Patent Publication Number: US-2006013216-A1

Title: Apparatus and method for supporting real-time services in a wireless network

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY  
      The present invention is related to that disclosed in U.S. Provisional Patent No. 60/587,620, filed Jul. 13, 2004, entitled “Support of Real-Time Services on the DO-A Systems” and U.S. Provisional Patent No. 60/656,345, filed Feb. 25, 2005, entitled “Support of Real-Time Services on the DO-A Systems”. U.S. Provisional Patent Nos. 60/587,620 and 60/656,345 are assigned to the assignee of the present application. The subject matter disclosed in U.S. Provisional Patent Nos. 60/587,620 and 60/656,345 is hereby incorporated by reference into the present disclosure as if fully set forth herein. The present application hereby claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Nos. 60/587,620 and 60/656,345.  
     TECHNICAL FIELD OF THE INVENTION  
      The present invention relates generally to wireless networks and, more specifically, to a technique for supporting real-time services such as voice-over-IP (VoIP) in a wireless network.  
     BACKGROUND OF THE INVENTION  
      Wireless telecommunications have advanced from a network of analog carriers to large digital networks using many different standards. Some standards are designed as both data and voice carriers, while others are primarily designed as data-only carriers, such as the Evolution Data-Only (EV-DO) standard.  
      The 1xEV-DO Revision A Standard (CDMA2000 High Rate Packet Data Air Interface Specification, 3GPP2 C.S0024-A, Version 1.0, March 2004, by the “3rd Generation Partnership Project 2”, hereby incorporated by reference) defines an entity called the Access Terminal (AT) that is more commonly called a Mobile Station (MS) in a wireless communication system, and an entity called the Access Network (AN) that is more commonly called a Base Station (BS). In preparation for transmitting on the Forward Traffic Channel, the AN takes a Physical Layer packet of one of several standard sizes in bits, modulates it into a symbol sequence, and then applies repetition and puncturing, as appropriate, to generate a modulated packet.  
      The AN then transmits a portion, or subpacket, of the modulated packet in a 1.67-millisecond slot. If the AT receives the subpacket with few enough symbol errors, it can demodulate and reconstruct the original Physical Layer packet without bit errors, in which case it sends an ACK back to the AN during the third slot after the subpacket transmission. If the AT cannot reconstruct the original packet correctly, it sends a NAK. If the AN does not receive an ACK, it transmits the next portion, or subpacket, of the modulated packet four slots after it transmitted the first subpacket.  
      The AT then tries to reconstruct the original packet without bit errors using both of the subpackets it has received. If it still cannot reconstruct the original packet correctly, it sends another NAK. By example in the standard, the AN continues to transmit subpackets four slots apart until it receives an ACK from the AT. If the AT can reconstruct the original packet without bit errors, in which case it sends an ACK back to the AN, before the AN has transmitted all the subpackets, this is called early termination. However, by example in the standard, if the AN never receives an ACK from the AT, it transmits all the subpackets that make up the modulated packet symbol sequence. This maximum number of subpackets is called the Nominal Transmit Duration, or Span, and is one of the attributes of the Transmission Format.  
      Another attribute is the original Physical Layer packet length in bits. The final attribute is the preamble length in chips. A preamble identifying which AT the packet is intended for is prepended to the first subpacket of each packet transmitted.  
      There are a total of 33 Transmission Formats (i.e., combinations of these three attributes) for the Forward Traffic Channel with the Enhanced Forward Traffic Channel MAC Protocol. They are grouped by what is called Data Rate Control (DRC) Index, one to four formats per DRC Index (for Single User use). The AT selects the DRC Index value according to the nominal data rate it estimates can currently be supported on the Forward Traffic Channel, and sends this estimate to the AN every slot. The AN then selects one of the up to four Transmission Formats associated with that DRC Index. In each group, the Span is the same for all the formats.  
      The preamble length is also the same for all formats. It is the packet length that distinguishes the formats in a group; the AT does what is called “blind reception” or “rate matching” to determine the original packet length.  
      A significant disadvantage of this approach stems from the example procedure to transmit the entire Span of subpackets when the AT is unable to reconstruct the original packet correctly, coupled with the large Span sizes associated with small DRC Indexes. This adversely affects the performance of real-time applications, such as XoIP, including but not limited to Voice-over-IP.  
      Therefore, there is a need in the art for an improved system and method for high-rate wireless packet transmission that is suitable for XoIP transmissions.  
     SUMMARY OF THE INVENTION  
