Abstract:
The present invention provides a method of selecting a random access preamble in a radio communication system operable at least on a physical layer and a medium access control layer. Random access preambles are divided into at least two groups, the groups depending on at least one of the following: radio conditions and a size of a message to be transmitted by user equipment. The method comprising: (a) the medium access control layer selecting one of the preamble groups; (b) the medium access control layer randomly selecting one random access preamble within the selected group; (c) the medium access control layer signaling the selected random access preamble to the physical layer; and (d) the physical layer generating and transmitting the generated random access preamble.

Description:
This application claims priority of U.S. Provisional Application No. 61/048,542 filed on 28 Apr. 2008, the content of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to random access channel (RACH) procedure in a cellular communications network, and in particular to selecting RACH preamble sequence. While it is described below in the context of a long term evolution (LTE) type cellular network for illustration purposes and because it happens to be well suited to that context, those skilled in the art will recognise that the invention disclosed herein can also be applied to various other types of cellular networks. 
     2. Discussion of the Related Art 
     Universal mobile telecommunications system (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The LTE of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardised UMTS. 
     The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement. 
       FIG. 1  is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice and packet data. 
     As illustrated in  FIG. 1 , the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN) and an evolved packet core (EPC) and one or more user equipment (UE) units  101 . The E-UTRAN may include one or more evolved NodeBs (eNodeB, or eNodeB)  103 , and a plurality of UEs  101  may be located in one cell. One or more E-UTRAN mobility management entity (MME)system architecture evolution (SAE) gateways  105  may be positioned at the end of the network and connected to an external network. 
     As used herein, “downlink” refers to communication from an eNodeB  103  to UE  101 , and “uplink” refers to communication from the UE  101  to an eNodeB  103 . UE  101  refers to communication equipment carried by a user and may be also be referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device. 
     An eNodeB  103  provides end points of a user plane and a control plane to the UE  101 . MMESAE gateway  105  provides an end point of a session and mobility management function for UE  101 . The eNodeB  103  and the MMESAE gateway  105  may be connected via an S 1  interface. 
     The eNodeB  103  is generally a fixed station that communicates with UE  101 , and may also be referred to as a base station (BS) or an access point. One eNodeB  103  may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNodeBs  103 . 
     The MME provides various functions including distribution of paging messages to eNodeBs  103 , security control, idle state mobility control, SAE bearer control, and ciphering and integrity protection of non-access stratum (NAS) signaling. The SAE gateway host provides assorted functions including termination of U-plane packets for paging reasons, and switching of the U-plane to support UE mobility. For clarity, MMESAE gateway  105  will be referred to herein simply as a “gateway,” but it is understood that this entity includes both an MME and an SAE gateway. 
     A plurality of nodes may be connected between the eNodeB  103  and the gateway  105  via the S 1  interface. The eNodeBs  103  may be connected to each other via an X 2  interface and neighbouring eNodeBs may have a meshed network structure that has the X 2  interface. 
       FIG. 2(   a ) is a block diagram depicting an architecture of a typical E-UTRAN and a typical EPC. As illustrated, eNodeB  103  may perform functions of selection for gateway  105 , routing toward the gateway during a radio resource control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of broadcast channel (BCCH) information, dynamic allocation of resources to UEs  101  in both uplink and downlink, configuration and provisioning of eNodeB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway  105  may perform functions of paging origination, LTE-IDLE state management, ciphering of the user plane, system architecture evolution (SAE) bearer control, and ciphering and integrity protection of non-access stratum (NAS) signaling. 
       FIGS. 2(   b ) and  2 ( c ) are block diagrams depicting the user plane protocol and the control plane protocol stack for the E-UMTS. As illustrated, the protocol layers may be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based upon the three lower layers of an open system interconnection (OSI) standard model that is well-known in the art of communication systems. 
     The physical layer, the first layer (L1), provides an information transmission service to an upper layer by using a physical channel. The physical layer is connected with a medium access control (MAC) layer located at a higher level through a transport channel, and data between the MAC layer and the physical layer is transferred via the transport channel. Between different physical layers, namely, between physical layers of a transmission side and a reception side, data is transferred via the physical channel. 
