Patent Publication Number: US-11381311-B2

Title: Cellular communication system having a set of small cells as a signal source

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/692,341 filed on Aug. 31, 2017, which claims the benefit under 35 USC § 119(a) of Korean Patent Application No. 10-2016-0116814, filed on Sep. 9, 2016, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     1. Field 
     The following description relates to a cellular communication technology, and more particularly, to a distributed antenna system (DAS) having a set of small cells as a source. 
     2. Description of Related Art 
     In mobile communications, wireless coverage is dependent, to large extent, on a base transceiver station (BTS). Due to high costs of installing BTSs, there has been an attempt to expand wireless coverage through a remote radio head (RRH) device. The RRH device is connected with a BTS via a fiber optic cable using common public radio interface (CPRI) protocols. 
     Further, a distributed antenna system (DAS) may be introduced in order to enhance in-building coverage. U.S. Pat. No. 9,042,732 discloses an example of the DAS. 
     A typical analog DAS includes a headend unit and remote units. The headend unit is connected to BTSs or RRH devices via coaxial cables, converts radio frequency (RF) signal inputs from the connected BTSs or RRH devices into optical RF signals, and outputs the optical RF signals, and the remote units are connected to the headend unit via optical fibers and distribute downlink optical RF signals to wireless terminals. 
     Such an analog DAS is difficult to reconfigure once it has been deployed, and cannot scale dynamically to a required service capacity. 
     Meanwhile, as mobile communication technology evolves into long term evolution (LTE)-Advanced, a small cell technology emerges as one approach to maximize usage efficiency within limited resources. Small cells are low-powered wireless access BTSs, which are each defined as having an operational range from at least 10 meters to a few hundred meters. Small cells are classified as femtocells, picocells, metrocells, and microcells according to their usage range and purpose of use. In addition, the classification of small cells may be made according to an installation site and purpose of service, and small cells for home, enterprise, urban areas, and rural areas may exist. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     The following description relates to a cellular communication system which uses an analog distributed antenna system (DAS) that is readily reconfigurable according to a required service capacity. 
     The following description relates to a cellular communication system which uses an analog DAS whose capacity is scalable according to a required service capacity. 
     The following description relates to a cellular communication system which uses an analog DAS that is readily reconfigurable according to a required service capacity while minimizing addition of hardware devices or components at the customer sites. 
     Further, the following description relates to a cellular communication system which uses an analog DAS that is remotely reconfigurable according to a required service capacity. 
     The proposed cellular communication system includes an analog DAS that has a set of small cells as a signal source. According to one aspect, the set of small cells has a variable architecture which may increase the number of users or capacity in association with the analog DAS. 
     The afore-mentioned aspect is achieved by a small cell signal source including: an upper small cell unit including upper protocol processors configured to process an upper first part of a protocol stack of a small cell; a lower small cell unit including lower protocol processors configured to process a remaining second part of the protocol stack of the small cell; and a first matching switch configured to respectively match the lower protocol processors to a plurality of remote units. According to one aspect, the cellular communication system includes a common controller configured to control activation of the plurality of upper protocol processors of the upper small cell unit and the plurality of lower protocol processors of the lower small cell unit according to a required service capacity and to control an operation of the first matching switch. 
     According to additional aspect, the upper protocol processor may process a media access control (MAC) layer or higher in the protocol stack of the small cell and the lower protocol processor may process a physical (PHY) layer of the protocol stack of the small cell. 
     According to another aspect, the plurality of upper protocol processors of the upper small cell unit may be implemented as software tasks whose executions are controlled by the common controller. Similarly, the plurality of lower protocol processors of the lower small cell unit may be implemented as software tasks whose executions are controlled by the common controller. 
     According to still another aspect, the common controller may be implemented by a part included in the upper small cell unit. 
     According to yet another aspect, the common controller may additionally activate the upper protocol processor when an increase in the number of users is required. 
     According to another aspect, the common controller may additionally activate the lower protocol processor when an increase in bandwidth is required, and accordingly, the common controller may control the operation of the first matching switch. 
     According to another aspect, the cellular communication system may further include a plurality of wireless converters installed between the lower protocol processors and input ports of the first matching switch, respectively, and configured to convert an output of the lower protocol processor into a radio frequency (RF) signal. According to an optional aspect, the common controller may control the plurality of wireless converters having the same service process so that different frequencies are modulated. 
     According to yet another aspect, the cellular communication system may further include a plurality of wireless converters respectively connected to output port of the first matching switch and configured to convert an output of each of the lower protocol processor into an RF signal. According to an optional aspect, the common controller may control the plurality of wireless converters having the same service process so that different frequencies are modulated. 
     In one general aspect, the first matching switch may be connected between outputs of the plurality of lower protocol processors and a plurality of inputs of a headend unit of an analog distributed antenna system. According to an optional aspect, when additional service capacity is required, the common controller may additionally activate the upper protocol processor and a lower protocol processor corresponding to the upper protocol processor, wherein the lower protocol processor transceiving an RF signal is activated in a band that is different from a current band, and the common controller controls the additionally activated lower protocol processor to be connected to the base-station signal driving unit. According to another optional aspect, when additional service capacity is required, the common controller may additionally activate the upper protocol processor and the lower protocol processor, control a base-station signal driving unit connected to the additionally activated lower protocol processor to perform band up/down-conversion, and control a remote unit corresponding to the base-station signal driving unit to perform reverse band conversion. 
     In another general aspect, the first matching switch may be connected between outputs of the plurality of upper protocol processors and inputs of the plurality of lower protocol processors. According to an optional aspect, when an additional service capacity is required, the common controller may additionally activate the lower protocol processor transceiving an RF signal in a band that is different from a current band. According to another optional aspect, when additional service capacity is required, the common controller may additionally activate the lower protocol processor, control a base-station signal driving unit connected to the additionally activated lower protocol processor to perform band up/down-conversion, and control a remote unit corresponding to the base-station signal driving unit to perform reverse band conversion. 
     Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of an analog distributed antenna system (DAS). 
         FIG. 2  is a block diagram illustrating an example of a configuration of a remote optical unit of  FIG. 1 . 
         FIG. 3  is a block diagram illustrating a configuration of a cellular communication system according to one exemplary embodiment. 
         FIG. 4  is a block diagram illustrating a configuration of a cellular communication system according to another exemplary embodiment. 
         FIG. 5  is a block diagram illustrating a configuration of a cellular communication system according to still another exemplary embodiment. 
         FIGS. 6 to 8  are block diagrams illustrating embodiments of a configuration of a lower small cell unit in the exemplary embodiment shown in  FIG. 3 or 5 . 
         FIGS. 9 to 11  are block diagrams illustrating embodiments of a remote optical unit in the exemplary embodiments shown in  FIGS. 3 to 5 . 
         FIG. 12  is a diagram for describing a first operation mode of a cellular communication system according to one exemplary embodiment. 
         FIG. 13  is a diagram for describing a second operation mode of a cellular communication system according to one exemplary embodiment. 
         FIG. 14  is a diagram for describing a third operation mode of the cellular communication system in accordance with the exemplary embodiment shown in  FIG. 4 or 5 . 
         FIG. 15  is a diagram illustrating another example of cellular communication system that implements the second operation mode described with reference to  FIG. 13 . 
         FIG. 16  is a diagram for describing a fifth operation mode of the cellular communication system in accordance with the exemplary embodiment illustrated in  FIG. 3 or 5 . 
         FIG. 17  is a diagram for describing a sixth operation mode of the cellular communication system in accordance with the exemplary embodiment illustrated in  FIG. 3 or 5 . 
         FIG. 18  is a diagram for describing a seventh operation mode of the cellular communication system in accordance with the exemplary embodiment illustrated in  FIG. 3 or 5 . 
     
    
    
     Throughout the drawings and the detailed description, the same drawing reference numerals should be understood as referring to the same elements, features, and structures unless otherwise described. The relative size and depiction of these elements may be exaggerated for clarity, illustration, and convenience. 
     DETAILED DESCRIPTION 
     The following description is provided to assist the reader in gaining a comprehensive understanding of the methods, apparatuses, and/or systems described herein. Also, although configurations of selectively described aspects or selectively described embodiments in below are illustrated as a single integrated configuration in the drawings, it should be understood that these configurations may be freely combined with each other as long as a technological contradiction of such a combination is not apparent for those skilled in the art unless otherwise described. It should also be noted that each block of the block diagrams may represent a physical element, and in some cases, may represent part of a function of one physical element or a logical representation of a function over a number of physical elements. A block or a portion of a block may sometimes be a set of program instructions. All or some of the blocks may be implemented by hardware, software, or a combination thereof. 