      To address the above-discussed deficiencies of the prior art, it is an object of the present invention to provide, for use in a wireless network, a mobile station capable of packet data communications, said mobile station comprising a circuit for receiving radio-frequency signals, including packet data communications, wherein the packet data communications includes a series of packets, at least some of which have a first subpacket and a plurality of subsequent subpackets; and a processor, connected to decode the packet data communications, wherein the processor is configured to examine each received subpacket to determine if the received subpacket includes preamble data, wherein if one of the subsequent subpackets of a received packet includes preamble data, then that subpacket is identified as the first subpacket of a new packet, and all other subpackets of the received packet are discarded.  
      It is another object of the present invention to provide, for use in a wireless network, a mobile station capable of packet data communications, said mobile station comprising a circuit for receiving radio-frequency signals, including packet data communications, wherein the packet data communications includes a series of packets, at least some packets having a first subpacket and a plurality of subsequent subpackets; and a processor, connected to decode the packet data communications, wherein the processor is configured to detect a last subpacket identifier, wherein if a last subpacket identifier is detected, then the next subpacket received is determined to be the first subpacket of a new packet.  
      It is another object of the present invention to provide, for use in a wireless network, a base station capable of packet data communications, said base station comprising a circuit for transmitting radio-frequency signals, including packet data communications, wherein the packet data communications includes a series of packets, at least some packets having a first subpacket and a plurality of subsequent subpackets; a circuit for receiving radio-frequency signals, including acknowledgements indicating that a transmitted packet has been successfully received and decoded, and including negative acknowledgements indicating that a transmitted packet has not been successfully received and decoded; and a processor, connected to encode and decode the packet data communications, wherein if multiple subpackets of a first packet have been sent without receiving a corresponding acknowledgement, then the base station is configured to discard the first packet and send the first subpacket of a second packet.  
      It is another object of the present invention to provide, for use in a wireless network, a base station capable of packet data communications, said base station comprising a circuit for transmitting radio-frequency signals, including packet data communications, wherein the packet data communications includes a series of packets, at least some packets having a first subpacket and a plurality of subsequent subpackets; a circuit for receiving radio-frequency signals, including acknowledgements indicating that a transmitted packet has been successfully received and decoded, and including negative acknowledgements indicating that a transmitted packet has not been successfully received and decoded; and a processor, connected to encode and decode the packet data communications, wherein if multiple subpackets of a first packet have been sent without receiving a corresponding acknowledgement, then the base station is configured the first send a subpacket including a last subpacket identifier and thereafter send the first subpacket of a second packet.  
      Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:  
       FIG. 1  illustrates an exemplary wireless network that supports real-time services, such as voice-over-IP (VoIP), according to the principles of the present invention;  
       FIG. 2  illustrates an exemplary base station according to an exemplary embodiment of the present invention;  
       FIG. 3  illustrates wireless mobile station according to an advantageous embodiment of the present invention;  
       FIGS. 4A and 4B  depict active and idle slots in accordance with an exemplary embodiment of the present invention; and  
       FIG. 5  depicts and active slot in accordance with an alternate embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged wireless network.  
      Preferred embodiments include techniques to support real time services, in particular Voice Over IP (VoIP), on the DO-A systems. Current DO-A systems are designed for non-real time applications. The systems are geared towards supporting higher data rates, higher latency applications.  
      Some of the issues addressed herein for supporting real time applications include (1) support of additional single user and multi-user transmission formats, and of a Span that is not limited to 1,2,4,8, 16, but can also support 3, 5, 7, 9, etc., (2) as an alternate embodiment, change in the MAC transmission format to transmit a last subpacket identifier.  
      Another issue is a better QoS guarantee. Different embodiments allow for support of different classes of QoS, dependent on the types of services. Another issue is a technique to increase the success of the transmission of packets, useful for boosted power applications.  
      The disclosed embodiments also enable an improved DO-A scheduler that helps to balance different QoS, such that VoIP (low delay, higher QoS) gets higher priority over NRT applications and such that NRT does not suffer too much and the overall throughput does not decrease. This also allows optimization of pre-scheduled, guaranteed users.  
      While the 1xEV-DO Revision A Standard uses the terms Access Terminal (AT) and Access Network (AN), the description herein will use the more common terms Mobile Station (MS) and Base Station (BS), as the techniques disclosed herein are not limited to systems complying with the 1xEV-DO Revision A Standard.  
