Patent Publication Number: US-8989155-B2

Title: Systems and methods for wireless backhaul in distributed-input distributed-output wireless systems

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of the following co-pending U.S. patent applications: 
     U.S. application Ser. No. 13/475,598, entitled “System and Methods To Enhance Spatial Diversity In Distributed-Input Distributed-Output Wireless Systems.” 
     U.S. application Ser. No. 13/464,648, entitled “System and Methods to Compensate for Doppler Effects in Distributed-Input Distributed Output Systems.” 
     U.S. application Ser. No. 12/917,257, entitled “Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering” 
     U.S. application Ser. No. 12/802,988, entitled “Interference Management, Handoff, Power Control And Link Adaptation In Distributed-Input Distributed-Output (DIDO) Communication Systems” 
     U.S. Pat. No. 8,170,081, issued May 1, 2012, entitled “System And Method For Adjusting DIDO Interference Cancellation Based On Signal Strength Measurements” 
     U.S. application Ser. No. 12/802,974, entitled “System And Method For Managing Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDO Clusters” 
     U.S. application Ser. No. 12/802,989, entitled “System And Method For Managing Handoff Of A Client Between Different Distributed-Input-Distributed-Output (DIDO) Networks Based On Detected Velocity Of The Client” 
     U.S. application Ser. No. 12/802,958, entitled “System And Method For Power Control And Antenna Grouping In A Distributed-Input-Distributed-Output (DIDO) Network” 
     U.S. application Ser. No. 12/802,975, entitled “System And Method For Link adaptation In DIDO Multicarrier Systems” 
     U.S. application Ser. No. 12/802,938, entitled “System And Method For DIDO Precoding Interpolation In Multicarrier Systems” 
     U.S. application Ser. No. 12/630,627, entitled “System and Method For Distributed Antenna Wireless Communications” 
     U.S. Pat. No. 7,599,420, issued Oct. 6, 2009, entitled “System and Method for Distributed Input Distributed Output Wireless Communication”; 
     U.S. Pat. No. 7,633,994, issued Dec. 15, 2009, entitled “System and Method for Distributed Input Distributed Output Wireless Communication”; 
     U.S. Pat. No. 7,636,381, issued Dec. 22, 2009, entitled “System and Method for Distributed Input Distributed Output Wireless Communication”; 
     U.S. Pat. No. 8,160,121, issued Apr. 17, 2012, entitled, “System and Method For Distributed Input-Distributed Output Wireless Communications”; 
     U.S. Pat. No. 7,711,030, issued May 4, 2010, entitled “System and Method For Spatial-Multiplexed Tropospheric Scatter Communications”; 
     U.S. Pat. No. 7,418,053, issued Aug. 26, 2008, entitled “System and Method for Distributed Input Distributed Output Wireless Communication”; 
     U.S. Pat. No. 7,885,354, issued Aug. 2, 2011, entitled “System and Method For Enhancing Near Vertical Incidence Skywave (“NVIS”) Communication Using Space-Time Coding.” 
    
    
     BACKGROUND 
     Prior art multi-user wireless systems add complexity and introduce limitations to wireless networks which result in a situation where a given user&#39;s experience (e.g. available bandwidth, latency, predictability, reliability) is impacted by the utilization of the spectrum by other users in the area. Given the increasing demands for aggregate bandwidth within wireless spectrum shared by multiple users, and the increasing growth of applications that can rely upon multi-user wireless network reliability, predictability and low latency for a given user, it is apparent that prior art multi-user wireless technology suffers from many limitations. Indeed, with the limited availability of spectrum suitable for particular types of wireless communications (e.g. at wavelengths that are efficient in penetrating building walls), prior art wireless techniques will be insufficient to meet the increasing demands for bandwidth that is reliable, predictable and low-latency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent publication with color drawing(s) will be provided by the U.S. Patent and Trademark Office upon request and payment of the necessary fee. 
