Patent Publication Number: US-2015085840-A1

Title: Time division long term evolution (td-lte) frame structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Patent Application No. 61/883,169 filed on Sep. 26, 2013, in the names of Ruoheng LIU, et al., the disclosure of which is expressly incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     Aspects of the present disclosure relate generally to wireless communication systems, and more particularly to modification of a time division long term evolution (TD-LTE) frame structure. 
     2. Background 
     Wireless communication systems are widely deployed to provide various telecommunication services such as telephony, video, data, messaging, and broadcasts. Typical wireless communication systems may employ multiple-access technologies capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmit power). Examples of such multiple-access technologies include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, orthogonal frequency division multiple access (OFDMA) systems, single-carrier frequency divisional multiple access (SC-FDMA) systems, and time division synchronous code division multiple access (TD-SCDMA) systems. 
     These multiple access technologies have been adopted in various telecommunication standards to provide a common protocol that enables different wireless devices to communicate on a municipal, national, regional, and even global level. An example of an emerging telecommunication standard is Long Term Evolution (LTE). LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS) mobile standard promulgated by Third Generation Partnership Project (3GPP). It is designed to better support mobile broadband Internet access by improving spectral efficiency, lower costs, improve services, make use of new spectrum, and better integrate with other open standards using OFDMA on the downlink (DL), SC-FDMA on the uplink (UL), and multiple-input multiple-output (MIMO) antenna technology. However, as the demand for mobile broadband access continues to increase, there exists a need for further improvements in LTE technology. Preferably, these improvements should be applicable to other multi-access technologies and the telecommunication standards that employ these technologies. 
     This has outlined, rather broadly, the features and technical advantages of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described below. It should be appreciated by those skilled in the art that this disclosure may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the teachings of the disclosure as set forth in the appended claims. The novel features, which are believed to be characteristic of the disclosure, both as to its organization and method of operation, together with further objects and advantages, will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure. 
     SUMMARY 
     In one aspect, a method of wireless communication is disclosed. The method includes communicating with a base station using a special subframe that extends a guard period over an uplink pilot time slot and one or more disabled, adjacent uplink subframes. The method also includes associating a control information subframe with a specific downlink subframe while accounting for both cell radius extension and loss of the one or more disabled, adjacent uplink subframes used to communicate the extended special subframe. 
     In another aspect, a method of wireless communication is disclosed. The method includes communicating with a user equipment (UE) using a special subframe that extends over a guard period over an uplink pilot time slot and one or more disabled, adjacent uplink subframes. The method also includes associating control information of a specific subframe with an uplink subframe while accounting for both cell radius extension and loss of the one or more disabled, adjacent uplink subframes used to communicate the extended special subframe. 
     Another aspect discloses a wireless communication apparatus having a memory and at least one processor coupled to the memory. The processor(s) is configured to communicate with a base station using a special subframe that extends a guard period over an uplink pilot time slot and one or more disabled, adjacent uplink subframes. The processor(s) is also configured to associate a control information subframe with a specific downlink subframe while accounting for both cell radius extension and loss of the one or more disabled, adjacent uplink subframes used to communicate the extended special subframe. 
     Another aspect discloses a wireless communication apparatus having a memory and at least one processor coupled to the memory. The processor(s) is configured to communicate with a user equipment (UE) using a special subframe that extends over a guard period over an uplink pilot time slot and one or more disabled, adjacent uplink subframes. The processor(s) is also configured to associate control information of a specific subframe with an uplink subframe while accounting for both cell radius extension and loss of the one or more disabled, adjacent uplink subframes used to communicate the extended special subframe. 
     Additional features and advantages of the disclosure will be described below. It should be appreciated by those skilled in the art that this disclosure may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the teachings of the disclosure as set forth in the appended claims. The novel features, which are believed to be characteristic of the disclosure, both as to its organization and method of operation, together with further objects and advantages, will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, nature, and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout. 
         FIG. 1  is a diagram illustrating an example of a network architecture. 
         FIG. 2  is a diagram illustrating an example of an access network. 
         FIG. 3  is a diagram illustrating an example of a downlink frame structure in LTE. 
         FIG. 4  is a diagram illustrating an example of an uplink frame structure in LTE. 
         FIG. 5  is a diagram illustrating an example of a radio protocol architecture for the user and control plane. 
         FIG. 6  is a diagram illustrating an example of an evolved Node B and user equipment in an access network. 
         FIG. 7  is a block diagram conceptually illustrating an example of an air to ground communication system according to an aspect of the present disclosure. 
         FIG. 8  is a diagram conceptually illustrating an example of an aircraft antenna system according to an aspect of the present disclosure. 
         FIG. 9  is a block diagram showing how timing advance coordinates communications of user equipments (UEs) positioned at different distances from a base station. 
         FIG. 10  is a timing diagram in which a guard period (T GP ) prevents overlap between downlink and uplink communications at UE. 
         FIG. 11  is a timing diagram in which a duration of a guard period (T GP ) is insufficient, resulting in an overlap between downlink and uplink communications at a base station. 
         FIG. 12  is a block diagram illustrating conventional TD-LTE radio frame configurations. 
         FIG. 13  is a table illustrating special subframe component lengths according to the various special subframe configurations based on a normal cyclic prefix (CP). 
         FIG. 14  illustrates a time-domain resource allocation of synchronization and broadcast channels within the subframes of a TD-LTE radio frame structure. 
         FIG. 15  is a block diagram illustrating a modified radio frame structure according one aspect of the present disclosure. 
         FIGS. 16A and 16B  are block diagrams illustrating configurations of a TD-LTE radio frame structure with a first extended special subframe to support a first extended cell radius according one aspect of the present disclosure. 
         FIGS. 17A and 17B  are block diagrams illustrating other configurations of a TD-LTE radio frame structure with a first extended special subframe to support the first extended cell radius according one aspect of the present disclosure. 
         FIGS. 18A and 18B  are block diagrams illustrating configurations of a TD-LTE radio frame structure with a second extended special subframe to support a second extended cell radius according one aspect of the present disclosure. 
         FIGS. 19A and 19B  are block diagrams illustrating other configurations of a TD-LTE radio frame structure with a second extended special subframe to support a second extended cell radius according one aspect of the present disclosure. 
         FIG. 20  is a table of the guard time overhead associated with a next generation air to ground (AG) system configuration for supporting the first extended cell radius and the second extend cell radius as compared to a conventional (non-extended) cell radius. 
         FIG. 21  illustrates categorization of an air cell in multiple zones to support extended cell radii according to one aspect of the present disclosure. 
         FIGS. 22A and 22B  are block diagram illustrating nested frame structures according to one aspect of the present disclosure. 
         FIG. 23  further illustrates categorization of an air cell in multiple zones to support extended cell radii according to another aspect of the present disclosure. 
         FIG. 24  is a table illustrating a maximum downlink hybrid automatic repeat request (HARQ) processes based on a next generation AG system configuration according to an aspect of the present disclosure. 
         FIGS. 25A and 25B  illustrate configurations of a time division long term evolution (TD-LTE) radio frame structure including tables of downlink association set indexes, which represent the timing of acknowledgement (ACK)/negative acknowledgement (NACK) feedback when communicating with an extended special subframe according to an aspect of the present disclosure. 
         FIGS. 26A and 26B  are tables illustrating a downlink HARQ processes and timing, which may be used for determining downlink association set index k, i.e., the timing of acknowledgement (ACK)/negative acknowledgement (NACK) feedback in a next generation AG system according to an aspect of the present disclosure. 
         FIG. 27  is a table illustrating a uplink hybrid automatic repeat request (HARQ) processes based on a next generation AG system configuration according to another aspect of the present disclosure. 
         FIGS. 28A and 28B  illustrate configurations of time division long term evolution (TD-LTE) radio frame structures including tables of uplink association indexes, which represent the timing of physical uplink shared channel (PUSCH) transmission when communicating with an extended special subframe according to another aspect of the present disclosure 
         FIGS. 29A and 29B  illustrate configurations of a time division long term evolution (TD-LTE) radio frame structure including the timing of uplink grants transmitted by a base station and the relative timing of the associated physical uplink shared channel (PUSCH) transmission when communicating with an extended special subframe according to another aspect of the present disclosure. 
         FIGS. 30A and 30B  illustrate configurations of time division long term evolution (TD-LTE) radio frame structures including the timing of physical HARQ indicator channel (PHICH) and the relative timing of the corresponded physical uplink shared channel (PUSCH) transmission when communicating with an extended special subframe according to another aspect of the present disclosure. 
         FIGS. 31A and 31B  illustrate configurations of time division long term evolution (TD-LTE) radio frame structures including the factor m i  of the number of physical HARQ indicator channel (PHICH) groups for each downlink subframe when communicating with an extended special subframe according to another aspect of the present disclosure. 
         FIG. 32  is a flow diagram illustrating a method for modification of a time division long term evolution (TD-LTE) frame structure according to one aspect of the present disclosure. 
         FIG. 33  is a flow diagram illustrating a method for modification of a time division long term evolution (TD-LTE) frame structure according to another aspect of the present disclosure. 
         FIG. 34  is a block diagram illustrating different modules, means and/or components in an exemplary apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below, in connection with the appended drawings, is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the various concepts. It will be apparent to those skilled in the art, however, that these concepts may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring such concepts. As described herein, the use of the term “and/or” is intended to represent an “inclusive OR”, and the use of the term “or” is intended to represent an “exclusive OR”. 
     Aspects of the telecommunication systems are presented with reference to various apparatus and methods. These apparatus and methods are described in the following detailed description and illustrated in the accompanying drawings by various blocks, modules, components, circuits, steps, processes, algorithms, etc. (collectively referred to as “elements”). These elements may be implemented using electronic hardware, computer software, or any combination thereof. Whether such elements are implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. 
     By way of example, an element, or any portion of an element, or any combination of elements may be implemented with a “processing system” that includes one or more processors. Examples of processors include microprocessors, microcontrollers, digital signal processors (DSPs), field programmable gate arrays (FPGAs), programmable logic devices (PLDs), state machines, gated logic, discrete hardware circuits, and other suitable hardware configured to perform the various functionality described throughout this disclosure. One or more processors in the processing system may execute software. Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. 
     Accordingly, in one or more exemplary embodiments, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or encoded as one or more instructions or code on a non-transitory computer-readable medium. Computer-readable media includes computer storage media. Storage media may be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer. Combinations of the above should also be included within the scope of computer-readable media. 
       FIG. 1  is a diagram illustrating an LTE network architecture  100 . The LTE network architecture  100  may be referred to as an Evolved Packet System (EPS)  100 . The EPS  100  may include one or more user equipment (UE)  102 , an evolved UMTS terrestrial radio access network (E-UTRAN)  104 , an evolved packet core (EPC)  110 , a home subscriber server (HSS)  120 , and an operator&#39;s IP services  122 . The EPS can interconnect with other access networks, but for simplicity those entities/interfaces are not shown. As shown, the EPS provides packet-switched services, however, as those skilled in the art will readily appreciate, the various concepts presented throughout this disclosure may be extended to networks providing circuit-switched services. 
     The E-UTRAN includes the evolved Node B (eNodeB)  106  and other eNodeBs  108 . The eNodeB  106  provides user and control plane protocol terminations toward the UE  102 . The eNodeB  106  may be connected to the other eNodeBs  108  via a backhaul (e.g., an X2 interface). The eNodeB  106  may also be referred to as a base station, a base transceiver station, a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS), an extended service set (ESS), or some other suitable terminology. The eNodeB  106  provides an access point to the EPC  110  for a UE  102 . Examples of UEs  102  include a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a laptop, a personal digital assistant (PDA), a satellite radio, a global positioning system, a multimedia device, a video device, a digital audio player (e.g., MP3 player), a camera, a game console, or any other similar functioning device. The UE  102  may also be referred to by those skilled in the art as a mobile station, a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless device, a wireless communications device, a remote device, a mobile subscriber station, an access terminal, a mobile terminal, a wireless terminal, a remote terminal, a handset, a user agent, a mobile client, a client, or some other suitable terminology. 
     The eNodeB  106  is connected to the EPC  110  via, e.g., an S1 interface. The EPC  110  includes a mobility management entity (MME)  112 , other MMEs  114 , a serving gateway  116 , and a packet data network (PDN) Gateway  118 . The MME  112  is the control node that processes the signaling between the UE  102  and the EPC  110 . Generally, the MME  112  provides bearer and connection management. All user IP packets are transferred through the serving gateway  116 , which itself is connected to the PDN Gateway  118 . The PDN Gateway  118  provides UE IP address allocation as well as other functions. The PDN Gateway  118  is connected to the Operator&#39;s IP Services  122 . The operator&#39;s IP services  122  may include the Internet, the Intranet, an IP multimedia subsystem (IMS), and a PS streaming service (PSS). 
       FIG. 2  is a diagram illustrating an example of an access network  200  in an LTE network architecture. In this example, the access network  200  is divided into a number of cellular regions (cells)  202 . One or more lower power class eNodeBs  208  may have cellular regions  210  that overlap with one or more of the cells  202 . A lower power class eNodeB  208  may be a remote radio head (RRH), a femto cell (e.g., home eNodeB (HeNB)), a pico cell, or a micro cell. The macro eNodeBs  204  are each assigned to a respective cell  202  and are configured to provide an access point to the EPC  110  for all the UEs  206  in the cells  202 . There is no centralized controller in this example of an access network  200 , but a centralized controller may be used in alternative configurations. The eNodeBs  204  are responsible for all radio related functions including radio bearer control, admission control, mobility control, scheduling, security, and connectivity to the serving gateway  116 . 
