Abstract:
The present invention relates to methods for establishing a connection between a user equipment and a wireless network. More particularly, the present invention relates to methods for selecting a preamble based on its power back-off metric in order to randomly access a wireless network while avoiding collisions with other user equipments attempting to access the network at the same time.

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 61/089,979, filed on Aug. 19, 2008, which is incorporated herein by this reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to methods of randomly accessing wireless communication networks. 
     BACKGROUND 
     Mobile communication systems enable a mobile terminal (a.k.a., user equipment (UE)) to access a network via a network node (e.g., a base station). In some systems, before the UE begins transmitting traffic to the network via the network node, the UE performs a random access (RA) procedure to request access to the network. For example, the UE transmits an access burst to the network node using a random access channel. 
     To distinguish between different UEs performing RA, the access burst transmitted by the UE contains a preamble randomly chosen by the UE that the network node may use to identify the UE. Generally, the UE will uniformly, randomly select a preamble from a set of preambles (e.g., 64 preambles) that was derived from one or more root sequences (e.g., Zadoff-Chu sequences) associated with the network node. 
     A set of one or more preambles may be derived from a root sequence by cyclic shifting of the root sequence. The number of preambles that can be derived from a root sequence depends on the maximum expected round trip time between the UE and the network node. For instance, if a root sequence had a length of 800 μs and a very short maximum expected round trip time, then 64 preambles could be derived from the root sequence if the cyclic shift length was less than or equal to 800/64 μs, or 12.5 μs. The maximum expected round trip time is not always short. Indeed, sometimes it may be rather large. Because the cyclic shift length must be large enough to avoid any ambiguity in preamble detection due to propagation round trip time, in some instances, multiple root sequences (e.g., 64 root sequences) are required to generate 64 unique preambles. 
     According to some standards, a number of root sequences are available to derive the preambles. For instance, according to the 3G Long-Term Evolution (LTE) standard, a total of 838 root sequences are available for use. Each network node in the network is typically associated with a subset of the 838 root sequences. While different nodes may have the same root sequences associated with them, it is generally advisable to assign different root sequences to nodes that are physically near each other to avoid ambiguity. 
     Not all root sequences have the same properties. For instance, different root sequences can have different power back-off metrics (PBM) (e.g., different cubic metric (CM), peak-to-average power ratio, out-of-band emissions, etc.). All preambles derived from a particular root sequence inherit the PBM properties of the particular root sequence. While it may be desirable for a UE to select a preamble having the “best” PBM characteristics when randomly accessing a network node, all UEs should not use the preamble with the best PBM characteristics because this would result in an increase in collisions. Thus, there exists a need in the art for a method of selecting a preamble with desirable PBM characteristics while at the same time not exacerbating the collision problem. 
     SUMMARY 
     In one aspect the present invention provides a method performed by a UE for randomly accessing a network node. In some embodiments, the method includes the following steps: (A) receiving from the network node sequence information for defining a set of sequences (e.g., a sequence index and a cyclic shift length where the sequence index is specific to a set of one or more network nodes of which the network node is a member), where each sequence in the set is associated with a power back-off metric (e.g., a cubic metric, a peak to average power ratio, or an out of band emission related metric); (B) selecting a sequence from the set of sequences; and (C) using the selected sequence or a sequence derived from the selected sequence to access the network node, wherein the step of selecting a sequence from the set of sequences comprises randomly selecting a sequence from the set of sequences using a non-uniform selection process such that the probability that a particular sequence is selected is a function of the power back-off metric associated with the particular sequence. 
     In some embodiments, the step of using the selected sequence or the sequence derived from the selected sequence to access the network node comprises transmitting to the network node the selected sequence or a preamble derived from the selected sequence. 
     In some embodiments, the step of selecting a sequence from the set of sequences comprises randomly selecting a sequence from the set of sequences using a non-uniform selection process such that the probability that a particular sequence is selected is a function of (i) the power back-off metric associated with the particular sequence and (ii) a value representing an amount of path loss experienced by the UE, such that if the value representing the amount of path loss experienced by the UE is greater than a threshold the non-uniform selection process favors certain sequences from the set, and if the value representing the amount of path loss experienced by the UE is less than a threshold the non-uniform selection process favors other sequences from the set of sequences wherein each other sequence is associated with a power back-off metric that is higher than the average or median power back-off metric of said certain sequences. 
