Abstract:
A method and apparatus for hybrid multi-layer transmission includes receiving a multi-layer signal from a source device, wherein the multi-layer signal includes a plurality of sublayers. A quantity of the plurality of sublayers is decoded and partial information relating to the decoded sublayers is transmitted to a destination device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/811,404 filed Apr. 12, 2013, the contents of which are hereby incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    Multiple relay networks, in which a source encoder communicates with a destination device through a number of relays, may be utilized for a wide range of applications. Activity in a multiple relay network may focus on Gaussian networks in which a first hop amounts to a Gaussian broadcast channel from a source device to relays, and a second hop to a multiple access channel between relays and receivers. Various transmission strategies, including decode-and-forward (DF), compress-and-forward (CF), amplify-and-forward (AF) and hybrid AF-DF, may be utilized for communication in such a network. 
         [0003]    In a variation of a classical multi-relay channel, relays may be connected to the destination through digital backhaul links of finite-capacity. For example, this model may be utilized in cloud radio cellular networks, in which the base stations (BSs) may act as relays connected to the central decoder via finite-capacity backhaul links. 
         [0004]    Pooling multiple relays into a distributed multiple-input multiple-output (MIMO) system includes a number of issues that may need to be addressed. High-performance operation of such systems may require a centralized data and channel processor, which may place significant throughput and latency requirements on the backhaul links which connect the relays to the centralized processor. For example, in cloud radio cellular networks, where base stations act as relays connected to the central decoder in the cloud, the backhaul problem may be acute because the links may have a finite capacity that may be insufficient for traditional approaches. 
       SUMMARY 
       [0005]    A method and apparatus for hybrid multi-layer transmission is disclosed. The method includes receiving a multi-layer signal from a source device, wherein the multi-layer signal includes a plurality of sublayers. A quantity of the plurality of sublayers is decoded and partial information relating to the decoded sublayers is transmitted to a destination device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    A more detailed understanding may be had from the following description, given by way of example in conjunction with the accompanying drawings wherein: 
           [0007]      FIG. 1A  is a system diagram of an example communications system in which one or more disclosed embodiments may be implemented; 
           [0008]      FIG. 1B  is a system diagram of an example wireless transmit/receive unit (WTRU) that may be used within the communications system illustrated in  FIG. 1A ; 
           [0009]      FIG. 1C  is a system diagram of an example radio access network and an example core network that may be used within the communications system illustrated in  FIG. 1A ; 
           [0010]      FIG. 2  is an example system diagram of a network including multiple relays in communication with and encoder and a decoder via out-of-band digital backhaul links within given capacities; 
           [0011]      FIG. 3  is a flow diagram of an example method of multilayer transmission with hybrid relaying; 
           [0012]      FIG. 4  is an example diagram depicting achievable rates versus the backhaul capacity C 1 =C 2  in a symmetric network with M=2, P=0 dB, and g 1 =g 2 =10 dB ; and 
           [0013]      FIG. 5  is an example diagram depicting achievable rates versus the back haul capacity C 1 =C 2  per relay with M=2, P=0 dB, and [g 1 , g 2 ]=[0,10 ]dB. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1A  is a diagram of an example communications system  100  in which one or more disclosed embodiments may be implemented. The communications system  100  may be a multiple access system that provides content, such as voice, data, video, messaging, broadcast, etc., to multiple wireless users. The communications system  100  may enable multiple wireless users to access such content through the sharing of system resources, including wireless bandwidth. For example, the communications systems  100  may employ one or more channel access methods, such as code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrier FDMA (SC-FDMA), and the like. 
         [0015]    As shown in  FIG. 1A , the communications system  100  may include wireless transmit/receive units (WTRUs)  102   a ,  102   b ,  102   c ,  102   d , a radio access network (RAN)  104 , a core network  106 , a public switched telephone network (PSTN)  108 , the Internet  110 , and other networks  112 , though it will be appreciated that the disclosed embodiments contemplate any number of WTRUs, base stations, networks, and/or network elements. Each of the WTRUs  102   a ,  102   b ,  102   c ,  102   d  may be any type of device configured to operate and/or communicate in a wireless environment. By way of example, the WTRUs  102   a ,  102   b ,  102   c ,  102   d  may be configured to transmit and/or receive wireless signals and may include user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a smartphone, a laptop, a netbook, a personal computer, a wireless sensor, consumer electronics, and the like. 