       FIG. 1  illustrates exemplary wireless network  100 , which supports real-time services, such as voice-over-IP (VoIP), according to the principles of the present invention. Wireless network  100  comprises a plurality of cell sites  121 - 123 , each containing one of the base stations, BS  101 , BS  102 , or BS  103 . Base stations  101 - 103  communicate with a plurality of mobile stations (MS)  111 - 114  over code division multiple access (CDMA) channels according to, for example, the IS-2000 standard (i.e., CDMA2000). In an advantageous embodiment of the present invention, mobile stations  111 - 114  are capable of receiving data traffic and/or voice traffic on two or more CDMA channels simultaneously. Mobile stations  111 - 114  may be any suitable wireless devices (e.g., conventional cell phones, PCS handsets, personal digital assistant (PDA) handsets, portable computers, telemetry devices) that are capable of communicating with base stations  101 - 103  via wireless links.  
      The present invention is not limited to mobile devices. The present invention also encompasses other types of wireless access terminals, including fixed wireless terminals. For the sake of simplicity, only mobile stations are shown and discussed hereafter. However, it should be understood that the use of the term “mobile station” in the claims and in the description below is intended to encompass both truly mobile devices (e.g., cell phones, wireless laptops) and stationary wireless terminals (e.g., a machine monitor with wireless capability).  
      Dotted lines show the approximate boundaries of cell sites  121 - 123  in which base stations  101 - 103  are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites may have other irregular shapes, depending on the cell configuration selected and natural and man-made obstructions.  
      As is well known in the art, each of cell sites  121 - 123  is comprised of a plurality of sectors, where a directional antenna coupled to the base station illuminates each sector. The embodiment of  FIG. 1  illustrates the base station in the center of the cell. Alternate embodiments may position the directional antennas in corners of the sectors. The system of the present invention is not limited to any particular cell site configuration.  
      In one embodiment of the present invention, each of BS  101 , BS  102  and BS  103  comprises a base station controller (BSC) and one or more base transceiver subsystem(s) (BTS). Base station controllers and base transceiver subsystems are well known to those skilled in the art. A base station controller is a device that manages wireless communications resources, including the base transceiver subsystems, for specified cells within a wireless communications network. A base transceiver subsystem comprises the RF transceivers, antennas, and other electrical equipment located in each cell site. This equipment may include air conditioning units, heating units, electrical supplies, telephone line interfaces and RF transmitters and RF receivers. For the purpose of simplicity and clarity in explaining the operation of the present invention, the base transceiver subsystems in each of cells  121 ,  122  and  123  and the base station controller associated with each base transceiver subsystem are collectively represented by BS  101 , BS  102  and BS  103 , respectively.  
      BS  101 , BS  102  and BS  103  transfer voice and data signals between each other and the public switched telephone network (PSTN) (not shown) via communication line  131  and mobile switching center (MSC)  140 . BS  101 , BS  102  and BS  103  also transfer data signals, such as packet data, with the Internet (not shown) via communication line  131  and packet data server node (PDSN)  150 . Packet control function (PCF) unit  190  controls the flow of data packets between base stations  101 - 103  and PDSN  150 . PCF unit  190  may be implemented as part of PDSN  150 , as part of MSC  140 , or as a stand-alone device that communicates with PDSN  150 , as shown in  FIG. 1 . Line  131  also provides the connection path for control signals transmitted between MSC  140  and BS  101 , BS  102  and BS  103  that establish connections for voice and data circuits between MSC  140  and BS  101 , BS  102  and BS  103 .  
      Communication line  131  may be any suitable connection means, including a T1 line, a T3 line, a fiber optic link, a network packet data backbone connection, or any other type of data connection. Line  131  links each vocoder in the BSC with switch elements in MSC  140 . The connections on line  131  may transmit analog voice signals or digital voice signals in pulse code modulated (PCM) format, Internet Protocol (IP) format, asynchronous transfer mode (ATM) format, or the like.  
      MSC  140  is a switching device that provides services and coordination between the subscribers in a wireless network and external networks, such as the PSTN or Internet. MSC  140  is well known to those skilled in the art. In some embodiments of the present invention, communications line  131  may be several different data links where each data link couples one of BS  101 , BS  102 , or BS  103  to MSC  140 .  
      In the exemplary wireless network  100 , MS  111  is located in cell site  121  and is in communication with BS  101 . MS  113  is located in cell site  122  and is in communication with BS  102 . MS  114  is located in cell site  123  and is in communication with BS  103 . MS  112  is also located close to the edge of cell site  123  and is moving in the direction of cell site  123 , as indicated by the direction arrow proximate MS  112 . At some point, as MS  112  moves into cell site  123  and out of cell site  121 , a hand-off will occur.  
