Abstract:
A power saving (PS) scheduling process is provided for scheduling uplink and downlink frame transmissions between a PS transmitter and at least one PS receiver using PSAD sequences in a wireless communication system. The power saving scheduling process involves determining a power saving schedule of transmission opportunities for communication between a PS transmitter and at least one PS receiver over a shared channel, wherein the schedule includes corresponding durations for uplink transmissions of frames from each PS receiver to the PS transmitter. A Power Saving Aggregation Descriptor (PSAD) frame containing said schedule is constructed. A PSAD sequence is initiated by transmitting the PSAD from the PS transmitter to each PS receiver.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to wireless networks, and in particular, to power saving for high throughput wireless local area networks (WLANs). 
       BACKGROUND OF THE INVENTION 
       [0002]    In many wireless communication systems, a frame structure is used for data transmission between a transmitter and a receiver. For example, the IEEE 802.11 standard uses frame aggregation in a Media Access Control (MAC) layer and a physical (PHY) layer. In a typical transmitter, a MAC layer receives a MAC Service Data Unit (MSDU) and attaches a MAC header thereto, in order to construct a MAC Protocol Data Unit (MPDU). The MAC header includes information such as a source address (SA) and a destination address (DA). The MPDU is a part of a PHY Service Data Unit (PSDU) and is transferred to a PHY layer in the transmitter to attach a PHY header (i.e., a PHY preamble) thereto to construct a PHY Protocol Data Unit (PPDU). The PHY header includes parameters for determining a transmission scheme including a coding/modulation scheme. 
         [0003]    Many battery powered devices such as cellular phones and consumer electronic (CE) devices are being provided with the capability to access wireless networks such as high throughput wireless (e.g., radio frequency) local area networks (WLANs). As such, an efficient method of scheduling uplink and downlink frame transmissions over a shared channel at the access point (AP) is needed for power saving at power saving stations (e.g., the battery powered devices). 
         [0004]    There are two approaches for a wireless station (STA) to access a shared wireless communication channel in a WLAN. One approach is a contention-free arbitration (CF) method. The other approach is a contention-based arbitration (CB) method. The CF access method utilizes a point coordinator function (PCF) to control access to the channel. When a PCF is established, the PCF polls registered STAs for communications and provides channel access to the STAs based polling results. The CB access method utilizes a random back off period to provide fairness in accessing the channel. In the CB period, a STA monitors the channel. If the channel has been silent for a pre-defined period of time, the STA waits a certain period of time, such that if the channel remains silent, the STA transmits on the channel. 
         [0005]    Power Save Aggregation (PSA) is a mechanism for scheduling transmission opportunities over a shared channel, which employs a power saving aggregation descriptor (PSAD) frame.  FIG. 1  illustrates the format of a conventional PSAD control frame  10  (described in IEEE Wireless LAN Edition (2003), “A compilation based on IEEE Std 802.11™—1999 (R2003) and its amendments,” incorporated herein by reference). The PSAD frame is a MAC control frame that provides a schedule of transmission opportunities (TXOP) to be used by a PSAD transmitter and PSAD receivers. The scheduled TXOPs begin immediately subsequent to the transmission of the PSAD frame. A high throughput (HT) station that is capable of using PSAD information indicates such capability in the PSAD frame. In an IEEE WLAN, such as IEEE 802.11, the PSAD frame is sent by the PSAD transmitter (i.e., an AP) to schedule both downlink and uplink transmissions between the AP and PSAD receivers (i.e., the PS stations). 
         [0006]    The PSAD frame  10  has a MAC header that includes the following subfields: Frame Control and Duration  12 , a Receiver Address (RA)  14 , a Transmitter Address (TA)  16  and a Basic Service Set Identifier (BSSID)  18 . A More PSAD Indicator bit  19  specifies whether there will be another PSAD sequence following a current PSAD sequence. A Descriptor End field  17  specifies the duration of the current PSAD sequence. The value of the sequence duration is an integer number of two Orthogonal Frequency-Division Multiplexing (OFDM) symbols (i.e., 8 μs). A STA ID field  15  specifies the station ID of a station associated to the AP. A down link transmission (DLT) Start Offset field  13  indicates the start of a DLT for a station relative to the end of the PSAD frame. The DLT offset is provided as an integer number of ½ OFDM symbols (i.e., 2 μs). If no DLT is scheduled for a station, but an uplink transmission (ULT) is scheduled for that station, then the DLT Start Offset field  13  is set to null (0). 