         [0016]    The communications systems  100  may also include a base station  114   a  and a base station  114   b . Each of the base stations  114   a ,  114   b  may be any type of device configured to wirelessly interface with at least one of the WTRUs  102   a ,  102   b ,  102   c ,  102   d  to facilitate access to one or more communication networks, such as the core network  106 , the Internet  110 , and/or the other networks  112 . By way of example, the base stations  114   a ,  114   b  may be a base transceiver station (BTS), a Node-B, an eNode B, a Home Node B, a Home eNode B, a site controller, an access point (AP), a wireless router, and the like. While the base stations  114   a ,  114   b  are each depicted as a single element, it will be appreciated that the base stations  114   a ,  114   b  may include any number of interconnected base stations and/or network elements. 
         [0017]    The base station  114   a  may be part of the RAN  104 , which may also include other base stations and/or network elements (not shown), such as a base station controller (BSC), a radio network controller (RNC), relay nodes, etc. The base station  114   a  and/or the base station  114   b  may be configured to transmit and/or receive wireless signals within a particular geographic region, which may be referred to as a cell (not shown). The cell may further be divided into cell sectors. For example, the cell associated with the base station  114   a  may be divided into three sectors. Thus, in one embodiment, the base station  114   a  may include three transceivers, i.e., one for each sector of the cell. In another embodiment, the base station  114   a  may employ multiple-input multiple-output (MIMO) technology and, therefore, may utilize multiple transceivers for each sector of the cell. 
         [0018]    The base stations  114   a ,  114   b  may communicate with one or more of the WTRUs  102   a ,  102   b ,  102   c ,  102   d  over an air interface  116 , which may be any suitable wireless communication link (e.g., radio frequency (RF), microwave, infrared (IR), ultraviolet (UV), visible light, etc.). The air interface  116  may be established using any suitable radio access technology (RAT). 
         [0019]    More specifically, as noted above, the communications system  100  may be a multiple access system and may employ one or more channel access schemes, such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA, and the like. For example, the base station  114   a  in the RAN  104  and the WTRUs  102   a ,  102   b ,  102   c  may implement a radio technology such as Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA), which may establish the air interface  116  using wideband CDMA (WCDMA). WCDMA may include communication protocols such as High-Speed Packet Access (HSPA) and/or Evolved HSPA (HSPA+). HSPA may include High-Speed Downlink Packet Access (HSDPA) and/or High-Speed Uplink Packet Access (HSUPA). 
         [0020]    In another embodiment, the base station  114   a  and the WTRUs  102   a ,  102   b ,  102   c  may implement a radio technology such as Evolved UMTS Terrestrial Radio Access (E-UTRA), which may establish the air interface  116  using Long Term Evolution (LTE) and/or LTE-Advanced (LTE-A). 
         [0021]    In other embodiments, the base station  114   a  and the WTRUs  102   a ,  102   b ,  102   c  may implement radio technologies such as IEEE 802.16 (i.e., Worldwide Interoperability for Microwave Access (WiMAX)), CDMA2000, CDMA2000 1X, CDMA2000 EV-DO, Interim Standard 2000 (IS-2000), Interim Standard 95 (IS-95), Interim Standard 856 (IS-856), Global System for Mobile communications (GSM), Enhanced Data rates for GSM Evolution (EDGE), GSM EDGE (GERAN), and the like. 
         [0022]    The base station  114   b  in  FIG. 1A  may be a wireless router, Home Node B, Home eNode B, or access point, for example, and may utilize any suitable RAT for facilitating wireless connectivity in a localized area, such as a place of business, a home, a vehicle, a campus, and the like. In one embodiment, the base station  114   b  and the WTRUs  102   c ,  102   d  may implement a radio technology such as IEEE 802.11 to establish a wireless local area network (WLAN). In another embodiment, the base station  114   b  and the WTRUs  102   c ,  102   d  may implement a radio technology such as IEEE 802.15 to establish a wireless personal area network (WPAN). In yet another embodiment, the base station  114   b  and the WTRUs  102   c ,  102   d  may utilize a cellular-based RAT (e.g., WCDMA, CDMA2000, GSM, LTE, LTE-A, etc.) to establish a picocell or femtocell. As shown in  FIG. 1A , the base station  114   b  may have a direct connection to the Internet  110 . Thus, the base station  114   b  may not be required to access the Internet  110  via the core network  106 . 