     The modulation and multiple access scheme employed by the access network  200  may vary depending on the particular telecommunications standard being deployed. In LTE applications, OFDM is used on the downlink and SC-FDMA is used on the uplink to support both frequency division duplexing (FDD) and time division duplexing (TDD). As those skilled in the art will readily appreciate from the detailed description to follow, the various concepts presented herein are well suited for LTE applications. However, these concepts may be readily extended to other telecommunication standards employing other modulation and multiple access techniques. By way of example, these concepts may be extended to evolution-data optimized (EV-DO) or ultra mobile broadband (UMB). EV-DO and UMB are air interface standards promulgated by the 3rd Generation Partnership Project 2 (3GPP2) as part of the CDMA2000 family of standards and employs CDMA to provide broadband Internet access to mobile stations. These concepts may also be extended to universal terrestrial radio access (UTRA) employing wideband-CDMA (W-CDMA) and other variants of CDMA, such as TD-SCDMA; global system for mobile communications (GSM) employing TDMA; and evolved UTRA (E-UTRA), ultra mobile broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, and flash-OFDM employing OFDMA. UTRA, E-UTRA, UMTS, LTE and GSM are described in documents from the 3GPP organization. CDMA2000 and UMB are described in documents from the 3GPP2 organization. The actual wireless communication standard and the multiple access technology employed will depend on the specific application and the overall design constraints imposed on the system. 
     The eNodeBs  204  may have multiple antennas supporting MIMO technology. The use of MIMO technology enables the eNodeBs  204  to exploit the spatial domain to support spatial multiplexing, beamforming, and transmit diversity. Spatial multiplexing may be used to transmit different streams of data simultaneously on the same frequency. The data streams may be transmitted to a single UE  206  to increase the data rate or to multiple UEs  206  to increase the overall system capacity. This is achieved by spatially precoding each data stream (i.e., applying a scaling of an amplitude and a phase) and then transmitting each spatially precoded stream through multiple transmit antennas on the downlink. The spatially precoded data streams arrive at the UE(s)  206  with different spatial signatures, which enables each of the UE(s)  206  to recover the one or more data streams destined for that UE  206 . On the uplink, each UE  206  transmits a spatially precoded data stream, which enables the eNodeB  204  to identify the source of each spatially precoded data stream. 
     Spatial multiplexing is generally used when channel conditions are good. When channel conditions are less favorable, beamforming may be used to focus the transmission energy in one or more directions. This may be achieved by spatially precoding the data for transmission through multiple antennas. To achieve good coverage at the edges of the cell, a single stream beamforming transmission may be used in combination with transmit diversity. 
     In the detailed description that follows, various aspects of an access network will be described with reference to a MIMO system supporting OFDM on the downlink. OFDM is a spread-spectrum technique that modulates data over a number of subcarriers within an OFDM symbol. The subcarriers are spaced apart at precise frequencies. The spacing provides “orthogonality” that enables a receiver to recover the data from the subcarriers. In the time domain, a guard interval (e.g., cyclic prefix) may be added to each OFDM symbol to combat inter-OFDM-symbol interference. The uplink may use SC-FDMA in the form of a DFT-spread OFDM signal to compensate for high peak-to-average power ratio (PAPR). 
       FIG. 3  is a diagram  300  illustrating an example of a downlink frame structure in LTE. A frame (10 ms) may be divided into 10 equally sized subframes. Each subframe may include two consecutive time slots. A resource grid may be used to represent two time slots, each time slot including a resource block. The resource grid is divided into multiple resource elements. In LTE, a resource block contains 12 consecutive subcarriers in the frequency domain and, for a normal cyclic prefix in each OFDM symbol, 7 consecutive OFDM symbols in the time domain, for a total of 84 resource elements. For an extended cyclic prefix, a resource block contains 6 consecutive OFDM symbols in the time domain, resulting in 72 resource elements. Some of the resource elements, as indicated as R  302 ,  304 , include downlink reference signals (DL-RS). The DL-RS include Cell-specific RS (CRS) (also sometimes called common RS)  302  and UE-specific RS (UE-RS)  304 . UE-RS  304  are transmitted only on the resource blocks upon which the corresponding physical downlink shared channel (PDSCH) is mapped. The number of bits carried by each resource element depends on the modulation scheme. Thus, the more resource blocks that a UE receives and the higher the modulation scheme, the higher the data rate for the UE. 
       FIG. 4  is a diagram  400  illustrating an example of an uplink frame structure in LTE. The available resource blocks for the uplink may be partitioned into a data section and a control section. The control section may be formed at the two edges of the system bandwidth and may have a configurable size. The resource blocks in the control section may be assigned to UEs for transmission of control information. The data section may include all resource blocks not included in the control section. The uplink frame structure results in the data section including contiguous subcarriers, which may allow a single UE to be assigned all of the contiguous subcarriers in the data section. 
     A UE may be assigned resource blocks  410   a ,  410   b  in the control section to transmit control information to an eNodeB. The UE may also be assigned resource blocks  420   a ,  420   b  in the data section to transmit data to the eNodeB. The UE may transmit control information in a physical uplink control channel (PUCCH) on the assigned resource blocks in the control section. The UE may transmit only data or both data and control information in a physical uplink shared channel (PUSCH) on the assigned resource blocks in the data section. An uplink transmission may span both slots of a subframe and may hop across frequency. 
     A set of resource blocks may be used to perform initial system access and achieve uplink synchronization in a physical random access channel (PRACH)  430 . The PRACH  430  carries a random sequence. Each random access preamble occupies a bandwidth corresponding to six consecutive resource blocks. The starting frequency is specified by the network. That is, the transmission of the random access preamble is restricted to certain time and frequency resources. There is no frequency hopping for the PRACH. The PRACH attempt is carried in a single subframe (1 ms) or in a sequence of few contiguous subframes and a UE can make only a single PRACH attempt per frame (10 ms). 
       FIG. 5  is a diagram  500  illustrating an example of a radio protocol architecture for the user and control planes in LTE. The radio protocol architecture for the UE and the eNodeB is shown with three layers: Layer 1, Layer 2, and Layer 3. Layer 1 (L1 layer) is the lowest layer and implements various physical layer signal processing functions. The L1 layer will be referred to herein as the physical layer  506 . Layer 2 (L2 layer)  508  is above the physical layer  506  and is responsible for the link between the UE and eNodeB over the physical layer  506 . 
     In the user plane, the L2 layer  508  includes a media access control (MAC) sublayer  510 , a radio link control (RLC) sublayer  512 , and a packet data convergence protocol (PDCP)  514  sublayer, which are terminated at the eNodeB on the network side. Although not shown, the UE may have several upper layers above the L2 layer  508  including a network layer (e.g., IP layer) that is terminated at the PDN gateway  118  on the network side, and an application layer that is terminated at the other end of the connection (e.g., far end UE, server, etc.). 
     The PDCP sublayer  514  provides multiplexing between different radio bearers and logical channels. The PDCP sublayer  514  also provides header compression for upper layer data packets to reduce radio transmission overhead, security by ciphering the data packets, and handover support for UEs between eNodeBs. The radio link control (RLC) sublayer  512  provides segmentation and reassembly of upper layer data packets, retransmission of lost data packets, and reordering of data packets to compensate for out-of-order reception due to hybrid automatic repeat request (HARQ). The MAC sublayer  510  provides multiplexing between logical and transport channels. The MAC sublayer  510  is also responsible for allocating the various radio resources (e.g., resource blocks) in one cell among the UEs. The MAC sublayer  510  is also responsible for HARQ operations. 
     In the control plane, the radio protocol architecture for the UE and eNodeB is substantially the same for the physical layer  506  and the L2 layer  508  with the exception that there is no header compression function for the control plane. The control plane also includes a radio resource control (RRC) sublayer  516  in Layer 3 (L3 layer). The radio resource control (RRC) sublayer  516  is responsible for obtaining radio resources (i.e., radio bearers) and for configuring the lower layers using radio resource control signaling between the eNodeB and the UE. 
       FIG. 6  is a block diagram of an eNodeB  610  in communication with a UE  650  in an access network. In the downlink, upper layer packets from the core network are provided to a controller/processor  675 . The controller/processor  675  implements the functionality of the L2 layer. In the downlink, the controller/processor  675  provides header compression, ciphering, packet segmentation and reordering, multiplexing between logical and transport channels, and radio resource allocations to the UE  650  based on various priority metrics. The controller/processor  675  is also responsible for HARQ operations, retransmission of lost packets, and signaling to the UE  650 . 
     The transmit processor  616  implements various signal processing functions for the L1 layer (i.e., physical layer). The signal processing functions includes coding and interleaving to facilitate forward error correction (FEC) at the UE  650  and mapping to signal constellations based on various modulation schemes (e.g., binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), M-phase-shift keying (M-PSK), M-quadrature amplitude modulation (M-QAM)). The coded and modulated symbols are then split into parallel streams. Each stream is then mapped to an OFDM subcarrier, multiplexed with a reference signal (e.g., pilot) in the time and/or frequency domain, and then combined together using an inverse fast Fourier transform (IFFT) to produce a physical channel carrying a time domain OFDM symbol stream. The OFDM stream is spatially precoded to produce multiple spatial streams. Channel estimates from a channel estimator  674  may be used to determine the coding and modulation scheme, as well as for spatial processing. The channel estimate may be derived from a reference signal and/or channel condition feedback transmitted by the UE  650 . Each spatial stream is then provided to a different antenna  620  via a separate transmitter  618 TX. Each transmitter  618 TX modulates an RF carrier with a respective spatial stream for transmission. 
     At the UE  650 , each receiver  654 RX receives a signal through its respective antenna  652 . Each receiver  654 RX recovers information modulated onto an RF carrier and provides the information to the receiver processor  656 . The receiver processor  656  implements various signal processing functions of the L1 layer. The receiver processor  656  performs spatial processing on the information to recover any spatial streams destined for the UE  650 . If multiple spatial streams are destined for the UE  650 , they may be combined by the receiver processor  656  into a single OFDM symbol stream. The receiver processor  656  then converts the OFDM symbol stream from the time-domain to the frequency domain using a fast Fourier transform (FFT). The frequency domain signal comprises a separate OFDM symbol stream for each subcarrier of the OFDM signal. The symbols on each subcarrier, and the reference signal, is recovered and demodulated by determining the most likely signal constellation points transmitted by the eNodeB  610 . These soft decisions may be based on channel estimates computed by the channel estimator  658 . The soft decisions are then decoded and deinterleaved to recover the data and control signals that were originally transmitted by the eNodeB  610  on the physical channel. The data and control signals are then provided to the controller/processor  659 . 
     The controller/processor  659  implements the L2 layer. The controller/processor can be associated with a memory  660  that stores program codes and data. The memory  660  may be referred to as a computer-readable medium. In the uplink, the controller/processor  659  provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, control signal processing to recover upper layer packets from the core network. The upper layer packets are then provided to a data sink  662 , which represents all the protocol layers above the L2 layer. Various control signals may also be provided to the data sink  662  for L3 processing. The controller/processor  659  is also responsible for error detection using an acknowledgement (ACK) and/or negative acknowledgement (NACK) protocol to support HARQ operations. 
     In the uplink, a data source  667  is used to provide upper layer packets to the controller/processor  659 . The data source  667  represents all protocol layers above the L2 layer. Similar to the functionality described in connection with the downlink transmission by the eNodeB  610 , the controller/processor  659  implements the L2 layer for the user plane and the control plane by providing header compression, ciphering, packet segmentation and reordering, and multiplexing between logical and transport channels based on radio resource allocations by the eNodeB  610 . The controller/processor  659  is also responsible for HARQ operations, retransmission of lost packets, and signaling to the eNodeB  610 . 
     Channel estimates derived by a channel estimator  658  from a reference signal or feedback transmitted by the eNodeB  610  may be used by the TX processor  668  to select the appropriate coding and modulation schemes, and to facilitate spatial processing. The spatial streams generated by the TX processor  668  are provided to different antenna  652  via separate transmitters  654 TX. Each transmitter  654 TX modulates an RF carrier with a respective spatial stream for transmission. 
     The uplink transmission is processed at the eNodeB  610  in a manner similar to that described in connection with the receiver function at the UE  650 . Each receiver  618 RX receives a signal through its respective antenna  620 . Each receiver  618 RX recovers information modulated onto an RF carrier and provides the information to a RX processor  670 . The RX processor  670  may implement the L1 layer. 
     The controller/processor  675  implements the L2 layer. The controller/processor  675  can be associated with a memory  676  that stores program codes and data. The memory  676  may be referred to as a computer-readable medium. In the uplink, the controller/processor  675  provides demultiplexing between transport and logical channels, packet reassembly, deciphering, header decompression, control signal processing to recover upper layer packets from the UE  650 . Upper layer packets from the controller/processor  675  may be provided to the core network. The controller/processor  675  is also responsible for error detection using an ACK and/or NACK protocol to support HARQ operations. 
     Time Division Long Term Evolution (TD-LTE) Frame Structure Modification 
     The spectrum available for Internet communication to aircraft by terrestrial air to ground (ATG) systems is limited for practical and economic reasons. Providing seamless communication with aircraft flying at high altitudes over a large area (such as the continental U.S.) involves spectrum that is available over the large area. That is, the spectrum assigned to the ATG system should be available nationwide. It has been problematic, however, to identify a portion of spectrum that is available nationwide, much less arranging to free up such a portion of spectrum that is allocated for other uses. 
     A large amount of spectrum is assigned to geostationary satellites for use in broadcast TV and two way fixed satellite service (FSS). In one aspect of the present disclosure, a high data rate aircraft to ground communications antenna system provides an aircraft with Internet service. 
