Abstract:
A method and apparatus are described. A wireless transmit/receive unit (WTRU) generates data bits and piggybacked acknowledgement/non-acknowledgement (PAN) bits and generates a plurality of symbols based on the data bits and the PAN bits. Each symbol of the plurality of symbols represents a plurality of bits and has a least significant bit (LSB) position, and no PAN bits are present in the LSB position of each of the plurality of symbols. The plurality of symbols are transmitted.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/196,385 filed Aug. 22, 2008, which claims the benefit of U.S. Provisional application No. 60/957,908, filed Aug. 24, 2007, the contents of which are hereby incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    This application is related to wireless communications. 
       BACKGROUND 
       [0003]    The Global System for Mobile Communication (GSM) is one of the most widely deployed communication standards for mobile wireless communication. In order to introduce packet-switched technology, general packet radio service (GPRS) was developed by the European Telecommunications Standards Institute (ETSI). One limitation of GPRS is that it does not support voice services. Other issues with GPRS include lack of higher data rates supported as well as poor link adaptation algorithms. Therefore, the third generation partnership project (3GPP) developed a new standard for GSM to support high rate data services, released in 1999 and known as enhanced data rates for GSM evolution (EDGE). 
         [0004]    A network configured according to these standards comprises a core network (CN), at least one wireless transmit/receive units (WTRU) attached to a radio access network (RAN), such as the GSM/EDGE radio access network (GERAN). The GERAN comprises a plurality of base transceiver stations (BTSs), each connected to and controlled by a base station controller (BSC). The combination of the BSCs and the corresponding BTSs is realized as the Base Station System (BSS). 
         [0005]    The radio link control/medium access control (RLC/MAC) protocol, which resides in the WTRU and the BSS, is responsible for reliable transmission of information between the WTRU and the network. In addition, the physical layer latency, (for example, packet transfer and serialization delays) is controlled by the RLC/MAC protocol. 
         [0006]    A goal for GERAN evolution is to develop new technology, new architecture and new methods for settings and configurations in wireless communication systems. One work item for GERAN evolution is latency reduction. Release 7 (R7) of the 3GPP GERAN standard introduces several features that may improve throughput and reduce latency of transmissions in the uplink (UL) and the downlink (DL). UL improvements are referred to as higher uplink performance for GERAN evolution (HUGE), and DL improvements are referred to as reduced symbol duration higher order modulation and turbo coding (REDHOT). Both of these improvements may generally be referred to as evolved general packet radio service 2 (EGPRS-2) features. 
         [0007]    The Latency Reduction feature includes two (2) technical approaches that may operate either in a stand-alone mode, or in conjunction with any of the other GERAN R7 improvements. One approach uses a fast positive acknowledgement/negative acknowledgement (ACK/NACK) reporting (FANR) mode. Another approach uses a reduced transmission time interval (RTTI) mode. A WTRU may operate in both FANR and RTTI modes of operation with legacy EGPRS modulation and coding schemes (MCSs), and with the newer EGPRS-2 modulation and coding schemes. 
         [0008]    REDHOT and HUGE provide increased data rates and throughput compared to legacy EGPRS DL and UL. These modes may be implemented through the use of higher order modulation schemes, such as sixteen quadrature amplitude modulation (16-QAM) and thirty two quadrature amplitude modulation (32-QAM). These modes may also involve the use of higher symbol rate transmissions and turbo-coding. Similar to legacy systems, REDHOT and HUGE involve an extended set of modulation and coding schemes that define new modified information formats in the bursts, various coding rates and coding techniques and the like. 
         [0009]    Prior to the introduction of FANR, ACK/NACK information was typically sent in an explicit message, referred to as an RLC/MAC control block, which contained a starting sequence number and a bitmap representing radio blocks. The reporting strategy (how and when reports are sent, and the like) was controlled by the network. The WTRU would send a Control Block as a response to a poll from the base station system (BSS). The poll will also include information about the UL transmission time (for example, when the WTRU is allowed to send its control block in the UL). During normal operation, when higher layer information is exchanged between the WTRU and the network, the information transfer occurs using RLC Data Blocks. 