       A better understanding of the present invention can be obtained from the following detailed description in conjunction with the drawings, in which: 
         FIG. 1  illustrates one embodiment of the MU-MAS consists of a centralized processor  101 , a network  102  and M transceiver stations  103  communicating wirelessly to N client devices UE 1 -UE 4 ; 
         FIG. 2  illustrates one embodiment consisting of multiple routers or gateways  202  and high-speed wired links  205  that interconnect the centralized processor  201  to  202  as well as routers or gateways  202  among themselves; 
         FIG. 3  illustrates one embodiment of the BTS-AP  408  retransmits the streams of bits to multiple BTS-RPs  409  and  410  over point-to-multipoint wireless links; 
         FIG. 4  illustrates one embodiment of the BTS-AP  408  retransmits the streams of bits to multiple BTS-RPs  409  and  410  over point-to-multipoint wireless links; 
         FIG. 5  illustrates one embodiment of a DIDO system utilized the mesh network [5, 7-8]; 
         FIG. 6  illustrates one example of BSN deployment in downtown San Francisco, Calif.; 
         FIG. 7  illustrates one embodiment shows two stations downtown San Francisco with point-to-multipoint links. Note that any BTS-RP can repeat the signal to other BTS-NPs as well as shown on the left side of that figure; 
         FIG. 8  illustrates an exemplary mesh network deployment with nodes distributed around downtown San Francisco; 
         FIG. 9  illustrates one embodiment of solar panels  901  are connected to the charge controller  902  (the image of a meter is for illustration purposes only, and not a requirement for a charge controller) that charges the battery; 
         FIG. 10  illustrates one embodiment of the invention utilizes the rectenna; 
         FIG. 11  illustrates one embodiment of the rectenna  1001  is combined with the antenna  1007  used for the point-to-point/multipoint wireless link to form the receiver; 
         FIG. 12  illustrates one embodiment of a typical DIDO network consisting of active BTSs  1201  transmitting streams of bits to the UEs  1203  and inactive BTSs; 
         FIG. 13  illustrates one embodiment of all UEs may be moving to the same area; 
         FIG. 14  illustrates one embodiment of power consumption is not an issue and there are no inactive BTSs in the neighborhood of the UEs that can be turned on, another solution is to increase the transmit power at the BTSs far away from the UE cluster 
     
    
    
     DETAILED DESCRIPTION 
     One solution to overcome many of the above prior art limitations is an embodiment of Distributed-Input Distributed-Output (DIDO) technology. DIDO technology is described in the following patents and patent applications, all of which are assigned the assignee of the present patent and are incorporated by reference. These patents and applications are sometimes referred to collectively herein as the “related patents and applications.” 
     U.S. application Ser. No. 13/475,598, entitled “Systems and Methods to Enhance Spatial Diversity in Distributed Input Distributed Output Wireless Systems.” 
     U.S. application Ser. No. 13/464,648, entitled “System and Methods to Compensate for Doppler Effects in Distributed-Input Distributed Output Systems.” 
     U.S. application Ser. No. 12/917,257, entitled “Systems And Methods To Coordinate Transmissions In Distributed Wireless Systems Via User Clustering” 
     U.S. application Ser. No. 12/802,988, entitled “Interference Management, Handoff, Power Control And Link Adaptation In Distributed-Input Distributed-Output (DIDO) Communication Systems” 
     U.S. Pat. No. 8,170,081, issued May 1, 2012, entitled “System And Method For Adjusting DIDO Interference Cancellation Based On Signal Strength Measurements” 
     U.S. application Ser. No. 12/802,974, entitled “System And Method For Managing Inter-Cluster Handoff Of Clients Which Traverse Multiple DIDO Clusters” 
     U.S. application Ser. No. 12/802,989, entitled “System And Method For Managing Handoff Of A Client Between Different Distributed-Input-Distributed-Output (DIDO) Networks Based On Detected Velocity Of The Client” 
     U.S. application Ser. No. 12/802,958, entitled “System And Method For Power Control And Antenna Grouping In A Distributed-Input-Distributed-Output (DIDO) Network” 
     U.S. application Ser. No. 12/802,975, entitled “System And Method For Link adaptation In DIDO Multicarrier Systems” 
     U.S. application Ser. No. 12/802,938, entitled “System And Method For DIDO Precoding Interpolation In Multicarrier Systems” 
     U.S. application Ser. No. 12/630,627, entitled “System and Method For Distributed Antenna Wireless Communications” 
     U.S. Pat. No. 7,599,420, issued Oct. 6, 2009, entitled “System and Method for Distributed Input Distributed Output Wireless Communication”; 
     U.S. Pat. No. 7,633,994, issued Dec. 15, 2009, entitled “System and Method for Distributed Input Distributed Output Wireless Communication”; 
     U.S. Pat. No. 7,636,381, issued Dec. 22, 2009, entitled “System and Method for Distributed Input Distributed Output Wireless Communication”; 
     U.S. Pat. No. 8,160,121, issued Apr. 17, 2012, entitled, “System and Method For Distributed Input-Distributed Output Wireless Communications”; 
     U.S. application Ser. No. 11/256,478, entitled “System and Method For Spatial-Multiplexed Tropospheric Scatter Communications”; 
     U.S. Pat. No. 7,418,053, issued Aug. 26, 2008, entitled “System and Method for Distributed Input Distributed Output Wireless Communication”; 
     U.S. application Ser. No. 10/817,731, entitled “System and Method For Enhancing Near Vertical Incidence Skywave (“NVIS”) Communication Using Space-Time Coding.” 
