Patent Publication Number: US-8121145-B2

Title: Contention-based random access method with autonomous carrier selection

Description:
BACKGROUND 
     The present invention relates to a contention-based multiple access protocol for an uplink channel and, more particularly, to a contention-based multiple access protocol that allows variable-rate and multi-packet transmission on the uplink channel by user terminals. 
     In a wireless packet data network, a plurality of user terminals transmit packet data to an access point over a shared uplink channel. A random access protocol is often used to share a portion of the uplink bandwidth among the user terminals. The random access protocol may be a reservation-based protocol or a contention-based protocol. In either case, the channel is typically divided in the time domain into a sequence of time slots. The user terminals share the channel by transmitting in different time slots. The channel may also use Orthogonal Frequency Division Multiplexing (OFDM) or Code Division Multiple Access (CDMA) to allow multiple user terminals to transmit in the same time slot. In such cases, multiple user terminals may transmit in the same time slot, but on different subcarrier frequencies or with different spreading codes. 
     A reservation-based random access protocol reserves resources for individual user terminals. The mobile stations request permission from the access point to transmit on the shared uplink channel. If the request is granted, the access point reserves resources for the user terminal and sends a grant message to the user terminal identifying the reserved resources. The reserved resources may, for example, comprise a time slot or portion of a time slot. After receiving permission, the user terminal transmits its data using the allocated resources. One shortcoming of reservation-based multiple access protocols is the delay incurred in the request/grant procedure. 
     In contention-based multiple access protocols, resources are not reserved and the user terminals compete with one another for access to the channel. One such protocol is called slotted ALOHA. In slotted ALOHA, the shared uplink channel is divided into a sequence of time slots. When a user terminal has data to transmit, it selects a time slot and begins its transmission at the start of the selected time slot. With single packet reception, the packet will be received by the access point if no other user terminal transmits in the same slot. However, if another user terminal transmits in the same time slot, a collision occurs and neither packet will be received. In the event of a collision, each user terminal backs off a random amount and retransmits in another time slot. It has been shown that the maximum throughput using slotted ALOHA is 0.36 packets per slot. This low throughput is the main disadvantage of the slotted ALOHA approach. 
     Multi-packet reception (MPR) can be used to significantly improve the throughput of the slotted ALOHA approach. With MPR, the access point can receive multiple packets in the same time slot and frequency without collision. A number of techniques can be used to enable MPR including use of multiple receive antennas at the access point, code multiple access (CDMA) techniques, and multi-user detection techniques. The recently proposed Dynamic Queue Protocol exploits MPR capability of the receiver to provide an efficient access scheme. 
     MPR protocols developed to date have concentrated on the symmetric case where all packets have the same probability of reception. To achieve symmetry, it is assumed that power control is employed so that all packets are received with the same power by the access point. This approach, however, results in a very low system throughput because the user terminals in advantageous channel conditions are penalized. 
     SUMMARY 
     The present invention provides a method and apparatus for controlling transmission from a plurality of user terminals to an access point on a random access channel in a wireless communication system. An access point may determine an allowed information rate for each user terminal and transmit a rate control parameter indicative of the allowed information rates to the user terminals. The rate control parameter may comprise, for example, an allowed number of fixed rate packets that the user terminal may transmit in a time slot of the random access channel, or a code rate for a variable rate packet. The access point also computes a packet transmission probability and transmits the packet transmission probability to the use terminals on a common control channel. The packet transmission probability controls the number of user terminals that will transmit in a given time slot to reduce collisions and increase the departure rate. The user terminals selectively transmit one or more packets to the access point in a time slot on the random access channel based on the allowed information rate and the packet transmission probability. In one exemplary embodiment, the user terminals may transmit up to an allowed number of packets in the same time slot based on the rate control parameter. In another embodiment, the user terminals transmit a variable rate packet determined based on the rate control parameter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary wireless communication system according to one embodiment. 
         FIG. 2  illustrates an exemplary access point in a wireless communication system. 
         FIG. 3  illustrates an exemplary user terminal in a wireless communication system. 
         FIG. 4  illustrates and exemplary method implemented by an access point in a wireless communication system. 
         FIG. 5  illustrates and exemplary method implemented by a user terminal in a wireless communication system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings,  FIG. 1  illustrates a communication network  10  comprising a plurality of user terminals  20  communicating with an access point  12  over a shared uplink channel. In the exemplary embodiment, access point  12  has multiple receive antennas  14 . Each user terminal  20  has a single transmit antenna  22 . However, the number of antennas  14 ,  22  is not a material aspect of the invention. The communication system may implement any know multiple access technology, including but not limited to code division multiple access (CDMA), time division multiple access (TDMA), and orthogonal frequency division multiplexing (OFDM). 