       FIG. 1  is a diagram illustrating an example of an analog distributed antenna system (DAS) to which the present invention is applicable. As shown in  FIG. 1 , the analog DAS includes a headend unit  100  and a plurality of remote units  150 . In the illustrated example, the remote units are represented as remote optical units  150 . The headend unit  100  and the plurality of remote optical units  150  are connected via optical cables. The term “remote unit” indicates that the headend unit may be connected to the plurality of remote optical units  150  via coaxial cables or other analog cables instead of the optical cables. The headend unit  100  mixes and converts a plurality of radio frequency (RF) signals from a plurality of base transceiver stations (BTSs) into an optical RF signal. Each of the remote optical units  150  is disposed at each site, converts the optical RF signal received from the headend unit into an electrical RF signal, and transmits the electrical RF signal to the antenna. A description regarding a downlink signal may be applied to an uplink signal in a symmetrical manner. 
     For example, the headend unit  100  includes a base station interface unit (BIU)  110  and an optical distribution unit (ODU)  130 . The BIU  110  includes a plurality of base-station signal driving units  111 , which receive a signal of each band from a BTS, up/down convert the band of the signal if necessary, and output the resultant signal, and a combine-divider  113 , which mixes RF signals input from the plurality of base-station signal driving units and output a resultant signal. In the drawing, according to the names used in an applicant&#39;s commercialized system, the base-station signal driving unit is denoted as a main driver BTS unit (MDBU) and the combine-divider  113  is denoted as a main combiner/divider unit (MCDU). In the present exemplary embodiment, the base-station signal driving units  111  are connected to an output of BTS equipment of each mobile communication service operator via coaxial cables. However, the present invention is not limited to the above, and the base-station signal driving units  111  may also be connected via optical cables. The base-station driving units  111  may deal with different frequency bands or different signal specifications according to specifications of a mobile communication service operator or specifications of BTS equipment. The combine-divider  113  has a plurality of outputs, through each of which a mixture of the RF signals output from the base-station signal driving units  111  is output according to needs of the remote optical units  150 . 
     The ODU  130  converts the RF signal output from the combine-divider  113  into an optical RF signal, and distributes and outputs the optical RF signal through a plurality of output ports. 
     The remote optical unit  150  receives the optical RF signals from the ODU  130 . Each output port of the ODU  130  may be connected with the remote optical unit  150 . For example, in  FIG. 1 , an ODU  130 - 1  is connected with remote optical units  150 - 1 ,  150 - 2 , and  150 - 3 . Also, an ODU  130 - 2  is connected with remote optical units  150 - 4  and  150 - 5 . 
       FIG. 2  is a block diagram illustrating an example of a configuration of a remote optical unit of  FIG. 1 . In one exemplary embodiment, the remote optical unit  150  includes three types of modules which are a photoelectric conversion unit  151  configured to convert the optical RF signals received from the ODU  130  into electrical RF signals, a plurality of remote drive units (RDUs)  156 - 1  and  156 - 2 , and a combine unit configured to combine signals generated from the RDUs. In one exemplary embodiment, the remote optical units  150  may include the plurality of RDUs. 
     Each of the RDUs  156 - 1  and  156 - 2  includes a bandpass filter  153  which filters only a required band in an RF signal on a downlink path, a modulation/demodulation unit  155  which up/down converts a band of the filtered RF signal when necessary and outputs a resultant signal, and a power amplifier  157 . Each of the RDUs  156 - 1  and  156 - 2  includes a low noise amplifier  154  which amplifies a signal from an antenna on an uplink path and a modulation/demodulation unit  152  which up/down converts a band of output from the low noise amplifier  154  and outputs a resultant signal when necessary. A plurality of RF signals of different bands may be multiplexed and transmitted through a single optical fiber. 
       FIG. 3  is a block diagram illustrating a configuration of a cellular communication system according to one exemplary embodiment. As illustrated in  FIG. 3 , the cellular communication system includes equipment for a small cell signal source  200 , a headend unit  100 , and remote units  150 - 1  and  150 - 2 . The small cell signal source  200  includes an upper small cell unit  210  and a lower small cell unit  230 . The upper small cell unit  210  includes a plurality of upper protocol processors  211 . Each upper protocol processor  211  processes a first part of a protocol stack of a small cell from an uppermost protocol of the protocol stack of the small cell. 
     The lower small cell unit  230  includes a plurality of lower protocol processors  231 . Each lower protocol processor  231  processes a remaining second part of the protocol stack of the small cell. 
     According to one aspect, the first part processed by the upper protocol processor  211  may be a media access control (MAC) layer and/or higher in the protocol stack of the small cell. In this case, the remaining second part processed by the lower protocol processor may be a physical (PHY) layer in the protocol stack of the small cell. Such function splitting is possible on various levels. 
     In regard to long term evolution (LTE) radio interface protocols, a number of technical materials have been available, for example, there is the article entitled “Overview of LTE Radio Interface and Radio Network Architecture for High Speed, High Capacity and Low Latency” which was published on pages 10-19 in NTT TOCOMO Technical Journal Vol. 13 No. 1. As described in the article, the LTE radio interface protocols may be classified as radio resource control (RRC), packet data convergence protocol (PDCP), radio link control (RLC), MAC, and PHY layer protocols. Among PDCP, RLC, and MAC layers that correspond to a second layer in seven layers of open system interconnection (OSI), the PDCP layer receives an RRC packet and processes header compression, a cipher of an RRC message, and integrity protection. The RLC layer segments and concatenates protocol data unit (PDU) packets from the PDCP layer to generate an RLC PDU with an appropriate packet size. In particular, the RLC layer performs automatic repeat request (ARQ) control according to a response from a receiving side. The MAC layer performs scheduling of shared channel resources. Also, the MAC layer processes a hybrid ARQ (HARQ). The PHY layer performs cell search and channel-dependent scheduling in a frequency domain, and performs a random access procedure when making a call. 
     Although the illustrated embodiment is described as an example of an LTE system, the present invention is not limited thereto and can be applied to general mobile communication technologies or a combination thereof, as is typical for a DAS. 
     Virtualization technique of a radio access network (RAN), which has been recently discussed in the Small Cell Forum (http://www.smallcellforum.org), may be considered as a reference technique, although it is not directly related to the present invention. According to this technique, some upper layers among RAN layers for mobile communication are virtualized and implemented in cloud space in a software manner, and the remaining layers are implemented as physical network functions. In this forum, the discussion on “function splitting” is in progress regarding which layers are to be virtualized and which layers are to be implemented as physical network functions among the RAN layers. According to the ongoing discussion, because splitting is performed on higher layers, transport costs are advantageously reduced, while RF gains are also reduced. Important factors for considering function splitting in RAN virtualization which can secure smooth communications between virtualized functional blocks and physical lower layer functional blocks include bandwidth and latency. According to the materials disclosed by the Small Cell Forum, function splitting between a MAC layer and a PHY layer may provide a trade-off in terms of bandwidth and latency. 
     Referring back to  FIG. 3 , the small cell signal source  200  of the cellular communication system in accordance with the exemplary embodiment includes a first matching switch  250  and a common controller  213 . The first matching switch  250  respectively matches lower protocol processors to the plurality of remote units  150  connected to the BIU by switching on or off an output path of each of the lower protocol processors. According to a required service capacity, the common controller  213  activates a plurality of upper protocol processors of the upper small cell unit and a plurality of lower protocol processors of the lower small cell unit, and accordingly, controls an operation of the first matching switch  250 . 
     In one exemplary embodiment, the upper protocol processors  211  may be implemented in a software manner, as well as in a circuitry manner. For example, each of the upper protocol processors  211  may be programmed in a general programming language (e.g., C/C++ language, etc.), then be compiled and executed as a task or a process on a central processing unit (CPU). In one exemplary embodiment, the lower protocol processors  231  may be implemented in a circuit manner. For example, each of the lower protocol processors  231  may be implemented in the form of a gate array or firmware. 
     According to one aspect, a plurality of upper protocol processors  211 - 1  to  211 - p  of the upper small cell unit  210  may be implemented as software tasks whose execution is controlled by the common controller  213 . Similarly, a plurality of lower protocol processors  231 - 1  to  231 - q  of the lower small cell unit  230  may be implemented as software tasks whose execution is controlled by the common controller  213 . As is already known, since the upper protocol processors  211 - 1  to  211 - p  are implemented as software tasks, they do not physically or logically reside within the upper small cell unit  210 , and no corresponding program modules exist on a program code. Rather, the upper protocol processors may be considered as entities which are newly generated and executed on memory under the control of a scheduler. Although, due to the technical difficulty in representation, it is stated that the small cell unit includes a plurality of upper protocol processors, such statement or similar expressions are intended to be construed to imply a case in which the plurality of upper protocol processors are implemented as software tasks, as well as a case in which the upper protocol processors are implemented physically, that is, as circuitry, in the upper small cell unit. 