     To reduce the size and complexity of the present patent application, the disclosure of some of the related patents and applications is not explicitly set forth below. Please see the related patents and applications for a full description of the disclosure. 
     1. SYSTEM DESCRIPTION 
     We describe a multiuser (MU) multiple antenna system (MAS) for wireless transmissions comprising wireless and/or wireline backhaul to connect multiple antennas. The MU-MAS consists of a centralized processor  101 , a network  102  and M transceiver stations  103  communicating wirelessly to N client devices UE 1 -UE 4 , as depicted in  FIG. 1 . 
     The centralized processor unit  101  receives N streams of information with different network contents (e.g., videos, web-pages, video games, text, voice, etc., streamed from Web servers or other network sources C 1 -C 5 ) intended for different client devices. Hereafter, we use the term “stream of information” to refer to any stream of data sent over the network containing information that can be demodulated or decoded as a standalone stream, according to certain modulation/coding scheme or protocol, to produce certain voice, data or video content. In one embodiment, the stream of information is a sequence of bits carrying network content that can be demodulated or decoded as a standalone stream. 
     The centralized processor  101  utilizes precoding transformation to combine (according to certain algorithm) the N streams of information from the network content C 1 -C 5  into M streams of bits. The precoding transformation can be linear (e.g., zero-forcing [22], block-diagonalization [20-21], matrix inversion, etc.) or non-linear (e.g., dirty-paper coding [11-13] or Tomlinson-Harashima precoding [14-15], lattice techniques or trellis precoding [16-17], vector perturbation techniques [18-19]). Hereafter, we use the term “stream of bits” to refer to any sequence of bits that does not necessarily contain any useful bit of information and as such cannot be demodulated or decoded as a standalone stream to retrieve the network content. In one embodiment of the invention, the stream of bits is the complex baseband signal produced by the centralized processor and quantized over given number of bits to be sent to one of the M transceiver stations  103 . 
     In one embodiment, the MAS is a distributed-input distributed-output (DIDO) system as described in our previous patent applications [0002-0018]. In this embodiment, the DIDO system consists of:
         User Equipment (UE)  104 : An RF transceiver for fixed or mobile clients  104  receiving data streams over the downlink (DL) channel from the DIDO backhaul and transmitting data to the DIDO backhaul via the uplink (UL) channel   Base Transceiver Station (BTS)  103 : The BTSs  103  interface the DIDO backhaul with the wireless channel. BTSs of one embodiment are access points  103  consisting of DAC/ADC and radio frequency (RF) chain to convert the baseband signal to RF. In some cases, the BTS is a simple RF transceiver equipped with power amplifier/antenna and the RF signal is carried to the BTS via RF-over-fiber technology as described in our previous patent applications.   Controller (CTR): A CTR is one particular type of BTS  103  designed for certain specialized features such as transmitting training signals for time/frequency synchronization of the BTSs and/or the UEs, receiving/transmitting control information from/to the UEs, receiving the channel state information (CSI) or channel quality information from the UEs. One or multiple CTR stations can be included in any DIDO system. When multiple CTRs are available, the information to or from those stations are combined to increase diversity and improve link quality. In one embodiment, the CSI is received from multiple CTRs via maximum ratio combining (MRC) techniques to improve CSI demodulation. In another embodiment, the control information is sent from multiple CTRs via maximum ratio transmission (MRT) to improve SNR at the receiver side. The scope of the invention is not limited to MRC or MRT, and any other diversity technique (such as antenna selection, etc.) can be employed to improve wireless links between CTRs and UEs.   Centralized Processor (CP): The CP is a DIDO server  101  interfacing the Internet or other types of external networks with the DIDO backhaul. In one embodiment, the CP computes the DIDO baseband processing and sends the waveforms to the distributed BTSs for DL transmission   Base Station Network (BSN): The BSN is the network  102  connecting the CP  101  to the distributed BTSs  103  carrying information for either the DL or the UL channel. The BSN is a wireline or a wireless network or a combination of the two. For example, the BSN is a DSL, cable, optical fiber network, or line-of-sight or non-line-of-sight wireless link. Furthermore, the BSN is a proprietary network, or a local area network, or the Internet.       