     The uplink channel is a contention-based random access channel. The multiple access protocol used is a variation of the slotted-ALOHA protocol. The uplink channel is subdivided into a plurality of time slots. To access the channel, the user terminal  20  selects a time slot and begins transmitting a data packet at the beginning of the time slot. The duration of the data packet equals one time slot. In the event of a collision, each user terminal  20  backs off a random amount and retransmits the data packet in another time slot. 
     In the exemplary embodiment, access point  12  employs multi-packet reception (MPR) to receive multiple data packets from one or more user terminals  20  in a single time slot. Because the access point  12  is equipped with multiple antennas  14 , the access point  12  may exploit spatial and multi-user diversity of the user terminals  20  to jointly decode and demodulate multiple data packets that are simultaneously transmitted from one or more user terminals  20  in the same time slot. In general, the number of simultaneous packets that may be successfully received by the access point  12  in a single time slot equals the number of receive antennas  14 . Those skilled in the art will appreciate that other MPR techniques may also be employed to increase the number of simultaneous packets. For example, CDMA and/or OFDM techniques may be used to enable MPR with only a single antenna  14 . 
     In a conventional system using slotted-ALOHA, power control is used to ensure that packets transmitted from all user terminals  20  are received with the same power by the access point  12 . Because the packet size and information rate is fixed for all packets, this approach results in a low system throughput because the user terminals  20  may not take advantage of favorable channel conditions to increase data rates. According to the present invention, two new techniques are employed in combination to achieve higher system throughput on the uplink channel. First, the present invention uses per antenna multiplexing so that each antenna  14  may receive up to K multiplexed packets in the same time slot. Secondly, the present invention employs rate control on the uplink channel to allow user terminals  20  with more advantageous channel conditions to transmit at higher overall information rates than user terminals  20  with less advantageous channel conditions. The rate control aspect allows the system to take advantage of multi-user diversity to improve system throughput. 
     Multiplexing packets received by each antenna  14  may be achieved by using direct-sequence (DS) or frequency-hopping (FH) spreading codes and/or by employing low-rate error control codes. With (pseudo-) random spreading and/or channel coding, up to KN multiplexed packets may be transmitted in the same time slot and received successfully by a receiver with N antennas employing multi-packet (or in other words, multi-user) detection. 
     Rate control is based on channel conditions reported to the access point  12  by the user terminals  20 . In one embodiment, the user terminals  20  report channel conditions to the access point  12  and the access point  12  determines the information rate for each user terminal  20  based on the reported channel conditions from the user terminal  20 . The access point  12  then transmits a rate control parameter indicative of the selected information rate to each user terminal  20 . The rate control parameter may be transmitted over an associated control channel for the random access channel. 
     The information rate controlled is the number of information bits transmitted by the user terminal  20  in a single time slot of the random access channel. In one embodiment, referred to herein as the multi-packet approach, a user terminal  20  may transmit multiple packets in a single time slot. The size and coding for all packets is the same so that each packet has the same number of information bits. The rate control parameter from the access point  12  may comprise the allowed number of packets that may be transmitted. The information rate for a user terminal  20  in a given time slot is an integer multiple of the information rate for a single packet. 
     In another embodiment, referred to herein as the variable rate approach, each user terminal  20  may transmit a single variable rate packet in a given time slot. In this embodiment, the packet size is the same for all packets. However, user terminals  20  with advantageous conditions may transmit with higher code rates and thus higher information rates (e.g., more information bits). User terminals with less advantageous channel conditions transmit with lower code rates and hence lower information rates (e.g., fewer information bits). The rate control parameter may comprise the selected code rate for the packet. The information rate in this embodiment varies proportionally with the code rate. 
     In both embodiments described above, system throughput may be maximized by controlling the number of user terminals  20  that will attempt transmission in a single time slot to reduce the number of collisions. In a slotted ALOHA system, each user terminal  20  attempts transmission in a time slot independently. Assuming that the number of user terminals  20  is large and the probability q of transmission of a packet in a given time slot is small, the actual transmission attempts will be Poisson distributed. Assuming an attempt rate g, the distribution of attempts a is given by: 
                     Pr   ⁡     (     a   ⁢           ⁢   attempts     )       =         e     -   g       ⁢     g   a         a   !               Eq   .           ⁢     (   1   )                 
The normalized packet departure rate, defined as the expected number of successful packet transmissions per time slot divided by the multiplexing factor K, is given by:
 