     In some embodiments, the step of randomly selecting a sequence from the set of sequences using a non-uniform selection process is performed only if one or more certain events are detected. The one or more certain events may include a path loss exceeding a threshold, the receipt of a hand off command, the UE being located at a cell edge, and/or the nth successive failure of a random access attempt, where n&gt;1. In some embodiments, the method also includes receiving a path loss threshold transmitted from the network node, wherein the one or more certain events comprises determining that a measured path loss exceeds the path loss threshold. 
     In some embodiments, the step of selecting a sequence from the set of sequences comprises randomly selecting a sequence from the set of sequences using a non-uniform selection process that favors certain sequences from the set of sequences, wherein each of said certain sequences is associated with a power back-off metric that is lower than an average or median power-back off metric for the set of sequences. 
     In some embodiments, the step of randomly selecting a sequence from the set of sequences using a non-uniform selection process comprises: forming a second set of sequences; and randomly selecting a sequence from only the second set of sequences. The step of forming the second set of sequences may include: for each sequence included in the first recited set of sequences, determining whether the sequence should be added to the second set of sequences, wherein the determination is based on, at least in part, the power back-off metric associated with the sequence, and adding the sequence to the second set of sequences in response to determining that the sequence should be added to the second set of sequences. 
     In some embodiments, the step of determining whether the sequence should be added to the second set of sequences comprises determining whether the power back-off metric associated with the sequence is below a threshold, wherein if the power back-off metric associated with the sequence is below the threshold, then the sequence should be added to the second set of sequences such that the second set of sequences contains only those sequences that are associated with a power back-off metric that is relatively small. 
     In some embodiments, the step of randomly selecting a sequence from the second set of sequences is performed such that each sequence in the second set of sequences has an equal probability of being randomly selected. 
     In some embodiments, the set of sequences is a set of root sequences, and the UE stores a set of power back-off metrics, wherein each power back-off metric is associated with a root sequence. 
     In another aspect, the present invention provides an improved communication device. In some embodiment, the improved communication device includes a receiver operable to receive sequence information; a data processing system configured to define a set of sequences, where each sequence in the set is associated with a power back-off metric, and select a sequence from the set of sequences; and a transmitter operable to transmit to a network node the selected sequence or a sequence derived from the selected sequence, wherein the data processing system is operable to randomly select a sequence from the set of sequences using a non-uniform selection process such that the probability that a particular sequence is selected is a function of the power back-off metric associated with the particular sequence. 
     The above and other aspects and embodiments are described below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements 
         FIG. 1  illustrates a portion of a mobile communication system. 
         FIG. 2  is a flowchart depicting a method for randomly accessing a wireless network according to embodiments of the present invention. 
         FIG. 3  is a flowchart depicting a method of randomly selecting a sequence from a set of sequences using a non-uniform selection process according to embodiments of the present invention. 
         FIG. 4  is a flowchart depicting a method, according to embodiments of the present invention, of determining whether a sequence with a low power back-off metric is preferred. 
         FIG. 5  is a flowchart depicting a method, according to embodiments of the present invention, of determining whether a sequence with a low power back-off metric is preferred. 
         FIG. 6  schematically depicts a UE according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 ,  FIG. 1  illustrates a portion of a mobile communication system  100 . As depicted in  FIG. 1 , mobile communication system  100  may comprise a plurality of network nodes  102   a ,  102   b  (e.g., base stations) for enabling a mobile terminal  104  (a.k.a., user equipment (UE)  104 ) to access network  110 . 
     As described above, in some communication systems, UE  104  must transmit to a network node  102  a request to access network  110  prior to transmitting traffic to network  110 . This request may be transmitted as an access burst on a random access channel (e.g., a Physical Random Access Channel (PRACH)). As also discussed above, when UE  104  transmits a message (e.g., an access burst) to a network node  102  using the random access channel, UE  104  should select a preamble to include in the message. As further discussed above, it would be advantageous if UE  104  can intelligently select a preamble without significantly increasing the likelihood of a preamble collision. 