     2. WIRELESS AND WIRELINE BACKHAUL 
     The embodiments of the present invention describe systems and methods for practical BSN deployment via wireless or wireline links (or combination of both) in DIDO systems. In one embodiment, the BSN is the wireline network in  FIG. 2  consisting of multiple routers or gateways  202  and high-speed wired links  205  that interconnect the centralized processor  201  to  202  as well as routers or gateways  202  among themselves. The wired links  205  carry the streams of bits to be sent to all the BTSs  204  connected to the same DIDO BSN. Routers and gateways are connected to switches or hubs  203  via wired links  206 . The wired links  206  carry only the streams of bits intended to the BTSs  204  connected to the same switch or hub  203 . The BTSs  204  send the streams of bits received from the centralized processor  201  simultaneously over the DIDO wireless link in a way that every UE recovers and demodulates its own stream of information. 
     The wired links  205  and  206  are comprised of various network technologies including, but not limited to, digital subscriber lines (DSL), cable modems, fiber rings, T1 lines, hybrid fiber coaxial (HFC) networks. Dedicated fiber typically has very large bandwidth and low latency, potentially less than a millisecond in a local region, but it is less widely deployed than DSL and cable modems. Today, DSL and cable modem connections typically have between 10-25 ms in last-mile latency in the United States, but they are very widely deployed. 
     The streams of bits sent over the BSN consist of baseband signals from the CP to the BTSs. Assuming each complex sample of the baseband signal is quantized over 32 bits (i.e., 16 for real and 16 for imaginary parts) the total bandwidth requirement for the BSN to operate the BTSs at 10 MSample/sec (i.e., a 10 MHz bandwidth) over the wireless DIDO links is 320 Mbps. Typically, only 16 or fewer bits of quantization are enough to represent the baseband signal with negligible error (especially if compression techniques are utilized to reduce the bandwidth requirements), thereby reducing the BSN throughput requirement to 160 Mbps or less. In one embodiment of the invention, the DIDO systems use compression techniques to reduce the amount of throughput required over the BSN backhaul. Moreover, DIDO technology has been proven to provide an order of magnitude increase in spectral efficiency over any existing wireless technology. Therefore, it is possible to relax the baseband throughput requirement at the BTS from 10 MSample/sec down to 5 Sample/sec or 1 Sample/sec, while providing comparable or higher per-user throughput over the wireless link than any conventional wireless communications systems. Hence, in practical DIDO deployments, the throughput requirement at the BSN can be as low as 16 Mbps to every BTS. 
     In another embodiment, c. When a wireless link is used between two BTSs, one BTS acts as an access point (BTS-AP)  308  that redistributes wirelessly the streams of bits to other remote BTSs. Every remote BTS receives its dedicated stream of bits from the BTS-AP and retransmits over the DIDO wireless link, acting as a repeater (BTS-RP)  309 . Note that the zigzagged single-arrowed lines in  FIG. 3  indicate wireless transmission streams of bits from the BTSs to the UEs over the DIDO links, whereas the straight double-arrowed lines indicate point-to-point wireless transmission over the BSN backhaul. 
     Some examples of wireless links used for the BSN backhaul are commercially available WiFi bridges operating in the ISM 2.4, 5.8 or 24 GHz bands [1-5], or wireless optics communications such as laser light transmission [6], or any other radio frequency (RF) or optics proprietary system that can provide high-throughput low-latency wireless network connections. Note that all of the above systems can achieve reliable connection speeds from 100 Mbps up to 1 Gbps or more, which is sufficient to enable high-speed wireless links between BTS-AP and BTS-RPs over the BSN. 