         [0023]    The RAN  104  may be in communication with the core network  106 , which may be any type of network configured to provide voice, data, applications, and/or voice over internet protocol (VoIP) services to one or more of the WTRUs  102   a ,  102   b ,  102   c ,  102   d . For example, the core network  106  may provide call control, billing services, mobile location-based services, pre-paid calling, Internet connectivity, video distribution, etc., and/or perform high-level security functions, such as user authentication. Although not shown in  FIG. 1A , it will be appreciated that the RAN  104  and/or the core network  106  may be in direct or indirect communication with other RANs that employ the same RAT as the RAN  104  or a different RAT. For example, in addition to being connected to the RAN  104 , which may be utilizing an E-UTRA radio technology, the core network  106  may also be in communication with another RAN (not shown) employing a GSM radio technology. 
         [0024]    The core network  106  may also serve as a gateway for the WTRUs  102   a ,  102   b ,  102   c ,  102   d  to access the PSTN  108 , the Internet  110 , and/or other networks  112 . The PSTN  108  may include circuit-switched telephone networks that provide plain old telephone service (POTS). The Internet  110  may include a global system of interconnected computer networks and devices that use common communication protocols, such as the transmission control protocol (TCP), user datagram protocol (UDP) and the internet protocol (IP) in the TCP/IP internet protocol suite. The networks  112  may include wired or wireless communications networks owned and/or operated by other service providers. For example, the networks  112  may include another core network connected to one or more RANs, which may employ the same RAT as the RAN  104  or a different RAT. 
         [0025]    Some or all of the WTRUs  102   a ,  102   b ,  102   c ,  102   d  in the communications system  100  may include multi-mode capabilities, i.e., the WTRUs  102   a ,  102   b ,  102   c ,  102   d  may include multiple transceivers for communicating with different wireless networks over different wireless links. For example, the WTRU  102   c  shown in  FIG. 1A  may be configured to communicate with the base station  114   a , which may employ a cellular-based radio technology, and with the base station  114   b , which may employ an IEEE 802 radio technology. 
         [0026]      FIG. 1B  is a system diagram of an example WTRU  102 . As shown in  FIG. 1B , the WTRU  102  may include a processor  118 , a transceiver  120 , a transmit/receive element  122 , a speaker/microphone  124 , a keypad  126 , a display/touchpad  128 , non-removable memory  130 , removable memory  132 , a power source  134 , a global positioning system (GPS) chipset  136 , and other peripherals  138 . It will be appreciated that the WTRU  102  may include any sub-combination of the foregoing elements while remaining consistent with an embodiment. 
         [0027]    The processor  118  may be a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Array (FPGAs) circuits, any other type of integrated circuit (IC), a state machine, and the like. The processor  118  may perform signal coding, data processing, power control, input/output processing, and/or any other functionality that enables the WTRU  102  to operate in a wireless environment. The processor  118  may be coupled to the transceiver  120 , which may be coupled to the transmit/receive element  122 . While  FIG. 1B  depicts the processor  118  and the transceiver  120  as separate components, it will be appreciated that the processor  118  and the transceiver  120  may be integrated together in an electronic package or chip. 
         [0028]    The transmit/receive element  122  may be configured to transmit signals to, or receive signals from, a base station (e.g., the base station  114   a ) over the air interface  116 . For example, in one embodiment, the transmit/receive element  122  may be an antenna configured to transmit and/or receive RF signals. In another embodiment, the transmit/receive element  122  may be an emitter/detector configured to transmit and/or receive IR, UV, or visible light signals, for example. In yet another embodiment, the transmit/receive element  122  may be configured to transmit and receive both RF and light signals. It will be appreciated that the transmit/receive element  122  may be configured to transmit and/or receive any combination of wireless signals. 
         [0029]    In addition, although the transmit/receive element  122  is depicted in  FIG. 1B  as a single element, the WTRU  102  may include any number of transmit/receive elements  122 . More specifically, the WTRU  102  may employ MIMO technology. Thus, in one embodiment, the WTRU  102  may include two or more transmit/receive elements  122  (e.g., multiple antennas) for transmitting and receiving wireless signals over the air interface  116 . 