     In particular, aspects of the present disclosure provide methods and apparatus for a next generation air to ground (Next-Gen AG) system. The Next-Gen AG system may include ground base stations (GBSs) in communication with aircraft transceivers (ATs) in airplanes that may use an uplink portion of spectrum assigned for satellite systems. A system  700  for Next-Gen AG communication according to an illustrative aspect of the present disclosure is shown in  FIG. 7 . 
     In this configuration, the Next-Gen AG system  700  includes a ground base station  710  that transmits and receives signals on a satellite uplink band using a forward link (FL)  708 - 1  and a reverse link (RL)  706 - 1 . A first aircraft  750 - 1  includes an aircraft antenna  800  and aircraft transceiver (AT)  650  ( FIG. 6 ) in communication with the ground base station  710 . The aircraft transceiver (AT)  650  may also receive and transmit signals on the satellite uplink band using the forward link  708 - 1  and the return link  706 - 1 . In this configuration, the aircraft antenna  800  may include a directional antenna, for example, as shown in  FIG. 8 . 
       FIG. 8  shows one example of an aircraft antenna  800  having aircraft antenna arrays  802  ( 802 - 1 , . . . ,  802 -N) operating at, for example, 14 gigahertz (GHz). Representatively, the aircraft antenna array  802 - 1  has twelve horn antennas  804  ( 804 - 1 , . . . ,  804 - 12 ) each covering 30° sectors in azimuth with an aperture size of approximately 2.0 inches×0.45 inches, and having a gain of &gt;10 dBi (dB isotropic). In one configuration, an overall diameter of the antenna array is roughly 8 inches. Although described with reference to an aircraft antenna array, any directional antenna may be provided according to the aspects of the present disclosure. While the described aspect of the present disclosure are provided with reference to aircraft, the present disclosure is not limited thereto. Aspect of the present disclosure may apply to any current or future airborne objects that communicate with a ground station. 
     In this configuration, the aircraft antenna  800  includes a multi-beam switchable array that is able to communicate with the ground base station  710  at any azimuth angle. As shown in  FIG. 7 , the aircraft antenna  800  is mounted below the fuselage with a small protrusion and aerodynamic profile to reduce or minimize wind drag. In one configuration, the antenna elevation coverage is from approximately 3° to 20° below horizon to provide, for example, the pointing directions for the antenna gain. The aircraft antenna  800  may include an array N of elements positioned such that each element directs a separate beam at different azimuth angles, each covering 360/N degrees, for example, as shown in  FIG. 8 . 
     Although  FIG. 8  illustrates the aircraft antenna arrays  802  in a twelve-beam array configuration, it should be recognized that other configurations are possible while remaining within the scope of the present disclosure. In particular, one example configuration includes four-antenna arrays in a four-beam array configuration. In another configuration, a directional antenna may be provided as part of the Next-Gen AG system  700  while remaining within the scope of the present disclosure. 
     Referring again to  FIG. 7 , a second aircraft  750 - 2  includes a system having an aircraft antenna  800  that communicates with an aircraft transceiver (AT)  650 , as shown in  FIG. 6 . The aircraft antenna  800  is in communication with the ground base station  710  and also receives and transmits signals on the satellite uplink band using a forward link  708 - 2  and a return link  706 - 2 . 
     A Next-Gen AG system, for example, as shown in  FIG. 7 , may provide broadband connectivity to flying aircraft using an aircraft transceiver (AT)  650 , as shown in  FIG. 6 . In this configuration, the aircraft transceiver may operate according to a time division long term evolution (TD-LTE) air interface. In a time division duplex (TDD) terminal (e.g., the AT  650 ), however, a timing-advanced uplink transmission should not overlap with reception of any preceding downlink. 
     For example, a TD-LTE air interface may operate according to an orthogonal uplink intra-cell multiple access scheme. In this example, transmissions from different UEs (e.g., AT  650 ) in a cell are time aligned at the receiver of the eNodeB (e.g., the ground base station  710 ) to maintain uplink multiple access orthogonality. In operation, a timing advance may be applied at the UE transmitter to provide time alignment of the uplink transmissions relative to the received downlink timing. Using a timing advance at the base station may counteract the various propagation delays between different UEs. 
       FIG. 9  is a block diagram  900  in which a UE A, a UE B and a UE C are positioned at different distances from a base station  910 . The differing distances from the base station  910 , however, result in varying propagation delays from the different UEs to the base station  910 . In this example, the UE transmissions are orthogonal when they arrive at the base station and are made synchronous in the time domain by performing timing advance (TA) signaling at the base station. Generally, the application of the timing advance at the base station synchronizes the UE transmissions within a fraction of the CP (cyclic prefix) length. A timing advance command may be sent as a medium access control (MAC) element with a 0.52 microsecond timing resolution and from 0 up to a maximum of 0.67 milliseconds in a baseline TD-LTE configuration. In this example, the UE A receives a timing advance (α), UE B receives a timing advance (β) and UE C receives a timing advance (γ) to enable time alignment at the receiver of the base station  910 . 
     In TD-LTE, switching between transmit/receive functions occurs from downlink to uplink (UE switching from reception to transmission) and from uplink to downlink (eNodeB (base station) switching from reception to transmission). To preserve the orthogonality of the LTE uplink, propagation delays between an eNodeB and the UEs are compensated by a timing advance. For a time division duplex (TDD) system, the timing-advanced uplink transmission should not overlap with reception of any preceding downlink. 
     A TD-LTE air interface may prevent overlap between downlink and uplink communication by specifying a transmission gap (e.g., a guard period (GP)) between the downlink and uplink communications. The guard period between reception (downlink) and transmission (uplink) may be specified to accommodate a greatest possible timing advance and any switching delay. The timing advance of the TD-LTE air interface is a function of the round-trip propagation delay. In addition, the total guard time for an uplink-downlink cycle of a TD-LTE air interface may be longer than the worst round-trip propagation delay supported by a cell. 
       FIG. 10  is a timing diagram  1000  in which a guard period (T GP )  1012  between a downlink communication  1008 - 1  and an uplink communication  1006 - 1  of an eNodeB is selected to prevent overlap between a downlink communication  1008 - 2  and an uplink communication  1006 - 2  of a UE  1050 . To prevent the overlap, the guard period (T GP ) should exceed both a round-trip propagation delay (2T P ) and a receive-to-transmit switching delay (T UE-Rx-Tx )  1016  at the UE  1050 , where T P  denotes the one-way propagation delay. For example, the guard period (T GP ) may be computed according the following equation: 
         T   GP &gt;2 T   P   +T   UE-Rx-Tx   (1)
 
     The 3GPP LTE specification, however, is limited to a guard period duration of approximately 0.72 milliseconds. This guard period duration presumes a maximum one-hundred (100) kilometer cell radius. In a Next-Gen AG system, however, a larger cell size (e.g., a cell radius of two-hundred fifty (250) to three hundred fifty (350) kilometers) may be specified. 
       FIG. 11  is a timing diagram  1100  in which a duration of the guard period (T GP )  1112  between a downlink communication  1008 - 1  and an uplink communication  1006 - 1  of an eNodeB  1010  is insufficient, resulting in an overlap  1120  between a downlink communication  1008 - 2  and an uplink communication  1006 - 2  of the UE  1050 . As a result, using the 3GPP defined TDD frame structures leads to uplink-downlink overlap and significant signal degradation and data loss within a Next-Gen AG system. 
     In one aspect of the present disclosure, the frame structure used by an air interface of a Next-Gen AG system structure is modified. In one configuration, a TD-LTE frame structure with a two (2) millisecond special subframe is specified to support a cell radius on the order of two-hundred (200) to two-hundred fifty (250) kilometers. In another configuration, a TD-LTE frame structure with a three (3) millisecond special subframe is specified to support a cell radius on the order of three-hundred (300) to three-hundred fifty (350) kilometers. In a further configuration, a nested frame structure provides co-existence between different uplink-downlink subframe configurations. In one aspect of the present disclosure, air cells are categorized into multiple zones based on the distance to a base station (e.g., an eNodeB  610 ). In this aspect of the disclosure, different uplink/downlink subframe configurations corresponding to different round-trip propagation delays are used to accommodate communication with each of the multiple zones. 
     The nested frame structure enables dynamic variation as an airborne object moves from one zone to another. For example, the nested frame structure enables dynamic switching between various special subframes lengths in each zone. This dynamic switching may be achieved with or without a break in the call. When it is achieved without breaking the call, the nested frame structure becomes a dynamic frame structure. In one configuration, the nested frame structure dynamically varies between a non-extended special subframe, a first extended special subframe and a second extended special subframe as the UE moves between difference zones of an air cell (e.g., Zone 0, Zone 1 and Zone 2 of  FIG. 23 ). 
       FIG. 12  is a block diagram illustrating a conventional TD-LTE radio frame structure  1200 . Representatively, the conventional TD-LTE radio frame structure  1200  includes a subframe number  1230 , an uplink-downlink configuration column  1232  and a downlink-to-uplink switch-point periodicity column  1234 . In this example, the TD-LTE radio frame structure spans ten (10) milliseconds and consists of ten (10) one (1) millisecond subframes (SF 0, . . . , SF 9). The various subframes may be configured as a downlink (D) subframe, an uplink (U) subframe or a special (S) subframe. In this example, SF 1 is configured as a special subframe in each of the seven (0, . . . , 6) uplink-downlink configurations; SF 6 is configured as a special subframe in uplink-downlink configurations 0, 1, 2 and 6. 
     The special subframe  1240  serves as a switching point between downlink and uplink communications. The special subframe  1240  includes a downlink pilot time slot (DwPTS) portion  1242 , a guard period (GP) portion  1244  and an uplink pilot time slot (UpPTS) portion  1246 . In operation, the DwPTS portion  1242  of the special subframe  1240  may be treated as a regular but shortened downlink subframe. The DwPTS portion  1242  usually contains a reference signal (RS), control information and a primary synchronization signal (PSS). The DwPTS portion may also carry data transmissions. The UpPTS portion  1246  of the special subframe  1240  may be used for either a sounding reference signal (e.g., a one (1) symbol length) or a special (random access channel (RACH) for a small cell size (e.g., a two (2) symbol length). 
     As shown in  FIG. 12 , the GP portion  1244  of the special subframe  1240  provides a switching point between downlink and uplink communications. A length of the GP portion  1244  of the special subframe  1240  is one of the factors in determining the maximum supportable cell size. In this example a maximum length of the GP portion  1244  is: 
       MaxGPLength=10 OFDM symbols+10 CPs=0.714 milliseconds  (2)
 
       FIG. 13  is a table  1300  illustrating special subframe component lengths according to the various special subframe configurations based on a normal cyclic prefix (CP). The table  1300  includes a special subframe configuration column  1332 , a DwPTS column  1342 , a GP column  1344  and an UpPTS column  1346  within a component length column  1336 . In this example, the component lengths are indicated in units of orthogonal frequency division multiplexing (OFDM) symbols. 
       FIG. 14  illustrates the time-domain resource allocation of synchronization and broadcast channels within the subframes of a TD-LTE radio frame structure  1400  based on a configuration index  1432  and a subframe number  1430 . In this example, a primary synchronization signal (PSS) is allocated within the third OFDM symbol of subframe 1 and subframe 6 (e.g., every five (5) milliseconds of either a downlink subframe or a DwPTS portion of a special subframe). A secondary synchronization signal (SSS) is allocated within the last OFDM symbol of subframe 0 and subframe 5 (e.g., every five (5) milliseconds of a downlink subframe). A physical broadcast channel (PBCH) is allocated within OFDM symbols 7-10 of subframe 0 (e.g., every ten (10) milliseconds). A system information block of type 1 (SIB 1) is allocated within subframe 5 (e.g., an even radio frame). 
     In one aspect of the present disclosure, the radio frame structure used by an air interface of a Next-Gen AG system structure is modified to accommodate a larger cell radius. As noted, a TD-LTE air interface may prevent overlap between uplink and downlink communication by specifying a transmission gap (e.g., a guard period (GP)) between the downlink and uplink communications. The 3GPP LTE specification, however, is limited to guard period durations on the order of 0.714 milliseconds (see equation (2)). This guard period duration presumes a maximum one-hundred (100) kilometer cell radius. In a Next-Gen AG system, however, a larger cell size (e.g., a cell radius of two-hundred fifty (250) to three hundred fifty (350) kilometers) is specified. 
     In one aspect of the present disclosure, a special subframe is redesigned to enable downlink to uplink switching with a large round trip delay (RTD). As noted above in  FIG. 10 , overlap is prevented by specifying a guard period (T GP ) that exceeds a round-trip propagation delay (2T P ) and a receive-to-transmit switching delay (T UE-Rx-Tx )  1016  at the UE  1050 , where T P  denotes the one-way propagation delay. The guard period (T GP ) may be computed according to equation (1). For example, assuming an expanded cell radius of two-hundred fifty (250) kilometers (km), the round trip propagation delay when an aircraft is at a cell edge, is given by: 
       2 Tp (250 km)=(2×250 km)/speed-of-light≈1.67 milliseconds  (3)
 
     Assuming an expanded cell radius of three-hundred fifty (350) kilometers (km), the round trip propagation delay when an aircraft is at a cell edge, is given by: 
       2 Tp (350 km)=(2×350 km)/speed-of-light≈2.33 milliseconds  (4)
 
     The 3GPP LTE specification, however, is limited to a smaller guard period duration (e.g., 0.714 milliseconds) to support a maximum one-hundred (100) kilometer cell radius. Based on equation (1), for a two-hundred fifty (250) kilometer cell radius, the guard period is computed as follows: 
         T   GP &gt;1.67 milliseconds+ T   UE-Rx-Tx   (5)
 
     For a three-hundred fifty (350) kilometer cell radius, the guard period is computed as follows: 
         T   GP &gt;2.33 milliseconds+ T   UE-Rx-Tx   (6)
 
       FIG. 15  is a block diagram illustrating a modified radio frame structure  1500  according to one aspect of the present disclosure. This configuration of the modified radio frame structure  1500  maintains the 3GPP synchronization/broadcast channel structure shown in  FIG. 14 . In this configuration, subframes 0, 1, 5 and 6 are either downlink or special subframes to allow primary synchronization signal (PSS), secondary synchronization signal (SSS), broadcast control channel (BCCH), dynamic broadcast channel (D-BCH) and system information block of type 1 (SIB1) transmissions. Maintaining the 3GPP synchronization/broadcast channel structure shown in  FIG. 14  avoids complex hardware changes. 