         [0007]    Further, the DLT duration field  11  indicates the length of a DLT for a station. The DLT duration field  11  is based on a number of ½ OFDM symbols. If no DLT is scheduled for a station, but a ULT is scheduled for that station, then the DLT Duration  11  is set to null (0). A ULT Start Offset field  20  indicates the start of the ULT. The first ULT is scheduled to begin after a short interframe space (SIFS) interval from the end of the last DLT described in the PSAD. The ULT Start Offset is based on an integer number of ½ OFDM symbols (i.e., 2 us). If no ULT is scheduled for a station, but a DLT is scheduled for that station, then the ULT Start Offset  20  is set to null (0). A ULT Duration field  21  indicates the length of a ULT for a station. The ULT Duration is based on a number of ½ OFDM symbols. If no ULT is scheduled for a station, but a DLT is scheduled for that station, then the ULT Duration  21  is set to null (0). A station cannot use the medium longer than the time allocated in the PSAD frame. 
         [0008]      FIG. 2  shows the timing structure of a PSAD sequence  25 . The PSAD frame  10  (which includes receiving station IDs, ULT and DLT offsets and durations) from the AP provides information to each station (e.g., STA 1 , . . . , STA n ) regarding the time-positions of corresponding frames. This allows each station to recognize the time-position of its frames  14 . 
         [0009]    However, in such existing power saving approaches using PSAD, the AP has no knowledge of the durations of uplink data frames from power saving (PS) stations. Though some approaches using traffic specification (TSPEC) provide the time, maximal and nominal length of frames in a data stream, nevertheless not all frames from PS stations are associated with a TSPEC. Even with a TSPEC, the AP has no knowledge of the actual sizes of the uplink frames with variable sizes, in order to properly reserve a shared channel for those transmissions. 
         [0010]    If the AP overestimates the ULT duration, throughput performance will be degraded since the shared channel remains reserved for longer than needed, and idles unnecessarily. If the AP underestimates the ULT duration, then PS stations experience buffer overflow. Further, Quality of Service (QoS) performance will be negatively affected. For example, delay and delay jitter will be increased. Moreover, conventional PSAD approaches do not specify PSAD utilization in the Contention-Free Period (CFP) and the Contention Period (CP) of transmission. 
         [0011]    There is, therefore, a need for a method and system to schedule uplink and downlink traffic for both PS and non-PS stations. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a power saving (PS) scheduling method and system for scheduling uplink and downlink frame transmissions between a PS transmitter and at least one PS receiver using PSAD sequences in a wireless communication system. 
         [0013]    In one embodiment, such power saving scheduling involves determining a power saving schedule of transmission opportunities for communication between a PS transmitter and at least one PS receiver over a shared channel, wherein the schedule includes corresponding durations for uplink transmissions of frames from each PS receiver to the PS transmitter; constructing a Power Saving Aggregation Descriptor (PSAD) frame containing said schedule; and initiating a PSAD sequence by transmitting the PSAD from the PS transmitter to each PS receiver. 
         [0014]    The PS transmitter precisely schedules uplink and downlink transmission times for each PS receiver using uplink information from each PS receiver without polling the receivers. The PS transmitter notifies the PS receivers to provide such uplink information using signaling in PSAD frames transmitted to the PS receivers. 
         [0015]    Precise duration for uplink traffic with variable frame sizes is allocated to achieve high efficiency of channel utilization. Therefore, the PS receivers enter lengthier sleep cycles in order to save more power and transmission bandwidth. 
         [0016]    These and other features, aspects and advantages of the present invention will become understood with reference to the following description, appended claims and accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  illustrates the format of a conventional PSAD control frame. 
           [0018]      FIG. 2  shows the structure of a conventional PSAD sequence. 
           [0019]      FIG. 3  shows a modified PSAD frame, according to an embodiment of the present invention. 
           [0020]      FIG. 4  shows the format of a Next Frame Notice (NFN) control frame, according to an embodiment of the present invention. 
           [0021]      FIG. 5  shows an example of a PSAD sequence as Piggyback, according to an embodiment of the present invention. 
           [0022]      FIG. 6  shows the structure and the sequence of a CFP and CP channel access with PSAD, according to an embodiment of the present invention. 
           [0023]      FIG. 7A  shows a flowchart of example scheduling procedures with PSAD and NFN at stations, according to an embodiment of the present invention. 