         [0030]    The transceiver  120  may be configured to modulate the signals that are to be transmitted by the transmit/receive element  122  and to demodulate the signals that are received by the transmit/receive element  122 . As noted above, the WTRU  102  may have multi-mode capabilities. Thus, the transceiver  120  may include multiple transceivers for enabling the WTRU  102  to communicate via multiple RATs, such as UTRA and IEEE 802.11, for example. 
         [0031]    The processor  118  of the WTRU  102  may be coupled to, and may receive user input data from, the speaker/microphone  124 , the keypad  126 , and/or the display/touchpad  128  (e.g., a liquid crystal display (LCD) display unit or organic light-emitting diode (OLED) display unit). The processor  118  may also output user data to the speaker/microphone  124 , the keypad  126 , and/or the display/touchpad  128 . In addition, the processor  118  may access information from, and store data in, any type of suitable memory, such as the non-removable memory  130  and/or the removable memory  132 . The non-removable memory  130  may include random-access memory (RAM), read-only memory (ROM), a hard disk, or any other type of memory storage device. The removable memory  132  may include a subscriber identity module (SIM) card, a memory stick, a secure digital (SD) memory card, and the like. In other embodiments, the processor  118  may access information from, and store data in, memory that is not physically located on the WTRU  102 , such as on a server or a home computer (not shown). 
         [0032]    The processor  118  may receive power from the power source  134 , and may be configured to distribute and/or control the power to the other components in the WTRU  102 . The power source  134  may be any suitable device for powering the WTRU  102 . For example, the power source  134  may include one or more dry cell batteries (e.g., nickel-cadmium (NiCd), nickel-zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion), etc.), solar cells, fuel cells, and the like. 
         [0033]    The processor  118  may also be coupled to the GPS chipset  136 , which may be configured to provide location information (e.g., longitude and latitude) regarding the current location of the WTRU  102 . In addition to, or in lieu of, the information from the GPS chipset  136 , the WTRU  102  may receive location information over the air interface  116  from a base station (e.g., base stations  114   a ,  114   b ) and/or determine its location based on the timing of the signals being received from two or more nearby base stations. It will be appreciated that the WTRU  102  may acquire location information by way of any suitable location-determination method while remaining consistent with an embodiment. 
         [0034]    The processor  118  may further be coupled to other peripherals  138 , which may include one or more software and/or hardware modules that provide additional features, functionality and/or wired or wireless connectivity. For example, the peripherals  138  may include an accelerometer, an e-compass, a satellite transceiver, a digital camera (for photographs or video), a universal serial bus (USB) port, a vibration device, a television transceiver, a hands free headset, a Bluetooth® module, a frequency modulated (FM) radio unit, a digital music player, a media player, a video game player module, an Internet browser, and the like. 
         [0035]      FIG. 1C  is a system diagram of the RAN  104  and the core network  106  according to an embodiment. As noted above, the RAN  104  may employ a UTRA radio technology to communicate with the WTRUs  102   a ,  102   b ,  102   c  over the air interface  116 . The RAN  104  may also be in communication with the core network  106 . As shown in  FIG. 1C , the RAN  104  may include Node-Bs  140   a ,  140   b ,  140   c , which may each include one or more transceivers for communicating with the WTRUs  102   a ,  102   b ,  102   c  over the air interface  116 . The Node-Bs  140   a ,  140   b ,  140   c  may each be associated with a particular cell (not shown) within the RAN  104 . The RAN  104  may also include RNCs  142   a ,  142   b . It will be appreciated that the RAN  104  may include any number of Node-Bs and RNCs while remaining consistent with an embodiment. 
         [0036]    As shown in  FIG. 1C , the Node-Bs  140   a ,  140   b  may be in communication with the RNC  142   a . Additionally, the Node-B  140   c  may be in communication with the RNC  142   b . The Node-Bs  140   a ,  140   b ,  140   c  may communicate with the respective RNCs  142   a ,  142   b  via an Iub interface. The RNCs  142   a ,  142   b  may be in communication with one another via an Iur interface. Each of the RNCs  142   a ,  142   b  may be configured to control the respective Node-Bs  140   a ,  140   b ,  140   c  to which it is connected. In addition, each of the RNCs  142   a ,  142   b  may be configured to carry out or support other functionality, such as outer loop power control, load control, admission control, packet scheduling, handover control, macrodiversity, security functions, data encryption, and the like. 
         [0037]    The core network  106  shown in  FIG. 1C  may include a media gateway (MGW)  144 , a mobile switching center (MSC)  146 , a serving GPRS support node (SGSN)  148 , and/or a gateway GPRS support node (GGSN)  150 . While each of the foregoing elements are depicted as part of the core network  106 , it will be appreciated that any one of these elements may be owned and/or operated by an entity other than the core network operator. 