     In another embodiment, the BTS-AP  408  retransmits the streams of bits to multiple BTS-RPs  409  and  410  over point-to-multipoint wireless links, as shown in  FIG. 4 . The BTS-AP employs the same wireless resource (i.e., same time and same frequency band) for all links to the BTS-RPs, and avoids interference between links by creating very narrow beams (via highly directional antennas or antenna arrays using beamforming techniques). Narrow beams can also be used at the BTS-RPs to improve link quality and reduce interference from other neighbor locations. When remote locations are too close to each other and it is impractical to direct narrow beams to them without them interfering, the same wireless resource can be shared between those BTS-RPs via different multiple access techniques such as TDMA, FDMA, OFDMA or CDMA. 
     To provide network service from the BTS-AP to the BTS-RPs, the present invention employs point-to-point or point-to-multipoint line-of-sight (LOS) links. In another embodiment, the LOS may not be available and the link employs beamforming, MRT, MIMO or other diversity techniques to improve link quality in non-LOS (NLOS) links. 
     Another way to extend coverage area of the wireless backhaul is via mesh networks [5, 7-9]. A practical mesh network in downtown San Francisco, Calif. has been deployed by Webpass [8] using Wi-Fi transceivers operating in the ISM band that can achieve speeds from 45 Mbps up to 200 Mbps. As described above, these speeds would be sufficient for carrying the streams of bits from the CP to the BTSs in a practical BSN deployment. In one embodiment of the invention, a DIDO system utilized the mesh network [5, 7-8] in  FIG. 5  to extend coverage to multiple BTSs distributed across a wide area. Every loop of the mesh network comprises of one or multiple BTS-APs to guarantee continuous connection to other BTS-RPs, even in the occurrence of temporary or permanent network failures. 
     One example of BSN deployment in downtown San Francisco, Calif., is depicted in  FIG. 6 . The circles indicate locations that have access to fiber or other types of high-speed wireline network connections. Some of those locations may be equipped with BTS-APs, or routers and switches using antennas  307  for point-to-point wireless transmission of the streams of bits to other BTSs. The solid dots denote BTS-RPs  309 , or routers and switches employing antennas  307  to receive the streams of bits and retransmit them over the DIDO wireless links. Note that the BTS-AP can also retransmit its own stream of bits wirelessly over the DIDO link, thereby acting as a repeater too. 
     In one embodiment of the invention, one or more of the highest BTS-APs in the BSN broadcast control information to all other DIDO BTSs. Control information consists of training sequences or known pilots at given frequency used to recover time and frequency offsets at the BTSs. For example, the main BTS-AP sends one training sequence known by all other BTSs, such that those BTSs can estimate time and frequency offsets, and use them for time and frequency synchronization. In this scenario, the BTSs do not need any global positioning system (GPS) receiver to keep time and frequency synchronization among each other. 
     We observe that, though the layout in  FIG. 6  looks similar to typical cellular systems (i.e., with one main tower sending wireless signal to multiple locations), its framework and functionalities are fundamentally different. In fact, the BTS-AP sends streams of bits to the BTS-RPs as opposed to streams of information in cellular systems. Streams of bits are sent over the BSN backhaul in our invention, whereas the streams of information are sent from the tower to the subscribers (i.e., last part of the communications link, following the backhaul) in a cellular system. Also, the links from the BTS-AP to every BTS-RP are fixed point-to-point, meaning they use high directional antennas such that interference to other BTS-RPs is removed and the link quality is improved. On the other hand, cellular systems transmit energy over the entire cell or sector or part of sector (via beamforming) and avoids interference across clients by using different multiple access techniques (e.g., TDMA, FDMA, OFDMA, CDMA, SDMA). 
     When only one BTS-AP is insufficient to serve multiple BTS-RPs spread across a wide area, additional BTS-APs can be used to establish other point-to-point/multipoint links to the BTS-RPs. For example,  FIG. 7  shows two stations downtown San Francisco with point-to-multipoint links. Note that any BTS-RP can repeat the signal to other BTS-NPs as well as shown on the left side of that figure. 