                       E   ⁡     [   D   ]       =       1   K     ⁢       ∑     k   =   0     KN     ⁢         ke     -   g       ⁢     g   k         k   !             ,           Eq   .           ⁢     (   2   )                 
where k represents the access demand and N represents the number of receive antennas  14 . For a conventional slotted ALOHA (e.g., N=1, K=1), the maximum departure rate is 1/e=0.3679 achieved at an attempt rate g=1. It may be shown that the maximum normalized departure rate for K=2 and N=1 is 0.42 achieved at an attempt rate of 1.618. Thus, a slotted ALOHA system that implements some form of per-antenna multiplexing achieves a higher maximum normalized departure rate than a system in which only one packet is received per antenna  14  in each time slot. This gain is due to the fact that two or more packets have to be transmitted to cause a collision in a conventional system, while three or more packets have to be transmitted to cause a collision when per-antenna multiplexing is used. Thus, the probability of collision is smaller when per antenna multiplexing is used. This gain is referred to herein as the statistical multiplexing gain. It may be shown that as K approaches infinity, the maximum normalized packet departure rate E[D] approaches 1.
 
     The maximum-possible throughput of a contention-based multiple access system is related to the maximum normalized departure rate by:
 
T max =ηE[D],  Eq. (3)
 
where η is the spectral efficiency of the multi-packet communication system in bits/s/Hz and r is the information rate per time slot. For a CDMA system with processing gain L and packet information rate r, the spectral efficiency η is given by:
 
     
       
         
           
             
               
                 
                   η 
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                       rK 
                       L 
                     
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     The spectral efficiency depends not only on the transmission medium but also on the type of receiver. Multi-user receivers, such as the optimum Maximum Likelihood Sequence Estimation (MLSE) receiver and the linear Minimum Mean Squared Error (MMSE) receiver, have a much higher spectral efficiency than the conventional single-user receiver. For a CDMA system with random spreading codes in flat Rayleigh fading and E b /N 0 =10 dB, the spectral efficiency for a linear MMSE receiver is maximized for K≈L. On the other hand, the spectral efficiency for an optimum MLSE receiver is maximized for K→∞. The maximum spectral efficiency for the MMSE receiver is less than a single-user system (K=1). On the other hand, the spectral efficiency for an optimum MLSE receiver for large K is greater than the single-user system. Note that the maximum-possible throughput of the contention-based multiple access system is directly proportional to the spectral efficiency as shown in Eq. (3). 
     To maximize system throughout, the access point  12  controls the attempt rate g so that the maximum departure rate is realized. More particularly, the access point  12  controls the attempt rate g by computing a packet transmission probability q and transmitting the packet transmission probability q to the user terminals  20  over a common control channel. The user terminals  20  transmit packets on the uplink channel based on the received packet transmission probability q. For example, if q=0.66, then two-thirds of the user terminals  20  with packets to send should transmit. An individual user terminal  20  may determine whether to transmit based on the outcome of a random event. For example, user terminal  20  may generate a random number between 0 and 1 and compare the random number to the packet transmission probability q. The decision to transmit is based on the outcome of the comparison. 
     The packet transmission probability q depends on the number user terminals  20  that have packets to send and the number of packets that each user terminal  20  is allowed to send. As the demand for access to the channel increases, the packet transmission probability q needed to maintain the desired attempt rate g decreases. The access demand, denoted as k, is the number of packets that may be transmitted in a time slot if all user terminals  20  with packets to send transmitted at the same time. The access point  12  may determine the access demand k based on status reports from the user terminals  20 . User terminals  20  having packets to send may report their status to the access point  12  over an uplink control channel. The access point  12  may then determine the access demand k from the status reports from all user terminals  20  and the information rates for the user terminals  20 . The packet transmission probability q may be updated periodically and transmitted to the user terminals  20  over a common control channel. 
     With the multi-packet approach, where user terminals  20  may send multiple packets per time slot, the transmission probability q may be computed according to: 
                       q   ⁡     (   k   )       =     max   ⁡     (         Kg   0     k     ,   1     )         ,           Eq   .           ⁢     (   5   )                 
where K represents the multiplexing factor (e.g., the number of packets that may be received by each antenna) and g 0  represents the desired attempt rate that maximizes system throughput. As noted above, the access point  12  calculates the packet transmission probability q and transmits the calculated packet transmission probability q to the user terminals  20  over a common control channel. Each user terminal  20  attempts packet transmission in a time slot with a probability equal to the transmission probability q. If a user terminal  20  is allowed to transmit multiple packets in a time slot, it may decide each packet transmission independently or it may decide all packet transmissions jointly.
 