         [0038]    The RNC  142   a  in the RAN  104  may be connected to the MSC  146  in the core network  106  via an IuCS interface. The MSC  146  may be connected to the MGW  144 . The MSC  146  and the MGW  144  may provide the WTRUs  102   a ,  102   b ,  102   c  with access to circuit-switched networks, such as the PSTN  108 , to facilitate communications between the WTRUs  102   a ,  102   b ,  102   c  and traditional land-line communications devices. 
         [0039]    The RNC  142   a  in the RAN  104  may also be connected to the SGSN  148  in the core network  106  via an IuPS interface. The SGSN  148  may be connected to the GGSN  150 . The SGSN  148  and the GGSN  150  may provide the WTRUs  102   a ,  102   b ,  102   c  with access to packet-switched networks, such as the Internet  110 , to facilitate communications between and the WTRUs  102   a ,  102   b ,  102   c  and IP-enabled devices. 
         [0040]    As noted above, the core network  106  may also be connected to the networks  112 , which may include other wired or wireless networks that are owned and/or operated by other service providers. 
         [0041]    As described in more detail below, a communication network may include a transmitter, (e.g., source/encoder), communicating with a receiver, (e.g., destination/decoder), through a number of out-of-band relays that are connected to the decoder through capacity-constrained digital backhaul links. A transmission and relaying strategy in which multi-layer transmission is used may leverage different decoding capabilities of the relays to enable hybrid DF and CF relaying. Each relay may forward part of a decoded message and a compressed version of the received signal. Utilizing a multi-layer strategy may facilitate decoding at the destination based on the information received from the relays. In an alternate broadcast coding approach, each layer may encode an independent message. As described below, each layer may encode a selected set of independent messages. 
         [0042]      FIG. 2  is an example system diagram of a network  200  including multiple relays in communication with an encoder and a decoder via out-of-band digital backhaul links within given capacities. As shown in the system  200  of  FIG. 2 , an encoder  210  communicates transmissions h (designated h 1 , h i , and h M ) to respective relays  220  (designated  220   1 ,  220   i , and  220   M ), which transmit a respective communication along backhaul links having capacity C (designated C 1 , C i , and C M ) to a decoder  230 .  FIG. 2  shows an example communication with the multiple relays  220  connected to the decoder  230  via out-of-band digital backhaul links within given capacities. In the example network  200 , h 1 =√{square root over (g 1 e jθ     1   )}, h i =√{square root over (g i e jθ     i   )}, and h M =√{square root over (g M e jθ     M   )}. For purposes of example, either the encoder  210 , the decoder  230 , or both, may be included in a base station. 
         [0043]    Accordingly, the network  200  depicts a variation of a multi-relay channel discussed, in which the relays  220  are connected to the destination, (i.e., decoder  230 ), through digital backhaul links of finite-capacity. One motivation for this model may come in the form of cloud radio cellular networks, in which the base stations may act as relays connected to a central decoder via the finite-capacity backhaul links. 
         [0044]    A transmission strategy that is based on multi-layer transmission and hybrid relaying may be utilized as described below. Hybrid relaying may be performed by having each relay  220  forward part of the decoded messages, which may amount to partial decode-and-forward (DF), along with a compressed version of the received signal, thus adhering also to the compress-and-forward (CF) paradigm. The multi-layer strategy used at the source may be designed so as to facilitate decoding at the destination based on the information received from the relays. 
         [0045]    The amount of information decodable at the relays  220  may depend on the generally different fading powers, (e.g., g 1  . . . , g M ). To leverage the different channel qualities, flexible decoding at the relays  220  may be enabled by adopting a multi-layer transmission strategy at the encoder  210 . For example, the transmitter, (i.e., encoder  210 ), splits its message into independent submessages or sublayers, (e.g., W 1 , . . . , W M+1 ), with corresponding rates R 1 , . . . , R M+1  in bit(s) per channel use (bit/c.u.), respectively. The idea is that message W 1  may be decoded by all relays  220 , message W 2  only by relays  2 , . . . , M, (i.e.,  220   2  . . .  220   M ), and so on. This way, relays  220  having better channel conditions may decode more information. Additionally, message W M+1  may be instead decoded only at the destination, (i.e., decoder  230 ). 
         [0046]    To encode these messages, the encoded signal may be given by 
         [0000]    
       