     Referring now to  FIG. 2 ,  FIG. 2  is a flow chart illustrating a process  200 , according to some embodiments, performed by UE  104  for randomly accessing a network node  102 . 
     Process  200  may begin in step  202 , where UE  104  determines whether it needs to randomly access a network node  102 . If it does, the process proceeds to step  204 , otherwise process  202  may be repeated again. 
     In step  204 , UE  104  obtains sequence information. For example, in step  202  UE  104  may receive the sequence information from a network node  102 . In some systems, each network node periodically broadcasts sequence information. For example, in some systems, each network node broadcasts a logical sequence index and a cyclic shift length. Additionally, in some systems (e.g., LTE) a high speed flag is also transmitted. 
     In step  206 , UE  104  uses the received sequence information to define a set of sequences. For example, in some embodiments, the set of defined sequences may consist of all of the root sequences that correspond to the received sequence information. In other embodiments, the set of defined sequences may consist of sixty four (64) preambles, where each preamble was derived from a root sequence included in the set of root sequences that correspond to the received sequence information. For instance, if the set of root sequences that correspond to the received sequence information consists of a single root sequence, then each of the 64 preambles are derived from that one root sequence. As another example, if the set of root sequences that correspond to the received sequence information consists of 64 root sequences, then each of the 64 preambles is derived from a different one of the root sequences. 
     In step  208 , UE  104  determines whether it should use a non-uniform or uniform selection process to select a sequence from the set of sequences (e.g., UE  104  determines whether it should select a sequence with a low power back-off metric or a high power back-off metric). Process  200  proceeds to step  210  if UE  104  determines it should use a non-uniform selection process, otherwise it proceeds to step  212 . 
     In step  210 , UE  104  randomly selects a sequence from the set of sequences defined in step  206  using a non-uniform selection process. In some circumstances, in step  210 , UE  104  randomly selects a sequence from the set of sequences defined in step  206  using a non-uniform selection process that favors sequences having a low power back-off metric. For example, if UE  104  determines that it is at a cell edge or has a high path loss, then UE  104  will select a sequence from the set of sequences using a selection process that favors sequences having a low power back-off metric (e.g., sequences associated with a power back-off metric that is lower than the average or median power back-off metric associated with the set of sequences). 
     In other circumstances, in step  210 , UE  104  randomly selects a sequence from the set of sequences defined in step  206  using a non-uniform selection process that favors sequences having a high power back-off metric. For example, if UE  104  determines that it is not at a cell edge or does not have a high path loss, then UE  104  may select a sequence from the set of sequences using a selection process that favors sequences having a high power back-off metric (e.g., sequences associated with a power back-off metric that is higher than the average or median power back-off metric associated with the set of sequences). 
     In step  212 , UE  104  randomly selects a sequence from the set of sequences defined in step  206  using a uniform selection process such that no sequences are favored in the selection process. 
     In step  214 , UE  104  transmits to the network node from which UE  104  received the sequence information a message containing the selected sequence (i.e., the selected preamble or a preamble derived from the selected root sequence depending on whether the set of sequences consists of preambles or root sequences). 
     In step  216 , UE  102  determines whether there was a transmission failure (e.g., whether the network node to which the message was transmitted successfully received the message). If there was no transmission failure, process  200  may end (or return back to step  202 ). If there was a transmission failure, process  200  proceeds to step  220 . 
     In step  220  a counter that keeps track of the number of transmission failures is incremented (this counter may have been initialized to zero prior to performing step  214 ). 
     Referring now to  FIG. 3 ,  FIG. 3  is a flow chart illustrating an exemplary process  300  for performing step  210 . Process  300  may begin in step  302  where a set of candidate sequences is initialized. A sequence is then selected from the set defined in step  206  and “removed” from the set (step  304 ). A sequence may be “removed” from the set by, for example, setting a flag indicating that the sequence has been selected. 