       FIG. 8  depicts an exemplary mesh network deployment with nodes distributed around downtown San Francisco. A few key advantages of the mesh type of architecture are:
         Serendipitous Deployment: The BTSs can be placed anywhere it is convenient. For the BTS-AP the only requirements are connection to power source and to high-speed wired network. The BTS-RP has no need for network connection, since that can be established wirelessly, and it can be placed anywhere there is roof access available. In contrast, in prior art wireless systems, such as cellular, BTSs are restricted in their physical placement relative to one another and physical obstacles, often resulting in expensive, inconvenient or unsightly placement, or if a required placement is not available resulting in loss of coverage.   Better Coverage: Since the BTSs can be installed in a serendipitous fashion, it is realistic to assume that from almost any location around the service area it is possible to find at least one BTS connected to the mesh network. Hence, when installing a new BTS-RP there is good chances to find other BTS-RPs or BTS-APs in that neighborhood to access the mesh network. Likewise, any UE at a give location can see at least one or more BTSs to deliver its stream of information.   Lower Power Consumption: Every BTS does not need to transmit the streams of bits wirelessly to BTSs placed faraway. In fact, in a mesh network, the BTS reaches only one or a few BTSs in its neighborhood, thereby reducing transmit power requirement and yielding significant improvement in power consumption.   Higher Robustness to Network Failures: In a mesh network, one or multiple BTS-APs are connected to the same loop such that the network connection is always in place even in extreme situations when one or more BTS-APs shut down due to network failures or temporary power blackouts.       

     In one embodiment of the invention, the BSN is a serendipitous network where its nodes (e.g., BTS, BTS-AP or BTS-RP) are installed wherever it is convenient. The convenience of installation of the BTSs in the network is evaluated based on:
         availability of high-speed low-latency network or Internet connection, whether wire, wireless, fiber or other connectivity;   whether it is feasible to obtain authorization to install BTSs in certain special locations (e.g., rooftops, power poles, light poles, monuments) or there is any restriction imposed by FCC emission limits;   how inexpensive it is to lease locations designated for installation of the BTSs. One of the economical advantages of DIDO over cellular systems is obtained by keeping CAPEX as well as OPEX of network deployment to a minimum. This is achieved by installing a multitude of extremely inexpensive BTSs, whose overall CAPEX and OPEX are much lower than cellphone towers required to cover the same service area.       

     Some embodiments discussed herein and in the related patents and applications require: i) access to a wireline network connection; ii) a power outlet. Removing these two requirements can significantly simplify the installation and maintenance of BTSs making the DIDO network more serendipitous. As discussed above, it is possible to remove the first requirement by utilizing BTS-AP and BTS-RP to create point-to-point/multipoint wireless links. To eliminate the second requirement, embodiments of the invention provide two solutions: i) exploiting solar power; ii) employing wireless power transfer. 
     In one embodiment of the invention, solar panels  901  are connected to the charge controller  902  (the image of a meter is for illustration purposes only, and not a requirement for a charge controller) that charges the battery  903 , as shown in  FIG. 9 . The battery provides DC current to the BTS  905  via the inverter  904 , that converts the battery voltage to the voltage defined by the specs of the BTS. For example, if the BTS accepts input voltage of 6V and the battery provides 12V, the inverter converts 12V into 6V. The BTS is equipped with the antenna  906  to transmit and receive radio waves to/from the UEs via the wireless DIDO link. Moreover, the BTS is connected to another antenna  907  that provides network connectivity via the point-to-point/multipoint wireless link. Note that the antennas  906  and  907  can be tuned at the same frequency or different frequencies, depending on the part of the spectrum used for the two different types of links. For example, the DIDO link to antenna  906  can be designed to operate at VHF or UHF, whereas the point-to-point/multipoint link to antenna  907  may use the ISM band at microwaves (i.e., 5.8 GHz for WiFi). In one embodiment of the present invention, those links are not limited to any particular frequency of operation. 
     As an example, let us assume the BTS draws 3 Amps (A) of current at 6VDC input voltage to transmit 1 W over the wireless link, with less that 10% efficiency (i.e., accounting for power loss in the circuitry and use of class A linear power amplifiers, which are typically very inefficient). If the battery is rated for 60 Ah, the presently preferred embodiment would discharge it only down to 50% (or 30 Ah) to preserve its lifespan. Then, it would take approximately 10 hours to discharge the battery down to 50%, when the BTS is continuously powered on and operating at 1 W of radiated power. Typical commercially available solar panels operate with efficiency of about 20% to produce about 12 W/ft 2 . Assuming there is enough real estate to host a 5 ft 2  solar panel, the total power produced by the solar panel  901  is 60 W. Since the battery voltage is typically 12VDC, the solar panel  901  provides 5 A of current to the battery through the charge controller. Hence it would take approximately 6 hours to recharge the battery from 50% to a 100% full charge with that solar panel. This is a typical example of a self-sustained system, where the charging rate is faster than the discharging rate. Note that a short charging rate and long discharging rate is particularly convenient for night operation, when the solar panel is inactive due to the lack of sunlight. In another embodiment, multiple batteries and a switch are used to switch across different batteries, allowing independent charging/discharging cycles throughout the day and maintaining consistent power supply during the night hours. 