     In another exemplary embodiment, some of logical modules of the upper protocol processor  211  may be implemented in a circuit manner. For example, a part of each of the upper protocol processors  211  may be implemented in the form of a gate array or firmware, and the remaining part may be implemented as a software task. Similarly, in one exemplary embodiment, some of logical modules of the lower protocol processor  231  may be implemented in a circuit manner. For example, a part of each of the lower protocol processor  231  may be implemented in the form of a gate array or firmware, and the remaining part may be implemented as a software task. 
     In another exemplary embodiment, the upper protocol processor  211  may be implemented as a software task, and the lower protocol processors  231 - 1  to  231 - q  may be implemented in a circuit manner. 
     In this case, the common controller  213  schedules execution of shared resources of the upper small cell unit  210 . According to another aspect, the common controller  213  may be implemented as one of components included in the upper small cell unit. In the illustrated exemplary embodiment, the small cell is an LTE small cell, and, for example, the common controller may be implemented as a function of a scheduler that processes scheduling of shared channel resources in the MAC layer. Additionally, the common controller  213  operates to mitigate interference which may occur when a plurality of PHY layers transmit different RF signals using the same frequency in a multi-cell or multi-sector environment. 
     The first matching switch  250  may be a physical switch that switches on or off output, or may be implemented as a software input/output interface for using an output of a program module as input or a program code, such as an input/output command. In the illustrated exemplary embodiment, the first matching switch  250  is included in the lower small cell unit  230 , but it is not limited thereto and may be configured as a separate device. 
     In the illustrated exemplary embodiment, the first matching switch  250  connects an output of each of the lower protocol processors  231  to a plurality of headend units  100  by switching on or off an output path of each of the lower protocol processors  231 . 
     According to one aspect, when the number of users increases in a region where a DAS provides services, that is, in a service region, the common controller  213  additionally activates the upper protocol processor  211  of the upper small cell unit. 
     According to another aspect, when additional bandwidth or capacity is required due to an increase in the use of the frequency band by users in a region where the DAS provides services, i.e., in a service region, the common controller  213  additionally activates the lower protocol processor  231  of the lower small cell unit  230 . The common controller  213  controls the first matching switch  250  to connect the activated lower protocol processor  231  to the remote unit that needs additional capacity. 
       FIG. 4  is a block diagram illustrating a configuration of a cellular communication system according to another exemplary embodiment. As illustrated in  FIG. 4 , the cellular communication system includes a small cell signal source  200 , a headend unit  100 , and a plurality of remote units  150 - 1  and  150 - 2 . The small cell signal source  200  includes an upper small cell unit  210 , a lower small cell unit  230 , and a second matching switch  250 ′. The upper small cell unit  210  includes a plurality of upper protocol processors  211 - 1  to  211 - n  that process an upper first part of a protocol stack of a small cell. The lower small cell unit  230  includes a plurality of lower protocol processors  231 - 1 ,  231 - 2 , and  231 -I that process a remaining second part of the protocol stack of the small cell. 
     The second matching switch  250 ′ switches on or off connections between each lower protocol processor of the lower small cell unit and each upper protocol processor of the upper small cell unit. The second matching switch  250 ′ may be a physical switch that switches on or off output, or may be implemented as a software input/output interface for using an output of a program module as input, or a program code such as an input/output command. Since configurations corresponding to those of  FIG. 4  are similar to those in  FIG. 3 , detailed descriptions thereof are omitted. 
     According to one aspect, when the number of users increases in a region where a DAS system provides services, i.e., in a service region, the common controller  213  additionally activates the upper protocol processor  211  of the upper small cell unit and the lower protocol processor  231  of the lower small cell unit, and in turn, controls an operation of the second matching switch  250 ′. According to another aspect, when additional bandwidth or capacity is required due to an increase in the use of the frequency band by users in a region where a DAS system provides services, i.e., in a service region, the common controller  213  additionally activates the lower protocol processor  231  of the lower small cell unit  230   
       FIG. 5  is a block diagram illustrating a configuration of a cellular communication system according to still another exemplary embodiment. As illustrated in  FIG. 5 , the cellular communication system includes a small cell signal source  200 , a headend unit  100 , and a plurality of remote units  150 - 1  and  150 - 2 . The small cell signal source  200  includes an upper small cell unit  210 , a lower small cell unit  230 , a first matching switch  250 , and a second matching switch  250 ′. The upper small cell unit  210  includes a plurality of upper protocol processors  211 - 1  to  211 - n  that process an upper first part of a protocol stack of a small cell. The lower small cell unit  230  includes a plurality of lower protocol processors  231 - 1 ,  231 - 2 , and  231 -I that process a remaining second part of the protocol stack of the small cell. 
     The first matching switch  250  switches on or off an output path of each lower protocol processor. The second matching switch  250 ′ switches on or off a connection between each lower protocol processor of the lower small cell unit and each upper protocol processor of the upper small cell unit. Each of the first matching switch  250  and the second matching switch  250 ′ may be a physical switch which switches on or off output, or may be implemented as a software input/output interface for using an output of a program module as input, or a program code such as an input/output command. Since configurations corresponding to those of  FIG. 5  are similar to those in  FIG. 3 , detailed descriptions thereof will be omitted. 
     According to one aspect, when the number of users increases in a region where a DAS system provides services, that is, in a service region, the common controller  213  additionally activates the upper protocol processor  211  of the upper small cell unit and the lower protocol processor  231  of the lower small cell unit, and in turn, controls an operation of the second matching switch  250 ′. 
     According to another aspect, when additional bandwidth or capacity is required due to an increase in the use of the frequency band by users in a region where a DAS system provides services, i.e., in a service region, the common controller  213  additionally activates the lower protocol processor  231  of the lower small cell unit  230 , and in turn, controls an operation of the first matching switch  250 . Thus, a plurality of lower protocol processors  231  may be connected to a single upper protocol processor  211 . 
     As should be apparent from the exemplary embodiments illustrated, one or both of the first matching switch  250  and the second matching switch  250 ′ may be provided.  FIG. 6  is a block diagram illustrating one embodiment of a configuration of the lower small cell unit  230  in the exemplary embodiment shown in  FIG. 3 or 5 . According to one aspect, in the lower small cell unit  230 , the lower protocol processor  231  includes wireless converters  231 ′- 1  to  231 ′- q . In the present embodiment, an output from the small cell is an RF signal, and the first matching switch  250  switches the RF signal from the lower protocol processor  231 . 
       FIG. 7  is a block diagram illustrating another embodiment of a configuration of the lower small cell unit  230  in the exemplary embodiment shown in  FIG. 3 or 5 . As shown in  FIG. 7 , the first matching switch includes wireless converters  251 - 1  to  251 - q  at a receiving side. The small cell outputs a digital signal, for example, an LTE IQ signal, and the first matching switch  250  converts the IQ signal into an RF signal by modulating the IQ signal in the receiving side, and then transmits the RF signal to the remote optical unit which matches the small cell. 
       FIG. 8  is a block diagram illustrating still another embodiment of a configuration of the lower small cell unit  230  in the exemplary embodiment shown in  FIG. 3 or 5 . As shown in  FIG. 8 , the first matching switch  250  includes the wireless converters  251 - 1  to  251 - q  at an output side. The small cell outputs a digital signal, for example, an LTE IQ signal, and the first matching switch  250  respectively connects the small cells to the remote optical units so that outputs of the small cells respectively match the remote optical units, and then the first matching switch  250  converts the IQ signal into an RF signal by modulating the IQ signal, and outputs the RF signal. 
       FIG. 9  is a block diagram illustrating one embodiment of a configuration of the remote optical units  150 - 1  and  150 - 2  in the exemplary embodiments shown in  FIGS. 3 to 5 . Referring to  FIG. 9 , the remote optical unit  150  includes three types of modules which are the photoelectric conversion unit  151  configured to convert optical RF signals received from ODUs  130  into electrical RF signals, the plurality of RDUs  156 - 1  and  156 - 2 , and a combine unit  159  configured to combine signals generated from the RDUs. In one exemplary embodiment, the remote optical unit  150  may include the plurality of RDUs  156 - 1  and  156 - 2 . The RDUs may have different configurations from each other. 
     The RDU  156 - 1  includes a bandpass filter  153 - 1  which filters only a required band in an RF signal on a downlink path, and a power amplifier  157 - 1  which amplifies the filtered RF signal. The RDU  156 - 1  includes a low noise amplifier  154 - 1  which amplifies a signal from an antenna on an uplink path. 