           [0024]      FIG. 7B  shows a flowchart of another example of scheduling procedures with PSAD and NFN at stations, according to an embodiment of the present invention. 
           [0025]      FIG. 8  shows an example partitioning structure for traffic from/to non-PS and PS stations in the CFP with PSAD, according to an embodiment of the present invention. 
           [0026]      FIG. 9  shows a block diagram of an example WLAN system, implementing a power saving scheduling mechanism according to an embodiment of the present invention. 
           [0027]      FIG. 10  shows an example protocol architecture for the AP and the STAs in  FIG. 9 , which implement a power saving scheduling mechanism, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The present invention provides a power saving scheduling mechanism for access to a shared channel in wireless networks such as WLANs. In one implementation, this involves scheduling uplink and downlink frame transmissions at an AP within the WLAN, with an objective of power saving. In addition, transmission traffic from/to PS stations and non-PS stations in the WLAN, is partitioned so that PS stations can reduce power (e.g., enter a power saving state or a sleep cycle) when traffic for non-PS stations is transmitted. The AP separates and schedules the traffic from/to PS and non-PS stations at different channel periods. During a channel period allocated for transmission traffic from/to non-PS stations, all PS stations can enter a sleep cycle. This provides the PS stations with longer time sleep cycles, and without frequent switches between active cycles and sleep cycles. 
         [0029]    The AP is enabled to precisely schedule uplink and downlink transmission times for each PS station using two control frames: a modified PSAD frame and a newly defined next frame notice (NFN) frame. This allows for the allocation of precise durations for uplink traffic with variable frame sizes to achieve high efficiency channel utilization. In addition, PS stations can enter sleep cycles for as long as possible in order to save more power. 
         [0030]      FIG. 3  shows a modified PSAD frame  30  according to an embodiment of the present invention, wherein a NFN indicator  32  (e.g., one reserved bit from PSAD reserved bits) provides a NFN for a corresponding STA. Other and/or additional fields and/or bits in the PSAD frame may be used for the NFN indication as well. 
         [0031]    The NFN indicator  32  is used to indicate whether the ULT Start Offset and ULT Duration of the corresponding STA are to specify the uplink transmission of a NFN control frame from a station to the AP.  FIG. 4  shows the format of a NFN control frame  40 , according to an embodiment of the present invention. The NFN control frame  40  is used by a station to inform the AP of the duration of a frame pending at the head of the station&#39;s transmission queue. The NFN control frame  40  has a MAC header  41  including the following fields: a Frame Control  42 , an Association ID (AID)  43 , a Transmitter Address (TA)  45 , and a Basic Service.Set Identifier (BSSID)  44 . The subfields  42 - 45  can be as described in the IEEE Wireless LAN Edition (2003), “A compilation based on IEEE Std 802.11™—1999 (R2003) and its amendments,” incorporated herein by reference. 
         [0032]    As shown in  FIG. 4  according to the present invention, the NFN control frame  40  further includes a Duration field  46  as a payload including 2 bytes and a CRC field  47 . The Duration field  46  indicates the duration of a frame pending at the head of the station&#39;s transmission queue. The Duration field  46  is based on an integer number of ½ OFDM symbols (e.g., 2 μs). The total length of the NFN control frame  40  is 22 bytes. 
         [0033]    As shown by the example communication sequence  50  in  FIG. 5 , an NFN control frame  40  can be transmitted from a station to the AP separately, or piggybacked or aggregated with other uplink data  52  or control frames such as an ACK  54 , transmitted from a station to the AP. 
         [0034]    Power saving is achieved by replacing the conventional polling for a CFP for channel access, with a scheduling mechanism based on modified PSAD frames  30  ( FIG. 3 ) and NFN control frames  40  ( FIG. 4 ). After a SIFS interval past a beacon signal, a CFP for channel access begins. During the CFP period, the AP (the PSAD transmitter) sends out the first modified PSAD frame  30  to PS stations (the PSAD receivers) to initiate a PSAD sequence. The modified PSAD frame  30  provides information to each PS station regarding the time and location of PPDUs in a PSAD sequence. This allows a PS station to recognize the position of its PPDU frames by only reading the modified PSAD frame  30 , and implement power saving by subsequently waking up only at the time-positions necessary to receive its frames. Additionally, the modified PSAD  30  can provide uplink transmission scheduling for any expected response or reverse data flow from the receiving PS stations. The uplink schedule is conveyed using the ULT Start Offset values in the modified PSAD frame  30 . 