     In regions characterized by poor exposure to sunlight (e.g., rainy and cloudy places or locations close to the North and South Poles during the months of short daylight duration, shaded areas, etc.) solar panels may not be a practical solution to power up the BTS. One alternative is wireless power transfer. Another embodiment of the invention utilizes the rectenna  1001  in  FIG. 10  to provide DC current to the battery  1003  through the charge controller  1002 . The wireless power is transmitted to the rectenna  1001  via a highly directional antenna at a different location. In one embodiment, the wireless power transmitter is one or more BTS-APs with highly directional antennas to form a very narrow beam aimed to the location of the rectenna  1001 . The rectenna  1001  itself is equipped with a highly directional antenna to increase the amount of received power. 
     In another embodiment, the rectenna  1001  is combined with the antenna  1007  used for the point-to-point/multipoint wireless link to form the receiver  1107  shown in  FIG. 11 . This is a compact design where the same antenna  1107  is used to demodulate digital content coming from the wireless network as well as storing power to feed the battery and so the BTS. 
     3. RECONFIGURABLE BTS CONFIGURATION 
     We have described systems and methods for wireless backhaul to provide network connection to all BTSs within the same BSN. Next, we describe systems and methods to reconfigure the network topology based on the UE distribution. In a recent report by Morgan Stanley [10] it was shown that traffic distribution in current cellular systems is highly heterogeneous: in typical cellular networks, only 20% of the base stations carry the 80% of the data traffic. This effect is due to high concentration of wireless subscribers in densely populated metro areas. In these areas, data congestion occurs when multiple subscribers try to access the cellular network at the same time during the busy hours of the day, resulting in dropped calls, limited connection speed and poor coverage. 
     One key advantage of DIDO is its ability to dynamically reconfigure the network to adapt to the changing spatial distribution of the subscribers in a certain area and variable distribution of traffic over time.  FIG. 12  shows a typical DIDO network consisting of active BTSs  1201  transmitting streams of bits to the UEs  1203  and inactive BTSs  1202  that are connected to the BSN but are not transmitting any data over the wireless link. The circle around the active BTSs indicate their coverage area. In this scenario, the UEs are uniformly distributed around the given area and the CP has activated all BTSs in the proximity of the UEs to guarantee good coverage. 
     During certain times of the day, however, the distribution of the UEs may change. For example, in the case of a public event, all UEs may be moving to the same area as depicted in  FIG. 13 . In this case, the CP recognizes the change in UE distributions and activates the BTSs in closer proximity to the UEs, while setting the others in standby. In one embodiment, the CP determines the pathloss from every BTSs to every UE base on the CSI feedback or any training or other control information sent from the UEs to the BTSs via the uplink wireless channel. Note that the number of active BTSs is kept constant as the UE configuration changes over time to guarantee there are enough degrees of freedom over the wireless link to create parallel non-interfering channels to all UEs. Moreover, setting some of the active BTSs to standby is one way to save power as well as keep computational complexity at the CP constant. In another embodiment, the active BTSs are not set to standby as the UE distribution changes, thereby guaranteeing better coverage at the expense of more power consumption and higher computational complexity at the CP. 
     If power consumption is not an issue and there are no inactive BTSs in the neighborhood of the UEs that can be turned on, another solution is to increase the transmit power at the BTSs far away from the UE cluster, as shown in  FIG. 14 . Increasing transmit power creates more interfering patterns across BTSs that are exploited by the DIDO precoder to create non-interfering data streams. 
     4. REFERENCES 
     
         
         [1] Ubuquiti, “airMAX”, 
         [2] Ubuquiti, “airFiber”, 
         [3] MikroTik, “Routerboard”, 
         [4] Ruckus wireless, “Long-range 802.11n Wi-Fi point-to-point/multipoint backhaul”, 
         [5] DigitalAir wireless, “Outdoor wireless”, 
         [6] DigitalAir wireless, “GeoDesy laser links 1.25 Gbps full duplex”, 
         [7] Netsukuku, 
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