     The RDU  156 - 2  includes a bandpass filter  153 - 2  which filters only a required band in the RF signal on the downlink path, and a power amplifier  157 - 2  which amplifies the filtered RF signal. The RDU  156 - 2  includes a low noise amplifier  154 - 2  which amplifies the signal from the antenna on the uplink path. Outputs from the two RDUs  156 - 1  and  156 - 2  are combined by the combine unit  159  and are transmitted to a single antenna. 
     In this case, the bandpass filter  153 - 1  of the RDU  156 - 1  and the bandpass filter  153 - 2  of the RDU  156 - 2  may have different pass bands. A plurality of RF signals with different bands may be multiplexed and transmitted through a single optical fiber. 
     In the illustrated exemplary embodiments, the remote optical unit  150  may have a suitable configuration capable of transmitting RF signals of two different bands which are output from the headend unit  100 . 
       FIG. 10  is a block diagram illustrating another embodiment of a configuration of the remote optical units  150 - 1  and  150 - 2  in the exemplary embodiments shown in  FIGS. 3 to 5 . Referring to  FIG. 10 , the remote optical unit  150  includes three types of modules which are the photoelectric conversion unit  151  configured to convert the optical RF signals received from the ODUs  130  into electrical RF signals, the plurality of RDUs  156 - 1  and  156 - 2 , and the combine unit  159  configured to combine signals generated from the RDUs. In the exemplary embodiment, the remote optical unit  150  may include the plurality of RDU modules  156 - 1  and  156 - 2 . The RDUs may have different configurations from each other. 
     The RDU  156 - 1  includes the bandpass filter  153 - 2  which filters only a required band in an RF signal on a downlink path, a demodulation unit  155 - 2  which demodulates the filtered RF signal by converting a band of the signal when necessary, and the power amplifier  157 - 2  which amplifies the demodulated signal. The RDU  156 - 1  includes the low noise amplifier  154 - 1  which amplifies a signal from an antenna on an uplink path, and a modulation unit  152 - 1  which modulates the amplified signal by converting a band of the signal when necessary. 
     The RDU  156 - 2  includes a bandpass filter  153 - 4  which filters only a required band in the RF signal on the downlink path, a demodulation unit  155 - 4  which demodulates the filtered RF signal by converting a band of the signal when necessary, and a power amplifier  157 - 4  which amplifies the demodulated signal. The RDU  156 - 2  includes a low noise amplifier  154 - 3  which amplifies the signal from the antenna on the uplink path and a modulation unit  152 - 3  which modulates the amplified signal by converting a band of the signal when necessary. The modulated signal outputs from the two RDUs  156 - 1  and  156 - 2  are in different bands. 
     According to one aspect, the headend unit may modulate RF signals of the same bands and transmit the modulated signals in order to increase coverage or capacity. These RF signals of different bands are transmitted through different remote optical units. Outputs of the two RDUs  156 - 1  and  156 - 2  having different frequencies are combined by the combine unit  159  and are transmitted to a single antenna. 
       FIG. 11  is a block diagram illustrating still another embodiment of a configuration of the remote optical units  150 - 1  and  150 - 2  in the exemplary embodiments shown in  FIGS. 3 to 5 . Referring to  FIG. 11 , the remote optical unit  150  includes three types of modules which are the photoelectric conversion unit  151  configured to convert the optical RF signals received from the ODUs  130  into electrical RF signals, the plurality of RDUs  156 - 1  and  156 - 2 , and the combine unit  159  configured to combine signals generated from the RDUs. In one exemplary embodiment, the remote optical unit  150  may include the plurality of RDU modules  156 - 1  and  156 - 2 . The RDUs may have different configurations from each other. 
     The RDU  156 - 1  includes the bandpass filter  153 - 1  which filters only a required band in an RF signal on a downlink path, and the power amplifier  157 - 1  which amplifies the filtered RF signal. The RDU  156 - 1  includes the low noise amplifier  154 - 1  which amplifies a signal from an antenna on an uplink path. 
     The RDU  156 - 2  includes the bandpass filter  153 - 4  which filters only a required band in the RF signal on the downlink path, the demodulation unit  155 - 4  which demodulates the filtered RF signal by converting a band of the signal when necessary, and the power amplifier  157 - 4  which amplifies the demodulated signal. The RDU  156 - 2  includes the low noise amplifier  154 - 3  which amplifies the signal from the antenna on the uplink path, and the modulation unit  152 - 3  which modulates the amplified signal by converting a band of the signal when necessary. Outputs of the two RDUs  156 - 1  and  156 - 2  are combined by the combine unit  159  and are transmitted to a single antenna. 
     In the illustrated exemplary embodiment, since the RDU  156 - 1  does not include a modulation/demodulation unit, the RDU  156 - 1  performs bandpass filtering on the RF signal output from the headend unit  100 , then amplifies the filtered signal and outputs the amplified signal. The RDU  156 - 2  extracts an RF signal of a specific frequency band using the bandpass filter  153 - 4 , demodulates the extracted RF signal through band-down conversion using the modulation/demodulation unit  155 - 4 , and outputs a resulting signal. 
     In the exemplary embodiments illustrated in  FIGS. 3 to 5 , scalability of the small cell is addressed from two perspectives. One is the number of users, and the other is bandwidth, i.e., total throughput. The proposed invention can improve both the number of users and the bandwidth/throughput. According to one aspect, the common controller  213  may additionally activate the upper protocol processor  211  in the upper small cell unit  210 , thereby increasing the number of users that can be supported. According to another aspect, the common controller  213  may allocate more computational resources, for example, memory or CPU resources, to the upper protocol processor  211  being executed in the upper small cell unit  210 , thereby enabling the increase of the number of users. 
     According to still another aspect, the common controller  213  may additionally activate the lower protocol processor  231  being executed in the lower small cell unit  230 , thereby increasing the bandwidth that can be supported. According to yet another aspect, the common controller  213  may allocate more computational resources, for example, memory or CPU resources, to the lower protocol processor  231  being executed in the lower small cell unit  230 , thereby enabling the increase of the bandwidth. In this case, the common controller  213  may need to appropriately control the first matching switch  250  or the second matching switch  250  in the lower small cell unit  230 . Hereinafter, the above-described aspects will be described in detail with reference to  FIGS. 12 to 16 . 
       FIG. 12  is a diagram for describing a first operation mode of a cellular communication system in accordance with the exemplary embodiment shown in  FIG. 3 . In the illustrated first operation mode, only one upper protocol processor  211 - 1  is being executed in the upper small cell unit  210 , and only one lower protocol processor  231 - 1  is being executed in the lower small cell unit  230 . The first matching switch  250  distributes output from the lower protocol processor  231 - 1 , i.e., a PHY layer  231 - 1 , into two signals and transmits the two signals to two input ports, which are an RF_1_SISO port and an RF_1_MIMO port of the BIU  110 . 
     In the illustrated exemplary embodiment, the BIU  110  includes two types of base-station signal driving units  111 - 1  and  111 - 2 , one of which does not include a demodulator but has the RF_1_SISO input port, and the other of which includes a demodulator and has the RF_1_MIMO input port. Alternatively, both of the base-station signal driving units  111 - 1  and  111 - 2  may have the same configurations and may include demodulator modules therein, but the base-station signal driving unit  111 - 1  may be configured not to use the included demodulator module and the base-station signal driving unit  111 - 2  may be configured to use the included demodulator module. In this case, a band of a signal output from the base-station signal driving unit  111 - 1  is represented as RF_1_SISO, and a band of a signal output from the base-station signal driving unit  111 - 2  is represented as RF_1_MIMO. 
     The common controller  213  has information about such a system configuration, and controls the first matching switch  250  according to the information so that interference among RF signals of the same frequency band can be avoided. 
     The two base-station signal driving units  111 - 1  and  111 - 2  of the BIU  110  each receive the distributed RF signal. The base-station signal driving unit  111 - 1 , which receives the RF signal through the RF_1_SISO input port, outputs the received RF signal intact to the combine-divider  113 . The base-station signal driving unit  111 - 2 , which receives the RF signal through the RF_1_MIMO input port, modulates the RF signal to up or down convert a band thereof, and outputs a resulting signal to the combine-divider  113 . The combine-divider  113  mixes the two RF signals and outputs a mixture of the RF signals to the ODU  130 . The reason why the base-station signal driving unit  111 - 2  modulates the RF signal is because otherwise the mixed signal output from the combine-divider  113  cannot be separated later. 