         [0035]    The AP determines the precise duration of each data frame for uplink traffic from a PS station to the AP. When the AP has TSPEC information of an uplink flow, and constant frame sizes and constant bit rates are used by that flow, then the AP can specify ULT Start Offset  31  and ULT Duration  34  in the modified PSAD frame  30 . 
         [0036]    If the uplink flow is variable in frame size or there is no TSPEC information for that uplink traffic at the AP, then the AP cannot specify the precise value for the ULT Duration  34 , without more. However, by transmitting a NFN control frame  40  to the AP, a PS station can inform the AP of the expected transmission duration of the next uplink frame (i.e., the frame pending at the head of the transmission queue for transmission to the AP next). 
         [0037]    Accordingly, for each PS station in the AP&#39;s polling list, if the AP can determine the precise ULT Duration  34  for the next uplink frame with TSPEC information, then the AP sets the ULT Duration value in the PSAD frame  30 ; otherwise, the AP sets the ULT Duration  34  as the transmission time of a NFN control frame  40  to require that PS station to inform the AP of the duration of the next uplink frame from the PS station. 
         [0038]      FIG. 6  illustrates an example PSAD sequencing  60  for channel access during a CFP and a CP, according to an embodiment of the present invention. A CFP  62  begins a SIFS period after a beacon signal  64 . There can be multiple PSAD sequences  61  within one CFP, wherein each PSAD sequence begins with the transmission of a modified PSAD frame  30  from the AP. During a CFP, downlink data from the AP to PS stations and non-PS stations are partitioned into different PSAD sequences. A first modified PSAD frame  30  is transmitted a SIFS interval after a beacon signal starts a CFP. After receipt of a PSAD frame  30  that initiates a PSAD sequence, a PS station can schedule its own sleep (power save) and wake (active) cycles during that PSAD sequence. The transmission times of subsequent PSAD frames can be calculated from the sequence duration field of a previous PSAD. 
         [0039]    If the last PSAD sequence completes before the maximum duration of the CFP (i.e., CFPMaxDuration), the AP transmits a CF-end frame  66  to complete the CFP period  62 . The More PSAD indicator field  35  ( FIG. 3 ) in the last PSAD frame  30  transmitted by the AP during the CFP  62 , informs the stations whether to expect a PSAD sequence at the beginning of a CP  68  that follows the CFP  62 . Therefore, if the AP wishes to send one or more frames to the PS stations during the CP  68 , then during the CFP  62  the AP sends a last PSAD frame  30  with the More PSAD indicator field  35  set to indicate to the receiving stations to expect a PSAD sequence at the start of the CP  68  following the CFP  62 . After the last PSAD sequence in the interval spanning the CFP  62  and the CP  68 , all of the PS stations can enter a sleep cycle until the start of the next CFP. 
         [0040]    Now also referring to the flowcharts in  FIGS. 7A-B , an example of a channel access scheduling processes  90 ,  95 , using the modified PSAD frame  30  ( FIG. 3 ) and NFN frame  40  ( FIG. 4 ), according to an embodiment of the present invention, is now described. The example channel access scheduling processes  90  in  FIG. 7A  is implemented by a power saving AP (PSAD transmitter) and the scheduling process  95  in  FIG. 7B  is implemented in one or more PS stations (PSAD receivers) in a WLAN, according to the following steps: 
         [0041]    At the power saving AP side ( FIG. 7A ):
       Step  100 : The AP prepares a modified PSAD frame  30  ( FIG. 3 ) for transmission to PS stations over a channel. The information in the PSAD frame  30  is based on: the AP&#39;s transmission queue occupation, TSPEC information of PS stations in the AP&#39;s polling list, and NFN information fed back from PS stations. If the AP decides to send another PSAD frame after the current PSAD frame, the AP sets the More PSAD indicator field  35  in the current PSAD frame  30  to “1.” For each PS station in the AP&#39;s polling list, if the AP cannot obtain any information about the uplink traffic from the PS station, then the AP sets the NFN bit  32  in the current PSAD frame  30  to “1” and specifies&#39; the ULT Duration  34  as the length of transmitting a NFN control frame  40  from a PS station.   Step  102 : The AP transmits the PSAD frame  30  over the channel in a CFP.   Step  104 : The AP transmits downlink frames to PS stations over the channel during DLT intervals (scheduled by the DLT Start Offset  37  and DLT Duration  39 ) in the PSAD frame  30 . Further, the AP receives uplink frames from the PS stations during scheduled ULT intervals (scheduled by the ULT Start Offset  31  and ULT Duration  34 ) in the PSAD frame  30 .   Step  106 : If additional PSAD frames are to be transmitted, the AP transmits the additional PSAD frames a SIFS interval after the current PSAD sequence. If delay and jitter requirements allow, the AP schedules downlink and uplink traffic for PS stations and non-PS stations in different PSAD sequences.   Step  108 : If the last PSAD sequence completes before the maximum duration of the CFP (CFPMaxDuration), the AP sends out a CF-end frame to complete the CFP period. The More PSAD indicator field  35  of the last PSAD frame during the CFP informs the PS stations whether there is a PSAD (or otherwise), at the beginning of the following CP.   Step  110 : Optionally, if the AP wishes to transmit additional downlink frames to PS stations during the CP, then the AP transmits a PSAD frame to announce the PSAD sequence at the start of the CP.   Step  112 : After the last PSAD sequence in the interval spanning the CFP and CP, all PS stations can enter sleep (power saving) cycles until the start of the next CFP.       