       FIG. 16A  is a block diagram illustrating one configuration of a TD-LTE radio frame structure with a first extended special subframe (e.g., two (2) milliseconds) to support a first extended cell radius on the order of two-hundred fifty (250) kilometers. The frame structure  1600  has a ten (10) millisecond periodicity that includes an extended special subframe 1650 that extends over subframe 1 and subframe 2. This frame structure  1600  supports Next-Gen AG system configurations A and B, as noted by the configuration index  1632 . In this configuration, the Next-Gen AG system configuration A is based on uplink-downlink configuration zero (0), as shown in  FIG. 12 . In addition, the Next-Gen AG system configuration B is based on uplink-downlink configuration three (3), as shown in  FIG. 12 . 
       FIG. 16B  further illustrates a modified special subframe  1640  to enable formation of the extended special subframe  1650  shown in  FIG. 16A . The modified special subframe  1640  includes a downlink pilot time slot (DwPTS) portion  1642  and a guard period (GP) portion  1644 . An uplink pilot time slot (UpPTS) portion  1646  and the adjacent uplink subframe (e.g., SF 2 and/or SF 7) are omitted (muted) to extend the guard period (GP) portion  1644  to form the extended special subframe  1650  ( FIG. 16A ). For example, the guard period (GP) portion  1644  may be combined with a GP portion of a muted, adjacent uplink subframe (e.g., SF 2, SF 7 and SF 12) to provide a twenty five (25) OFDM symbol length (e.g. 1.785 ms), depending on whether a normal or extended cyclic prefix is used. In this configuration, the DwPTS portion  1642  of the modified special subframe  1640  is treated as a regular, but shortened downlink subframe. For example, the DwPTS portion  1642  may have a three (3) OFDM symbol length, used to transmit a reference signal (RS), control information, a primary synchronization signal (PSS), and the like. 
     In this configuration, special subframe configuration zero (0) is applied while muting the UpPTS portion  1646 . For example, the UpPTS portion  1646  may be muted by not scheduling any sounding reference signals. In Next Gen AG system configuration B, uplink subframe 2, adjacent to special subframe 1 is muted to provide the extended special subframe  1650  as a two (2) millisecond extended special subframe. In this example, the uplink subframe 2 is muted by not scheduling any uplink data transmissions during uplink subframe 2. Muting the uplink subframe 2 may also involve moving any acknowledgement (ACK)/negative acknowledgement (NACK) feedback to a next suitable subframe. Also, any channel quality information (CQI), precoding matrix indicator, and/or rank indicator information is not reported during the uplink subframe 2. In addition, no sounding reference signal (SRS), scheduling request (SR), and/or physical random access channel (PRACH) transmission are performed during the uplink subframe 2. In Next-Gen AG system configuration A, both uplink subframe 2, adjacent to special subframe 1, and uplink subframe 7, adjacent to special subframe 6 are muted to provide the extended special subframe  1650 . 
       FIG. 17A  illustrates another configuration of a TD-LTE frame structure  1700  with a first extended special subframe (e.g., two (2) milliseconds) also specified to support the first extended cell radius (e.g., two-hundred (200) to two-hundred fifty (250) kilometers). The TD-LTE frame structure  1700  has a twenty (20) millisecond periodicity with an extended special subframe  1750  that extends over special subframe 1 and uplink subframe 2. In this configuration, the extended special subframe  1750  includes a downlink pilot time slot (DwPTS) portion  1752  and an extended guard period (GP) portion  1754 . This TD-LTE frame structure  1700  supports Next-Gen AG system configuration C, as noted by the configuration index  1732 . In this configuration, the Next-Gen AG system configuration C dynamically switches between uplink-downlink configuration zero (0), and uplink-downlink configuration three (3), as shown in 
       FIG. 12 . For example, even subframes may use uplink-downlink configuration zero (0) and odd subframes may use uplink-downlink configuration three (3). 
       FIG. 17B  further illustrates a modified special subframe  1740  to enable formation of the extended special subframe  1750 , shown in  FIG. 17A . The modified special subframe  1740  includes a downlink pilot time slot (DwPTS) portion  1742  and a guard period (GP) portion  1744 . An uplink pilot time slot (UpPTS) portion  1746  and an adjacent uplink subframe (e.g., SF 2, SF 7 and/or SF 12) are omitted (e.g., muted) to extend the guard period (GP) portion  1744  to form the extended special subframe  1750  ( FIG. 17A ). In this configuration, the DwPTS portion  1742  of the modified special subframe  1740  is treated as a regular, but shortened downlink subframe. For example, the DwPTS portion  1742  may have a three (3) OFDM symbol length to transmit a reference signal (RS), control information, a primary synchronization signal (PSS), and the like. In this example, the guard period (GP) portion  1744  may be combined with a GP portion of a muted, adjacent uplink subframe (e.g., SF 2, SF 7 and SF 12) to provide a twenty five (25) OFDM symbol length (e.g. 1.785 ms). In one configuration, a maximum timing advance of approximately 1.67 milliseconds is applied at the base station (e.g., an eNodeB  610 ) to synchronize communication. 
     In this configuration, special subframe configuration zero (0) is also applied while muting the UpPTS portion  1746 . The UpPTS portion  1746  may be muted by not scheduling any sounding reference signals. For example, the uplink subframe 2, adjacent to special subframe 1 is muted to provide the extended special subframe  1750  as a two (2) millisecond extended special subframe. The uplink subframe 2 may be muted by not scheduling any uplink data transmissions during uplink subframe 2. Muting the uplink subframe 2 may also involve moving any acknowledgement (ACK)/negative acknowledgement (NACK) feedback to a next suitable subframe. Also, any channel quality information (CQI), precoding matrix indicator, and/or rank indicator information is not reported during uplink subframe 2. In addition, no sounding reference signal (SRS), scheduling request (SR), and/or physical random access channel (PRACH) transmission are performed during uplink subframe 2. 
       FIG. 18A  illustrates another configuration of a TD-LTE frame structure  1800  with a second extended special subframe (e.g., three (3) milliseconds) specified to support a second extended cell radius on the order of three-hundred (300) to three-hundred fifty (350) kilometers. The TD-LTE frame structure  1800  has a ten (10) millisecond periodicity with an extended special subframe  1850  that extends over subframe 1, subframe 2 and subframe 3. In this configuration, the extended special subframe  1850  includes a downlink pilot time slot (DwPTS) portion  1852  and an extended guard period (GP) portion  1854 . This TD-LTE frame structure  1800  supports Next-Gen AG system configurations D and E, as noted by the configuration index  1832 . In this configuration, the Next-Gen AG system configuration D is based on uplink-downlink configuration zero (0), as shown in  FIG. 12 . In addition, the Next-Gen AG system configuration E is based on uplink-downlink configuration three (3), as shown in  FIG. 12 . 
       FIG. 18B  illustrates a modified special subframe  1840  to enable formation of the extended special subframe  1850 , shown in  FIG. 18A . The modified special subframe  1840  also includes a downlink pilot time slot (DwPTS) portion  1842  and a guard period (GP) portion  1844 . An uplink pilot time slot (UpPTS) portion  1846  and the two contiguous, adjacent uplink subframes (e.g., SF 2 and SF 3, SF 7 and SF 8) are omitted (e.g., muted) to extend the guard period (GP) portion  1844  to form the extended special subframe  1850  ( FIG. 18A ). For example, the guard period (GP) portion  1844  may be combined with a GP portion of a muted, adjacent uplink subframe (e.g., SF 2 and SF 3, SF 7 and SF 8) to provide a thirty nine (39) OFDM symbol length (e.g. 2.72 milliseconds). In this configuration, the DwPTS portion  1842  of the modified special subframe  1840  is also treated as a regular, but shortened downlink subframe. For example, the DwPTS portion  1842  may have a three (3) OFDM symbol length to transmit a reference signal (RS), control information, a primary synchronization signal (PSS), and the like. 
     In this configuration, special subframe configuration zero (0) is also applied while muting the UpPTS portion  1846 . In this example, the UpPTS portion  1846  is muted by not scheduling any sounding reference signals. Representatively, uplink subframe 2 and uplink subframe 3 adjacent to special subframe 1 are muted to provide the extended special subframe  1850  as a three (3) millisecond extended special subframe. In this example, uplink subframe 2 and uplink subframe 3 are muted by not scheduling any uplink data transmissions during uplink subframes 2 and 3. Muting uplink subframes 2 and 3 may also involve moving any acknowledgement (ACK)/negative acknowledgement (NACK) feedback to a next suitable subframe. Also, any channel quality information (CQI), precoding matrix indicator, and/or rank indicator information is not reported during uplink subframes 2 and 3. In addition, no sounding reference signal (SRS), scheduling request (SR), and/or physical random access channel (PRACH) transmission are performed during the uplink subframes 2 and 3. 
       FIG. 19A  illustrates another configuration of a TD-LTE frame structure  1900  with a three (3) millisecond special subframe specified to support the second extended cell radius (e.g., three-hundred fifty (350) to four-hundred (400) kilometers). The TD-LTE frame structure  1900  has a twenty (20) millisecond periodicity with an extended special subframe  1950  that extends over subframes 1 to 3, 6 to 8 and 11 to 13. In this configuration, the extended special subframe  1950  includes a downlink pilot time slot (DwPTS) portion  1952  and an extended guard period (GP) portion  1954 . This TD-LTE frame structure  1900  supports Next-Gen AG system configuration F, as noted by the configuration index  1932 . In this configuration, the Next-Gen AG system configuration F dynamically switches between uplink-downlink configuration zero (0), and uplink-downlink configuration three (3), as shown in  FIG. 12 . For example, even subframes may use uplink-downlink configuration zero (0) and odd subframes may use uplink-downlink configuration three (3). 
       FIG. 19B  illustrates a modified special subframe  1940  to enable formation of the extended special subframe  1950 , shown in  FIG. 19A . The modified special subframe  1940  includes a downlink pilot time slot (DwPTS) portion  1942  and a guard period (GP) portion  1944 . An uplink pilot time slot (UpPTS) portion  1946  and the two contiguous, adjacent uplink subframes (e.g., SF 2 and SF 3, SF 7 and SF 8, SF 12 and SF 13) are omitted (e.g., muted) to extend the guard period (GP) portion  1944  to form the extended special subframe  1950  ( FIG. 19A ). In this configuration, the DwPTS portion  1942  of the modified special subframe  1940  is treated as a regular, but shortened downlink subframe. For example, the DwPTS portion  1942  may have a three (3) OFDM symbol length, used to transmit a reference signal (RS), control information, a primary synchronization signal (PSS), and the like. In this example, the guard period (GP) portion  1944  may be combined with a GP portion of a muted, adjacent uplink subframe (e.g., SF 2 and SF 3, SF 7 and SF 8, SF 12 and SF 13) to provide a thirty nine (39) OFDM symbol length (e.g. 2.72 milliseconds). In one configuration, a maximum timing advance of approximately 2.66 milliseconds is applied at the base station (e.g., an eNodeB  610 ) to synchronize communication. 
     In this configuration, special subframe configuration zero (0) is also applied while muting the UpPTS portion  1946 . The UpPTS portion  1946  may be muted by not scheduling any sounding reference signals. For example, uplink subframes 2 and 3 adjacent to special subframe 1 are muted to provide the extended special subframe  1950  as a three (3) millisecond extended special subframe. In addition, uplink subframes 7 and 8 as well as uplink subframes 12 and 13 are muted. Uplink subframe 2 and 3, 7 and 8, and 12 and 13 may be muted by not scheduling any uplink data transmissions during these uplink subframes. Muting these uplink subframes may also involve moving any acknowledgement (ACK)/negative acknowledgement (NACK) feedback to a next suitable subframe. Also, any channel quality information (CQI), precoding matrix indicator, and/or rank indicator information is not reported during these uplink subframes. In addition, no sounding reference signal (SRS), scheduling request (SR), and/or physical random access channel (PRACH) transmission are performed during these uplink subframes. 
       FIG. 20  is a table  2000  of the guard time overhead associated with the Next-Gen AG system configurations for supporting the first extended cell radius and the second extend cell radius as compared to a conventional (non-extended) cell radius. As noted above, the 3GPP LTE specification is limited to a guard time duration of approximately 0.72 milliseconds (e.g., 10 OFDM symbols). This guard period duration presumes a maximum one-hundred (100) kilometer cell radius, referred to herein as a non-extended cell radius. In a Next-Gen AG system, however, extended cell radii (e.g., a cell radius of two-hundred fifty (250) to three hundred fifty (350) kilometers) are specified. A guard time for a first extended cell radius (e.g., two-hundred fifty (250) kilometers) is approximately 1.78 milliseconds (e.g., twenty five (25) OFDM symbols). A guard time for a second extended cell radius (e.g., three-hundred fifty (350) kilometers) is approximately 2.72 milliseconds (e.g., thirty nine (39) OFDM symbols). 