     The ODU  130  converts the mixed signal into an optical RF signal and distributes the optical RF signal. The remote optical units  150 - 1  and  150 - 2  receive the distributed optical RF signal. In the illustrated exemplary embodiment, the remote optical unit  150 - 1  has a suitable configuration capable of transmitting an RF signal of an RF_1_SISO band, which is output from the base-station signal driving unit  111 - 1  of the BIU  110 . In addition, the remote optical unit  150 - 2  has a suitable configuration capable of transmitting an RF signal of an RF_1_MIMO band, which is output from the base-station signal driving unit  111 - 2  of the BIU  110 . That is, the remote optical unit  150 - 1  does not include a modulation/demodulation unit, while the remote optical unit  150 - 2  includes a modulation/demodulation unit to convert the RF signal of the RF_1_MIMO band into an RF signal of the RF_1_SISO band. 
     The remote optical unit  150 - 1  photoelectrically converts the received optical RF signal into an electrical RF signal, then extracts an RF signal of a specified frequency band, i.e., the RF_1_SISO band, using the bandpass filter, amplifies the RF signal, and transmits the amplified signal to the antenna. The remote optical unit  150 - 2  photoelectrically converts the received optical RF signal into an electrical RF signal, then extracts an RF signal of another specified frequency band, i.e., the RF_1_MIMO band, using the bandpass filter, up or down converts a band of the extracted RF signal to a specified frequency band, amplifies the converted signal, and outputs a resulting signal to the antenna. Consequently, the two remote optical units  150 - 1  and  150 - 2  transmit the same data through RF signals of the same band. Since the two remote optical units  150 - 1  and  150 - 2  provide services to different floors of a building, coverage may be expanded. 
       FIG. 13  is a diagram for describing a second operation mode of the cellular communication system in accordance with the exemplary embodiment shown in  FIG. 3 . The second operation mode described herein is an example of a response that the system in accordance with the proposed invention may make when it is necessary to increase the number of users and the bandwidth/capacity in the first operation mode illustrated in  FIG. 12 . 
     According to one aspect, when it is necessary to increase bandwidth or capacity, the common controller  213  controls an operation of the first matching switch and controls whether the headend unit performs modulation. In the illustrated second operation mode, the common controller  213  generates and executes new tasks such that the common controller  213  generates and executes a new upper protocol processor  211 - 2  and a new lower protocol processor  231 - 2  and connects both of the processors. In this case, MAC+#2 processes different data as the new upper protocol processor  211 - 2  from data processed by MAC+#1. In the illustrated second operation mode, the new lower protocol processor  231 - 2  outputs a signal of the same frequency band as that of the existing lower protocol processor  231 - 1 . 
     The first matching switch  250  disconnects the existing connection between the lower protocol processor PHY #1  231 - 1  and the RF_1_MIMO input port of the base-station signal driving unit  111 - 2 , and connects the newly generated and executed lower protocol processor PHY #2  231 - 2  to the RF_1_MIMO input port of the base-station signal driving unit  111 - 2 . The two upper protocol processors  211 - 1  and  211 - 2  and the two lower protocol processors  231 - 1  and  231 - 2  are executed, which is generally identical to the case of two small cells being executed. 
     The two base-station signal driving units  111 - 1  and  111 - 2  of the BIU  110  receive RF signals through two input ports. The base-station signal driving unit  111 - 1  outputs the received RF signal intact to the combine-divider  113 . The base-station signal driving unit  111 - 2  modulates the received RF signal to up or down convert a band thereof, and outputs a resulting signal to the combine-divider  113 . The reason why the base-station signal driving unit  111 - 2  modulates the RF signal is because otherwise the mixed signal output from the combine-divider  113  cannot be separated later. 
     The ODU  130  converts the mixed signal into an optical RF signal and distributes the optical RF signal. The remote optical units  150 - 1  and  150 - 2  receive the distributed optical RF signal. The remote optical unit  150 - 1  photoelectrically converts the received optical RF signal into an electrical RF signal, then extracts an RF signal of a specified frequency band, i.e., the RF_1_SISO band, using the bandpass filter, amplifies the RF signal, and transmits the amplified signal to the antenna. The remote optical unit  150 - 2  photoelectrically converts the received optical RF signal into an electrical RF signal, then extracts an RF signal of another specified frequency band, i.e., the RF_1_MIMO band, using the bandpass filter, up or down converts a band of the extracted RF signal to a specified frequency band, amplifies a converted signal, and outputs a resulting signal to the antenna. Consequently, since the two remote optical units  150 - 1  and  150 - 2  transmit different LTE data through RF signals of the same band, it is possible to form two mobile communication cells and to double the number of users and the bandwidth/capacity in comparison to the first operation mode illustrated in  FIG. 12 . 
       FIG. 14  is a diagram for describing a third operation mode of the cellular communication system in accordance with the exemplary embodiment shown in  FIG. 4 or 5 . The third operation mode described herein is an example of a response that the system in accordance with the proposed invention may make when a necessity to increase bandwidth/capacity used by users arises in the first operation mode illustrated in  FIG. 12 . 
     According to one aspect, when it is necessary to increase bandwidth/capacity, the common controller  213  additionally activates the lower protocol processor, controls an operation of the first matching switch, and controls whether the lower protocol processor performs modulation. In the illustrated third operation mode, the common controller  213  generates and executes new tasks such that the common controller  213  generates and executes a new lower protocol processor  231 - 2 . The common controller  213  controls the second matching switch  250 ′ to connect the newly generated lower protocol processor  231 - 2  to the upper protocol processor  211 - 1 . 
     In the illustrated third operation mode, the new lower protocol processor  231 - 2  outputs a signal of the same frequency band as that of the existing lower protocol processor  231 - 1 . 
     The first matching switch  250  disconnects the existing connection between the lower protocol processor PHY #1  231 - 1  and the RF_1_MIMO input port of the base-station signal driving unit  111 - 2 , and connects the newly generated and executed lower protocol processor PHY #2  231 - 2  to the RF_1_MIMO input port of the base-station signal driving unit  111 - 2 . 
     The base-station signal driving units  111 - 1  and  111 - 2  of the BIU  110  receive RF signals through two input ports. The base-station signal driving unit  111 - 1  outputs the received RF signal intact to the combine-divider  113 . The base-station signal driving unit  111 - 2  modulates the received RF signal to up or down convert a band thereof, and outputs a resulting signal to the combine-divider  113 . The reason why the base-station signal driving unit  111 - 2  modulates the RF signal is because otherwise the mixed signal output from the combine-divider  113  cannot be separated later. 
     The ODU  130  converts the mixed signal into an optical RF signal and distributes the optical RF signal. The remote optical units  150 - 1  and  150 - 2  receive the distributed optical RF signal. The remote optical unit  150 - 1  photoelectrically converts the received optical RF signal into an electrical RF signal, then extracts an RF signal of a specified frequency band, i.e., the RF_1_SISO band, using the bandpass filter, amplifies the RF signal, and transmits the amplified signal to the antenna. The remote optical unit  150 - 2  photoelectrically converts the received optical RF signal into an electrical RF signal, then extracts an RF signal of another specified frequency band, i.e., the RF_1_MIMO band, using the bandpass filter, up or down converts a band of the extracted RF signal to a specified frequency band, amplifies the converted signal, and outputs a resulting signal to the antenna. Consequently, the bandwidths of both the remote optical units  150 - 1  and  150 - 2  can be widened more than that of the first operation mode of  FIG. 12 . 
     In the first to third operation modes shown in  FIGS. 12 to 14 , extending coverage is dependent on the modulation/demodulation in the base-station signal driving unit of the headend unit and the modulation/demodulation in the corresponding remote unit. However, aspects of the proposed invention are not limited to the above, that is, by controlling the wireless converters, as shown in  FIGS. 6 to 8 , to convert a signal into a different frequency band that is used by the same service operator, the modulation/demodulation in the headend side may not be necessary. Although such an aspect may be applied to any operation mode which has been described with reference to  FIGS. 12 to 14  or will be described below, an exemplary operation in a case in which the aspect is applied to the second operation mode shown in  FIG. 13  will be described herein. 
       FIG. 15  is a diagram illustrating an example of another cellular communication system that implements the second operation mode described with reference to  FIG. 13 . The example shown in  FIG. 15  is substantially the same as the exemplary embodiment of  FIG. 13 , but the base-station signal driving unit  111 - 2  and the corresponding remote unit  150 - 2  do not need a modulation/demodulation unit. Rather, the lower protocol processors of the lower baseband unit  230  are connected with the wireless converters  231 ′- 1  and  231 ′- 2 , respectively. As should be apparent to those skilled in the art, the respective lower protocol processors of the exemplary embodiments illustrated in  FIGS. 12 to 14  require wireless converters, which are simply omitted in the drawings. The difference between the above embodiments and the present embodiment is that modulation/demodulation frequencies of the wireless converters are controlled by the common controller  213 . 
