Abstract:
The present invention is directed to improvements for distributed antenna systems and more particularly to methods and systems for improving uplink communications. In one embodiment, Aggregation Point Noise Blocking provides for blocking or filtering the noise contributed by one or more of the branches coupled to an aggregation point that are not carrying the signal from a particular terminal. Signal activity from a given terminal on a particular branch can be identified and that information can be used to selectively block or filter the signal noise contributed by the other branches to an aggregation point. The selective blocking or filtering can also include an attenuation function to attenuate the signal and provide dynamic range smoothing. In another embodiment the signal can be regenerated to produce a signal that has a restored or very high SNR. An uplink regeneration block can be provided which demodulates the uplink signal to recover the transmitted information and then re-modulates the transmitted information to regenerate a signal at a restored or very high SNR.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims any and all benefits as provided by law of U.S. Provisional Application No. 60/890,587 filed Feb. 19, 2007, which is hereby incorporated by reference in its entirety. 
    
    
     This application is related to U.S. application Ser. No. 12/033,226, filed on Feb. 19, 2008, which is hereby incorporated by reference in its entirety. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable 
     REFERENCE TO MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND 
     1. Technical Field of the Invention 
     The present invention is directed to improvements for distributed antenna systems and more particularly to methods and systems for improving uplink communications. 
     2. Description of the Prior Art 
     Distributed Antenna Systems (DAS) are used to provide and/or enhance coverage for wireless services such as Cellular Telephony and Medical Telemetry inside buildings and campuses. The general architecture of a DAS is depicted in  FIG. 1 . 
     The DAS  100  typically includes a source  102  (a transmitter, receiver or transceiver for sending and/or receiving a signal), a main aggregation point  104 , one or more remote or intermediate aggregation points  122 ,  124 ,  126  and two or more antennae  132 ,  134 ,  136  connected to the remote/intermediate aggregation points. The DAS can also include one or more terminals  140  for transmitting signals to and receiving signals from one or more of the antennae. 
     In referring to the signal flows in DAS systems, the term Downlink signal refers to the signal being transmitted by the source transmitter (e.g. cellular base station) to the terminals and the term Uplink signal refers to the signals being transmitted by the terminals to the source receiver. 
     Most wireless services have both an uplink and a downlink, but some have only a downlink (e.g. a mobile video broadcast service) or only an uplink (e.g. certain types of medical telemetry). 
     One measure of signal quality for a wireless signal is its Signal-to-Noise-Ratio (SNR). It represents the ratio of the relative power level of the desired signal to the power level of the undesired noise in the bandwidth specific to that signal. The higher the SNR, the “better” the signal is. Every receiver requires a minimal level of SNR in order to be able to correctly demodulate and/or decode the received signal. In the vicinity of the transmitter (such as a wireless terminal) which emits the signal, the SNR would typically be very high. 
     On its path to the receiver, the SNR of the uplink signal can decrease in one of two ways. In one way, the level of the signal is attenuated while the noise level remains constant. This is typical of the propagation of the signal in the air and through any passive elements of a DAS system. In another way, the level of the signal remains constant or is increased, but the noise level increases even more. This is typical of the propagation of a signal through active elements in a DAS such as amplifiers. 
     It should be noted that when multiple uplink paths are combined, as is the case in virtually every DAS, the resulting noise level is a combination of the noise levels of the different paths, while the resulting signal level would typically be equal to the highest signal level encountered on any single path (the typical situation would be for the signal level to be high on one branch of the DAS, corresponding to the area where the terminal generating the signal is located, and very low or non-existent on other branches). As a result, aggregating multiple uplink paths increases the noise level and therefore reduces the SNR of the uplink signal. 
     A qualitative analysis of the degradation of the uplink SNR can be achieved by tracing the path of the uplink signal from terminal to receiver. A generic diagram of an uplink path  200  is depicted in  FIG. 2 . Each such stage of the signal path can have an impact on the SNR. 
     Propagation through the air  212  attenuates the level of the signal, and the attenuation is greater for higher frequency signals than it is for lower frequency signals. The noise level, in the best case, is the inherent environmental noise, usually referred to as Thermal Noise. Typically, the level of this noise is −174 dBm/Hz. In some cases, the environment may exhibit a higher level of noise in the specific frequency range in which the signal of interest operates. 