     The MAC layer of Layer 2 (L2) provides services to a radio link control (RLC) layer (which is a higher layer) via a logical channel. The RLC layer of Layer 2 (L2) supports the transmission of data with reliability. It should be noted that the RLC layer illustrated in  FIGS. 2(   b ) and  2 ( c ) is depicted because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself is not required. The packet data convergence protocol (PDCP) layer of Layer 2 (L2) performs a header compression function that reduces unnecessary control information such that data being transmitted by employing Internet protocol (IP) packets, such as IPv4 or IPv6, can be efficiently sent over a radio (wireless) interface that has a relatively small bandwidth. 
     A radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane and controls logical channels, transport channels and the physical channels in relation to the configuration, reconfiguration, and release of the radio bearers (RBs). Here, the RB signifies a service provided by the second layer (L2) for data transmission between the terminal and the E-UTRAN. 
     As illustrated in  FIG. 2(   b ), the RLC and MAC layers (terminated in an eNodeB  103  on the network side) may perform functions such as scheduling, automatic repeat request (ARQ), and hybrid automatic repeat request (HARQ). The PDCP layer (terminated in eNodeB  103  on the network side) may perform the user plane functions such as header compression, integrity protection, and ciphering. 
     As illustrated in  FIG. 2(   c ), the RLC and MAC layers (terminated in an eNodeB  103  on the network side) perform the same functions as for the control plane. As illustrated, the RRC layer (terminated in an eNodeB  103  on the network side) may perform functions such as broadcasting, paging, RRC connection management, RB control, mobility functions, and UE measurement reporting and controlling. The NAS control protocol (terminated in the MME of gateway  105  on the network side) may perform functions such as an SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for the signaling between the gateway and UE  101 . The control plane also comprises PDCP between RLC and RRC. 
     The NAS control protocol may use three different states; first, a LTE_DETACHED state if there is no RRC entity; second, a LTE_IDLE state if there is no RRC connection while storing minimal UE information; and third, an LTE_ACTIVE state if the RRC connection is established. Also, the RRC state may be divided into two different states such as an RRC_IDLE and an RRC_CONNECTED. 
     In RRC_IDLE state, the UE  101  may receive broadcasts of system information and paging information while the UE  101  specifies a discontinuous reception (DRX) configured by NAS, and the UE has been allocated an identification (ID) which uniquely identifies the UE in a tracking area. Also, in RRC-IDLE state, no RRC context is stored in the eNodeB  103 . 
     In RRC_CONNECTED state, the UE  101  has an E-UTRAN RRC connection and a context in the E-UTRAN, such that transmitting and/or receiving data to/from the network (eNodeB) becomes possible. Also, the UE  101  can report channel quality information and feedback information to the eNodeB  103 . 
     In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE  101  belongs. Therefore, the network can transmit and/or receive data to/from the UE  101 , the network can control mobility (handover) of the UE  101 , and the network can perform cell measurements for a neighbouring cell. 
     In RRC_IDLE mode, the UE  101  specifies the paging discontinuous reception (DRX) cycle. Specifically, the UE  101  monitors a paging signal at a specific paging occasion of every UE specific paging DRX cycle. 
     The procedure where the UE  101  sends a first message to the network is referred to as initial access. For this the common uplink channel called random access channel is used. In most systems the initial access starts from UE with the connection request message including the reason of the request, and the answer from the network indicating the allocation of radio resources for the requested reason. Thus, RACH is an uplink common channel used for transmitting control information and user data. It is applied in random access, and used for low-rate data transmissions from the higher layer. 
     In 3GPP TS 25.331 there are several reasons, called establishment cause, for sending a connection request message. The following are listed: originating conversational/streaming/background/subscribed traffic call, terminating conversational/streaming/interactive/background call, emergency call, inter radio access technology (RAT) cell re-selection, inter-RAT cell change order, registration, detach, originating high/low priority signaling, call re-establishment and terminating high/low priority signaling. 