     At step  306 , the power back-off metric (PBM) associated with the selected sequence is determined. If the PBM of the selected sequence is identified as being below a certain pre-defined threshold value (T 2 ) at step  308 , then the selected sequence is added to the set of candidate sequences (step  310 ), otherwise process  300  proceeds to step  312 . In step  312 , UE  104  determines whether the set of sequences defined in step  206  is “empty” (i.e., whether all of the sequences in the set have been selected). If the set is not empty, the process loops back to step  304 . If, the set is empty, then a sequence can be randomly selected from the set of candidate sequences using, for example, a uniform selection process (step  314 ). In this manner, UE  104  randomly selects a sequence from the set of sequences defined in step  206  using a non-uniform selection process that favors sequences having a lower power back-off metric. 
     Process  300  is an example process for performing step  210 . Other processes for performing step  210  are contemplated. For example, step  210  may be implemented by randomly selecting a sequence from the set defined in step  206  such that the probability that a particular sequence is selected is function of the PBM associated with the sequence (e.g., sequences with a low PBM may be weighted more heavily in the selection process than sequences that do not have a low PBM such that the sequences with a low PBM are selected more of the time than sequences with high PBM). Additionally, the probability that a particular sequence is selected may also be function of a value representing an amount of path loss. 
     Referring now to  FIG. 4 ,  FIG. 4  is a flow chart illustrating an exemplary process  400  for performing step  208 . Process  400  may begin in step  402  where it is determined whether UE  104  is being handed off from one node to another node. If the UE is being handed off, then the method proceeds to step  210 , which prefers sequences with low power back-off metrics. The reason that the method goes to step  210  during a hand off is because, when a UE is being handed off from one network node to another, it is normally on the edge of the node&#39;s transmission radius and, therefore, it is important to have a sequence with a low power back-off metric. If the UE is not being handed of (i.e., if it is just being turned on of has been in standby mode for an extended period of time), then the UE determines whether the measured path loss is greater than a certain pass loss threshold value (shown here as T 2 ) at step  404 . If the path loss is greater than the threshold, then the method advances to step  210 . If, however, the path loss is less than the threshold value, then the method determines whether the counter incremented in step  220 , which is indicative of the number of transmission attempts to the network node, is greater than a certain threshold T 1  (step  406 ). If the counter is greater than T 1 , then the method advances to step  210 . However, if the counter is below T 1 , then the method advances to step  212 . The path loss threshold value T 2  may be a configuration parameter stored in UE  104  prior to UE  104  performing process  200  and/or it may be communicated to UE  104  by a network node  102 . 
     Referring now to  FIG. 5 ,  FIG. 5  is a flow chart illustrating an exemplary process  500  for performing step  208 . Process  500  may begin in step  502  where UE  104  determines whether its is being handed off or not. If it is, then the path loss is determined and if the path loss is greater than a certain threshold T 2  as determined at step  504 , then the method advances to step  210 . If, however, the path loss is less than the threshold value, then the method determines whether the counter incremented in step  220  is greater than the threshold T 1  (step  506 ). If the counter is greater than T 1 , then the method advances to step  210 . However, if the counter is below T 1 , then the method advances to step  212 . 
     Referring now to  FIG. 6 ,  FIG. 6  is a functional block diagram of UE  104  according to some embodiments of the invention. As shown, UE  104  may comprise a data processing system  602  (e.g., one or more microprocessors), a data storage system  606  (e.g., one or more non-volatile storage devices) and computer software  608  stored on the storage system  606 . Data  610  (e.g., the above mentioned threshold values and root sequences) may also be stored in storage system  606 . UE  104  also includes transmit/receive (Tx/Rx) circuitry  604  for transmitting data to and receiving data from network nodes  102 . 
     Software  608  is configured such that when data processing system executes software  608 , UE  104  performs steps described above (e.g., the steps described above with reference to the flow charts shown in  FIGS. 2-5 ). For example, software  608  may include: (1) computer instructions configured to obtain sequence information for defining a set of sequences (root sequences or preambles) and (2) computer instructions configured to randomly select a sequence from the set of sequences using a non-uniform selection process such that the probability that a particular sequence is selected is a function of a power back-off metric associated with the particular sequence. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above described exemplary embodiments. 
     Additionally, while the processes described above and illustrated in the drawings are shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and some steps may be performed in parallel.