Patent Publication Number: US-2009225686-A1

Title: Packet Communications Methods and Apparatus Using Half-Duplex Uplink and Downlink Transmission Intervals that Support Packet Retransmission and Expansions

Description:
FIELD OF THE INVENTION 
     This invention relates to communications apparatus and methods and, more particularly, to wireless mobile packet communications apparatus and methods. 
     BACKGROUND OF THE INVENTION 
     The evolution of the mobile cellular standards, such as GSM and WCDMA, has been guided by a desire to provide high capacity and high throughput to individual users in order to support advanced services, such as video and multimedia applications. A proposal for a new flexible cellular system with such capabilities is the Super 3G or Long-Term 3G Evolution (LTE), which may be seen as an evolution of the 3G WCDMA standard. LTE is a packet-switched system in which users share a broadband channel and which allows for flexible resource allocation in which a single user may be provided with very high peak rates. 
     Among features under consideration for LTE standardization is Frequency Division Duplex (FDD) operation in which uplink (UL) and downlink (DL) between a mobile terminal and a base station use different frequency bands and transmission and reception by the terminal occur simultaneously. In order to obtain sufficient isolation between transmission and reception, such operation may require use of an antenna duplexer at the mobile terminal, which may incur undesirable losses. Depending on the carrier spacing between UL and DL and the operating frequencies, the duplexer loss may vary between 1.5 and 4 dB. 
     Power consumption has become a problem for such advanced communication systems, as battery technology generally has not kept up with the pace of the increased power required for high data rate services. Moreover, heat dissipation associated with new techniques raises new challenges for mobile phone design. Extra losses in the TRX chain are often directly reflected in the terminal power consumption. 
     Introducing low power modes with low duty cycles is one way to reduce terminal power consumption. For example, to reduce power consumption in voice applications, a terminal may be activated only to send and receive a VoIP packet every 20 ms or so and, in between these windows, the terminal may be placed into a low-power sleep mode. 
     SUMMARY OF THE INVENTION 
     Some embodiments of the present invention provide methods of operating a wireless communications system wherein packet transmissions between a base station and terminals are performed using half-duplex uplink and downlink transmission intervals with adaptive intra-interval packet retransmission and packet expansion, e.g., packet expansion. The uplink and downlink transmission intervals used for communication with a terminal may be offset commensurate with a degree of symmetry of information flow between the terminal and the base station. The offset between the uplink and downlink transmission intervals may be adaptively adjusted based on a type of information being communicated between terminals and the base station. For example, different offsets may be used for voice and data communications. In some embodiments of the present invention, uplink and downlink transmission intervals for communications with a terminal have substantially equal durations. For example, voice packet transmissions between the base station and the terminal may use uplink and downlink transmission intervals having substantially equal durations. 
     Further embodiments of the present invention provide a wireless base station including a radio interface circuit configured to support packet transmissions with terminals using half-duplex uplink and downlink transmission intervals with adaptive intra-interval packet retransmission and packet expansion. The radio interface circuit may be configured to support uplink and downlink transmission intervals for communication with a terminal that are offset commensurate with a degree of symmetry of information flow between the terminal and the base station. The radio interface circuit may be configured to adaptively adjust an offset between the uplink and downlink transmission intervals based on a type of information being communicated between terminals and the base station. For example, the radio interface circuit may be configured to apply different offsets for voice and data communications. The radio interface circuit may be configured to support uplink and downlink transmission intervals for communications with a terminal that have substantially equal durations. For example, the radio interface circuit may be configured to support voice packet transmissions between the base station and the terminal using uplink and downlink transmission intervals having substantially equal durations. 
     In still further embodiments of the present invention, a wireless terminal includes a radio interface circuit configured to support packet transmissions with a base station using half-duplex uplink and downlink transmission intervals with adaptive intra-interval packet retransmission and packet expansion. The radio interface circuit may be configured to support uplink and downlink transmission intervals that are offset commensurate with a degree of symmetry of information flow between the terminal and the base station. The radio interface circuit may be configured to adaptively adjust an offset between the uplink and downlink transmission intervals based on a type of information being communicated between the terminal and the base station. For example, the radio interface circuit may be configured to apply different offsets for voice and data communications. The radio interface circuit may be configured to support uplink and downlink transmission intervals having substantially equal durations. For example, the radio interface circuit may be configured to support voice packet transmissions between the base station and the terminal using uplink and downlink transmission intervals having substantially equal durations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-3  are schematic diagrams illustrating mobile terminal and base station apparatus and operations according to some embodiments of the present invention. 