     Generally, most of the SNR deterioration occurs in this segment of the path, since the signal attenuation in the air  212  is typically much worse than in any passive segment of the DAS (e.g. coax cable). One way to decrease the signal attenuation in this segment is to shorten the maximal distance between a terminal and the closest DAS antenna to the terminal, or increase the number of antennas covering each area (and thus increase their density and decrease the distance between them). However, there is a cost penalty associated with doing this. 
     The passive section  214  of the DAS  200  is defined as the series of passive elements (antenna, cables, filters, combiners, attenuators, etc.) that precede the first active gain element (amplifier) in the uplink path. Propagation through the passive section  214  of the DAS  200  typically attenuates the level of the signal. However, the antenna may increase the signal level, if it has positive gain in the frequency range of interest. In a well designed DAS  200 , the passive section  214  will maintain the level of noise at the Thermal level and might possibly reduce the noise to the Thermal level if, for example, it was higher coming into this section. On balance, the passive section  214  will typically decrease the SNR of the signal, more severely impacting signals of higher frequency. 
     Within the active portion  126  of the DAS  200 , the level of the signal can be controlled by adjusting the gain of the amplifiers in the different system elements. An amplifier however, introduces a minimal noise level which is typically higher than the Thermal noise level and increases commensurate with the gain of the amplifier, and thereby will always decrease the SNR of the signal. 
     At each aggregation point  218 , the noise levels of the different branches are combined. Since typically each uplink signal originating from a specific terminal will be present on only one of the DAS  200  branches being combined in the aggregation point  218 , the signal level of the combined signal (assuming unity gain) will be the same. The signal level does not change, while the noise level increases, and therefore the SNR is impaired. In an aggregation point  218  with N branches the increase in noise level, and therefore the decrease in SNR, expressed in dB would be 10 log N. 
     SUMMARY 
     The design and configuration of DAS systems can differ in the combinations of cabling they use (fiber, coax, CAT-5/6, etc.), the points in the signal path that are active (i.e include power amplification) as compared to those which are passive, as well as other aspects. The approach described in this document applies in general to any DAS architecture. 
     In accordance with the inventions, the following terminology is typically used: 
     Path Loss: the attenuation (or deterioration in SNR) that the signal undergoes as it propagates from the terminal to the nearest DAS antenna (expressed in dB). 
     DAS uplink Noise Figure (NF): the impact of the DAS on the SNR of the uplink signal (expressed in dB). The DAS NF is assumed to include the antenna gain. 
     Link Budget: the maximal SNR deterioration allowed in the path from the terminal to the receiver (expressed in dB). The sum of the Path Loss and the DAS uplink NF must not exceed the Link Budget. 
     We define the Uplink Performance of a DAS system as the degree to which it degrades the SNR of an uplink signal. This is usually referred to as the uplink Noise Figure (NF) of the system. For a given Link Budget, improved Uplink Performance can be leveraged in one of or a combination of the following two ways. 
     One way is to increase the spacing between the antennas, and thus decrease the number of antennas required to cover a given area. The increase in Path Losses, can be offset by the decrease in uplink NF to maintain the Link Budget and reduce the cost of the DAS. 
     The second way is to increase the total area that can be covered by a DAS system connected to a single receiver/transmitter. This follows from the fact that adding another branch to the DAS (to extend the coverage area) increases the NF of the DAS. However, if the NF of each branch is improved, the cumulative NF decreases and more branches can be added. Theoretically, any improvement of 3 dB in the NF of a single branch would allow for a doubling of the coverage area. 
     The benefits of increasing the coverage area of a DAS include: 1) For networking technologies that support sectorization (such as cellular networks), the ability to extend the coverage area associated with a single sector means that potentially less base-station equipment is required to cover a given area; 2) Some networking technologies, such as certain types of medical telemetry, do not support sectorization, meaning in essence that the whole coverage area must be connected to a single receiver/transmitter. In this case, the ability to increase the coverage area associated with a single receiver directly impacts the total coverage area in which this wireless service can be made available. 