         
           
             
               
                 
                   
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                           k 
                           = 
                           1 
                         
                         
                           M 
                           + 
                           1 
                         
                       
                        
                       
                         
                           
                             P 
                             k 
                           
                         
                          
                         
                           X 
                           k 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
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                   1 
                 
               
             
           
         
       
     
         [0000]    where the signals X 1 , . . . , X M+1  are independent and distributed as XN(0,1), and the power coefficients P 1 , . . . , P M+1  are subject to the power constraint Σ k=1   M+1 P k ≦P. The signal X 1  may encode message W 1 , signal X 2  may encode both messages W 1  and W 2 , and so on. Accordingly, signal X k  may encode messages W 1 , . . . , W k  for k=1, . . . , M. Signal X k  may not only encode message W k , and signal X M+1  may encode message W M+1 . 
         [0047]    Therefore, Relay  1 , (i.e.,  220   1 ), may decode message W 1  from X 1 , while relay  2 , (i.e.,  220   2 ), may first decode message W 1  from X 1 , and then message W 2  from X 2  using its knowledge of W 1  and so on. Accordingly, relay k may decode messages W 1 , . . . , W k  for k=1, . . . , M. From standard information-theoretic considerations, the following conditions may be sufficient to guarantee that rates R k  are decodable by the relays 
         [0000]        R   k   ≦I ( X   k   ; Y   k   |X   1   , . . . , X   k−1 ),   Equation 2
 
         [0000]    for k=1, . . . , M. This may be because, in accordance with Equations 1 and 2, with k=1, namely R 1 ≦I(X 1 ; Y 1 ), may ensure that not only relay  1 , but all relays may decode message W 1 . Generalizing, the inequality for a given k may guarantee that not only relay k may decode message W k  after having decoded W 1 , . . . , W k 1 , but also all relays k+1, . . . , M may decode message W k . The signal X M+1 , and thus message W M+1  may be decoded by the destination, (i.e., decoder  230 ), only. 
         [0048]    As discussed above, relay i, (i.e.,  220   i ), may decode messages W 1 , . . . , W i . Accordingly, each ith relay  220  may transmit partial information about the decoded messages to the destination decoder  230  via the backhaul links. In other words, each relay  220  may transmit specific subsets of the bits that make up the decoded messages. The rate at which this partial information may be transmitted to the destination decoder  230  may be selected so as to enable the decoder  230  to decode messages W 1 , . . . , W M  jointly based on all the signals received from the relays  220 . C i   DF ≦C i  may be denoted as the portion of the backhaul capacity devoted to the transmission of the messages decoded by relay i. 
         [0049]    Beside the rate allocated to the transmission of each part of the decoded messages, relay i may utilize the residual backhaul link to transmit a compressed version Ŷ i  of the received signal Y i . The compression strategy at relay i may be characterized by the test channel p(ŷ i |y i ) according to conventional rate-distortion theory. Moreover, since the received signals at different relays  220  may be correlated with each other, a distributed source coding strategy may be utilized. Successive decoding may be used via, for example, Wyner-Ziv compression, with a given order Ŷ n(1) → . . . →Ŷ n(M) , where π(i) may be a given permutation of the relays  220  indices M. Thus, the decoder  230  may successfully retrieve the descriptions) Ŷ 1 , . . . , Ŷ M  if the conditions 
         [0000]        I ( Y   π(i)   ; Ŷ   π(i)   |Ŷ   {π(1), . . . , π(i−1)} )≦ C   π(i)   CF    Equation 3
 