       FIG. 2  illustrates exemplary base station  101  in greater detail according to an exemplary embodiment of the present invention. Base station  101  comprises base station controller (BSC)  210  and base transceiver station (BTS)  220 . Base station controllers and base transceiver stations were described previously in connection with  FIG. 1 . BSC  210  manages the resources in cell site  121 , including BTS  220 . BTS  120  comprises BTS controller  225 , channel controller  235  (which contains representative channel element  240 ), transceiver interface (IF)  245 , RF transceiver  250 , and antenna array  255 .  
      In a preferred embodiment, base station  101  operates according to the 1xEV-DO Revision A Standard, as modified according to the teachings herein. Those of skill in the art will recognize that other wireless standards and protocols can be used by base station  101 , similarly modified according to the teachings herein.  
      BTS controller  225  comprises processing circuitry and memory capable of executing an operating program that controls the overall operation of BTS  220  and communicates with BSC  210 . Under normal conditions, BTS controller  225  directs the operation of channel controller  235 , which contains a number of channel elements, including channel element  240 , that perform bi-directional communications in the forward channel and the reverse channel. A “forward” channel refers to outbound signals from the base station to the mobile station and a “reverse” channel refers to inbound signals from the mobile station to the base station. Transceiver IF  245  transfers the bi-directional channel signals between channel controller  240  and RF transceiver  250 .  
      According to various embodiments, either BTS controller  225  or channel controller  235  is configured to transmit subpackets on a forward channel, as described herein, and receive ACK or NAK signals from mobile stations in response. Further, in various embodiments, either BTS controller  225  or channel controller  235  is configured to transmit last subpacket identifiers, described below, to indicate when the last subpacket of a current packet has been sent.  
      Antenna array  255  transmits forward channel signals received from RF transceiver  250  to mobile stations in the coverage area of BS  101 . Antenna array  255  also sends to RF transceiver  250  reverse channel signals received from mobile stations in the coverage area of BS  101 . In a preferred embodiment of the present invention, antenna array  255  is multi-sector antenna, such as a three-sector antenna in which each antenna sector is responsible for transmitting and receiving in a 120° arc of coverage area. Additionally, RF transceiver  250  may contain an antenna selection unit to select among different antennas in antenna array  255  during both transmit and receive operations.  
       FIG. 3  illustrates wireless mobile station  111  according to an advantageous embodiment of the present invention. Wireless mobile station  111  comprises antenna  305 , radio frequency (RF) transceiver  310 , transmit (TX) processing circuitry  315 , microphone  320 , and receive (RX) processing circuitry  325 . MS  111  also comprises speaker  330 , main processor  340 , input/output (I/O) interface (IF)  345 , keypad  350 , display  355 , and memory  360 . Memory  360  further comprises basic operating system (OS) program  361 .  
      In a preferred embodiment, wireless mobile station  111  operates according to the 1xEV-DO Revision A Standard, as modified according to the teachings herein. Those of skill in the art will recognize that other wireless standards and protocols can be used by wireless mobile station  111 , similarly modified according to the teachings herein.  
      Radio frequency (RF) transceiver  310  receives from antenna  305  an incoming RF signal transmitted by a base station of wireless network  100 . Radio frequency (RF) transceiver  310  down-converts the incoming RF signal to produce an intermediate frequency (IF) or a baseband signal. The IF or baseband signal is sent to receiver (RX) processing circuitry  325  that produces a processed baseband signal by filtering, decoding, and/or digitizing the baseband or IF signal. Receiver (RX) processing circuitry  325  transmits the processed baseband signal to speaker  330  (i.e., voice data) or to main processor  340  for further processing (e.g., web browsing).  
      Transmitter (TX) processing circuitry  315  receives analog or digital voice data from microphone  320  or other outgoing baseband data (e.g., web data, e-mail, interactive video game data) from main processor  340 . Transmitter (TX) processing circuitry  315  encodes, multiplexes, and/or digitizes the outgoing baseband data to produce a processed baseband or IF signal. Radio frequency (RF) transceiver  310  receives the outgoing processed baseband or IF signal from transmitter (TX) processing circuitry  315 . Radio frequency (RF) transceiver  310  up-converts the baseband or IF signal to a radio frequency (RF) signal that is transmitted via antenna  305 .  
      In an advantageous embodiment of the present invention, main processor  340  is a microprocessor or microcontroller. Memory  360  is coupled to main processor  340 . According to an advantageous embodiment of the present invention, part of memory  360  comprises a random access memory (RAM) and another part of memory  360  comprises a Flash memory, which acts as a read-only memory (ROM).  