         [0010]    A drawback of the current ACK/NACK reporting protocols is that a full control block is needed every time ACK/NACK information is sent. Therefore, a large overhead is required when ACK/NACK information is frequently needed for delay sensitive services. 
         [0011]    Consequently, within the framework of GERAN evolution, a new ACK/NACK state machine that uses ACK/NACK reports “piggybacked” on RLC Data Blocks in the opposite link direction was introduced. 
         [0012]    This protocol has the potential to significantly reduce the retransmission delay without significant overhead. These piggybacked ACK/NACK (PAN) reports are bitmaps, designed as a combination of block sequence numbers (BSNs) which specify outstanding radio blocks bitmaps giving ACK/NACK information of radio blocks, and size bits or extension bits specifying the size of the ACK/NACK information. PANs are used to transmit an ACK/NACK bitmap within a radio block carrying RLC data. 
         [0013]    This allows for ACK/NACK information to consist either of one single PAN or to be split into several multiple segment PANs. This allows for a decrease in latency and round-trip times due to increased flexibility of sending ACK/NACK reports independently from data transmissions to a particular wireless transmit/receive unit (WTRU) without necessitating special RLC/MAC control blocks, while maintaining general principles of RLC window operation. 
         [0014]      FIG. 1  shows a conventional radio block. Currently, a PAN field may be inserted into a RLC/MAC radio block using modulation and coding schemes (MCSs) for EGPRS or new MCSs provided by REDHOT/HUGE (EGPRS-2). In both of these scenarios, the radio block consists of a separately encoded RLC/MAC header  105  that is decodable independent from the RLC data payload; an RLC data payload  110  and a PAN field  115  that is separately decodable from the RLC/MAC header and RLC data payload. 
         [0015]    Some legacy EGPRS radio blocks and some new REDHOT/HUGE radio blocks may contain more than one RLC data Protocol Data Unit (PDU) per radio block. The PAN is mapped on the burst together with the data. The placement of the PAN before interleaving is dependent on the interleaving depth of the data block. Since all PANs have low code rates, a maximized interleaving depth is preferred. 
         [0016]    The insertion of the PAN field  115  into the radio block requires heavier puncturing of the actual RLC data payload. In essence, since the overall number of bits that may be placed into the radio block is fixed, more encoded data bits must be removed from the RLC data payload once a PAN is inserted. Since the RLC/MAC header coding remains unchanged even when a PAN is inserted, the coding rate of the data portion should be increased. However, this may be detrimental to link performance and effectiveness of the link adaptation algorithm, because the increased channel coding rate and reduced number of channel bits of the affected RLC data payload  110  of the radio block may lead to more transmission errors and less protection of the data. 
         [0017]    Another problem is that the RLC/MAC header  105 , the RLC data payload  110  and the PAN field  115  are all independently channel coded. For example, a PAN field, which contains M=20 information bits and N=6 cyclic redundancy check (CRC) bits, is coded into 80 channel coded bits yielding a coding rate of approximately 0.33. Therefore, balancing error performance of the RLC/MAC header  105 , the RLC data payload  110  and the PAN field  115  is essential to good performance of the radio block. 
         [0018]    The different error performances of the portions making up the RLC MAC radio block  110  are shown in  FIG. 2 . For example, if the error rate of the RLC/MAC header  105  becomes too high, more transmissions are lost due to the receiver (WTRU or base station) failing to decode the RLC/MAC header  105 , rather than errors in the RLC data payload  110 . The protection of the PAN field  115  is also questionable, as well as the mapping of the PAN field  115 . 
         [0019]    In the conventional RLC/MAC radio block of  FIG. 1 , the RLC/MAC header  105 , the RLC data payload  110  and the PAN field  115  are interleaved together. Their channel-coded bits carried by the modulation symbols are spread across four (4) radio bursts such that bits belonging to the PAN field  115 , for example, are not necessarily contiguous. Applying a power offset just to a subset of PAN-carrying symbols may create extra leaking of transmit (Tx) power into the adjacent carriers due to radio frequency (RF) non-linearity from “normal” symbols transiting to symbols sent at higher offset power at the configured standard peak-to-average ratio (PAR) back-off for the given modulation order. This may result in intolerable out-of-band emission levels. 