     Another factor that impacts the uplink performance of a system is its Dynamic Range, the range of power levels, lowest to highest, that the system can handle. In a system composed of a well-designed DAS and a receiver, the receiver would typically have the more restricted dynamic range. The effect that a restricted dynamic range can have on a DAS deployment can be explained as follows. The strongest uplink signal is created when the terminal is as close as would be physically allowed to one of the DAS antennas (the Path Loss is the smallest possible). The weakest uplink signal is created when the terminal is as far away as is physically allowed from its nearest DAS antenna. The difference between the strongest signal and the weakest signal as described above must not exceed the dynamic range of the system. Since there is no way to limit the strongest signal, the only available way is to ensure the weakest signal is not too weak. As explained above, this can be accomplished by decreasing the distance between the antennas, with the associated negative financial impact. 
     A method that would allow a DAS to “smooth” out the differences between strong and weak uplink signals would help in overcoming the dynamic range limitations of the receiver and thus would allow for increasing the antenna spacing and the associated positive financial impact. Thus, improving the Uplink Performance of a DAS, and more particularly, its Dynamic Range behavior can provide tangible economical and functional benefits to a DAS. 
     In accordance with the invention, the SNR at an aggregation point can be improved by blocking or filtering the noise attributed to one or more of the branches that do not carry the signal. This can be accomplished by detecting whether there is signal activity from a particular terminal on any branch connected to an aggregation point and selectively blocking or filtering the input noise to be aggregated from one or more of the other branches connected to the aggregation point. 
     In accordance with the invention, the SNR of the uplink path can be improved by regenerating the signal at one or more points along the uplink path. This can be accomplished by using an Uplink Regeneration Block (URB) which can demodulate the signal and then re-modulate the signal at a very high SNR. While URBs can be located anywhere in the uplink signal path, there is a cost/performance tradeoff that can be used as a guide for determining the location and how may URBs are used. To reduce costs, the URBs can be located upstream of aggregation points in order to reduce the number of URB&#39;s used (lower cost) and improve the SNR of the aggregated signal, but to improve performance, the URBs should be located as far downstream as possible before the SNR has been deteriorated by the aggregation points (however many more URBs may be required). 
     These and other capabilities of the invention, along with the invention itself, will be more fully understood after a review of the following figures, detailed description, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a diagram of a distributed antenna system. 
         FIG. 2  is a diagrammatic view of an uplink path. 
         FIG. 3  is a diagrammatic view of an Aggregation Point Noise Blocking system according to the present invention. 
         FIG. 4  shows a flow chart of a method for selectively blocking noise at an aggregation point according to the present invention. 
         FIG. 5  is a diagrammatic view of an Uplink Regeneration Block according to the present invention. 
         FIG. 6  shows a flow chart of a method for regenerating a signal according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with the invention, two methods and systems for improving the Uplink Performance of a DAS are disclosed. Both methods allow for dynamic range “smoothing”, and thus further contribute to improving the uplink performance. 
     The Aggregation Point Noise Blocking (APNB) method and system minimizes noise accumulation in aggregation points of the DAS. 
     The Signal Regeneration method and system, includes regeneration of the signal along the uplink path of the DAS is order to improve the signal SNR to the values typical to a signal from a nearby transmitter. 
     Aggregation Point Noise Blocking 
     As explained above, at the aggregation points in the DAS, the noise from the various branches being aggregated is combined, thereby increasing the noise level, while the signal from a specific terminal, which typically would be present only on a single branch, does not increase accordingly. The result is a decrease in SNR which is proportional to the number of branches being aggregated. 
     According to the APNB invention, at any point in time the signal from a particular terminal is present only on one of the branches and there is no need to combine this signal with noise from other branches, so the noise from one or more of these branches can be blocked. The result is the elimination of the noise aggregation phenomena and its impact on the SNR. 
     In the case of a single terminal roaming the coverage area of the DAS, this “blocking” function would simply be the selection at every point in time of the single branch on which the terminal signal is present and the complete blocking of all other branches. However, a practical wireless system support multiple terminals, each of which can be located in a coverage area associated with a different branch of the aggregation point, and therefore a more granular “blocking” method can be used. The mechanism can be tailored to the specific wireless signals being handled by the DAS, and in particular to the technique being used to multiplex the signals of multiple terminals within the associated bandwidth. 