     Reason “originating call” means that the UE  101  wants to setup a connection, for instance a speech connection. Reason “terminating call” means that the UE  101  answers to paging. Reason “registration” means that the user wants to register only to location update. 
     To send the information over the air the physical random access procedure is used. The physical random access transmission is under the control of higher layer protocol which performs some important functions related to priority and load control. These procedures differ in detail but GSM, UMTS and LTE radio systems have some similarities between them. 
     The UE  101  selects randomly an access resource and transmits a RACH preamble part of a random access procedure to the network. A preamble is a short signal that is sent before the transmission of the RACH connection request message. The UE  101  repeatedly transmits the preamble by increasing the transmission power every time the preamble is sent until it the network indicates the detection of the preamble. Then the message part is sent at the level of power equal to the last preamble transmission power plus an offset signalled by the network. 
     From the physical layer perspective, random access procedure includes successful message  1  transmission of one preamble sequence selected among available preamble sequences and message  2  reception of random access response. This is illustrated in  FIG. 3 . In the following description instead of referring to preamble sequences, simply preambles are referred to, but it is to be understood that they mean the same thing. 
     Since the UE  101  in good channel conditions supports larger message  3  size than the UE  101  in bad channel conditions, it has been agreed that message  3  is dynamic in size and that the size is conveyed by the preamble in message  1 . Therefore, the available preambles are grouped into two sets based on the size of the message  3  to be transmitted on the uplink. The selection of one of the two groups is done depending on the UE radio conditions and the thresholds required for selecting one of the two groups. Then a random access preamble is selected randomly within the selected group. In current systems, such as the UMTS, the random access preamble selection is done by the physical layer due to the fact that RACH retransmission procedure is faster than  10  ms interaction between physical layer (L1) and upper layers (L2/L3). 
     An object of the present invention is to improve the RACH procedure. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a method of selecting a random access preamble in a radio communication system operable at least on a physical layer and a medium access control layer, random access preambles being divided into at least two groups, the groups depending on at least one of the following: radio conditions and a size of a message to be transmitted by user equipment, the method comprising:
         the medium access control layer selecting one of the preamble groups;   the medium access control layer randomly selecting one random access preamble within the selected group;   the medium access control layer signaling the selected random access preamble to the physical layer; and   the physical layer transmitting the selected random access preamble.       

     Thus, the L1 is simplified as a result of implementing the teachings of the present invention. L1 complexity is moved to upper layers (L2/L3), and additional complexity in upper layers is negligible. Moreover, the L1 behaviour can be the same for the situations, where ACK is received for the transmitted preamble, no ACK is received or where no preamble is received within a RACH response window. The proposed solution also leads to a substantially independent L1-L2 interaction. In accordance with the present invention, L1 is not aware whether the preamble transmission is first transmission or retransmission. L1 just sends preamble over the air and then after the end of time window response, it passes the received information to L2. 
     Preamble retransmissions could be based on latest configuration. In UMTS, all access burst retransmissions are based on the first defined configuration. By this proposal, for each preamble retransmission, a parameter, such as the maximum allowed power that UE  101  can set, a target preamble power, PRACH configuration (time and frequency position) and preamble format, obtained in broadcast channel (BCH) can be re-taken into account. This means that the L1 does not transmit by using an old configuration. For instance, if 8 retransmissions are allowed and 1 random access occasion per 10 ms, in prior art solutions UE  101  may transmit by using a configuration which is 80 ms old. The reason for this is the fact that once the RACH procedure is initialised in UMTS, the next L1/L2 interaction is done when the maximum number of retransmissions is reached or ACK or NACK is received. But in LTE, L1/L2 interaction is done for each preamble transmission/retransmission. 
     According to a second aspect of the present invention there is provided user equipment for selecting a random access preamble in a radio communication system, the user equipment being operable at least on a physical layer and a medium access control layer, random access preambles being divided into at least two groups, the groups depending on at least one of the following: radio conditions and a size of a message to be transmitted by the user equipment, wherein:
         the medium access control layer is operable to select one of the preamble groups;   the medium access control layer is operable to randomly select one random access preamble within the selected group;   the medium access control layer is operable to signal the selected random access preamble to the physical layer; and   the physical layer is operable to transmit the selected random access preamble.       