      Main processor  340  executes basic operating system (OS) program  361  stored in memory  360  in order to control the overall operation of wireless mobile station  111 . In one such operation, main processor  340  controls the reception of forward channel signals and the transmission of reverse channel signals by radio frequency (RF) transceiver  310 , receiver (RX) processing circuitry  325 , and transmitter (TX) processing circuitry  315 , in accordance with well-known principles.  
      Main processor  340  is capable of executing other processes and programs resident in memory  360 . Main processor  340  can move data into or out of memory  360 , as required by an executing process. Main processor  340  is also coupled to I/O interface  345 . I/O interface  345  provides mobile station  111  with the ability to connect to other devices such as laptop computers and handheld computers. I/O interface  345  is the communication path between these accessories and main controller  340 .  
      Main processor  340  is also coupled to keypad  350  and display unit  355 . The operator of mobile station  111  uses keypad  350  to enter data into mobile station  111 . Display  355  may be a liquid crystal display capable of rendering text and/or at least limited graphics from web sites. Alternate embodiments may use other types of displays.  
      In a preferred embodiment, main processor  340  is configured to process received data packets according to 1xEV-DO Revision A Standard, known to those of skill in the art, modified as described herein.  
      In a preferred embodiment, main processor  340  is also configured to identify a preamble if present in any slot of a received data packet, indicating that the MS is receiving a first subpacket of a new packet intended for that MS, as described below. Further, in some embodiments, main processor  340  is also configured to identify a last subpacket identifier, when received.  
      A significant disadvantage of known approaches, as described above, stems from the example procedure to transmit the entire Span of subpackets when the MS is unable to reconstruct the original packet correctly, coupled with the large Span sizes associated with small DRC Indexes.  
      Since the subpacket transmissions are separated by 4 slots, the maximum inter-packet transmission interval—the time from when the first subpacket of one packet is transmitted until the first subpacket of the next packet can be transmitted—equals the Span (subpackets per packet)×4 slots per inter-subpacket interval×1.67 ms per slot. For the DRC Indexes 0x1, 0x2, and 0x3 (&amp; 0x5), the Spans are 16, 8, and 4, respectively, making the maximum inter-packet interval 106.67, 53.33, and 26.67 milliseconds, respectively. For some real-time, Quality of Service (QoS) applications such as Voice over IP (VoIP) or other XoIP application that have a stringent latency requirement and/or a high packet offered rate, one or both of these inter-packet intervals may be too long.  
      Furthermore, one out of every four slots—25% of the transmit resources—are used up for the entire time that a packet is waiting for an ACK, using up valuable transmit capacity. In other words, it may be better for the BS to give up transmitting subpackets after, say, 8 or fewer subpackets (53.33 ms or less), or even after only two sub packets (13.33 ms), even if the MS has been unable to reconstruct the original packet correctly, in order to transmit the next packet in the sequence. If the packet is going to arrive at the endpoint too late and would get discarded anyhow, it is better to discard it as soon as possible to free up resources.  
      A preferred embodiment therefore provides for the BS to stop transmitting subpackets of the current packet on the DO-A Forward Traffic Channel before having received an ACK from the MS, in situations that warrant not waiting for the entire Span (maximum number) of subpackets to be transmitted.  
      This “forced early termination” will allow the BS to go ahead and transmit the first subpacket of a new packet in the slot that otherwise a subpacket of the current packet would have occupied. The new packet can be for the MS for which the current packet was intended or another MS.  
      Preferably, every MS—including the MS for which the current packet was intended—must be looking for a preamble, indicating this is the first subpacket of a new packet intended for that MS, in every slot—even the slots that normally would contain subpackets subsequent to the first subpacket of the current packet. The MSs other than the MS for which the current packet was intended must already be following this procedure (looking for a preamble), because they are unaware of the Span of the packet whose subpacket is currently being transmitted.  
      Preferably, the MS that is currently receiving subpackets of a packet must look for not only the next subpacket of that packet but also a preamble in the first subpacket of a new packet.  
      The forced early termination technique described herein gives the BS flexibility to handle real-time, QoS applications better, and also to handle more of them. It can reduce the latency without increasing the packet loss rate, and/or it can increase the number of concurrently active MSs without increasing the latency and packet loss rate.  
      Alternate embodiments include a dynamic algorithm for forced early termination. In other words, do not early-terminate in some situations, and early-terminate after greater or less than 8 transmissions, depending on the situation. For example, the more packets queued for an MS, the fewer transmissions of the current packet before forced early termination. Or, the more the congestion in general, the fewer transmissions per packet. A dynamic algorithm can also favor higher priority users over others (even those that would have to perform retransmissions at a higher protocol layer) in the face of congestion.  