     A power control algorithm may be used at the access point  12  to ensure that packets are received from all user terminals  20  with the same minimum power. User terminals  20  may employ different and preferably orthogonal spreading codes for packet transmission. Since the number of user terminals  20  in a contention-based multiple-access system may far exceed the processing gain, the spreading codes assigned to all of the user terminals  20  may not be mutually orthogonal. However, the spreading codes assigned to a user terminal  20  transmitting multiple packets in a time slot may be chosen to be orthogonal to limit self interference. 
     In the variable rate approach, where different user terminals  20  transmit at different code rates, the packet transmission probability q is computed as a function of the normalized access demand  k . If there are k packets of type j to be transmitted in the same time slot, the normalized access demand  k  is given by: 
                       k   _     =       ∑     j   =   1     J     ⁢       k   j       K   j           ,           Eq   .           ⁢     (   6   )                 
where K j  is the multiplexing factor for type j packets. Given an access demand  k , which may be determined by the access point  12  based on status reports form the user terminals  20 , the packet transmission probability may be computed according to:
 
                       q   ⁡     (     k   _     )       =     max   ⁡     (           g   _     0       k   _       ,   1     )         ,           Eq   .           ⁢     (   7   )                 
where  g   0  is the value of the total normalized attempt rate that maximizes system throughput. The normalized attempt rate  g   0  is given by:
 
                     g   _     =       ∑     j   =   1     J     ⁢       g   j     /     K   j                 Eq   .           ⁢     (   8   )                 
For a system with J=2, K 1 =1, K 2 =2, the maximum normalized packet departure rate is roughly independent of the individual attempt rates (g 1 ,g 2 ). Thus, the same transmission probability q may be used for packets of all types. The transmission probability q may be communicated by the access point  12  to all user terminals  20 . Each user terminal  20  attempts packet transmission in a time slot with a probability equal to the transmission probability q.
 