 
         [0049]    At a power saving station side ( FIG. 7B ):
       Step  200 : PS station receives a PSAD frame  30  from the PA over the channel.   Step  201 : The PS station checks if the PSAD frame  30  specifies ULT and DLT data transfer schedules for the PS station. If none are scheduled, then proceed to step  202 , otherwise proceed to step  204 .   Step  202 : The PS station enters a sleep cycle during the current PSAD sequence.   Step  204 : The PS station remains active to receive or transmit data frames during the periods specified by the PSAD frame, and sleeps during other periods.   Step  206 : The PS station determines if the NFN bit in the received PSAD frame  30  set to “1” for the PS station? If yes, proceed to step  208 , otherwise transmit uplink data frames to AP as scheduled, and proceed to step  210 .   Step  208 : Instead of sending an uplink data frame, the PS station checks its uplink queue status (e.g., to obtain: the precise length of the frame at the head of its transmission queue, number of frames in the queue, queue buffer remaining free, etc.), and constructs and transmits a NFN control frame  40  ( FIG. 4 ) to the AP during a scheduled ULT period. The NFN control frame  40  notifies the AP of the length of the frame at the head of the queue in the PS station. Using the information carried in the NFN frame  40 , the AP can specify the precise uplink data duration for this PS station in a next PSAD frame  30 .   Step  210 : After a PS station receives a CF-end frame from the AP, or when a CFPMaxDuration time passes without receiving a CF-end, the PS station enters the CP. If the PS station does not receive PSAD frames at the start of the CP, then the PS station enters a sleep cycle until the start of the next CFP.       
 
         [0057]      FIG. 8  shows an example partitioning structure  250  for traffic from/to non-PS and PS stations in a CFP  252 , according to an embodiment of the present invention. Transmission traffic from/to PS and non-PS stations is partitioned such that PS stations can reduce power (e.g., sleep) when traffic for non-PS stations is transmitted over the channel. The AP separates and schedules the traffic from/to PS and non-PS stations at different channel periods. During channel periods  254  for transmission of traffic from/to non-PS stations, all PS stations can enter a sleep state. During channel periods  256  for transmission of traffic from/to PS stations, the involved PS station(s) remain active. With the partitioning scheme, the PS stations are provided with longer sleep cycles without frequent switches between active and sleep states. 
         [0058]    The present invention is applicable to wireless networks in general, examples of which include WLAN, wireless personal area network (WPAN), wireless metropolitan area network (WMAN), different kinds of cellular networks, etc. 
         [0059]      FIG. 9  shows a block diagram of an example WLAN system  300  implementing a power saving scheduling mechanism according to an embodiment of the present invention. The system  300  includes an access point (AP)  302  and n stations (STAs)  304 , wherein some stations such as a cellular phone and a wireless camera are PS STAs. In the presence of an AP, usually STAs do not communicate with one another directly if the WLAN works at the infrastructure mode. All frames are transmitted to the AP, and the AP transmits them to their destined STAs. Since the AP is forwarding all frames, the STAs are no longer required to be in range of one another. The only requirement is that the STAs be within range of the AP. In  FIG. 9 , as an example, if STA  1  sends a frame to STA  2 , STA  1  first sends the frame to the AP, and the AP forwards the frame to STA  2 . The radio medium is shared among different stations and the APs using an algorithm called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) during the contention Period (CP). 