      The techniques disclosed herein can be applied to not only evolution data-only (e.g., EV-DO-A) systems but also other systems, including but not limited to hybrid ARQ systems such as evolution data and voice (EV-DV).  
      Some embodiments described above are implemented using active and idle slots on the forward channel, as illustrated in  FIGS. 4A and 4B .  
       FIG. 4A  depicts an active slot in accordance with this embodiment, including data slot  405 , MAC slot  410 , and pilot slot  415 . This Figure shows 1024 chips, which generally corresponds to one-half a full active slot.  
       FIG. 4B  depicts an idle slot in accordance with a preferred embodiment, including MAC slot  415  and pilot slot  420 .  
      Another alternative embodiment includes the BS transmitting an explicit “last subpacket identifier”. For example, a few (e.g., 1 or 4 or 16) chips can be taken from each of the 4 400-chip data segments of the Forward Link slot to explicitly tell the MS that this is the last subpacket of the current packet that the BS is going to transmit, even if the MS is still unable to reconstruct the packet after processing this subpacket. This eliminates the requirement for the MS to look for a preamble in slots in which it was expecting subsequent subpackets of the current packet.  
      The alternate embodiment adds a last subpacket identifier to the Forward Link Slot Structure, as shown in  FIG. 5 . A system in accordance with this alternate embodiment takes 4 chips from each of the 4 400-chip data segments of the Forward Link slot, as shown in  FIG. 5 . This takes 4 chips per segment×4 segments per slot/16 chips per symbol=1 whole symbol per slot from each of the 16 demultiplexed symbol streams. If the base station sets the value equal to ‘1’, the mobile terminal can determine that the received sub packet is the last sub packet.  
       FIG. 5  depicts an active slot in accordance with this embodiment, including data slot  505 , MAC slot  510 , and pilot slot  515 . Last subpacket identifier slot  520  indicates whether there is a packet following this sub-packet for this particular mobile station. This Figure shows 1024 chips, which generally corresponds to one-half a full active slot.  
      An idle slot, in accordance with this alternate embodiment, is the same as the typical idle slot show in  FIG. 4B .  
      In some embodiments, to guarantee reliable delivery, the last sub-packet identifiers are transmitted with power boosting. This ensures that the access terminals receive the packets.  
      According to a preferred embodiment, to ensure the guaranteed reception of the last sub packet identifier, a factor of 4 redundancy is used. All four last subpacket identifiers carry the same information. The mobile station, once it receives the information, performs the OR operation of the last sub-packet identifiers to determine whether there is a sub-packet following the current sub-packet.  
      As can be seen from Table 1 and Table 2, below, the maximum number of slots required to transmit the data in the current DO-A systems are fixed. Except for 1-slot transmissions, the maximum number of slots to be used has to be multiples of 2. This tends to cause the scheduling of the DO—systems to be more rigid. Hence, if the resources are available for a non-even slot, the base station still can&#39;t schedule that particular user. For supporting lower data rates, the number of slots required to transmit the data ranges from 4 to 16. All this contributes to the latency in the systems.  
                           TABLE 1                       TRANSMISSION FORMAT                   (Physical Layer Packet Size (bits),       Nominal Transmit Duration (slots),   Pilot   MAC   DATA       Preamble Length (chips))   Chips   Chips   Chips                                                (128, 16, 1,024)   3,072   4,096   24,576       (128, 8, 512)   1,536   2,048   12,288       (128, 4, 1024)   768   1,024   5,376       (128, 4, 256)   768   1,024   6,144       (128, 2, 128)   384   512   3,072       (128, 1, 64)   192   256   1,536       (256, 16, 1024)   3,072   4,096   24,576       (256, 8, 512)   1,536   2,048   12,288       (256, 4, 1024)   768   1,024   5,376       (256, 4, 256)   768   1,024   6,144       (256, 2, 128)   384   512   3,072       (256, 1, 64)   192   256   1,536       (512, 16, 1024)   3,072   4,096   24,576       (512, 8, 512)   1,536   2,048   12,288       (512, 4, 1024)   768   1,024   5,376       (512, 4, 256)   768   1,024   6,144       (512, 4, 128)   768   1,024   6,272       (512, 2, 128)   384   512   3,072       (512, 2, 64)   384   512   3,136       (512, 1, 64)   192   256   1,536       (1024, 16, 1024)   3,072   4,096   24,576       (1024, 8, 512)   1,536   2,048   12,288       (1024, 4, 256)   768   1,024   6,144       (1024, 4, 128)   768   1,024   6,272       (1024, 2, 128)   384   512   3,072       (1024, 2, 64)   384   512   3,136       (1024, 1, 64)   192   256   1,536       (2048, 4, 128)   768   1,024   3,072       (2048, 2, 64)   384   512   3,136       (2048, 1, 64)   192   256   1,536       (3072, 2, 64)   384   512   3,136       (3072, 1, 64)   192   256   1,536       (4096, 2, 64)   384   512   3,136       (4096, 1, 64)   192   256   1,536       (5120, 2, 64)   384   512   3,136       (5120, 1, 64)   192   256   1,536                  
 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
               