         [0000]    are satisfied for all i=1, . . . , M, where C i   CF ≦C i  may be defined as the capacity allocated by relay i to communicate the compressed received signal Ŷ i  to the decoder  230 . It may be recalled that Equation 3 is the rate needed to compress Y π(i)  as Ŷ π(i)  given that the destination has side information given by the previously decompressed signals Ŷ π(1) , . . . , Ŷ π(i−1) . 
         [0050]    A Gaussian test channel p(ŷ i |y i ), may be utilized so that the compressed signal Ŷ i  may be expressed as: 
         [0000]        Ŷ   i   =Y   i   +Q   i ,   Equation 4
 
         [0000]    where the compression noise Q i : XN (0, σ i   2 )) may be independent of the received signal Y i  to be compressed. 
         [0051]    The destination decoder  230  may first recover the descriptions Ŷ i , . . . , Ŷ M  from the signals received by the relays  220 . This step may be dependent that the conditions in Equation 3 are satisfied. Having obtained Ŷ M ={Ŷ 1 , . . . , Ŷ M }, the destination, (i.e., decoder  230 ), may jointly decode the messages W 1 , . . . , W M  based on the partial information about these messages received from the relays  220  and on the compressed received signals Ŷ M . Finally, message W M+1  may be decoded. 
         [0052]      FIG. 3  is a flow diagram of an example method  300  of multilayer transmission with hybrid relaying.  FIG. 3  is an example of a hybrid DF-CF relaying. 
         [0053]    In step  310 , the channel state is acquired. For example, relay  220   i , (relay i), may estimate channel h i  for i=1, . . . , M. The relay i may report its channel hi to the decoder  230  for i=1, . . . , M. 
         [0054]    Transmission and compression strategies may be determined in step  320 . For example, the decoder  230  may compute power allocations P 1 , . . . , P M+1 , compression strategies β 1 , . . . , β M , and the ordering π for decompression as described above. The decoder  230  may inform the encoder  210  about the obtained power allocations P 1 , . . . , P M+1 . Additionally, the decoder  230  may inform relay  220 , (relay i), about the obtained compression strategy β i  for i=1, . . . , M. The rate R k  and corresponding modulation and coding strategy ENC k  to be used for layer k for k=1, . . . , M+1 may be computed by the decoder  230  and it may inform the encoder  210  and relay  220   i , (relay i) about the rate R i  and coding strategy ENC i  for i=1, . . . , M, as well as informing the encoder  210  about the rate R M+1  and coding strategy ENC M+1 . 
         [0055]    In step  330 , the encoder  210  transmits communications to the relays  220 . For example, the encoder  210 , for a message W k 1∈{1, . . . , 2 nR     k   } for k=1, . . . , M+1, may build codewords {X k,t } t=1   n =ENC k (W 1 , . . . , W k ) for k=1, . . . , M and {X M+1,t } t=1   n =ENC M+1 (W M+1 ). The encoder  210  may transmit the signal 
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         [0000]    in channel use for t=1, . . . , n. Relay  220   i , (relay i) may receive signal Y i,t =h i X t +Z t  for t=1, . . . , n. 
         [0056]    In step  340 , the relays  220  decode the communications, generate sequences and transmit information to the decoder  230 . For example, Relay  220   i , (relay i) may decode messages W 1 , . . . , W i  based on the sequence {Y i,t } t=1   n  for i=1, . . . , M, and may generate the sequence {Ŷ i,t } t=1   n  with each signal Ŷ i,t  obtained by quantizing Y i,t  with noise Q i,t ˜CN(0, β i   −1 −1), for example, Ŷ i,t =Y i,t +Q i,t  for i=1, . . . , M. Relay  220   i , (relay i) may also transmit partial information about the decoded messages W 1 , . . . , W i  and the index associated with the sequence {Ŷ i,t } t=1   n  to the decoder  230  via backhaul link of capacity C i  for i=1, . . . , M. 