         FIGS. 4A-C  and  5 A-C are timing diagrams illustrating offset of uplink and downlink for half-duplex communications with retransmission and adaptive modulation and/or coding according to further embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying figures, in which embodiments of the invention are shown. This invention may, however, be embodied in many alternate forms and should not be construed as limited to the embodiments set forth herein. 
     Accordingly, while the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims. Like numbers refer to like elements throughout the description of the figures. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising,” “includes” and/or “including” (and variants thereof) when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Moreover, when an element is referred to as being “responsive” to another element/step (and variants thereof), it can be directly responsive to the other element/step, or intervening elements/steps may be present. In contrast, when an element/step is referred to as being “directly responsive” to another element/step (and variants thereof), there are no intervening elements/steps present. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items and may be abbreviated as “/”. 
     It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. 
     The present invention is described below with reference to schematic diagrams illustrating methods, apparatus (systems and/or devices) and/or computer program products according to embodiments of the invention. It is understood that a block of the diagrams, and combinations of blocks in the diagrams can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, and/or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer and/or other programmable data processing apparatus, create means (functionality) and/or structure for implementing the functions/acts specified in the diagrams. These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the function/act as specified in the diagrams. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the diagrams. 
     Accordingly, the present invention may be embodied in hardware and/or in software (including firmware, resident software, micro-code, etc.). Furthermore, the present invention may take the form of a computer program product on a computer-usable or computer-readable storage medium having computer-usable or computer-readable program code embodied in the medium for use by or in connection with an instruction execution system. In the context of this document, a computer-usable or computer-readable medium may be any medium that can contain, store, communicate or transport the program for use by or in connection with the instruction execution system, apparatus, or device. 
     The computer-usable or computer-readable medium may be, for example, an electronic, magnetic, optical, electromagnetic or semiconductor system, apparatus or device. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: a portable computer diskette, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), and a portable optical and/or magnetic media, such as a flash disk or CD-ROM. 
     Exemplary embodiments described herein relate to communications between terminals and base stations. It will be appreciated that “terminals” may include, but are not limited to, personal communications devices, such as cellular handsets, wireless-enabled personal digital assistants (PDAs) and similar devices, as well as wireless-enabled computers, such as wireless-enabled laptop, notebook and subnotebook computers. Such terminals may also be referred to as “user equipment” (UE) herein. “Base station” refers to a node that serves as a radio access point for terminals to a wireless communications system, including, but not limited to, cellular base stations, wireless access points, and the like. 
     Some embodiments of the present invention arise from a realization that use of a lossy duplexer can be avoided by using half-duplex uplink and downlink transmission intervals that support adaptive use of intra-interval packet retransmission (e.g., in an interference-limited condition) and packet expansion (e.g. packet expansion due to radio link adaptation, such as adaptive modulation and/or coding that may be used in a noise-limited condition). A time offset between uplink and downlink may be fixed and/or may be varied based on the type of communications, for example, based on a degree of symmetry of data flow between the uplink and the downlink. For example, for voice applications, uplink and downlink transmissions may have substantially equal durations to support the substantially symmetric nature of voice communications. For example, in an LTE implementation having a repetition period of 20 ms, the uplink and downlink transmission intervals may be offset by about one-half of the repetition period, e.g., around 10 ms. In further embodiments, an asymmetrical offset may be used for other applications, e.g., data applications, such as web browsing. 
     As discussed above, full duplex operation may require the use of a lossy duplexer. Half-duplex operation may be better in this respect because transmission and reception do not occur simultaneously, which can eliminate the need for a duplexer. For symmetric services, such as Voice over IP (VoIP), half-duplex operation may be achieved using time staggering of uplink and downlink. However, small time staggering may not allow sufficient time for packet expansion due to link adaptation in noise-limited conditions (e.g., at the cell edge) and the application of retransmission in interference-limited conditions (e.g., closer to the base station). 
     For example, in some embodiments of the present invention, the uplink and downlink transmission intervals in an LTE system may be offset by about half a repetition period. For VoIP in LTE, a 20 ms repetition period is used. Therefore, for half-duplex VoIP communications between user equipment (UE) and a base station, a time offset between UL and DL packets of about half of the repetition period or 10 ms may be used. In systems like LTE, there may be a limit to the peak output power (21 dBm or 24 dBm depending on user equipment (UE) power class), and increases in performance through use of increased UE output power may be limited when the UE is at the cell edge. In such situations, Adaptive Modulation and Coding (AMC) may be used to improve the link budget. AMC, however, may increase the VoIP packet length. 