               
                   
                 Number of Values Per Physical Layer Packet 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 TDM Chips 
               
               
                 Data 
                   
                   
                   
                   
                 (Preamble, 
               
               
                 Rate 
                   
                   
                 Code 
                 Modulation 
                 Pilot, 
               
               
                 (kbps) 
                 Slots 
                 Bits 
                 Rate 
                 Type 
                 Mac, Data) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 38.4 
                 16 
                 1,024 
                 1/5 
                 QPSK 
                 1,024 
               
               
                   
                   
                   
                   
                   
                 3,072 
               
               
                   
                   
                   
                   
                   
                 4,096 
               
               
                   
                   
                   
                   
                   
                 24,576 
               
               
                 76.8 
                 8 
                 1,024 
                 1/5 
                 QPSK 
                 512 
               
               
                   
                   
                   
                   
                   
                 1,536 
               
               
                   
                   
                   
                   
                   
                 2,048 
               
               
                   
                   
                   
                   
                   
                 12,288 
               
               
                 153.6 
                 4 
                 1,024 
                 1/5 
                 QPSK 
                 256 
               
               
                   
                   
                   
                   
                   
                 768 
               
               
                   
                   
                   
                   
                   
                 1,024 
               
               
                   
                   
                   
                   
                   
                 6,144 
               
               
                 307.2 
                 2 
                 1,024 
                 1/5 
                 QPSK 
                 128 
               
               
                   
                   
                   
                   
                   
                 384 
               
               
                   
                   
                   
                   
                   
                 512 
               
               
                   
                   
                   
                   
                   
                 3,072 
               
               
                 614.4 
                 1 
                 1,024 
                 1/3 
                 QPSK 
                 64 
               
               
                   
                   
                   
                   
                   
                 192 
               
               
                   
                   
                   
                   
                   
                 256 
               
               
                   
                   
                   
                   
                   
                 1,536 
               
               
                 307.2 
                 4 
                 2,048 
                 1/3 
                 QPSK 
                 128 
               
               
                   
                   
                   
                   
                   
                 768 
               
               
                   
                   
                   
                   
                   
                 1,024 
               
               
                   
                   
                   
                   
                   
                 6,272 
               
               
                 614.4 
                 2 
                 2,048 
                 1/3 
                 QPSK 
                 64 
               
               
                   
                   
                   
                   
                   
                 384 
               
               
                   
                   
                   
                   
                   
                 512 
               
               
                   
                   
                   
                   
                   
                 3,136 
               
               
                 1,228.8 
                 1 
                 2,048 
                 1/3 
                 QPSK 
                 64 
               
               
                   
                   
                   
                   
                   
                 192 
               
               
                   