     The table  2000  illustrates that supporting extended cell radii results in reduced system throughput as noted by the guard time (GT) overhead column. The system throughput loss due to the guard time overhead is in proportion to the coverage range (1:2.5:3.5). Supporting the extended cell radii involves a tradeoff between system throughput, uplink/downlink fairness (see DL-to-UL ratio column) and implementation complexity. The table  2000  illustrates that the Next-Gen AG system configurations B and F involve less guard time overhead, but with an unbalanced ratio of downlink/uplink flows. In addition, complexity varies between implementing an extended special subframe with a ten (10) millisecond periodicity and an extended special subframe with a twenty (20) millisecond periodicity. It should be noted that the DL-to-UL ratio column of the table  2000  does not include DwPTS in the special sub frame. 
     In a further configuration, a nested frame structure provides co-existence between different uplink-downlink subframe configurations. In one aspect of the present disclosure, air cells may be categorized into multiple zones based on the distance to a base station (e.g., an eNodeB  610 ). In this aspect of the disclosure, different uplink/downlink subframe configurations corresponding to different round-trip propagation delays may be used to accommodate communication with each of the multiple zones. 
       FIG. 21  illustrates categorization of an air cell  2100  into multiple zones to support extended cell radii according to one aspect of the present disclosure. In this configuration, the air cell  2100  includes an non-extended zone (Zone 0) for aircraft transceivers (ATs) that are less than eighty (80) to one-hundred (100) kilometers from a base station (e.g., eNodeB). The air cell  2100  also includes a first extended zone (Zone 1) for aircraft transceivers (ATs) that are less than two-hundred (200) to two-hundred fifty (250) kilometers from a base station (e.g., eNodeB). The air cell  2100  further includes a second extended zone (Zone 2) for aircraft transceivers (ATs) that are greater than two-hundred (200) to two-hundred fifty (250) kilometers from a base station (e.g., eNodeB). In this example, a first aircraft transceiver AT 1 is in the first zone (Zone 1) and a second aircraft transceiver AT 2 is in the second zone (Zone 2). In another scenario, the Airborne Object could be within Zone 0, and thus does not apply extended special subframe at all. In this scenario, the nested frame structure could dynamically change from applying an extended special subframe to applying a non-extended special subframe in co-ordination with a base station. 
     Categorizing the air cell  2100  into multiple zones to support extended cell radii involves a tradeoff between system capacity and cell coverage. Using a two (2) millisecond extended special subframe ( FIGS. 16A-17B ) involves less guard time overhead (e.g., reasonable system throughput), but cell coverage is limited to 250 kilometers. Using a three (3) millisecond extended special subframe ( FIGS. 18A-19B ) provides larger cell coverage with less system throughput (e.g., more guard time overhead). By subdividing the air cell  2100  into multiple zones, one aspect of the present disclosure enables coexistence between the two (2) millisecond extended special subframe and the three (3) millisecond extended special subframe by providing a nested frame structure, for example, as shown in  FIGS. 22A and 22B . Although described with reference to specific distances, the various zones of the present disclosure are not limited to these specific distances. 
     Referring again to  FIG. 21 , in one configuration, the base station (eNodeB) applies the two (2) millisecond extended special subframe when an aircraft transceiver (AT) is detected with a first extended cell radius. For example, the eNodeB applies a first extended special subframe (e.g., Next-Gen AG system configuration C) for communication with AT 1, which is detected within Zone 1. Similarly, the eNodeB applies a second extended special subframe (e.g., Next-Gen AG system configuration F) for communication with AT 2, which is detected within Zone 2. Based on this configuration, most aircraft are within Zone 1 and operate with high system capacity by using the first extended special subframe. Conversely, only a few cell-edge aircrafts are within Zone 2 in which a longer guard time is applied to prevent overlap between downlink and uplink transmissions. 
       FIG. 22A  is a block diagram illustrating a nested frame structure  2200  according to one aspect of the present disclosure. This configuration of a nested frame structure  2200  enables support for both a first extended special subframe  2250  and a second extended special subframe  2252 . The nested frame structure  2200  may switch between a first extended special subframe  2250  that extends over subframes SF 1 and SF 2 (SF 6 and SF 7, SF 11 and SF 12) and a second extended special subframe  2452  that extends over subframes SF 1 to SF 3 (SF 6 to SF 8 and SF 11 to SF 13). This nested frame structure  2200  supports switching between Next-Gen AG system configurations C and F, as noted by the configuration index  2232 . In this configuration, the Next-Gen AG system configurations C and F dynamically switch between uplink-downlink configuration zero (0) and uplink-downlink configuration three (3), as shown in  FIG. 12 . For example, even subframes may use uplink-downlink configuration zero (0) and odd subframes may use uplink-downlink configuration three (3). 
       FIG. 22B  further illustrates an extended special subframe  2240  according to another aspect of the present disclosure. The extended special subframe  2240  includes a downlink pilot time slot (DwPTS) portion  2242  and a guard period (GP) portion  2244 . An uplink pilot time slot (UpPTS) portion  2246  is omitted (e.g., muted) to extend the guard period (GP) portion  2244  of the extended special subframe  2240 . In this configuration, the DwPTS portion  2242  of the extended special subframe  2240  is treated as a regular, but shortened downlink subframe. 
     In this configuration, the special subframe configuration zero (0) is also applied while muting the UpPTS portion  2246 . The UpPTS portion  2246  may be muted by not scheduling any sounding reference signals. In this example, when an aircraft is in Zone 1, uplink subframes SF 2, SF 7 and SF 12 are muted to provide the extended special subframe  2240 . In this example the extended special subframe is configured as the first extended special subframe  2250  having a two (2) millisecond duration, as shown in  FIG. 22A . In addition, when an aircraft is in Zone 2, uplink subframes SF 2 and SF 3, SF 7 and SF 8, as well as uplink subframes SF 12 and SF 13 are muted to provide the second extended special subframe  2252  having a three (3) millisecond duration, as shown in  FIG. 22A . 
     The uplink subframes may be muted by not scheduling any uplink data transmissions during these uplink subframes. Muting these uplink subframes may also involve moving any acknowledgement (ACK)/negative acknowledgement (NACK) feedback to a next suitable subframe. Also, any channel quality information (CQI), precoding matrix indicator, and/or rank indicator information is not reported during these muted, uplink subframes. In addition, no sounding reference signal (SRS), scheduling request (SR), and/or random access channel (RACH) transmission are performed during these uplink subframes. 
       FIG. 23  illustrates a further categorization of air cells  2300  ( 2300 - 1 ,  2300 - 2  and  2300 - 3 ) into multiple zones to support extended cell radii according to one aspect of the present disclosure. In this configuration, the air cells  2300  include a first zone (Zone 1) for aircraft transceivers (ATs) that are less than two-hundred fifty (250) kilometers from a base station (e.g., eNodeB). The air cells  2300  also include a second zone (Zone 2) for aircraft transceivers (ATs) that are greater than two-hundred fifty (250) kilometers from a base station (e.g., eNodeB). In this example, a first aircraft transceiver AT 1 is in a first zone (Zone 1) of a first air cell  2300 - 1 , and a second aircraft transceiver AT 2 is in a second zone (Zone 2) at a cell-edge of a third air cell  2300 - 3 . 
     Using the nested frame structure  2200  by a base station involves categorization of aircraft within the various zones of the air cells  2300 . The base station uses the instantaneous location of all serving aircraft to categorize the aircraft within the various zones of the air cells  2300 . In one configuration, position location logic at each served aircraft transceiver (AT) communicates a zone index to the base station via a physical uplink shared channel (PUSCH), a physical uplink control channel (PUCCH), a physical uplink random access channel (PRACH) or other like uplink channels. In another configuration, position location logic of the base station computes a zone index of each served aircraft transceiver (AT). The position location logic may be a global position system (GPS), differential GPS, or other position detection scheme. 
     In this example, the first air cell  2300 - 1  is supported by eNodeB A, the second air cell  2300 - 2  is supported by eNodeB B, and the third air cell  2300 - 2  is supported by eNodeB C. In addition, a first aircraft transceiver AT 1 is less than two-hundred fifty (250) kilometers from the eNodeB A, while a second aircraft transceiver AT 2 is greater than two-hundred fifty (250) kilometers from eNodeB C at the cell-edge of the third air cell  2300 - 3 . Due to the increased timing advance applied at the base station for supporting the extended special subframes, uplink transmissions from aircrafts (e.g., AT 1) in Zone 1 may generate interference to neighbor cell&#39;s downlink transmission to aircraft (e.g., AT 2) in Zone 2. 
     In this configuration, uplink-to-downlink interference is mitigated by the directional antenna pattern at AT 1 and AT 2. That is, the interference over thermal noise (IoT) is quite small due to the roll-off in azimuth and elevation angle of the aircraft antenna relative to the boresight. In another configuration, the size of Zone 1 is reduced to avoid the uplink-to-downlink overlap. In a further configuration, the base station adjusts the uplink scheduling depending on the aircraft location. In this example, uplink transmission of AT 1 in Zone 1 are scheduled in subframes SF 3, SF 4, SF 8, SF 9, SF 13 and SF 14, as shown in  FIG. 22 . When AT 2 is in Zone 2, subframes SF 3, SF 8 and SF 13 are muted. 
     Reliable communication within a next generation air to ground (Next-Gen AG) system may involve techniques for retransmitting data when the data is not successfully received at a target location. For example, an automatic repeat request (ARQ) protocol may be used by an aircraft that receives data (e.g., UE  650 ) to request retransmission of various portions of the data when an initial transmission from a base station (e.g., eNodeB) is unsuccessful. Hybrid ARQ (HARQ) combines retransmission of data with error correction techniques and/or other techniques for improving the robustness of transmissions conducted within the Next-Gen AG system. 
     In physical layer specifications such as TD-LTE, a UE and an eNodeB may employ a HARQ scheme to improve data throughput and increase transmission reliability. The HARQ scheme provides transmission reliability by temporarily storing decision metrics that can be combined with subsequent decision metrics from data retransmissions. The decision metric may refer to a posterior probability or likelihood (soft value) of transmitted bits being a “0” or a “1” including, but not limited to, log-likelihood ratios (LLRs). Groups of such decision metrics may be used by a decoder to decode a transmitted sequence (e.g., a transport block). 
     TD-LTE provides physical layer support for HARQ on the physical downlink shared channel (PDSCH) and the physical uplink shared channel (PUSCH). In addition, TD-LTE provides physical layer support for sending associated acknowledgment feedback over separate control channels. In a Next-Gen AG system, transmission conducted pursuant to HARQ is performed in the context of one or more HARQ processes. These HARQ process can be managed by a HARQ controller at the aircraft (e.g., UE  650 ) and or similar mechanisms of the base station (e.g., eNodeB  610 ). A maximum number of HARQ processes is determined by an uplink/downlink configuration. 
     In a Next-Gen AG system, however, an extended special subframe is communicated by transmitting a special subframe that extends over an uplink pilot time slot and one or more disabled, adjacent uplink subframes. These adjacent uplink subframes may be disabled (e.g., muted) by not scheduling any uplink data transmissions during these adjacent, uplink subframes. Muting these adjacent uplink subframes may also involve moving any acknowledgement (ACK)/negative acknowledgement (NACK) feedback to a next suitable subframe. 
     In one configuration, ACK/NACK feedback for a physical downlink shared channel (PDSCH) transmission is moved to a next suitable uplink subframe by adjusting a downlink association set K. In addition, support of an increased, minimum response time is specified to deal with the larger propagation delay in the Next-Gen AG system due to the extended cell radii. In addition, retransmissions for asynchronous HARQ may be rescheduled via the physical downlink control channel (PDCCH) to allow for more flexible scheduling. 
     The absence of an ACK/NACK feedback in the extended special subframes and increased minimum response time (due to the extended cell radii) also involve a modification in the maximum number of HARQ processes. For example, as shown in table  2400  of  FIG. 24 , the maximum number of downlink HARQ processes may vary according to an uplink/downlink configuration index of the Next-Gen AG system. 
     Retransmissions from HARQ processes are triggered by receipt of ACK/NACK feedback (e.g., NACK feedback). Conventionally, a UE transmits ACK/NACK feedback in uplink subframe n in response to a PDSCH transmission within subframes n-k, e.g., the minimum value of k is 4. This allows for at least 3 milliseconds processing time at the UE. In one configuration, a longer ACK/NACK response time (e.g., k≧6) may be specified to meet an aircraft transceiver (AT) processing time and an increased propagation delay due to the extended cell radii in the Next-Gen AG system. In addition, a processing time, e.g., three milliseconds, at the base station may be maintained. 
       FIG. 25A  illustrates a configuration of a TD-LTE radio frame structure  2500 - 1  including tables of downlink association set indexes, which represent the timing of ACK/NACK feedback when communicating with an extended special subframe specified to support the noted, extended cell radii. The TD-LTE radio frame structure  2500 - 1  has a ten (10) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2 (e.g., Next-Gen AG system configurations A and B) and also SF 3 (e.g., Next-Gen AG system configurations D and E). The TD-LTE radio frame structure  2500 - 1  also includes extended special subframes that extend over subframes SF 6, SF 7 (e.g., configuration A) and SF 8 (e.g., configuration D). 