         [0057]    In step  350 , the decoder  230  performs decompression and decoding, the decoder  230  may first recover the signals for {Ŷ i,t } t=1   n  for i=1, . . . , M with the ordering {Ŷ i,t } t=1   n → . . . →{Ŷ π(M),t } t=1   n  based on the indices collected from the relays  220 . The decoder  230  may decode jointly the message W 1 , . . . , W M  based on the partial information received from the relays  220  and on the compressed signals {Ŷ i,t } t=1   n  for i=1, . . . , M. Finally, the decoder  230  may decode the message W M+1  based on the signals {Ŷ i,t } t=1   n  for i=1, . . . , M and the decoded messages W 1 , . . . , W M . 
         [0058]    Below are examples of numerical results of a multi-layer transmission scheme with hybrid relaying described above as compared to conventional schemes. For reference, achievable rates may also be compared with the cutset upper bound 
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         [0059]    For purposes of example, the case with two relays may be focused on, for example, M=2. Single-layer schemes may be marked with the label ‘SL’ and multi-layer schemes with ‘ML’. For CF related schemes, the optimal ordering π opt  may be found via exhaustive search and may be observed to be π=(1,2) for all the simulated cases. 
         [0060]      FIG. 4  is an example diagram  400  depicting achievable rates versus the backhaul capacity C 1 =C 2  in a symmetric network with M=2, P=0 dB, and g 1 =g 2 =10 dB. As shown in  FIG. 4 , the performance in a symmetric setting may be examined by plotting the rate versus the backhaul capacities C 1 =C 2  when P=0 dB and g 1 =g 2 =10 dB. In this symmetric set-up, the optimized hybrid scheme may end up reducing to either the DF or the CF strategy at small and large backhaul capacity, respectively. The single-layer and multi-layer strategies may not be distinguishable since they show the same performance when the relays experience the same fading power, for example, g 1 =g 2 . This may be a result of multi-layer strategies being relevant only when two relays have different decoding capabilities. 
         [0061]      FIG. 5  is an example diagram  500  depicting achievable rates versus the back haul capacity C 1 =C 2  per relay with M=2, P=0 dB, and [g 1 , g 2 ]=[0,10] dB. As shown in  FIG. 5 , the performance may be observed versus the backhaul C 1 =C 2  with P=0 dB and asymmetric channel powers [g 1 , g 2 ]=[0,10] dB. Unlike the symmetric setting in  FIG. 4 , the multi-layer strategy may be beneficial compared to the single-layer (SL) transmission for both DF and Hybrid schemes. Moreover, unlike the setting of  FIG. 4 , the hybrid relaying strategy may show a performance advantage with respect to all other schemes. This may be the case for intermediate values of the backhaul capacities C 1 =C 2 . It may also be mentioned that, as C 1 =C 2  increases, the performance of DF schemes may be limited by the capacity of the better decoder, namely log 2 (1+10)=3.46 bit/c.u., while CF, and thus also the hybrid strategy, are able, for C 1 =C 2  large enough, to achieve the cutset bound. 
         [0062]    Although features and elements are described above in particular combinations, one of ordinary skill in the art will appreciate that each feature or element can be used alone or in any combination with the other features and elements. In addition, the methods described herein may be implemented in a computer program, software, or firmware incorporated in a computer-readable medium for execution by a computer or processor. Examples of computer-readable media include electronic signals (transmitted over wired or wireless connections) and computer-readable storage media. Examples of computer-readable storage media include, but are not limited to, a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). A processor in association with software may be used to implement a radio frequency transceiver for use in a WTRU, UE, terminal, base station, RNC, or any host computer.