         [0020]    It is therefore desirable to have a method and apparatus for linking performance and error resilience of different portions of a radio block and matching portions of a radio block to their respective requirements for PAN filed inclusion, when compared to transmission without PAN field inclusion, without changing the number of channel coded bits. 
       SUMMARY 
       [0021]    A method and apparatus are described. A wireless transmit/receive unit (WTRU) generates data bits and piggybacked acknowledgement/non-acknowledgement (PAN) bits and generates a plurality of symbols based on the data bits and the PAN bits. Each symbol of the plurality of symbols represents a plurality of bits and has a least significant bit (LSB) position, and no PAN bits are present in the LSB position of each of the plurality of symbols. The plurality of symbols are transmitted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    A more detailed understanding may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein: 
           [0023]      FIG. 1  is a conventional RLC/MAC block structure for EGPRS data transfer; 
           [0024]      FIG. 2  depicts the error ratios of different portions of a RLC/MAC radio block without bit swapping. 
           [0025]      FIG. 3  shows the structure of a radio block without PAN bit swapping compared to the structure of a radio block with PAN bit swapping. 
           [0026]      FIG. 4  is a block diagram of a wireless communication system including a WTRU and a base station used to transmit and receive radio blocks with piggybacked ACK/NACK fields. 
           [0027]      FIG. 5  is a flow diagram of a procedure performed by the WTRU of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    When referred to herein, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. When referred to herein, the terminology “base station” includes but is not limited to a Node-B, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment. 
         [0029]      FIG. 3  shows the structure of a burst  300 A. The burst  300 A includes PAN bits  305 , header bits  310 , and data bits  315 . PAN bits  305  are interspersed throughout the burst and may be found in all bit positions of a symbol. It is noted that while the burst  300 A is representative of eight phase shift keying (8-PSK) modulation (that is, three bits per symbol), the PAN bit swapping technique disclosed herein may be applied to any modulation order. Due to the nature of phase shift keying modulation, those skilled in the art will recognize that the third bit position  350  of each symbol is more prone to error than the first two bit positions  340  of each symbol. 
         [0030]      FIG. 3  also shows the structure of modulated information bits after PAN bit swapping ( 300 B) is applied, according to one embodiment. PAN bits  305  in unreliable bit positions  350  of the each symbol (in the illustrated case of 8-PSK, the third bit position of each symbol) are “swapped” with data bits  315  in more reliable bit positions  340 . For example, PAN bit  305 A is shown in burst  300 A in the third bit position of a symbol. After PAN bit swapping, PAN bit  305 A has been swapped with a data bit  315  from a more reliable bit position. PAN bit  305 B is now located in a more reliable position. After channel coding, the burst is also accompanied by a training sequence  320 , two stealing flags (SF)  325 , and, in the DL direction, an uplink state flag (USF)  330  fields. 
         [0031]    It is noted that PAN bit swapping as disclosed herein improves the reliability of PAN bits  305 . However, as a trade off, data bits  315  that are swapped with PAN bits  305  are less reliable. Due to the importance of PAN bits  305  and data retransmission techniques, this trade off is generally acceptable. 
         [0032]    Additionally, areas in the middle of the burst  300 A, such as the training sequence  320 , are less prone to bad channel conditions. Therefore, it may be advantageous to swap PAN bits  305  with other bits that are close to the training sequence  320 . It would likewise be advantageous to swap PAN bits  305  with other bits in more desirable locations of the radio block. 
         [0033]    The PAN bit swapping described with reference to  FIG. 3  may also be applied to higher order modulation. More reliable (that is, most significant bits or outer constellation points) of sixteen quadrature amplitude modulation (16-QAM) and thirty two quadrature amplitude modulation (32-QAM) may be used for PAN bit swapping. Of course, PAN bit swapping as disclosed may be used with any modulation technique having multiple bits per symbol. 
         [0034]    In addition to PAN bit swapping, one or more power offsets may be applied to one or more individual portions of the burst  300 A to improve performance. The power offsets may be applied individually or in combination to the header  310 , data  315 , PAN  305 , training sequence  320 , stealing flag (SF)  325 , and/or uplink state flag (USF)  330  fields, in order to balance the individual error performance of each of the portions. The power offset or may be adjusted during system operation to take into account varying radio conditions, interference levels, power headroom, or presence and absence of individual fields by the radio transmitter. Accordingly, different power offset values may be applied to the different fields. By selective application of power offsets to certain portions of a radio block, link performance may be increased while creating only minimal interference to other receivers. 
