bin of interest can be modified by the processor applying a multiplier to the bin. In other aspects, a complex weight can be applied to the bins of interest. A complex weight can be used to modify the gain and phase of a bin by performing a complex multiplication of the bin and the complex weight. 
     The foregoing description of aspects and features of the disclosure, including illustrated examples, has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Numerous modifications, adaptations, and uses thereof will be apparent to those skilled in the art without departing from the scope of this disclosure. Aspects and features from each disclosed example can be combined with any other example. The illustrative examples described above are given to introduce the reader to the general subject matter discussed here and are not intended to limit the scope of the disclosed concepts.