     One embodiment of the invention includes Frequency-based Multiplexing, a method and system in which each terminal is assigned a unique, non-overlapping frequency (“channel”) on which to transmit its uplink. All channels are assumed to have the same bandwidth. The blocking mechanism appropriate for Frequency-based systems would be one in which the system determines which channels are active on each branch and blocks (filters) the parts of the band associated with channels that are not active on that branch. Since a channel is assumed to be active on a single branch, it follows that, for every channel, the associated frequency slice will be passed through from a single branch and blocked on all other branches. The result will be the elimination of the noise accumulation from multiple branches. 
     Another embodiment of the invention includes Time-based Multiplexing, a method and system in which a single frequency channel is used by all terminals but each accesses the channel at a different time or time slot. Time-based systems may differ in whether the allocation of channel time to each terminal is structured and slotted or more “random”. Their shared property is that at every point in time only a single terminal is transmitting. The blocking mechanism appropriate for Time-based systems would be one in which the system determines at every point in time the branch on which the active terminal for that time frame is present, and blocks all other branches. As before, the result will be the elimination of the noise accumulation from multiple branches. 
     Another embodiment of the invention includes Hybrid Frequency/Time-based Multiplexing, a method and system in which a time-based multiplexing scheme is employed concurrently (and independently) over multiple frequency channels. Thus each terminal will be active at certain times over a specific frequency channel. Examples of such systems include the iDEN, TDMA and GSM wireless telephony systems (which have a structured time multiplexing nature) and 802.11-based wireless LAN systems (which use a dynamic, contention-based scheme to ensure that at most a single terminal is active on every channel at every point in time). The blocking mechanism appropriate for hybrid frequency/time multiplexing systems is a hybrid of the respective blocking mechanisms for the frequency-based and time-based systems. In other words, at every point in time the system must determine which channels are active on which branches, and for the duration of that activity cycle must block the frequency associated with that channel on the other branches. 
     In accordance with one embodiment of the invention, the APNB method and system can provide the following functions, Activity Identification, Selective Noise Blocking, and Signal Attenuation. The Activity Identification function can determine which frequency channel is active on which branch at every point in time or for every time slot. This can include the recognition that it is possible that a signal emanating from a specific terminal can be present on more than one branch as when the terminal is located on the border between coverage areas associated with different branches. In this case the branch having the strongest signal for this terminal can be selected. The Selective Noise Blocking (SNB) function can provide that for every active channel, passing the signal from the “selected” branch through the aggregation point, and blocking the corresponding frequency range from other branches. The Signal Attenuation function can assist in dynamic range “smoothing” by attenuating strong signals. 
     The Activity Identification (AID) function can be tailored to the characteristics of the signal and the mobility characteristics of the terminals. For example, where the terminals in the system are of limited mobility, a manual configuration method can be used. In this configuration, an operator of the system manually configures the channels active on each branch, and for a time-based or hybrid time/frequency based scheme the time slot or time-slot/channel assignments associated with each branch. 
     In one embodiment of the invention, for example for a system having truly mobile terminals, an automatic scanning implementation can be used. In this embodiment, each branch is scanned for activity according to predefined time-slot and frequency assignments. Alternatively, each of the branches can be scanned simultaneously for activity on each of the possible frequencies that can be used. The nature of the activity detected, including the frequency band or channel, the time-slot (as necessary) and the signal strength (or an analogous measure of signal strength) or signal to noise ratio (SNR) can be sent to the controller. The controller can collect the activity information from each branch and store the information in memory. The controller can use an algorithm to determine, for each time period or time slot, which branches to block as a function of the activity detected on each branch. Where activity for a time period or time-slot is detected on only one branch, the other branches can be blocked. Where activity for a time period or time-slot is detected on more than one branch, but the activity is on the same frequency or channel, the branch corresponding to the activity with the greater signal strength or SNR can be allowed to pass to the combiner and the other channels blocked. Where the activity for a time period or time-slot is detected on more than one branch and includes multiple frequencies or channels, the controller can control each of the branches to selectively block or filter the branch to allocate only one branch for each frequency or channel over which activity was detected. In some alternative embodiments where signals from the same terminal are detected on more than one branch, it can be desirable to allow for the two or more signals (one from each of the active branches) to pass through to the combiner. 