     Mobile communication service operators use multiple frequencies for the same service in order to support as much bandwidth and/or as many users as possible. Terminal devices search all of multiple frequencies for services of the same service operator. The common controller  213  controls modulation/demodulation frequencies of the wireless converters in order to increase the number of users and/or the bandwidth. For example, in the exemplary embodiment shown in  FIG. 15 , the upper protocol processor  211 - 1  and the lower protocol processor  231 - 2  are additionally activated for the same service operator, and the wireless converter  231 ′- 1  and the wireless converter  231 ′- 2  are controlled to select two of the multiple frequencies used by the service operator and modulate/demodulate the selected frequencies. In this case, the bandpass filter  153 - 1  of the remote unit  150 - 1  is tuned to a band of the wireless converter  231 ′- 1 , and the bandpass filter  153 - 2  of the remote unit  150 - 2  is tuned to a band of the wireless converter  231 ′- 2 . As such, the coverage as well as the bandwidth and the number of users can be substantially increased for the same service operator. Although the exemplary embodiment of  FIG. 15  employs the configuration illustrated in  FIG. 6 , it should be understood by those skilled in the art that the configuration of  FIG. 7 or 8  can be applied to  FIG. 13  through the exemplary embodiment of  FIG. 15 . 
     Similar schemes use the configurations illustrated in  FIGS. 6 to 8 , and may be applied to the exemplary embodiment of  FIG. 14 . 
       FIG. 16  is a diagram for describing a fifth operation mode of the cellular communication system in accordance with the exemplary embodiment illustrated in  FIG. 3 or 5 . When it is necessary to increase the number of users or the throughput in the first operation mode as illustrated in  FIG. 12 , the fifth operation mode may be initiated. Alternatively, the fifth operation mode may be initiated in order to increase the throughput in the second operation mode illustrated in  FIG. 13 . In the fifth operation mode, the common controller  213  generates and executes the additional upper protocol processor  211 - 2  in the upper small cell unit  210 , generates and executes the additional lower protocol processor  231 - 2  in the lower small cell unit  230 , and connects the upper protocol processor  211 - 2  and the lower protocol processor  231 - 2 . In the case where PHY #1 and PHY #2 are hardware circuits, the common controller  213  transmits only an output of the newly executed upper protocol processor  211 - 2  to PHY #2. 
     MAC+#1, which is the existing upper protocol processor  211 - 1 , outputs data D 1 , while MAC+#2, which is the newly executed upper protocol processor  211 - 2 , outputs data D 2 . PHY #1, which is the existing lower protocol processor  231 - 1 , transmits a signal of an RF_1 band, while PHY #2, which is the newly executed lower protocol processor  231 - 2 , outputs a signal of an RF_2 band. 
     The first matching switch  250  distributes each output of each lower protocol processor  231 - 1  and  231 - 2 , i.e., PHY #1 and PHY #2  231 - 1  and  231 - 2 , into two, and transmits the distributed output to one pair of the RF_1_SISO port and the RF_1_MIMO port and the other pair of an RF_2_SISO port and an RF_2_MIMO port, which are pairs of input ports of the BIU  110 . 
     In the illustrated exemplary embodiment, the BIU  110  includes two activated base-station signal driving units  111 - 1  and  111 - 4  that receive RF signals of the RF_1 band as an input such that one base-station signal driving unit does not include a modulator but has the RF_1_SISO input port while the other includes a modulator and has the RF_1_MIMO input port. In this case, a band of the signal output from the base-station signal driving unit  111 - 1  is represented as RF_1_SISO, and a band of the signal output from the base-station signal driving unit  111 - 4  is represented as RF_1_MIMO. 
     In the illustrated exemplary embodiment, the BIU  110  includes two activated base-station signal driving units  111 - 2  and  111 - 3  that receive RF signals of the RF_2 band as inputs such that one base-station signal driving unit does not include a modulator but has the RF  2  SISO input port while the other includes a modulator and has the RF  2  MIMO input port. In this case, a band of the signal output the base-station signal driving unit  111 - 2  is represented as RF_2_SISO, and a band of the signal output from the base-station signal driving unit  111 - 3  is represented as RF_2_MIMO. 
     The common controller  213  has information about such a system configuration, and controls the first matching switch  250  according to the information so that interference among RF signals of the same frequency band can be avoided. 
     The two base-station signal driving units  111 - 1  and  111 - 4  of the BIU  110  each receive the distributed RF signal. The base-station signal driving unit  111 - 1 , which receives the RF signal through the RF_1_SISO input port, outputs the received RF signal intact to the combine-divider  113 . The base-station signal driving unit  111 - 4 , which receives the RF signal through the RF_1_MIMO input port, modulates the RF signal to up or down convert a band thereof, and outputs a resulting signal to the combine-divider  113 . 
     The two base-station signal driving unit  111 - 2  and  111 - 3  of the BIU  110  each receive the distributed RF signal. The base-station signal driving unit  111 - 2 , which receives the RF signal through the RF_2_SISO input port, outputs the received RF signal intact to the combine-divider  113 . The base-station signal driving unit  111 - 3 , which receives the RF signal through the RF_2_MIMO input port, modulates the RF signal to up or down convert a band thereof, and outputs a resulting signal to the combine-divider  113 . 
     The combine-divider  113  mixes the four signals and outputs a mixture of the signals to the ODU  130 . The ODU  130  converts the mixed signal into an optical RF signal and distributes the optical RF signal. 
     The remote optical units  150 - 1  and  150 - 2  receive the distributed optical RF signal. 
     In the illustrated exemplary embodiment, the remote optical unit  150 - 1  may be configured as the exemplary embodiment of  FIG. 9  and the remote optical unit  150 - 2  may be configured as the exemplary embodiment of  FIG. 10 . The operations of the remote optical units  150 - 1  and  150 - 2  in accordance with the exemplary embodiment of  FIG. 18  will be described with reference to  FIGS. 9 and 10 . 
     Referring to  FIG. 9 , the remote optical unit  150 - 1  processes signals of two channels and has a configuration suitable to transmit RF signals of two bands, i.e., the RF_1_SISO band and the RF_2_SISO band, which are output from the base-station signal driving units  111 - 1  and  111 - 2  of the BIU  110 . 
     An upper channel of the remote optical unit  150 - 1  extracts an RF signal of a specified frequency band, i.e., the RF_1_SISO band, from a photoelectrically converted RF signal output from the photoelectric conversion unit ( 151 ) through the bandpass filter  153 - 1 , amplifies the extracted RF signal, and outputs a resulting signal to the antenna. A lower channel of the remote optical unit  150 - 1  extracts an RF signal of a specified frequency band, i.e., the RF_2_SISO band, from the photoelectrically converted RF signal output from the photoelectric conversion unit ( 151 ) through the bandpass filter  153 - 2  amplifies the extracted RF signal, and outputs a resulting signal to the antenna. The RF signal of RF_1_SISO band and the RF signal of RF_2_SISO band are mixed in a forward direction by the combine unit  159  and then transmitted to the antenna. Descriptions of an uplink will be omitted. 
     Referring to  FIG. 10 , the remote optical unit  150 - 2  processes signals of two channels and has a configuration suitable to transmit RF signals of two bands, i.e., the RF_1_MIMO band and the RF_2_MIMO band, which are output from the base-station signal driving units  111 - 3  and  111 - 4  of the BIU  110 . More specifically, the remote optical unit  150 - 1  does not include a modulation/demodulation unit, whereas the remote optical unit  150 - 2  includes the modulation/demodulation unit  155 - 2 , which converts the RF signal of the RF_1_MIMO band into an RF signal of the RF_1 band, and the modulation/demodulation unit  155 - 4 , which converts the RF signal of the RF_2_MIMO band into an RF signal of the RF_2 band. 
     An upper channel of the remote optical unit  150 - 2  extracts an RF signal of another specified frequency band, i.e., the RF_1_MIMO band, from a photoelectrically converted RF signal output from the photoelectric conversion unit ( 151 ) through the bandpass filter  153 - 2 , demodulates the extracted RF signal to a specified frequency band, i.e., the RF_1 band, through band-down conversion by the modulation/demodulation unit  155 - 2 , amplifies the demodulated RF signal in the amplifier  157 - 2 , and outputs a resulting signal to the antenna. A lower channel of the remote optical unit  150 - 2  extracts an RF signal of another frequency band, i.e., the RF_2_MIMO band, from the photoelectrically converted RF signal through the bandpass filter  153 - 4 , demodulates the extracted RF signal to a specified frequency band, i.e., the RF_2 band, through band-down conversion by the modulation/demodulation unit  155 - 4 , amplifies the demodulated RF signal in the amplifier  157 - 4 , and outputs a resulting signal to the antenna. The RF signal of the RF_1_MIMO band and the RF signal of the RF_2_MIMO band are mixed in the forward direction by the combine unit  159  and then transmitted to the antenna. Descriptions of an uplink will be omitted. 