     Because of the time offset, both the UL and DL can increase the packet length (up to 10 ms each in the example) and a balanced link budget can be achieved. If the UE moves closer to the base station, the reverse operation may be carried out, which may reduce the packet length. In such conditions, the remaining time not used for the VoIP packet can be used for retransmissions in a hybrid (H) ARQ scheme. If the UE enters an interference-limited area, e.g., near the base station, retransmissions may be more effective than AMC. 
     The timing offset may be determined by the scheduling mechanism in the LTE base station and can be easily altered. The UL and DL may be imbalanced (e.g., have different link budgets in UL and DL caused by several reasons like better receiver sensitivity and/or higher transmit power in the base station) and the offset between UL and DL may be changed accordingly. If the link budget in the DL is better, the DL may need less than 50% of the 20 ms period, not only because the packets can be shortened for noise-limited conditions, but also because fewer retransmissions may be needed. In interference-limited situations, the UL may be better due to multi-user detection capabilities in the base station. In such situations, the UL may need less than 50% of the 20 ms repetition period. 
       FIGS. 1-3  illustrate a mobile terminal  100  and base station  200  according to some embodiments of the present invention. Referring to  FIG. 1 , the mobile terminal  100  and base station communicate over an uplink  10  and a downlink  20  having half-duplex transmission intervals that support intra-interval packet retransmission and packet expansion due to link adaptation. Referring to  FIG. 2 , the mobile terminal  100  may include radio interface circuitry  110  and user interface circuitry  130  operatively associated with a controller  120 , for example, a microprocessor, microcontroller or other control circuitry. Referring to  FIG. 3 , the base station  200  may include radio interface circuitry  210  and network interface circuitry  230  operatively associated with a controller  220 , for example, a microprocessor, microcontroller or other control circuitry. 
     As further illustrated in  FIGS. 2 and 3 , the respective radio interface circuits  110 ,  210  are configured to support the uplink  10  and downlink  20 . The radio interface circuitry  110 ,  210  may include, for example, baseband processors, mixers, power amplifiers, antennas and other signal transmission, reception and processing components. As further illustrated, the radio interface circuitry  110 ,  210  further include control circuitry configured to support adaptive application of packet retransmission  112 ,  212  and link adaptation  114 ,  214 , along with circuitry that controls uplink (UL) and downlink (DL) timing offset control  116 ,  216 . The packet retransmission circuitry  112 ,  212  may be configured, for example, to implement an automatic repeat request (ARQ) mechanism wherein a receiving device, i.e., either the mobile terminal  100  or the base station  200 , may request and obtain full or partial retransmission of packets to correct packet reception errors. The link adaptation circuitry  114 ,  214  may be configured, for example, to adaptively change the modulation scheme and/or the error correction coding scheme, which may be used, for example, to provide improved reception in noise-limited conditions, such as when the mobile terminal  100  is located a substantial distance from the base station  200  (e.g., at a cell edge). 
     The UL/DL offset control circuitry  116 ,  216  may be configured to control relative timing of transmission windows for the uplink  10  and downlink  20  in support of the packet retransmission circuitry  112 ,  212  and the link adaptation circuitry  114 ,  214 . In particular, the UL/DL offset control circuitry  116 ,  216  may provide durations for transmission intervals for uplink  10  and the downlink  20  sufficient to allow packet retransmission under a first propagation condition, for example, an interference-limited condition as might occur when the mobile terminal  100  is near the base station  200 , and for packet expansion due to AMC under a second propagation condition, for example, a noise-limited condition as might occur when the mobile terminal  100  is relative distant from the base station  200 . In some embodiments, the UL/DL timing offset control circuitry  116 ,  216  my be configured to vary a timing offset between the uplink  10  and the downlink  20  based on the type of communications being conducted between the mobile terminal  100  and the base station  200 . 