         [0035]    Referring to  FIG. 4 , an exemplary method  400  of applying a power offset as described above begins with initiating a transmission, (step  410 ). It is then determined if PAN bits are included in the radio block, (step  420 ). Depending on system operation, PAN bits may always be included so this step may be unnecessary. If PAN bits are present, the PAN bits located in unreliable bit positions are swapped with bits in more reliable bit positions, (step  430 ), as described above. Next, a power offset may be calculated for each various bits and/or regions of the radio block (for example, header field, PAN bits, training sequence, stealing flag), (step  440 ). Finally, the calculated power offset is applied to the radio block, (step  450 ). 
         [0036]    In the method  400 , the calculated power offset may, for example, counter-balance the effect of an increased coding rate for data bits. The calculated power offset may be applied semi-statically, using periodic adjustments, or may be adjusted during system operation to take into account varying radio conditions and/or interference levels and/or power headroom. 
         [0037]    A WTRU may independently calculate the power offset values based on predetermined criteria or measured values, or the WTRU may receive power offset values from the network. The network may adjust or configure the offset values based on link adaptation mechanisms. For example, the offset value may be signaled to a WTRU in a separate control block, (for example a packet power control/timing advance, packet time slot reconfigure or packet UL ACK/NACK message). Alternatively, other RLC/MAC control blocks may also be modified to convey this type of information. 
         [0038]    When PAN bit swapping and power offsets are used in combination, PAN bits may be swapped with other bits of a single radio burst among the four (4) radio bursts that make up a radio block, and a power offset may be applied to the entire radio burst containing the PAN bits. This approach avoids varying power levels within a burst. Alternatively, the PAN bits may also be swapped with bits of a subset of the four (4) radio bursts that make up the radio block. The power offset may then be applied to the bursts carrying the PAN bits. These methods may also be applied to the other bits, such as the header, data bits, and the like. 
         [0039]      FIG. 5  shows a WTRU  500  and a base station  505  each configured to implement the above disclosed methods. The WTRU  500  includes a transmitter  510 , a receiver  515 , and a processor  520 . The transmitter  510  and receiver  520  are coupled to an antenna  525  and the processor  520 . The WTRU  500  communicates with the base station  505  in an uplink direction  530  and a downlink direction  535  via an air interface. The processor  520  includes a modulator/demodulator  540 , an interleaver/deinterleaver  545 , and a constellation mapper/demapper  550 . The processor  520  is configured to produce radio blocks for transmission and process received radio blocks as described above. The interleaver/deinterleaver  545  is configured to interleave and deinterleave bits in a radio block, and to swap PAN bits with data bits as disclosed. The constellation mapper/demapper  550  is configured to code and decode symbols based on a modulation technique, such as QPSK, 16-QAM, 32-QAM, or the like, and to swap PAN bits with data bits as disclosed in cooperation with the interleaver/deinterleaver  545 . The modulator/demodulator  540  is configured to modulate the prepared radio block for uplink transmission via the transmitter  510  and to demodulate received radio blocks in the downlink via the receiver  515 . 
         [0040]    The processor  520  of the WTRU  510  is further configured to apply power offsets to various regions of the radio blocks, as disclosed. The processor  520 , in combination with the transmitter  510 , may adjust the transmission power according to calculated or received power offset values, either semi-statically or based on changing channel conditions, as described above. The processor  520  is further configured to receive, via the receiver  515 , power offset values from the base station  505 . 
         [0041]    The base station  505  may contain similar functionality as described above with reference to the WTRU  500 . A processor of the base station may be configured to generate power offset commands as disclosed, and to swap PAN bits as disclosed. 
         [0042]    Although the features and elements are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements. The methods or flow charts provided in the present invention may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs). 
         [0043]    Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine. A processor in association with software may be used to implement a radio frequency transceiver for use in a WTRU, user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, and an organic light-emitting diode.