     In this configuration, ACK/NACK feedback in uplink subframe SF n corresponds to a physical downlink shared channel (PDSCH) transmission in downlink subframe SF n-k. In this configuration, k is determined according to downlink association set K in which the value of k is adjusted so that ACK/NACK feedback for downlink subframes is moved to a next suitable uplink subframe. The downlink association set K, including the adjusted k values may, for example, replace Table 10.1-1 in 3GPP TS 36.213. 
     In Next-Gen AG system configuration A, a value of k=8 is indicated for uplink subframes SF 3 and SF 8. This means that ACK/NACK feedback for downlink subframe SF 5 of the previous radio frame (not shown) is provided in uplink subframe SF 3 of the current radio frame. In addition, ACK/NACK feedback for downlink subframe SF 0 of the current radio frame  2500 - 1  is provided in uplink subframe SF 8. 
     Similarly, in Next-Gen AG system configuration D, a value of k=9 is indicated for uplink subframes SF 4 and SF 9. This means that ACK/NACK feedback for downlink subframe SF 5 of the previous radio frame (not shown) is provided in uplink subframe SF 4 of the current radio frame. In addition, ACK/NACK feedback for downlink subframe SF 0 of the current radio frame  2500 - 1  is provided in uplink subframe SF 9. 
     Note that the number of downlink subframes are less than or equal to the number of uplink subframes with Next-Gen AG system configurations A and D. Hence, there is at most one ACK/NACK feedback for PDSCH transmissions in each of uplink subframes for Next-Gen AG system configurations A and D. By contrast, providing ACK/NACK feedback for PDSCH transmissions with Next-Gen AG system configurations B and E involves multiple ACK/NACK feedbacks in a single uplink subframe (e.g., SF 3 and SF 4) since the number of downlink subframes are more than the number of uplink subframes. In addition, the number of uplink subframes is reduced in Next-Gen AG system due to the extended special subframes. This process is further illustrated in  FIGS. 26A and 26B , in which ACK/NACK Feedback Tables  2600 - 1  and  2600 - 2  further illustrate the process for determining downlink association set index k to provide the ACK/NACK feedback for PDSCH transmission in Next-Gen AG system configurations B and E. This process is further illustrated in  FIGS. 26A and 26B , in which ACK/NACK Feedback Tables  2600 - 1  and  2600 - 2  further illustrate the process for determining k to provide the ACK/NACK feedback for Next-Gen AG system configurations B and E. 
     ACK/NACK Feedback Table  2600 - 1  of  FIG. 26A  illustrates the process for determining downlink association set index k and the maximum downlink HARQ processes for Next-Gen AG system configuration B. For example, in Next-Gen AG system configuration B, values of k=14, k=13 and k=8 are specified for uplink subframe SF 3 of the TD-LTE radio frame structure  2500 - 1 . This means that ACK/NACK feedbacks for downlink subframe SF 9 (corresponding to k=14) from 2 radio frames ahead, and downlink subframes SF 0 (corresponding to k=13) and SF 5 (corresponding to k=8) of the previous radio frame are provided in uplink subframe SF 3 of the current radio frame. In this example, k=12 is not in the downlink association set because it is assumed that no data is sent during a special subframe SF 1 of the TD-LTE radio frame structure  2610 . That is, although control data (e.g., uplink grants) may be sent during the DwPTS (e.g., the first 3 OFDM symbols) of a special subframe, PDSCH is not sent during the DwPTS of the special subframe. 
     In Next-Gen AG system configuration B, values of k=8, k=7 and k=6 are specified for uplink subframe SF 4 of the TD-LTE radio frame structure  2500 - 1 . This means that ACK/NACK feedbacks for downlink subframes SF 6 (corresponding to k=8), SF 7 (corresponding to k=7) and SF 8 (corresponding to k=6) of the previous radio frame is provided in uplink subframe SF 4 of the current radio frame. 
     ACK/NACK Feedback Table  2600 - 2  of  FIG. 26B  illustrates the process for determining downlink association set index k and the maximum downlink HARQ processes for Next-Gen AG system configuration E. In Next-Gen AG system configuration E, values of k=15, k=14, k=9, k=8, k=7 and k=6 are specified for uplink subframe SF 4. This means that ACK/NACK feedbacks for downlink subframe SF 9 (corresponding to k=15) from 2 radio frames ahead, and downlink subframes SF 0 (corresponding to k=14), SF 5 (corresponding to k=9), SF 6 (corresponding to k=8), SF 7 (corresponding to k=7) and SF 8 (corresponding to k=6) of the previous radio frame are provided in uplink subframe SF 4 of the current radio frame. 
       FIG. 25B  illustrates a configuration of a TD-LTE radio frame structure  2500 - 2  including tables of downlink association set indexes, which represent the timing of ACK/NACK feedback when communicating with an extended special subframe specified to support extended cell radii. The TD-LTE radio frame structure  2500 - 2  has a twenty (20) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2, subframes SF 6 and SF 7, and subframes SF 11 and SF 12 (e.g., Next-Gen AG system configuration C). In addition, extended special subframes extend over subframes SF 1 to SF 3, subframes SF 6 to SF 8, and subframes SF 11 to SF 13 in Next-Gen AG system configuration F. 
     In Next-Gen AG system configuration C, values of k=13 and k=8 are specified for uplink subframe SF 3 and values of k=8 and k=7 are specified for uplink subframe SF 4 of the TD-LTE radio frame structure  2500 - 2 . This means that ACK/NACK feedbacks for downlink subframes SF 10 (corresponding to k=13) and SF 15 (corresponding to k=8) of the previous radio frame (not shown) are provided in uplink subframe SF 3 of the current radio frame. In addition, ACK/NACK feedback for downlink subframes SF 16 (corresponding to k=8) and SF 17 (corresponding to k=7) of the previous radio frame are provided in uplink subframe SF 4 of the current radio frame. 
     In this example, values of k=10, k=9 are specified for uplink subframe SF 8; a value of k=9 is specified for uplink subframe SF 9; and a value of k=8 is specified for uplink subframe SF 13 of the TD-LTE radio frame structure  2500 - 2 . This means that ACK/NACK feedbacks for downlink subframes SF 18 (corresponding to k=10) and SF 19 (corresponding to k=9) of the previous radio frame are provided in uplink subframe SF 8 of the current radio frame. In addition, ACK/NACK feedback for downlink subframe SF 0 (corresponding to k=9) of the current radio frame is provided in uplink subframe SF 9 of the current radio frame. Similarly, ACK/NACK feedback for downlink subframe SF 5 (corresponding to k=8) of the current radio frame is provided in uplink subframe SF 13. 
     In Next-Gen AG system configuration F, values of k=14, k=9, k=8 and k=7 are specified for uplink subframe SF 4. This means that ACK/NACK feedback for downlink subframes SF 10 (corresponding to k=13), SF 15 (corresponding to k=9), SF 16 (corresponding to k=8) and SF 17 (corresponding to k=7) of the previous radio frame (not shown) are provided in uplink subframe SF 4 of the current radio frame. In addition, values of k=11, k=10 and k=9 are specified for uplink subframe SF 9 and a value of k=9 is specified for uplink subframe SF 14 of the TD-LTE radio frame structure  2500 - 2 . This means that ACK/NACK feedback for downlink subframes SF 18 (corresponding to k=11) and SF 19 (corresponding to k=10) of the previous radio frame and ACK/NACK feedback for downlink subframe SF 0 (corresponding to k=9) of the current radio frame are provided in uplink subframe SF 9. In addition, ACK/NACK feedback for downlink subframe SF 5 (corresponding to k=9) of the current radio frame is provided in uplink subframe SF 14. 
     As noted, an extended special subframe is communicated in a Next-Gen AG system by transmitting a special subframe that extends over an uplink pilot time slot and one or more disabled, adjacent uplink subframes. These adjacent uplink subframes may be disabled (e.g., muted) by not scheduling any uplink data transmissions during these adjacent, uplink subframes. Muting these adjacent uplink subframes may also involve suspending any uplink grants associated with a muted uplink subframe. In addition, the relative timing of between an uplink grant and the corresponding PUSCH transmission may be modified to ensure an increased minimum response time from an aircraft transceiver due to the extended cell radii. 
     In one configuration, the uplink grant related modification is achieved by adjusting an uplink association index K PUSCH . In addition, an increased, minimum response time is specified to deal with the larger propagation delays in the Next-Gen AG system due to the extended cell radii. Furthermore, retransmissions for synchronous HARQ may be indicated via the physical HARQ indicator channel (PDCCH) to allow for a more simplified implementation with reduced signaling overhead. 
     The use of extended special subframes also involves a reduction in the number of uplink HARQ processes. For example, as shown in table  2700  of  FIG. 27 , the number of uplink HARQ processes may vary according to an uplink/downlink configuration index of the Next-Gen AG system. For example, as shown in table  2700  of  FIG. 27 , the number of HARQ processes may vary according to an uplink/downlink configuration index of the Next-Gen AG system. In this example, a maximum number of HARQ processes may be limited to seven (7). In this configuration, retransmission is indicated via a physical HARQ indicator channel (PHICH) or a new uplink grant on a physical downlink control channel (PDCCH). 
     In one configuration, a physical uplink shared channel response time (e.g., k&gt;6) may be specified to meet an aircraft transceiver (AT) processing time and an increased propagation delay due to the extended zones in the Next-Gen AG system. In addition, a processing time (e.g., three milliseconds) at the base station is presumed in the Next-Gen AG system. 
       FIG. 28A  illustrates a configuration of a TD-LTE radio frame structure  2800 - 1  including physical uplink shared channel (PUSCH) data transmission when communicating with an extended special subframe. The TD-LTE radio frame structure  2800 - 1  also has a ten (10) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2 (e.g., Next-Gen AG system configurations A and B) and also SF 3 (e.g., Next-Gen AG system configurations D and E). The TD-LTE radio frame structure  2800 - 1  also includes extended special subframes that extend over subframes SF 6, SF 7 (e.g., configuration A) and SF 8 (e.g., configuration D). 
     In this configuration, a physical uplink shared channel (PUSCH) transmission of data in uplink subframe SF n corresponds to an uplink grant sent in a subframe SF n-K PUSCH . That is, a UE may transmit a new data package or retransmit an old package on a physical uplink shared channel (PUSCH) in an uplink subframe SF n. In this configuration, the PUSCH transmission in the uplink subframe SF n corresponds to a scheduling command transmitted on a physical downlink control channel (PDCCH) in a subframe SF n-K PUSCH . The PUSCH transmission in the uplink subframe SF n may also correspond to a NACK transmitted on a physical HARQ indicator channel (PHICH) in a subframe SF n-K PUSCH . The uplink subframe SF n may also correspond to a NACK transmitted on a physical HARQ indicator channel (PHICH) in a subframe SF n-K PUSCH . The uplink grant/NACK may be sent in a downlink subframe or a special subframe. In this configuration, K PUSCH  is determined according to an Uplink Association Index Table in which the value of K PUSCH  is adjusted so that no uplink grant/NACK is associated with a muted uplink subframe. The uplink grant/NACK may be sent in a downlink subframe or an extended special subframe. In this configuration, K PUSCH  is determined according to an Uplink Association Index Table in which the value of K PUSCH  is adjusted so that no uplink grant/NACK is associated with an extended special subframe. The Uplink Association Index Table, including the adjusted K PUSCH  values may, for example, replace Table 5.1.1.1-1 (K PUSCH ) and Table 7.3-Y (k′) in 3GPP TS 36.213. 
       FIG. 28A  further illustrates the Next-Gen AG system configuration A in which a value of K PUSCH =8 is indicated for uplink subframes SF 3, SF 4, SF 8 and SF 9. Based on this value of K PUSCH , a PUSCH transmission (e.g., a new data package or a retransmitted package) is transmitted in uplink subframe SF 3 in response to an uplink grant/NACK in a downlink subframe SF 5 from a previous radio frame (not shown). In addition, a PUSCH transmission is transmitted in uplink subframe SF 4 in response to an uplink grant/NACK in a special subframe SF 6 from the previous radio frame. Similarly, a PUSCH transmission is transmitted in uplink subframe SF 8 in response to an uplink grant/NACK in a downlink subframe SF 0 from the current radio frame. In addition, a PUSCH transmission is transmitted in uplink subframe SF 9 in response to an uplink grant/NACK in a special subframe SF 1 from the current radio frame. 
     In Next-Gen AG system configuration B, a value of K PUSCH =6 is specified for uplink subframes SF 3 and SF 4. Based on this value of K PUSCH , a PUSCH transmission is transmitted in uplink subframe SF 3 in response to an uplink grant/NACK in a downlink subframe SF 7 from a previous radio frame (not shown). In addition, a PUSCH transmission is transmitted in uplink subframe SF 4 in response to an uplink grant/NACK in a downlink subframe SF 8 from the previous radio frame. Similarly, in Next-Gen AG system configuration E, a value of K PUSCH =6 is specified for uplink subframe SF 4. Based on this value of K PUSCH , a PUSCH transmission is transmitted in uplink subframe SF 4 in response to an uplink grant/NACK in a downlink subframe SF 8 from the previous radio frame. 
     In Next-Gen AG system configuration D, a value of K PUSCH =8 is specified for uplink subframes SF 4 and SF 9. Based on this value of K PUSCH , a PUSCH transmission is transmitted in uplink subframe SF 4 in response to an uplink grant/NACK in a special subframe SF 6 from a previous radio frame (not shown). In addition, a PUSCH transmission is transmitted in uplink subframe SF 9 in response to an uplink grant/NACK in a special subframe SF 1 from the current radio frame. 