     VoIP performance is an important issue in the new cellular packet-switched systems like LTE and WiMAX. In particular, the talk time, which is generally directly coupled to the power consumption, is under investigation. Half-duplex systems may be preferable, as no duplexer, which may introduce loss in the transmit and receive chains, is required. Both LTE and WiMAX have advanced link adaptation schemes in order to improve coverage and reduce the effects of interference. In particular, Hybrid ARQ (HARQ) is used for retransmission of data, or sending incremental data to help error recovery schemes in the receivers. Furthermore, link adaptation is obtained using AMC, which switches between more spectrally efficient but more sensitive modulation schemes like 64-QAM and more robust but less efficient schemes like QPSK. In addition, with AMC, stronger or weaker coding schemes can be applied, for example, using punctured convolutional codes. 
     At the cell edge, the UE may be in noise-limited conditions. The received signal strength (RSS) may be just above the noise floor. Improvement of the link performance or extending the coverage can be achieved with AMC, i.e., by using simpler constellation schemes and/or by additional coding bits in order to increase the energy per bit. But since the peak output power is fixed, this may result in a longer time per bit. So for the same amount of information bits to be sent, longer packets may be needed. In a balanced link, UL and DL may have about the same link budget. Assuming symmetric traffic like voice (VoIP), the same AMC may be applied in UL and DL. In order to allow for the largest packet length extension, the time offset between UL and DL half-duplex operation may be set to about half the repetition period, which is 20 ms for most voice schemes including VoIP. So, for a voice service, 10 ms may be an optimal timing offset between UL and DL in a balanced link. 
     This is illustrated in  FIGS. 4A-C . Referring to  FIG. 4A , near the cell edge, more energy per packet  410  generally will be required, which may be accomplished by increasing the packet length. Referring to  FIGS. 4B and 4C , as the UE moves closer to the base station, shorter packets  410  can be used. However, the time freed up can now be used for retransmissions in response to acknowledgments  420 . Retransmissions may be more effective in interference-limited conditions, which are often found closer to the base station. Some embodiments provide an advantageous way to optimize the links with short packets with many retransmissions in an interference-limited environment, and long packets with few retransmissions in a noise-limited environment. 
     It is shown in  FIGS. 4A-C  that, after transmission of each packet  410 , there is an acknowledgement packet  420  transmitted in the opposite direction, if the time window allows. This signalling is used in the HARQ protocol. Preferably, the acknowledgement packet  420  follows the end of the data packet  410  it refers to with a fixed time delay. In the illustrated example, the time offset between the UL and DL VoIP is fixed at 10 ms. In current specifications, reserving slots in LTE is done through persistent scheduling, with the persistent scheduling mechanism relating to the leading edge of the first VoIP packet. Retransmission is done through dynamic scheduling, that is, the first UE acknowledgement slot is also reserved, but subsequent retransmissions with signalling are not. Using a fixed time offset according to some embodiments of the present invention may be attractive, as the persistent scheduling need not change when the packet length increases. Therefore, the scheduling does not have to be updated each time the UE position changes. Indeed, in-band signalling or blind detection can be applied to indicate the change in AMC and, therefore, the change in packet length. 
     According to further embodiments of the present invention, the uplink and downlink transmission intervals may be asymmetric. For example, in the LTE downlink, larger peak power can be used resulting in a larger link budget. This difference in link budget can be reflected in the half-duplex offset between UL and DL as is shown in  FIGS. 5A-C . In this case, the DL is allocated a shorter part of the 20 ms period than the UL, while providing retransmission and link adaptation in a manner similar to that shown in  FIGS. 4A-C . Because more power can be allocated in the DL, the packets  410  transmitted on the DL may be shorter than those transmitted on the UL, while still having an increased energy per bit. Furthermore, less retransmission may be required to achieve the same residual packet error rate (PER) as in the UL. 
     It will be further understood that relative offset between UL and DL transmission intervals may be fixed and/or have varying degrees of adaptability. For example, in some embodiments, such as in some voice-only or data/only applications, a fixed offset between UL and DL transmission intervals may be provided. In other embodiments, a system may transition between different transmission interval offsets for different modes, i.e., different offsets for voice and data modes, but the interval offset may remain fixed while in a particular mode. In further embodiments, however, DL/UL transmission interval offsets may be more adaptive, for example, UL/DL transmission interval offsets in a data application may be varied based on the type of data application and/or other parameters. It will also be understood that embodiments according to the present invention include embodiments in which UL and DL transmissions use the same or at least some of the same frequencies (e.g., in certain Time Division Duplex (TDD) or orthogonal frequency division multiplex (OFDM) embodiments), as well as embodiments in which the UL and DL transmissions do not share frequencies (e.g., in certain FDD embodiments). 
     In the drawings and specification, there have been disclosed embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.