    
    
     
       BRIEF DESCRIPTION THE DRAWINGS 
       Other objects, features and advantages of the invention will become apparent when reading the following description on non-limiting exemplary embodiments with reference to the accompanying drawings. 
         FIG. 1  is a block diagram illustrating network structure of an E-UMTS (or LTE) system. 
         FIGS. 2(   a ),  2 ( b ) and  2 ( c ) are block diagrams depicting logic architecture of typical network entities of the LTE system ( FIG. 2(   a )), a user-plane (U-plane) protocol stack ( FIG. 2(   b )) and a control-plane (C-plane) protocol stack ( FIG. 2(   c )). 
         FIG. 3  is a flow chart illustrating the RACH procedure. 
         FIG. 4  is a flow chart illustrating a method of random access preamble selection in accordance with a first embodiment of the present invention. 
         FIG. 5  is a flow chart illustrating a method of random access preamble selection in accordance with a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Two embodiments of the present invention are next described in more detail with reference to the attached figures. These embodiments are described in the context of LTE, but the invention is by no means limited to this environment. 
     In LTE the random access procedure is classified into two categories: non-synchronised random access and synchronised random access. Only non-synchronised random access is considered in the following. 
     The non-synchronised access is used when the uplink connection of the UE  101  is not time synchronised or when the UE  101  loses uplink synchronisation. The non-synchronised access allows the network to estimate and, if needed, to adjust the UE  101  transmission timing. Therefore, the non synchronised random access preamble is at least used for time alignment, and signature detection. The message payload may comprise any additional associated signaling information, e.g., a random ID, path/loss/channel quality indicator (CQI), access purpose, and so forth. 
     The UE  101  randomly selects a signature out of a group of signatures to distinguish between different UEs that attempt an access simultaneously. The preamble shall have good auto correlation properties in order for the eNodeB  103  to obtain an accurate timing estimate. In addition, the different preambles should have good cross-correlation properties in order for the eNodeB  103  to distinguish between simultaneous access attempts for different UEs using different signatures. A constant amplitude zero auto-correlation (CAZAC) sequence is used as a preamble signature sequence to achieve good detection probability. 
     Prior to initiation of the non-synchronised physical random access procedure, Layer 1 shall receive the following information from the higher layers:
         Available random access channels (number, frequency position, time period and timing offset);   Available preamble formats and their mapping to implicit messages;   Available Zadoff-Chu (ZC) root sequences and indices;   Initial preamble transmission power;   Power ramping step size (0 dB step size is allowed); and   Maximum number of preamble retransmissions.       

     From the physical layer perspective, the L1 random access procedure encompasses successful transmission of messages  1  and  2 , i.e. random access preamble and random access response, respectively. The remaining messages are scheduled for transmission by the higher layer on a shared data channel and thus not considered part of L1 random access procedure. A random access channel is a 1.08 MHz portion of a subframe or set of consecutive subframes reserved for random access preamble transmissions. 
     The following steps are required for the L1 random access procedure: 
     1. Prior to initiation of the non-synchronised physical random-access procedure, L1 shall receive the information indicated above from the higher layers. 
     2. A random access channel is randomly selected from the available non-synchronised random access channels. A preamble is then randomly selected from the available preamble set based on the implicit message to be transmitted. The random function shall be such that each of the allowed selections is chosen with equal probability. 
     3. The initial preamble transmission power level (which is set by the MAC) is determined using an open loop power control procedure. The transmission counter is set to the maximum number of preamble retransmissions. Preamble transmission then occurs using the selected random access channel, preamble, and preamble transmission power. 
     4. If no response (message  2 ) corresponding to the transmitted preamble (message  1 ) is detected then another random access channel and preamble are randomly selected. If the maximum transmission power and the maximum number of retransmissions have not been reached, then preamble retransmission occurs. Otherwise the L1 status (“No acknowledgment on non-synchronised random access”) is passed to the higher layers (MAC) and the physical random access procedure is exited. 