       FIG. 28B  illustrates a configuration of a TD-LTE radio frame structure  2800 - 2  including physical uplink shared channel (PUSCH) data transmission when communicating with an extended special subframe. The TD-LTE radio frame structure  2800 - 2  has a twenty (20) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2, subframes SF 6 and SF 7, and subframes SF 11 and SF 12 (e.g., Next-Gen AG system configuration C). In addition, extended special subframes extend over subframes SF 1 to SF 3, subframes SF 6 to SF 8, and subframes SF 11 to SF 13 (e.g., Next-Gen AG system configuration F). 
     In Next-Gen AG system configuration C, a value of K PUSCH =8 is specified for uplink subframes SF 3, SF 8, SF 9 SF 13 and SF 14. In addition, a value of K PUSCH =7 is specified for an uplink subframe SF 4. Based on the value of K PUSCH =8, a PUSCH transmission is transmitted in uplink subframe SF 3 in response to an uplink grant/NACK in a downlink subframe SF 15 from a previous radio frame (not shown). Based on the value of K PUSCH =7, a PUSCH transmission is transmitted in uplink subframe SF 4 in response to an uplink grant/NACK in a downlink subframe SF 17 from the previous radio frame. 
     In addition, a PUSCH transmission is transmitted in uplink subframe SF 8 in response to an uplink grant/NACK in a downlink subframe SF 0 from the current radio frame. Similarly, a PUSCH transmission is transmitted in uplink subframe SF 9 in response to an uplink grant/NACK in a special subframe SF 1 of the current radio frame. In addition, a PUSCH transmission is transmitted in uplink subframe SF 13 in response to an uplink grant/NACK in a downlink subframe SF 5 of the current radio frame. Also, a PUSCH transmission is transmitted in uplink subframe SF 14 in response to an uplink grant/NACK in a special subframe SF 6 of the current radio frame. 
     In Next-Gen AG system configuration F, a value of K PUSCH =7 is specified for uplink subframe SF 4. Based on the value of K PUSCH =7, a PUSCH transmission is transmitted in uplink subframe SF 4 in response to an uplink grant/NACK in a downlink subframe SF 17 from a previous radio frame (not shown). Based on the value of K PUSCH =8, a PUSCH transmission is transmitted in uplink subframe SF 9 in response to an uplink grant/NACK in a special subframe SF 1 from the current radio frame. In addition, a PUSCH transmission is transmitted in uplink subframe SF 14 in response to an uplink grant/NACK in a special subframe SF 6 from the current radio frame. 
       FIG. 29A  and  FIG. 29B  illustrate a configuration of a TD-LTE radio frame structure  2900 - 1  including the timing of uplink grants transmitted by a base station (e.g., eNodeB) for physical uplink shared channel (PUSCH) data transmission when communicating with an extended special subframe. The TD-LTE radio frame structure  2900 - 1  also has a ten (10) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2 (e.g., Next-Gen AG system configurations A and B) and also SF 3 (e.g., Next-Gen AG system configurations D and E). The TD-LTE radio frame structure  2900 - 1  also includes extended special subframes that extend over subframes SF 6 and SF 7 (e.g., configuration A) and SF 8 (e.g., configuration D). 
     In this aspect, a physical uplink shared channel (PUSCH) transmission of data in an uplink subframe SF n+K 1  is in response to an uplink grant/NACK transmitted in a subframe SF n. That is, a UE may detect a PDCCH transmission that includes a uplink grant and/or detect a PHICH transmission that includes a NACK in a subframe SF n that is intended for the UE. In response, the UE sends the corresponding PUSCH transmission in the uplink subframe SF n+K 1 . In this aspect, the K 1  values may, for example, replace Table 8-2 in 3GPP TS 36.213. Those tables in  FIGS. 29A and 29B  represent the same information as the tables in  FIGS. 28A and 28B  about the relative timing between an uplink grant/NACK and the corresponding PUSCH transmission with K 1 =K PUSCH , but in a different way of descriptions. 
       FIG. 29A  further illustrates Next-Gen AG system configuration A in which a value of K 1 =8 is specified for downlink subframes SF 0 and SF 5 as well as special subframes SF 1 and SF 6. Similarly, in Next-Gen AG system configuration D, the value of K 1 =8 is specified for special subframes SF 1 and SF 6. Based on this value of K 1 , a data package is transmitted in uplink subframe SF 8 of the TD-LTE radio frame structure  2900 - 1  in response to an uplink grant in the downlink subframe SF 0. In addition, a data package is transmitted in uplink subframe SF 9 of the TD-LTE radio frame structure  2900 - 1  in response to an uplink grant in the special subframe SF 1. Similarly, in Next-Gen AG system configuration D, a data package is also transmitted in uplink subframe SF 9 of the TD-LTE radio frame structure  2900 - 1  in response to an uplink grant in the special subframe SF 1. 
     In Next-Gen AG system configuration A, a data package is transmitted in an uplink subframe SF 3 of a subsequent TD-LTE radio frame structure (not shown) in response to an uplink grant in the downlink subframe SF 5. In addition, a data package is transmitted in an uplink subframe SF 4 of the subsequent TD-LTE radio frame structure in response to an uplink grant in the special subframe SF 6. Similarly, in Next-Gen AG system configuration D, a data package is transmitted in an uplink subframe SF 4 of the subsequent TD-LTE radio frame structure in response to an uplink grant in the special subframe SF 6. 
     In Next-Gen AG system configuration B, a value of K 1 =6 is specified for downlink subframes SF 7 and SF 8. Based on this value of K 1 , a data package is transmitted in an uplink subframe SF 3 of a subsequent TD-LTE radio frame structure (not shown) in response to the uplink grant in the downlink subframe SF 8. In addition, a data package is transmitted in an uplink subframe SF 4 of the subsequent TD-LTE radio frame structure in response to an uplink grant in a downlink subframe SF 8. Similarly, in Next-Gen AG system configuration E, a value of K 1 =6 is specified for a downlink subframe SF 8. Based on this value of K 1 , a data package is transmitted in an uplink subframe SF 4 of a subsequent TD-LTE radio frame structure (not shown) in response to an uplink grant in the downlink subframe SF 8. 
       FIG. 29B  illustrates a configuration of a TD-LTE radio frame structure  2900 - 2  including uplink grants for physical uplink shared channel (PUSCH) data transmission when communicating with an extended special subframe. The TD-LTE radio frame structure  2900 - 2  has a twenty (20) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2, subframes SF 6 and SF 7, and subframes SF 11 and SF 12 (e.g., Next-Gen AG system configuration C). In addition, extended special subframes extend over subframes SF 1 to SF 3, subframes SF 6 to SF 8, and subframes SF 11 to SF 13 (e.g., Next-Gen AG system configuration F). 
     In Next-Gen AG system configuration C, a value of K 1 =8 is specified for downlink subframes SF 0, SF 5 and SF 15 as well as special subframes SF 1 and SF 6. In addition, a value of K 1 =7 is specified for a downlink subframe SF 17. Based on the value of K 1 =8, a data package is transmitted in an uplink subframe SF 8 of the TD-LTE radio frame structure  2900 - 2  in response to an uplink grant in the downlink subframe SF 0. In addition, a data package is transmitted in an uplink subframe SF 9 of the TD-LTE radio frame structure  2900 - 2  in response to an uplink grant in the special subframe SF 0. A data package is also transmitted in an uplink subframe SF 13 of the TD-LTE radio frame structure  2900 - 2  in response to an uplink grant in the downlink subframe SF 5. 
     In this example, a data package is also transmitted in an uplink subframe SF 14 of the TD-LTE radio frame structure  2900 - 2  in response to an uplink grant in the special subframe SF 6. In addition, a data package is transmitted in an uplink subframe SF 3 of a subsequent TD-LTE radio frame structure (not shown) in response to an uplink grant in the downlink subframe SF 15. Based on the value of K 1 =7, a data package is also transmitted in an uplink subframe SF 4 of the subsequent TD-LTE radio frame structure in response to an uplink grant in the downlink subframe SF 17. 
     In Next-Gen AG system configuration F, a value of K 1 =8 is specified for uplink subframes SF 1 and SF 6. In addition, a value of K 1 =7 is specified for a downlink subframe SF 17. Based on the value of K 1 =8, a data package is transmitted in an uplink subframe SF 9 of the TD-LTE radio frame structure  2900 - 2  in response to an uplink grant in the special subframe SF 1. A data package is also transmitted in an uplink subframe SF 14 of the TD-LTE radio frame structure  2900 - 2  in response to an uplink grant in the special subframe SF 6. Based on the value of K 1 =7, a data package is transmitted in an uplink subframe SF 4 of a subsequent TD-LTE radio frame structure (not shown) in response to an uplink grant in the downlink subframe SF 17. 
       FIG. 30A  illustrates a configuration of a TD-LTE radio frame structure  2500 - 1  including the timing of ACK/NACK feedback received on a physical HARQ indicator channel (PHICH) when communicating with an extended special subframe. The TD-LTE radio frame structure  3000 - 1  also has a ten (10) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2 (e.g., Next-Gen AG system configurations A and B) and also SF 3 (e.g., Next-Gen AG system configurations D and E). The TD-LTE radio frame structure  3000 - 1  also includes extended special subframes that extend over subframes SF 6 and SF 7 (e.g., configuration A) and SF 8 (e.g., configuration D). 
     In this configuration, ACK/NACK feedback is received on the physical HARQ indicator channel (PHICH) assigned to a UE in a subframe n. This ACK/NACK feedback is associated with a physical uplink shared channel (PUSCH) transmission of data in an uplink subframe SF n-K 2 . That is, the UE may detect ACK/NACK feedback on the PHICH assigned to the UE. In response, the UE associates the detected ACK/NACK feedback with the PUSCH transmission in the uplink subframe SF-k 2 . In this configuration, the K 2  values may, for example, replace Table 8.3-1 in 3GPP TS 36.213. In this synchronous HARQ implementation, the K 2  values may be determined by a round trip time (RTT) and the K 1  values of, for example,  FIGS. 29A and 29B  as follows: 
         K   2   =RTT−K   1   (7)
 
       FIG. 30A  illustrates various K 2  values for Next-Gen AG system configuration A, B, D and E according to one aspect of the present disclosure. In Next-Gen AG system configuration A, a value of K 2 =7 is specified for downlink subframes SF 0 and SF 5 as well as special subframes SF 1 and SF 6. Similarly, in Next-Gen AG system configuration D, the value of K 2 =7 is specified for special subframes SF 1 and SF 6. Based on this value of K 2 , ACK/NACK feedback received in the downlink subframe SF 0 of the current radio frame is associated with a PUSCH transmitted (e.g., retransmitted) in an uplink subframe SF 3 of a previous radio frame (not shown). In addition, ACK/NACK feedback received in the special subframe SF 1 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 4 of the previous radio frame. Similarly, in Next-Gen AG system configuration D, ACK/NACK feedback received in the special subframe SF 1 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 4 of the previous radio frame. 
     In Next-Gen AG system configuration A, ACK/NACK feedback received in the downlink subframe SF 5 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 8 of the previous radio frame. In addition, ACK/NACK feedback received in the special subframe SF 6 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 9 of the previous radio frame. Similarly, in Next-Gen AG system configuration D, ACK/NACK feedback received in the special subframe SF 6 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 9 of the previous radio frame. 
     In Next-Gen AG system configuration B, a value of K 2 =4 is specified for downlink subframes SF 7 and SF 8. Based on this value of K 2 , ACK/NACK feedback received in the downlink subframe SF 7 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 3 of the current radio frame. In addition, ACK/NACK feedback received in the downlink subframe SF 8 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 4 of current radio frame. In addition, ACK/NACK feedback received in the downlink subframe SF 8 of the TD-LTE radio frame structure  3000 - 1  is associated with a data package transmitted in an uplink subframe SF 4 of the TD-LTE radio frame structure  3000 - 1 . Similarly, in Next-Gen AG system configuration E, a value of K 2 =6 is specified for a downlink subframe SF 8. Based on this value of K 2 , ACK/NACK feedback received in downlink subframe SF 8 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 4 of the current radio frame. 
       FIG. 30B  illustrates a configuration of a TD-LTE radio frame structure  3000 - 2  including ACK/NACK feedback received on a physical HARQ indicator channel (PHICH) when communicating with an extended special subframe. The TD-LTE radio frame structure  3000 - 2  has a twenty (20) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2, subframes SF 6 and SF 7, and subframes SF 11 and SF 12 (e.g., Next-Gen AG system configuration C). In addition, extended special subframes extend over subframes SF 1 to SF 3, subframes SF 6 to SF 8, and subframes SF 11 to SF 13 (e.g., Next-Gen AG system configuration F). 
     In Next-Gen AG system configuration C, a value of K 2 =12 is specified for downlink subframes SF 0, SF 5 and SF 15 as well as special subframes SF 1 and SF 6. In addition, a value of K 2 =13 is specified for a downlink subframe SF 17. Based on the value of K 2 =12, ACK/NACK feedback received in downlink subframe SF 0 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 8 of a previous radio frame structure (not shown). In addition, ACK/NACK feedback received in the special subframe SF 1 of the TD-LTE radio frame structure  3000 - 1  is associated with a PUSCH transmitted in an uplink subframe SF 9 of the previous radio frame structure. 
     In this example, ACK/NACK feedback received in downlink subframe SF 5 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 13 of a previous radio frame. In addition, ACK/NACK feedback received in the special subframe SF 6 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 14 of the previous frame. In addition, ACK/NACK feedback received in downlink subframe SF 15 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 3 of the current radio frame. Based on the value of K 2 =13, ACK/NACK feedback received in downlink subframe SF 17 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 4 of the current radio frame. 