     For each branch in the aggregation point, an Analog to Digital Converter (ADC) and a suitable Digital Signal Processor (DSP) can be used to scan the relevant frequency band for activity (e.g. a signal is present on one or more of known frequency channels) or the cessation of activity. The level of each active signal can be measured. A single ADC/DSP time-shared between the different branches can be used. 
     A central processing unit or control function can compare the results derived from each branch and create an integrated “activity map”. Where a specific channel is “active” on more than one branch, the central processing unit can select the branch having the strongest signal or highest power for that channel. Depending on the application, the controller can be implemented as a hardware based controller, such as part of a digital signal processor or a field programmable gate array, or as a high speed software controlled processor. 
     The scanning rate can depend on the characteristics of the signal. For frequency-based multiplexing the rate can be relatively slow, while for time-based or hybrid multiplexed systems the rate can be matched to the time-slot duration of the system. 
     Selective Noise Blocking (SNB) 
     Based on the “activity map” generated as described above, the system can determine for each and every point in time, which frequency ranges to pass through from a branch where activity was detected and which frequency ranges to block or filter from other branches where no activity was detected. In one embodiment of the invention, for each branch, an ADC is used to digitize the relevant frequency range. A DSP is then used to implement digital filters that pass through the selected frequency channels for this branch and block the rest. The signals from the different branches can be digitally combined, and then a Digital to Analog Converter (DAC) used to recreate an analog signal. 
       FIG. 3  shows a high level block diagram of an embodiment of the invention, an APNB  300  at an aggregation point and  FIG. 4  shows a flow chart showing the process for selective noise blocking according to an embodiment of the invention. The APNB  300  can include an ADC  310  connected to each branch  302 ,  304 ,  306  to be aggregated, an AID block  320  connected to each ADC  310  to detect signal activity and send signal activity information to the control block  360 , a SNB block  330  connected to each AID block  320  to selectively block or filter the signal if no signal activity is detected, a control block  360  for receiving the signal activity map information from the AID blocks  320  and providing the signal activity map information to the SNB blocks  330 , a digital combiner  340  for combining the processed and filtered digital signals from each branch and a DAC  350  for converting the combined digital signal to an analog signal  352  to be set upstream. At stage  402  the APNB block  300 , the input signal from each branch is processed similarly. At stage  410 , each received analog signal  302 ,  304 ,  306  is converted to a digital signal by the ADC  3   10 . At stage  420 , the digital signal is processed by the AID block  320  to determine if signal activity from a terminal is detected. At stage  422 , the absence or presence of signal activity can be sent to and stored in the control block  360  as the activity map. At stage  430 , the digital signal is then processed by the SNB block  330  to selectively block or filter the signal noise in the digital signal as a function of the absence or presence of signal activity detected or as a function of the activity map or a control signal from the control block  360 . At stage  430 , the SNB block  330  can also attenuate the digital signal to provide for dynamic range smoothing. At stage  440 , each of the digital branch signals is combined in the digital combiner  340  to produce a combined digital signal. At stage  450 , the combined digital signal is converted to an analog signal by DAG  350  and sent upstream. The resulting analog signal has an improved SNR because some or most of the noise from the inactive branches is not aggregated into the resulting signal. 
     It is possible to use the same ADC/DSP elements for both the Activity Identification function and the Selective Noise Blocking function, as well as the digital combining function. In one embodiment, a digital “delay line” can be inserted between the AID block  320  function and the SNB block  330  Function in order to provide the AID block  320  function with time to process the signal or signals before providing the information required for the SNB block  330  function to operate. This can be done using buffering capability associated with the DSP processing blocks. 
     Dynamic Range “Smoothing” 
     The APNB  300  mechanism can help “smooth” the dynamic range of the incoming signals by attenuating strong signals. This functionality can be integrated in the implementation described above for the SNB block  330  function. Specifically, the channel power measurements obtained at stage  420  in the AID block  320  function can be used by the SNB block  330  to determine at stage  430  whether a “pass-through” channel needs to be attenuated and if so by how much. 