     5. If a response (message  2 ) corresponding to the transmitted preamble (message  1 ) is detected, then the L1 status (“ACK on non-synchronised random access received”) is passed to the higher layers (MAC) and the physical random access procedure is exited. 
     The following information is assumed to be available before performing a preamble selection:
         The logical root sequence index;   The cyclic shift N CS  describing the correlation size of a Zadoff-Chu sequence;   The high speed flag for determining whether unrestricted or restricted preamble set shall be used;   The size of the random access preamble groups; and   The threshold required for selecting one of the access preamble groups.       

     In order to find out which sequence is transmitted by the UE  101 , the eNodeB  103  correlates received sequence with each of 64 sequences that are available. For instance, if the Zadoff-Chu sequence is ABCDEF (size is 6 samples), then say that the size of the cyclic shift is 2. It means that the eNodeB  103  and the UE  101  will generate 3 preambles: (AB), (BC), (EF). Then the UE  101  chooses one preamble randomly, e.g. preamble EF. The eNodeB  103  will then correlate the received preamble with all preambles and only correlation with preamble EF will be successful. 
     The preamble selection is in current solutions done by L1. However, the present invention proposes a solution where it is done by L2. The first embodiment of the present invention is next described with reference to the flow chart of  FIG. 4 . 
     In step  401  a set of available preambles per cell are obtained by L2 according to the equations described in 3GPP TS 36.211 v8.2.0 (2008-03) in Section 5.7.2. There are 64 preambles available in each cell. The set of 64 preambles in a cell is found by including first, in the order of increasing cyclic shift, all the available cyclic shifts of a root Zadoff-Chu sequence with the logical root index broadcast as part of the system information. In case the set of available preambles per cell cannot be obtained from a single root Zadoff-Chu sequence, additional preambles are obtained from the root sequences with the consecutive logical root indexes until all available 64 preambles are obtained. 
     In step  403  the available preambles are further divided into groups according to the size of random access preamble groups. In this example the number of the generated groups is two. The division depends on at least radio channel conditions and/or message size to be transmitted by the UE  101 . In step  405  one of the access preamble groups is selected depending on the size of the message  3  to be transmitted on the UL and on the radio conditions compared to the signalled threshold. 
     In step  407  the MAC layer selects randomly one random access preamble within the selected group. In step  409  the MAC layer signals the selected random access preamble to the physical layer. In step  411  the MAC layer indicating to the physical layer logical root sequence index of the selected preamble, preamble start position within the logical root sequence index of the selected preamble and cyclic shift. The physical layer then in step  413  maps the logical root sequence index to a physical root sequence index as explained in 3GPP TS 36.211 v8.2.0 (2008-03) in Section 5.7.2. In step  415  the physical layer generates the random access preamble based on the physical root index, start position and cyclic shift. Finally in step  417  the physical layer transmits the generated preamble. 
     Thus, in the above embodiment the required parameters on L1 for the selected preamble generation are:
         The logical root sequence index of the selected preamble;   The preamble start position within the logical root sequence index of selected preamble; and   The cyclic shift N CS .       

     The second embodiment of the present invention is next described with reference to the flow chart of  FIG. 5 . In this embodiment the required parameters on L1 are different. In this embodiment, steps  501 ,  503 ,  505 ,  507 ,  509 ,  515  and  517  are identical to steps  401 ,  403 ,  405 ,  407 ,  409 ,  415  and  417 , respectively. However, in step  511  the MAC layer indicates to the physical layer only the random access preamble index corresponding to the selected preamble. Then in step  513  the physical layer maps the random access preamble index to a physical root sequence index, preamble start position within the logical root sequence index of the selected preamble and cyclic shift. 
     Two embodiments of the present invention have been disclosed above in the illustrative case of a 3GPP LTE system. Those skilled in the wireless communication art will appreciate that various modifications can be brought to these embodiments without departing from the invention and from the attached claims. They will also appreciate that the invention is applicable to communications systems other than 3GPP LTE systems.