     In Next-Gen AG system configuration F, a value of K 2 =12 is specified for special subframes SF 1 and SF 6. In addition, a value of K 2 =13 is specified for a downlink subframe SF 17. Based on the value of K 2 =12, ACK/NACK feedback received in the special subframe SF 1 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 9 of the previous the current radio frame. ACK/NACK feedback received in the special subframe SF 6 of the current radio frame is also associated with a PUSCH transmitted in an uplink subframe SF 14 of the previous the current radio frame. Based on the value of K 2 =13, ACK/NACK feedback received in the downlink subframe SF 17 of the current radio frame is associated with a PUSCH transmitted in an uplink subframe SF 4 of the current radio frame. 
       FIG. 31A  illustrates a configuration of a TD-LTE radio frame structure  3100 - 1  including the factor m i  of the number of physical HARQ indicator channel (PHICH) groups for each downlink subframe when communicating with an extended special subframe. The TD-LTE radio frame structure  3100 - 1  also has a ten (10) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2 (e.g., Next-Gen AG system configurations A and B) and also SF 3 (e.g., Next-Gen AG system configurations D and E). The TD-LTE radio frame structure  3100 - 1  also includes extended special subframes that extend over subframes SF 6 and SF 7 (e.g., configuration A) and SF 8 (e.g., configuration D). 
     In this configuration, the TD-LTE radio frame structure  3100 - 1  indicates whether the physical HARQ indicator channel (PHICH) assigned to a downlink or special subframe n. This ACK/NACK feedback is associated with a physical uplink shared channel (PUSCH) transmission of data in an uplink subframe SF n-K 2 , as shown in  FIG. 31A . In the Next-Gen AG system, the number of a PHICH group may vary between downlink subframes (or DwPTS) and is given by: 
         m   i   ·N   PHICH   group   (8)
 
     The PHICH group factor m i  is shown in the TD-LTE radio frame structure  3100 - 1 . In this configuration, PHICH group factor m i  may, for example, replace Table 6.9-1 (m i ) in 3GPP TS 36.211. In this HARQ implementation, the PHICH index I PHICH  is set to zero (I PHICH =0) because no multiple ACK/NACKs are configured for any downlink subframe for a single UE, for example, as specified in section 9.1.2 in 3GPP TS 36.213. 
       FIG. 31A  illustrates various m i  values for Next-Gen AG system configurations A, B, D and E according to one aspect of the present disclosure. In Next-Gen AG system configuration A, a value of m i =1 is specified for downlink subframes SF 0 and SF 5 as well as special subframes SF 1 and SF 6, indicating that PHICH is being assigned in theses subframes. Similarly, in Next-Gen AG system configuration D, the value of m i =1 is specified for special subframes SF 1 and SF 6. This means that PHICH is assigned in the special subframes SF 1 and SF 6. A value of m i =0, however, is specified for the downlink subframes SF 1 and SF 6 of in Next-Gen AG system configuration D. As a result, this implies that there is no PHICH being assigned in the downlink subframes SF 1 and SF 6. 
     In Next-Gen AG system configuration B, a value of m i =0 is specified for downlink subframes SF 0, SF 5, SF 6 and SF 9 and a special subframe SF 1 of the TD-LTE radio frame structure  3100 - 1 . This implies that there is no PHICH being assigned in the downlink subframes SF 0, SF 5, SF 6 and SF 9 and the special subframe SF 1. Similarly, in Next-Gen AG system configuration E, a value of m i =0 is specified for downlink subframes SF 0, SF 5, SF 6, SF 7 and SF 9 as well as a special subframe SF 1 of the TD-LTE radio frame structure  3100 - 1 . This implies that there is no PHICH being assigned in the downlink subframes SF 0, SF 5, SF 6, SF 7 and SF 9 as well as the special subframe SF 1. 
     In Next-Gen AG system configuration B, a value of m i =1 is specified for downlink subframes SF 7 and SF 8 of the TD-LTE radio frame structure  3100 - 1 . This means that PHICH is assigned in the downlink subframes SF 7 and SF 8. Similarly, in Next-Gen AG system configuration E, a value of m i =1 is specified for downlink subframe SF 8 of the TD-LTE radio frame structure  3100 - 1 . This means that PHICH is assigned in the downlink subframe SF 8. 
       FIG. 31B  illustrates a configuration of a TD-LTE radio frame structure  3100 —including the factor m i  of the number of physical HARQ indicator channel (PHICH) groups for each downlink subframe when communicating with an extended special subframe. The TD-LTE radio frame structure  3100 - 2  has a twenty (20) millisecond periodicity with extended special subframes that extend over subframes SF 1, SF 2, subframes SF 6 and SF 7, and subframes SF 11 and SF 12 (e.g., Next-Gen AG system configuration C). In addition, extended special subframes extend over subframes SF 1 to SF 3, subframes SF 6 to SF 8, and subframes SF 11 to SF 13 (e.g., Next-Gen AG system configuration F). 
     In Next-Gen AG system configuration C, a value of m i =1 is specified for downlink subframes SF 0, SF 5, SF 15 and SF 17, as well as special subframes SF 1 and SF 6 of the TD-LTE radio frame structure  3100 - 2 . This means that PHICH is assigned in the downlink subframes SF 1, SF 5, SF 15 and SF 17, as well as the special subframes SF 1 and SF 6. Similarly, in Next-Gen AG system configuration F, a value of m i =1 is specified for special subframes SF 1 and SF 6 as well as a downlink subframe SF 17 of the TD-LTE radio frame structure  3100 - 1 . This means that PHICH is assigned in the special subframes SF 1 and SF 6 as well as the downlink subframe SF 17. 
     In Next-Gen AG system configuration C, a value of m i =0 is specified for downlink subframes SF 10, SF 16, SF 18 and SF 19, as well as a special subframe SF 11 of the TD-LTE radio frame structure  3100 - 2 . This implies that there is no PHICH being assigned in the downlink subframes SF 10, SF 16, SF 18 and SF 19, as well as the special subframe SF 11. Similarly, in Next-Gen AG system configuration F, a value of m i =0 is specified for downlink subframes SF 0, SF 5, SF 10, SF 15, SF 16, SF 18 and SF 19 as well as a special subframe SF 11 of the TD-LTE radio frame structure  3100 - 2 . This implies that there is no PHICH being assigned in the downlink subframes SF 0, SF 5, SF 10, SF 15, SF 16, SF 18 and SF 19 as well as the special subframe SF 11. 
       FIG. 32  illustrates a method  3200  for modification of a time division long term evolution (TD-LTE) frame structure according to an aspect of the present disclosure. In block  3210 , an eNodeB communicates with the UE using a special subframe that extends a guard period over an uplink pilot time slot and one or more disabled, adjacent uplink subframes. In one configuration, this extended special subframe is used to communicate with a UE when a position of a UE is detected as being within a first extended cell radius or a second extended cell radius outside of a non-extended cell radius (e.g., less than one-hundred (100) kilometers). For example, a first extended cell radius may be greater than one-hundred (100) kilometers and less than or equal to two-hundred fifty kilometers. A second extended cell radius may be greater than two-hundred fifty kilometers. 
     For example, the eNodeB may communicate using a first extended special subframe when the position of the UE is within the first extended cell radius. In this example, (see  FIGS. 16A to 17B ) the eNodeB may also communicate using a second extended special subframe (see  FIGS. 18A to 19B ) when the position of the UE is within the second extended cell radius. In this example, a length of the second extended special subframe is greater than a length of the first extended special subframe because the second extended cell radius is greater than the first extended cell radius. 
     Referring again to  FIG. 32 , at process block  3212 , a control information is associated with a specific downlink subframe while accounting for cell radius extension and loss of the one or more disabled, adjacent uplink subframes used to communicate the extended special subframe. In this configuration, the control information may be acknowledgement (ACK)/negative acknowledgement (NACK) feedback communicated during an uplink subframe. In this configuration, the ACK/NACK feedback communicated during the uplink subframe n corresponds to a PDSCH transmission in downlink subframe n-k. 
     For example, as shown in  FIG. 25A , the specific downlink subframes (e.g., SF 9, SF 0 and SF 5) are determined according to an downlink association set index value (e.g., 14, 13, 8) within the uplink subframe (e.g., SF 3) according to a Next-Gen AG system configuration (e.g., B), so that the ACK/NACK feedback corresponding to PDSCH transmissions in the specific downlink subframes (SF 9, SF 0 and SF 5) can be communicated in the uplink subframe (SF 3). Alternatively, a PUSCH transmission in uplink subframe n corresponds to an uplink grant and/or a negative acknowledgement (NACK) communicated during a downlink subframe n-k. For example, as shown in  FIG. 28A , an index value (e.g., 8) within an uplink subframe (e.g., SF 3) according to a Next-Gen AG system configuration (e.g., A) enables a UE to determine a downlink subframe (e.g., SF 5) that communicates an uplink grant or ACK/NACK feedback for the uplink subframe. 
       FIG. 33  illustrates a method  3300  for modification of a time division long term evolution (TD-LTE) frame structure according to another aspect of the present disclosure. In block  3310 , an eNodeB communicates with the UE using a special subframe that extends over an uplink pilot time slot and one or more disabled, adjacent uplink subframes. At process block  3312 , control information within a specific subframe is associated with an uplink subframe while accounting for loss of the one or more disabled, adjacent uplink subframes used to communicate the extended special subframe. In one configuration, the specific subframe may be a downlink subframe or a special subframe of an extended special subframe. In this configuration, the uplink subframe is determined according to an index value within the specific subframe. 
     For example, as shown in  FIG. 30A , an index value (e.g., 7) within a specific subframe (e.g., SF 1) according to a Next-Gen AG system configuration (e.g., A) enables a UE to determine an uplink subframe (e.g., SF 4) to which an uplink grant communicated during the specific subframe corresponds. Alternatively, as shown in  FIG. 31A , an uplink subframe (e.g., SF 8) to which ACK/NACK feedback communicated during a specific subframe (e.g., SF 0) corresponds is determined according to an index value (e.g., 8) within the specific subframe (e.g., SF 0) according to a Next-Gen AG system configuration (e.g., A). 
       FIG. 34  is a diagram illustrating an example of a hardware implementation for an apparatus  3400  employing a Next-Gen AG system  3414  according to one aspect of the present disclosure. The Next-Gen AG system  3414  may be implemented with a bus architecture, represented generally by a bus  3424 . The bus  3424  may include any number of interconnecting buses and bridges depending on the specific application of the Next-Gen AG system  3414  and the overall design constraints. The bus  3424  links together various circuits including one or more processors and/or hardware modules, represented by a processor  3426 , a communicating module  3402 , an associating module  3404 , and a computer-readable medium  3428 . The bus  3424  may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further. 
     The apparatus also includes a Next-Gen AG system  3414  coupled to a transceiver  3422 . The transceiver  3422  is coupled to one or more antennas  3420 . The transceiver  3422  provides a means for communicating with various other apparatus over a transmission medium. The Next-Gen AG system  3414  includes the processor  3426  coupled to the computer-readable medium  3428 . The processor  3426  is responsible for general processing, including the execution of software stored on the computer-readable medium  3428 . The software, when executed by the processor  3426 , causes the Next-Gen AG system  3414  to perform the various functions described supra for any particular apparatus. The computer-readable medium  3428  may also be used for storing data that is manipulated by the processor  3426  when executing software. 
     The Next-Gen AG system  3414  includes the communicating module  3402  for communicating with the UE using a special subframe that extends over an uplink pilot time slot and one or more disabled, adjacent uplink subframes. The Next-Gen AG system  3414  further includes the associating module  3404  for associating a control information subframe with a specific down link subframe while accounting for loss of the one or more disabled, adjacent uplink subframes used to communicate the extended special subframe. Alternatively, the associating module  3404  is configured for associating control information of a specific subframe with an uplink subframe while accounting for loss of the one or more disabled, adjacent uplink subframes used to communicate the extended special subframe. The communicating module  3402  and the associating module  3404  may be software modules running in the processor  3426 , resident/stored in the computer-readable medium  3428 , one or more hardware modules coupled to the processor  3426 , or some combination thereof. The Next-Gen AG system  3414  may be a component of the eNodeB  610  and/or the UE  650 . 
     In one configuration, the apparatus  3400  for wireless communication includes means for communicating with and means for associating. The means may be the communicating module  3402 , the associating module  3404  and/or the Next-Gen AG system  3414  of the apparatus  3400  configured to perform the functions recited by the communicating means and the associating means. In one aspect of the present disclosure, the communicating means may be the controller/processor  675  and/or memory  676 , the transmit processor  616 , and/or the transmitter  618  TX configured to perform the functions recited by the communicating means. In this aspect of the disclosure, the associating means may be the controller/processor  675  and/or memory  676  configured to perform the functions recited by the associating means. In another aspect, the aforementioned means may be any module or any apparatus configured to perform the functions recited by the aforementioned means. 
     The examples above describe aspects implemented in a TD-LTE system. Nevertheless, the scope of the disclosure is not so limited. Various aspects may be adapted for use with other communication systems, such as those that employ any of a variety of communication protocols including, but not limited to, CDMA systems, TDMA systems, FDMA systems, and OFDMA systems. 
     Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the disclosure herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. 
     The various illustrative logical blocks, modules, and circuits described in connection with the disclosure herein may be implemented or performed with a general-purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The steps of a method or algorithm described in connection with the disclosure herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal. 
     In one or more exemplary designs, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage media may be any available media that can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code means in the form of instructions or data structures and that can be accessed by a general-purpose or special-purpose computer, or a general-purpose or special-purpose processor. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.