     Enhancing Performance with Coherent Combining 
     In systems where signals can often be expected to be present on more than one branch of the aggregation point (for example, a system with a high density of antennas and where the aggregation point in question is “near” the antennas in the aggregation hierarchy), it is possible to improve the signal SNR even more by performing coherent combining of the signals from multiple branches. This can provide for improved performance in DAS systems were the terminals are highly mobile and move among several antennae. Referring to  FIG. 3  above, this functionality could be integrated into the “digital combining block”. 
     Signal Regeneration 
     In accordance with the invention, the system can regenerate signals along the uplink path such that their SNR is restored to levels typical of signals just being launched from the transmitter or the terminal. An Uplink Regeneration Block (URB) can include an Input Port, through which uplink signals are received at the RF level, and an Output Port, through which the regenerated signals are transmitted, again at the RF level. The URB can demodulate all active signals present on the Input Port. The specific processing will depend on the characteristics of the signal, but in general the functionality would be a subset of the functionality of the receiver typically associated with this signal. Depending on the modulation scheme used, the URB can be used to recover symbols rather than bits and reduce the required processing power. The URB can Re-modulate all demodulated signals and transmit them on the Output Port. The specific processing depends on the characteristics of the signal and the functionality of the previous block, but in general the functionality would be a subset of the functionality of (potentially multiple instances of) the transmitter typically associated with this signal. All regenerated signals can be transmitted at a predefined power level that can be predetermined in order to optimize the performance of the “real” receiver located at the “root” of the DAS. 
       FIG. 5  shows a high-level functional diagram of the Uplink Regeneration Block  500  (URB) and  FIG. 6  shows a flow chart of the uplink regeneration process, according to an embodiment of the invention. The URB  500  can include a Demodulation Block  510  for recovering the transmitted information  512  and a Re-modulation Block  520  for, using the recovered information  512 , re-generating the modulated signal  522  with a very high SNR. At stage  602 , the Demodulation Block  510  can receive the upstream signal  502  and, at stage  610 , demodulate the upstream signal  502  to recover the transmitted information  512 . At stage  620 , the Re-modulation block  520  can use the recovered information  512  to re-generate a modulated signal  522  that has a very high SNR which is sent upstream at stage  622 . 
     In an alternative embodiment, the URB  500  can include a single DSP-enabled processing block (e.g. DSP processor with associated periphery or FPGA with embedded DSP capabilities) to implement both the demodulation and re-modulation. The type and number of DSPs can depend on the signal characteristics and the processing power requirements of the different signals. If the required processing power exceeds that of a single processor, multiple DSPs can be employed in various architectures and configurations. 
     The regenerated signals can have very high SNR, comparable to a signal sent by the transmitter at the transmitting terminal. By transmitting the signals with the same power level and that level being optimized for the remainder of the uplink path, the Dynamic Range problem can be minimized. 
     The URBs can be deployed at any point along the uplink path in the DAS. There is, however, an associated cost/performance tradeoff associated with the deployment of URBs at various locations in the uplink path in the DAS. Locating the URBs upstream of aggregation points, after having combined several uplink branches, can reduce the number of URBs used as compared to having a URB on every branch prior to aggregation. Thus, from a cost perspective, the URBs can be located as upstream as possible. However, the greater improvement in Uplink Performance can be achieved by having the URBs further downstream, before the SNR of the received signal has been deteriorated by noise aggregation in the DAS. The reason is that the signals being regenerated by the URB are expected to have such a high SNR that the degradation caused by the DAS system would not impair at all the ability of the “main” receiver at the “root” of the DAS to correctly decode the signals and the main receiver is no longer the limiting factor of the system. Instead, the limiting factor would be the ability of the demodulation block in the URB to decode the signals, based on the SNR of the signals at its Input Port. From a system performance perspective therefore, the effective NF of the DAS could be considered the NF of the segment of the DAS up to the location of the URB. Thus placing the URB as far downstream as possible, before the SNR has been degraded by the DAS aggregation points, is likely to provide better performance but at a higher cost because more URB&#39;s may be used. 
     Other embodiments are within the scope and spirit of the invention. For example, due to the nature of software, functions described above can be implemented using software, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. 
     Further, while the description above refers to the invention, the description can include more than one invention.