     Packet rates may be controlled by varying the spreading factor in a CDMA system. User terminals  20  with high received power may be assigned codes with low spreading factor, while user terminals  20  with low received power may be assigned codes with high spreading factor. A power control algorithm may be optionally used to ensure that all packets are received with the same power and spreading factor product. 
       FIG. 2  illustrates an exemplary access point  12  for implementing the rate control techniques described herein. As previously noted, access point  12  has multiple receive antennas  14  to receive signals transmitted on the uplink channel from the user terminals  20 . Receive antennas  14  are coupled to a transceiver  15  with a multi-user detection (MUD) receiver  16  that jointly detects the signals from the user terminals  20 . The MUD receiver  16  may, for example, comprise a minimum means squared error (MMSE) receiver. The receiver  16  decodes and demodulates the received signals from the user terminals  20 . A random access controller  18  determines the information rates for the individual user terminals  20  and the packet transmission probability. The random access controller  18  also implements any power control algorithms that are used. The user terminals  20  provide channel quality feedback (e.g., CQI reports) to the access point  12 , which are used by the access point to determine information rates for the user terminals  20 . The user terminals  20  also provide status reports indicating whether the user terminals  20  have packets to send on the uplink channel. 
       FIG. 3  illustrates an exemplary user terminal  20 . User terminal  20  includes a single antenna  22  coupled to a transceiver  24 . Transceiver  24  includes a transmitter  26  for transmitting signals on the random access channel. The transmitter  26  encodes and modulates signals for transmission on the random access channel. A random access controller  28  controls the information rate of the user terminals  20  based on a rate control parameter received from the access point  12 . The random access controller  28  also implements the decision making for determining whether to transmit in a given time slot based on the packet transmission probability. 
       FIG. 4  illustrates an exemplary method  100  implemented by a wireless access point  12  for controlling packet transmission on a random access channel by one or more user terminals  20 . As previously noted, access point  12  receives channel conditions and status reports from user terminals  20  over uplink control channels. Based on the channel condition reports, the access point  12  determines the allowed information rates for the user terminals  20  (block  102 ). User terminals  20  with favorable channel conditions are allowed a higher information rate than similar user terminals  20  with less favorable channel conditions. The random access controller  18  may optionally transmit a rate control parameter to each user terminal  20  indicating the allowed information rates for each respective user terminal  20  (block  104 ). The rate control parameter may comprise the number of allowed packets in the multi-rate approach, or the code rate in the variable rate approach. The rate control parameter may be transmitted on an associated control channel. 
     After determining the information rates for the user terminals  20 , the access point  12  then determines the aggregate access demand for all user terminals  20  having packets to send to the access terminal  12  on the random access channel (block  106 ). The computation of the access demand only considers those user terminals  20  having packets to send. In the multi-packet embodiment, the aggregate demand is the total number of packets that the user terminals  20  are allowed to send in a time slot. In the variable rate approach, the aggregate demand is the total normalized number of packets that the user terminals  20  may send in a time slot, taking into account the varying information rates for different user terminals  20 . 
     Once the aggregate demand is known, access point  12  computes a packet transmission probability (block  108 ). The packet transmission probability determines how many of the user terminals  20 , on average, will transmit in a given time slot. For the multi-packet approach, the packet transmission probability is computed according to Eq. 5. For the variable rate approach, the packet transmission probability is computed according to Eq. 7. The access point  12  transmits the packet transmission probability to the user terminals  20  over a common control channel (block  110 ). 
     The access point  12  may compute the individual information rates for the user terminals  20  and the packet transmission probability on a periodic basis. For example, the information rates and packet transmission probability may be updated every n time slots. The number n of time slots between updates may be selected to achieve an acceptable trade-off between system throughput and system overhead. Also, those skilled in the art will appreciate that the number of time slots n between updates may be varied, depending on system load. For example, when the system is lightly loaded and system overhead is less of a concern, the number of time slots n may be reduced to make updates more frequent. 
       FIG. 5  illustrates an exemplary method  150  implemented by a user terminal  20  for transmitting packets on a random access channel. The user terminal  20  receives a rate control parameter from the access point  12  (block  152 ). For the multi-packet approach, the rate control parameter indicates an allowed number of packets that may be transmitted by the user terminal  20  in a time slot. The number of packets transmitted determines the information rate of the user terminal  20  and is computed based on channel quality reports provided by the user terminal  20 . For the variable rate approach, different user terminals  20  may transmit at different information rates. The rate control parameter from the access point  12  indicates an allowed information rate for a variable rate packet to be transmitted in a time slot on the random access channel. The information rate is determined by the access point based on channel quality reports sent from the user terminal  20  to the access point  12 . 
     The user terminal  20  also receives a packet transmission probability from the access point  12  over a common control channel (block  154 ). As noted above, the packet transmission probability is based on the maximum number of packets or normalized packets that may be sent in a time slot by user terminals  20  having packets to send. The user terminal  20  selectively transmits one or more packets in a time slot based on the rate control parameter and the packet transmission probability (block  156 ). In the multi-packet approach, the rate control parameter comprises an allowed number of packets that can be transmitted in a time slot. The user terminal  20  selectively transmits up to the allowed number of packets based on the packet transmission probability. The decision to transmit can be made separately for each packet when a user terminal  20  is allowed to send multiple packets, or a single decision may be made for all packets. In the variable rate approach, the rate control parameter is the code rate or information rate for a packet transmitted on the RACH. The user terminal selectively transmits a single packet with an information rate determined based on the rate control parameter. Again, the decision to transmit is made based on the packet transmission probability. 
     The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.