     As a result, both of the remote optical units  150 - 1  and  150 - 2  transmit the data D 1  through the RF signal of the RF_1 band and the data D 2  through the RF signal of the RF_2 band. That is, since different data D 1  and D 2  are transmitted using two frequency bands RF_1 and RF_2, it is the same as functions of two cells or a single cell sectorized into two sectors. 
       FIG. 17  is a diagram for describing a sixth operation mode of the cellular communication system in accordance with the exemplary embodiment illustrated in  FIG. 3 or 5 . When it is necessary to additionally increase the throughput in the fifth operation mode as illustrated in  FIG. 16 , the sixth operation mode may be initiated. In the sixth operation mode, the common controller  213  generates and executes another additional upper protocol processor  211 - 3  in the upper small cell unit  210 , generates and executes another additional lower protocol processor  231 - 3  in the lower small cell unit  230 , and connects the upper protocol processor  211 - 3  and the lower protocol processor  231 - 3 . In the case where PHY #1, PHY #2, and PHY #3 are hardware circuits, the common controller  213  transmits only an output of the newly executed upper protocol processor  211 - 3  to PHY #3. 
     MAC+#1 and MAC+#2, which are the existing upper protocol processors  211 - 1  and  211 - 2 , output data D 1  and data D 2 , respectively, while MAC+#3, which is the newly executed upper protocol processor  211 - 3 , outputs data D 3 . PHY #1, which is the existing lower protocol processor  231 - 1 , transmits a signal of the RF_1 band, while PHY #3, which is the newly executed lower protocol processor  231 - 3 , outputs a signal of the RF_2 band as does the existing lower protocol processor  213 - 2 . 
     In the same manner as above, the first matching switch  250  distributes an output of the lower protocol processor  231 - 1 , i.e., PHY #1, into two signals, and transmits the distributed output to the RF_1_SISO port and the RF_1_MIMO port, which are input ports of the BIU  110 . Also, the first matching switch  250  connects output of the lower protocol processor  231 - 2 , i.e., PHY #2, with the RF_2_SISO port, which is an input port of the BIU  110 . In addition, the first matching switch  250  disconnects the existing connection between the lower protocol processor  231 - 2 , i.e., PHY #2, and the RF_2_MIMO input port of the BIU  110 , and connects the newly generated and executed lower protocol processor  231 - 3 , i.e., PHY #2  231 - 2 , with the RF_2_MIMO input port of the BIU  110 . 
     In the illustrated exemplary embodiment, the BIU  110  includes two activated base-station signal driving units  111 - 1  and  111 - 4  that receive RF signals of the RF_1 band as an input such that one base-station signal driving unit does not include a modulator but has the RF_1_SISO input port while the other includes a modulator and has the RF_1_MIMO input port. In this case, a band of the signal output from the base-station signal driving unit  111 - 1  is represented as RF_1_SISO, and a band of the signal output from the base-station signal driving unit  111 - 4  is represented as RF_1_MIMO. 
     In the illustrated exemplary embodiment, the BIU  110  includes two activated base-station signal driving units  111 - 2  and  111 - 3  that receive RF signals of the RF_2 band as an input such that one base-station signal driving unit does not include a modulator/demodulator but has the RF_2_SISO input port while the other includes a modulator/demodulator and has the RF_2_MIMO input port. In this case, a band of the signal output from the base-station signal driving unit  111 - 2  is represented as RF_2_SISO, and a band of the signal output from the base-station signal driving unit  111 - 3  is represented as RF_2_MIMO. 
     The common controller  213  has information about such a system configuration, and controls the first matching switch  250  according to the information so that interference among RF signals of the same frequency band can be avoided. 
     The two base-station signal driving unit  111 - 1  and  111 - 4  of the BIU  110  each receive the distributed RF signal. The base-station signal driving unit  111 - 1 , which receives the RF signal through the RF_1_SISO input port, outputs the received RF signal intact to the combine-divider  113 . The base-station signal driving unit  111 - 4 , which receives the RF signal through the RF_1_MIMO input port, modulates the RF signal to up or down convert a band thereof, and outputs a resulting signal to the combine-divider  113 . 
     The two base-station signal driving unit  111 - 2  and  111 - 3  of the BIU  110  receive the RF signal from PHY #2 and PHY #3, respectively. The base-station signal driving unit  111 - 2 , which receives the RF signal through the RF_2_SISO input port, outputs the received RF signal intact to the combine-divider  113 . The base-station signal driving unit  111 - 3 , which receives the RF signal through the RF_2_MIMO input port, modulates the RF signal to up or down convert a band thereof, and outputs a resulting signal to the combine-divider  113 . 
     The combine-divider  113  mixes the four signals and outputs a mixture of the signals to the ODU  130 . The ODU  130  converts the mixed signal into an optical RF signal and distributes the optical RF signal. 
     In the illustrated exemplary embodiment, the remote optical unit  150 - 1  may be configured as the exemplary embodiment of  FIG. 9  and the remote optical unit  150 - 2  may be configured as the exemplary embodiment of  FIG. 10 . The operations of the remote optical units  150 - 1  and  150 - 2  in accordance with the exemplary embodiment of  FIG. 18  will be described with reference to  FIGS. 9 and 10 . 
     Referring to  FIG. 9 , the remote optical unit  150 - 1  processes signals of two channels and has a suitable configuration capable of transmitting RF signals of two bands, i.e., the RF_1_SISO band and the RF_2_SISO band, which are output from the base-station signal driving units  111 - 1  and  111 - 2  of the BIU  110 . 
     An upper channel of the remote optical unit  150 - 1  extracts an RF signal of a specified frequency band, i.e., the RF_1_SISO band, from a photoelectrically converted RF signal output from the photoelectric conversion unit ( 151 ) through the bandpass filter  153 - 1 , amplifies the extracted RF signal, and outputs a resulting signal to the antenna. A lower channel of the remote optical unit  150 - 1  extracts an RF signal of a specified frequency band, i.e., the RF_2_SISO band, from the photoelectrically converted RF signal output from the photoelectric conversion unit ( 151 ) through the bandpass filter  153 - 2 , amplifies the extracted RF signal, and outputs a resulting signal to the antenna. The RF signal of RF_1_SISO band and the RF signal of RF_2_SISO band are mixed in the forward direction by the combine unit  159  and then transmitted to the antenna. Descriptions of an uplink will be omitted. 
     Referring to  FIG. 10 , the remote optical unit  150 - 2  processes signals of two channels and has a suitable configuration capable of transmitting RF signals of two bands, i.e., the RF_1_MIMO band and the RF_2_MIMO band, which are output from the base-station signal driving units  111 - 3  and  111 - 4  of the BIU  110 . More specifically, the remote optical unit  150 - 1  does not include a modulation/demodulation unit, whereas the remote optical unit  150 - 2  includes the modulation/demodulation unit  155 - 2 , which converts the RF signal of the RF_1_MIMO band into an RF signal of the RF_1 band, and the modulation/demodulation unit  155 - 4 , which converts the RF signal of the RF_2_MIMO band into an RF signal of the RF_2 band. 
     An upper channel of the remote optical unit  150 - 2  extracts an RF signal of another specified frequency band, i.e., the RF_1_MIMO band, from a photoelectrically converted RF signal output from the photoelectric conversion unit ( 151 ) through the bandpass filter  153 - 2 , demodulates the extracted RF signal to a specified frequency band, i.e., the RF_1 band through band-down conversion by the modulation/demodulation unit  155 - 2 , amplifies the demodulated RF signal in the amplifier  157 - 2 , and outputs a resulting signal to the antenna. A lower channel of the remote optical unit  150 - 2  extracts an RF signal of another frequency band, i.e., the RF_2_MIMO band, from the photoelectrically converted RF signal through the bandpass filter  153 - 4 , demodulates the extracted RF signal to a specified frequency band, i.e., the RF_2 band through band-down conversion by the modulation/demodulation unit  155 - 4 , amplifies a demodulated RF signal in the amplifier  157 - 4 , and outputs a resulting signal to the antenna. The RF signal of RF_1_MIMO band and the RF signal of RF_2_MIMO band are mixed in the forward direction by the combine unit  159  and then transmitted to the antenna. Descriptions of an uplink will be omitted. 