         [0060]      FIG. 10  shows an example protocol architecture  400  for the PS AP and the PS STAs in  FIG. 9 , which implements a power saving scheduling mechanism, according to an embodiment of the present invention. The protocol architecture  400  includes a PS AP  402  and one or more PS STAs  404 . The AP  402  comprises a physical (PHY) layer  406 , and a media access control (MAC) layer  408 . The PHY layer  406  implements a type of IEEE 802.11 communication standard for transmitting data over a channel. The MAC layer  408  comprises a scheduler function  410  and a PSAD constructor  412 . The scheduler function  410  provides schedules for downlink and uplink transmissions, and the PSAD constructor  412  constructs PSAD frames including such schedules. 
         [0061]    Each STA  404  includes a PHY layer  414  corresponding to the PHY layer  406  of the AP  402 . Each STA further includes a MAC layer  416  that comprises a PSAD analyzer  418  and a NFN module  417 . The PSAD analyzer  418  analyzes received PSADs for managing scheduled communications with the AP  402 . The NFN module  417  generates NFN control frames  40  for transmission to the AP  402 . The scheduler function  410 , the PSAD constructor  412 , the PSAD analyzer  418  and the NFN module  417  are logical modules, which implement an example of the power saving mechanism according to the present invention, as described. 
         [0062]    Although in the description of  FIG. 10  the STAs and the AP have been shown separately, each is a type of wireless communication station capable of transmitting and/or receiving over a wireless channel in a wireless communication system such as a WLAN. Therefore, a wireless communication station herein can function as a transmitter, a receiver, an initiator and/or a responder. It then follows that an AP can function as a transmitter, a receiver, an initiator and/or a responder. Similarly, a STA can function as a transmitter, a receiver, an initiator and/or a responder. 
         [0063]    As such according to the present invention, transmissions in a CFP are completely scheduled with PSAD and NFN frames, eliminating the need for an IEEE 802.11 PCF polling mechanism in the CFP. Traffic sent to PS stations and non-PS stations can be partitioned so that PS stations can have more sleep time. Using NFN frames fed back from the PS stations, the AP can specify precise ULT Durations in a PSAD to avoid channel bandwidth idling and waste. During a CFP, fine granular power saving scheduling can be performed using multiple PSADs. Several approaches for feedback of NFN frame information to the AP can be utilized including, for example, a NFN control frame format, QoS control field in the MAC header, piggyback with other frames, etc. 
         [0064]    Compared to the conventional approaches, the present invention provides PS stations more time to sleep, and improves transmission efficiency by precisely scheduling frame transmission. The AP can assign precise ULT durations because stations can feedback the actual frame sizes of the uplink traffic using NFN frames  40 . This further allows power saving for PS stations whose traffic flows have no TSPEC to specify traffic characteristics. 
         [0065]    Alternatively, the need for transmission of a NFN frame  40  from a PS receiver to the PS transmitter can be automatically detected by the PS receiver without relying on a NFN field  32  in a PSAD. The PS transmitter can use other ways of notifying a PS receiver of the need for transmitting a NFN frame  40  to the PS transmitter. Because an NFN frame  40  has a predetermined fixed length (e.g., 22 bytes), in one example a PS AP calculates a ULT duration for a NFN frame  40  based modulation and coding rate of an uplink transmission from a PS STA. The PS AP schedules a ULT duration in a conventional PSAD  10  for transmission of such a NFN frame  40  from that PS STA, and transmits the PSAD  10 . Then, the PS STA checks its assigned ULT duration in the received PSAD  10 , and if that ULT duration matches the uplink transmission time needed for transmission of a NFN frame  40 , then the PS STA detects that the PS AP needs a NFN frame  40  from the PS STA (indicating length of next uplink frame from the PS STA). In that case, the PS STA transmits a NFN frame  40  to the PS AP during a scheduled uplink transmission period. Using such auto-detection, the conventional PSAD frame  10  can be used instead of the modified PSAD frame  30  to notify a PS STA of the need for a NFN control from  40 , according to an alternative power saving scheduling mechanism according to the present invention. A power saving station includes a device in which power consumption is reduced. 
         [0066]    As is known to those skilled in the art, the aforementioned example architectures described above, according to the present invention, can be implemented in many ways, such as program instructions for execution by a processor, as logic circuits, as an application specific integrated circuit, as firmware, etc. 
         [0067]    The present invention has been described in considerable detail with reference to certain preferred versions thereof; however, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.