                   
                   
                   
                   
                 256 
               
               
                   
                   
                   
                   
                   
                 1,536 
               
               
                 921.6 
                 2 
                 3,072 
                 1/3 
                 8-PSK 
                 64 
               
               
                   
                   
                   
                   
                   
                 384 
               
               
                   
                   
                   
                   
                   
                 512 
               
               
                   
                   
                   
                   
                   
                 3,136 
               
               
                 1,843.2 
                 1 
                 3,072 
                 1/3 
                 8-PSK 
                 64 
               
               
                   
                   
                   
                   
                   
                 192 
               
               
                   
                   
                   
                   
                   
                 256 
               
               
                   
                   
                   
                   
                   
                 1,536 
               
               
                 1,228.8 
                 2 
                 4,096 
                 1/3 
                 16-QAM 
                 64 
               
               
                   
                   
                   
                   
                   
                 384 
               
               
                   
                   
                   
                   
                   
                 512 
               
               
                   
                   
                   
                   
                   
                 3,136 
               
               
                 2457.6 
                 1 
                 4,096 
                 1/3 
                 16-QAM 
                 64 
               
               
                   
                   
                   
                   
                   
                 192 
               
               
                   
                   
                   
                   
                   
                 256 
               
               
                   
                   
                   
                   
                   
                 1,536 
               
               
                   
               
            
           
         
       
     
      For lower DRCs, as per the current standards, the delay for transmitting the packets becomes very large, because of Span. In the preferred embodiments, however, since the reconstruction and transmission of the packets are independent of the Span mechanism, the delay is considerably reduced. The base station can schedule the user every other slot for faster delivery for applications like VoIP and thus reduce the latency considerably. Thus, the scheduling of the DO-A systems gets improved resulting in faster delivery of the packets.  
      For the DRC Indexes 0x1, 0x2, and 0x3 (&amp; 0x5), the Spans are 16, 8, and 4, respectively, making the maximum inter-packet interval 106.67, 53.33, and 26.67 milliseconds, respectively. For some Quality of Service applications such as Voice over IP (VoIP) that have a high packet offered rate, one or all of these inter-packet intervals may be too long. In other words, it may be better for the BS to give up transmitting sub packets after, say, only two sub packets (13.33 ms), even if the MS has been unable to reconstruct the original packet correctly, in order to transmit the next packet in the sequence. The problem with the current standard is that it prescribes the BS to continue to transmit sub packets up to the specified Span, in the absence of receiving an ACK, and the AN must use the Span associated with the DRC Index (if the AT selected larger than warranted DRC Indexes, the packet error rate would increase unacceptably).  
      The reason for the OR operation is again to ensure that the mobile stations do not miss the packets intended for it.  
      According to a preferred embodiment, the base station can schedule the user anytime it wants, without having to wait for the even interval that the mobile station can receive the packets on. This gives freedom to the base station in scheduling the users and hence reduces the scheduling latency.  
      Further, since there is no need to transmit the packets only in their pre-designed fixed slots, the latency caused because of the Span is also reduced.  
      Preferably, the disclosed method mechanism of packet reconstruction can be turned ON or OFF, with the flag transmitted in the overhead messages.  
      According to an alternative embodiment, one chip is taken from each of the four 400-chip data segments of the Forward Link slot. This takes 1 chip per segment×4 segments per slot/16 chips per symbol=¼ symbol per slot from each of the 16 demultiplexed symbol streams in which the 16-ary Walsh Covers are added. This takes 1 chip per segment/400 total chips per segment=only ¼% of the payload data rate.  
      According to another alternative embodiment, 16 chips are taken from each of the 400-chip data segments of the Forward Link slot. This takes 16 chips per segment/16 chips per symbol=1 whole symbol from each 400-chip data segment of each of the 16 demultiplexed symbol streams, which is more acceptable from a design standpoint. However, it takes  16  chips per segment/400 total chips per segment=4% of the payload data rate.  
      Other alternative embodiments essentially add another channel to the Forward Link Structure. These would not take away any payload data rate. Another alternative embodiment includes expanding the 128-ary Walsh Cover for the MACindex to become 256-ary. This requires the current QPSK modulation to become 8PSK. Another alternative embodiment includes adding another channel input to the Walsh Chip Level Summer or to the TDM 3:1 combiner, in addition to the current MAC Channel P-ARQ and DRCLock bits.  
      Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.