     As a result, the remote optical unit  150 - 1  transmits the data D 1  through the RF signals of the RF_1 band and the data D 2  through the RF signals of the RF_2 band, while the remote optical unit  150 - 2  transmits the data D 1  through the RF signals of the RF_1 band and the data D 3  through the RF signals of the RF_2 band. That is, since different data D 1 , D 2 , and D 3  are transmitted using two frequency bands RF_1 and RF_2, it is the same as functions of three cells or a single cell sectorized into three sectors. 
       FIG. 18  is a diagram for describing a seventh operation mode of the cellular communication system in accordance with the exemplary embodiment illustrated in  FIG. 3 or 5 . When it is necessary to additionally increase the throughput in the sixth operation mode as illustrated in  FIG. 17 , the seventh operation mode may be initiated. In the seventh operation mode, the common controller  213  generates and executes another additional upper protocol processor  211 - 4  in the upper small cell unit  210 , generates and executes another additional lower protocol processor  231 - 4  in the lower small cell unit  230 , and connects the upper protocol processor  211 - 4  and the lower protocol processor  231 - 4 . In the case where PHY #1, PHY #2, PHY #3, and PHY #4 are hardware circuits, the common controller  213  transmits only an output of the newly executed upper protocol processor  211 - 4  to PHY #4. 
     MAC+#1, MAC+#2, and MAC+#3, which are the existing upper protocol processors  211 - 1 ,  211 - 2 , and  211 - 3 , output data D 1 , data D 2 , and data D 3 , respectively, while MAC+#4, which is the newly executed upper protocol processor  211 - 4 , outputs data D 4 . PHY #1, which is the existing lower protocol processor  231 - 1 , and PHY #4, which is the newly executed lower protocol processor  231 - 4 , both transmit signals of the RF_1 band, while the remaining existing lower protocol processors  231 - 2  and  231 - 3  output signals of the RF_2 band. 
     The first matching switch  250  disconnects the existing connection between the lower protocol processor  231 - 1 , i.e., PHY #1  231 - 1 , and the RF_1_MIMO input port of the BIU  110 , and connects the newly executed lower protocol processor  231 - 4 , i.e., PHY #4  231 - 4 , with the RF_1_MIMO input port of the BIU  110 . Connections of the remaining two input ports are maintained in the same state as the sixth operation mode. 
     The configurations of the BIU  110  and the remote optical units  150 - 1  and  150 - 2  are similar to those in the sixth operation mode. The common controller  213  has information about such a system configuration, and controls the first matching switch  250  according to the information so that interference among RF signals of the same frequency band can be avoided. 
     The four base-station signal driving units  111 - 1 ,  111 - 2 ,  111 - 3 , and  111 - 4  of the BIU  110  receive an RF_1_SISO RF signal, an RF_2_SISO RF signal, an RF_2_MIMO RF signal, and an RF_1_MIMO RF signal, respectively. The base-station signal driving unit  111 - 1 , which receives the RF signal through the RF_1_SISO input port, outputs the RF signal intact to the combine-divider  113 . The base-station signal driving unit  111 - 2 , which receives the RF signal through the RF_2_SISO input port, outputs the RF signal intact to the combine-divider  113 . The base-station signal driving unit  111 - 3 , which receives the RF signal through the RF_2_MIMO input port, modulates the received RF signal to up or down convert a band thereof and outputs a resulting signal to the combine-divider  113 . The base-station signal driving unit  111 - 4 , which receives the RF signal through the RF_1_MIMO input port, modulates the RF signal to up or down convert a band thereof and outputs a resulting signal to the combine-divider  113 . 
     The combine-divider  113  mixes the four signals and outputs a mixture of the signals to the ODU  130 . The ODU  130  converts the mixed signal into an optical RF signal and distributes the optical RF signal. 
     In the illustrated exemplary embodiment, the remote optical unit  150 - 1  may be configured as the exemplary embodiment of  FIG. 9  and the remote optical unit  150 - 2  may be configured as the exemplary embodiment of  FIG. 10 . The operations of the remote optical units  150 - 1  and  150 - 2  in accordance with the exemplary embodiment of  FIG. 18  will be described with reference to  FIGS. 9 and 10 . 
     Referring to  FIG. 9 , the remote optical unit  150 - 1  processes signals of two channels and has a suitable configuration capable of transmitting RF signals of two bands, i.e., the RF_1_SISO band and the RF_2_SISO band, which are output from the base-station signal driving units  111 - 1  and  111 - 2  of the BIU  110 . 
     An upper channel of the remote optical unit  150 - 1  extracts an RF signal of a specified frequency band, i.e., the RF_1_SISO band, from a photoelectrically converted RF signal output from the photoelectric conversion unit ( 151 ) through the bandpass filter  153 - 1 , amplifies the extracted RF signal, and outputs a resulting signal to the antenna. A lower channel of the remote optical unit  150 - 1  extracts an RF signal of a specified frequency band, i.e., the RF_2_SISO band, from the photoelectrically converted RF signal output from the photoelectric conversion unit ( 151 ) through the bandpass filter  153 - 2 , amplifies the extracted RF signal, and outputs a resulting signal to the antenna. The RF signal of the RF_1_SISO band and the RF signal of the RF_2_SISO band are mixed in the forward direction by the combine unit  159  and then transmitted to the antenna. Descriptions of an uplink will be omitted. 
     Referring to  FIG. 10 , the remote optical unit  150 - 2  processes signals of two channels and has a suitable configuration capable of transmitting RF signals of two bands, i.e., the RF_1_MIMO band and the RF_2_MIMO band, which are output from the base-station signal driving units  111 - 3  and  111 - 4  of the BIU  110 . More specifically, the remote optical unit  150 - 1  does not include a modulation/demodulation unit, whereas the remote optical unit  150 - 2  includes the modulation/demodulation unit  155 - 2 , which converts the RF signal of the RF_1_MIMO band into an RF signal of the RF_1 band, and the modulation/demodulation unit  155 - 4 , which converts the RF signal of the RF_2_MIMO band into an RF signal of the RF_2 band. 
     An upper channel of the remote optical unit  150 - 2  extracts an RF signal of another specified frequency band, i.e., the RF_1_MIMO band, from a photoelectrically converted RF signal output from the photoelectric conversion unit ( 151 ) through the bandpass filter  153 - 2 , demodulates the extracted RF signal to a specified frequency band, i.e., the RF_1 band through band-down conversion by the modulation/demodulation unit  155 - 2 , amplifies the demodulated RF signal in the amplifier  157 - 2 , and outputs a resulting signal to the antenna. A lower channel of the remote optical unit  150 - 2  extracts an RF signal of another frequency band, i.e., the RF_2_MIMO band, from the photoelectrically converted RF signal through the bandpass filter  153 - 4 , demodulates the extracted RF signal to a specified frequency band, i.e., the RF_2 band through band-down conversion by the modulation/demodulation unit  155 - 4 , amplifies the demodulated RF signal in the amplifier  157 - 4 , and outputs a resulting signal to the antenna. The RF signal of RF_1_MIMO band and the RF signal of RF_2_MIMO band are mixed in the forward direction by the combine unit  159  and then transmitted to the antenna. Descriptions of an uplink will be omitted. 
     As a result, the remote optical unit  150 - 1  transmits the data D 1  through the RF signals of the RF_1 band and the data D 2  through the RF signals of the RF_2 band, while the remote optical unit  150 - 2  transmits the data D 4  through the RF signals of the RF_1 band and the data D 3  through the RF signals of the RF_2 band. That is, since different data D 1 , D 2 , D 3 , and D 4  are sent out using two frequency bands RF_1 and RF_2, it is the same as functions of four cells or a single cell sectorized into four sectors. 
     That is, a band of the RF signal output from the first matching switch  250  of the lower small cell unit  230  matches a frequency of the bandpass filter of the RDU in the matched remote optical unit. In addition, in a case in which the band of the RF signal is converted in the base-station signal driving unit  111  of the BIU  110 , the band matches the frequency of the bandpass filter of the RDU inside the matched remote optical unit. 
     By generalizing the aspects of the proposed invention, it is possible to flexibly increase throughput when service capacity is additionally required. In other words, by varying a passband of the bandpass filter in each remote unit, the common controller additionally activates and allocates another physical layer, or controls the base-station signal driving unit to up or down convert a band of the signal to a band of the bandpass filter of a remote unit to be connected thereto in order to increase throughput for a physical layer that uses the same frequency band, and thereby an increase in throughput is generally achieved. 
     According to the proposed invention, it is possible to provide an expanded cellular communication using an existing analog DAS. 
     A number of examples have been described above. Nevertheless, it should be understood that various modifications may be made. For example, suitable results may be achieved if the described techniques are performed in a different order and/or if components in a described system, architecture, device, or circuit are combined in a different manner and/or replaced or supplemented by other components or their equivalents. Accordingly, other implementations are within the scope of the following claims.