755

Figure 17.5 Preamble timing at eNodeB for different random-access users.

Figure 17.6 Random-access-preamble generation.

is described. From each root Zadoff–Chu sequence XZC(u)(k), m − 1 cyclically shifted sequences are obtained by cyclic shifts of MZC/m each, where MZC is the length of the root Zadoff–Chu sequence.

Cyclically shifted ZC sequences possess several attractive properties. The amplitude of the sequences is constant, which ensures efficient power amplifier utilization and maintains the low PAR properties of the single-carrier uplink. The sequences also have ideal cyclic auto-correlation, which is important for obtaining an accurate timing estimation at the eNodeB. Finally, the cross-correlation between different preambles based on cyclic shifts of the same ZC sequence is zero at the receiver as long as the time cyclic shift N/m used when generating the preambles is larger than the maximum round-trip propagation time plus the maximum delay spread of the channel. Therefore, thanks to the ideal cross-correlation property, there is no intra-cell interference from multiple random-access attempts using preambles derived from the same Zadoff–Chu root sequence.

The generation of the random-access preamble is illustrated in Figure 17.6. Although the figure illustrates generation in the time-domain, frequency-domain generation can equally well be used in an implementation. Also, to allow for frequency-domain processing at the base station (discussed further below), a cyclic prefix is included in the preamble generation.

0.8 ms sampling window

Figure 17.7 Random-access-preamble detection in the frequency domain.

Preamble sequences are partitioned into groups of 64 sequences each. As part of the system configuration, each cell is allocated one such group by defining one or several root Zadoff–Chu sequences and the cyclic shifts required to generate the set of preambles. The number of groups is sufficiently large to avoid the need for careful sequence planning between cells.

When performing a random-access attempt, the terminal selects one sequence at random from the set of sequences allocated to the cell the terminal is trying to access. As long as no other terminal is performing a random-access attempt using the same sequence at the same time instant, no collisions will occur and the attempt will, with a high likelihood, be detected by the network.

The base-station processing is implementation specific, but thanks to the cyclic prefix included in the preamble, low-complexity frequency-domain processing is possible. An example hereof is shown in Figure 17.7. Samples over a window are collected and converted it into the frequency-domain representation using an FFT. The window length is 0.8 ms, which is equal to the length of the ZC sequence without a cyclic prefix. This allows to handle timing uncertainties up to 0.1 ms and matches the guard time defined.

The output of the FFT, representing the received signal in the frequency domain, is multiplied with the complex-conjugate frequency-domain representation of the root Zadoff–Chu sequence and the results is fed through an IFFT. By observing the IFFT outputs, it is possible to detect which of the shifts of the Zadoff–Chu root sequence has been transmitted and its delay. Basically, a peak of the IFFT output in

interval i corresponds to the i-th cyclically shifted sequence and the delay is given by the position of the peak within the interval. This frequency-domain implementation is computationally efficient and allows detection of multiple random-access attempts using different cyclic shifted sequences generated from the same root Zadoff–Chu sequence; in case of multiple attempts there will simply be a peak in each of the corresponding intervals.

17.2.2Step 2: Random access response

In response to the detected random access attempt, the network will, as the second step of the random-access procedure, transmit a message on the DL-SCH, containing:

•The index of the random-access preamble sequence the network detected and for which the response is valid.

•The timing correction calculated by the random-access-preamble receiver.

•A scheduling grant, indicating resources the terminal shall use for the transmission of the message in the third step.

•A temporary identity used for further communication between the terminal and the network.

In case the network detected multiple random-access attempts (from different terminals), the individual response messages of multiple mobile terminals can be combined in a single transmission. Therefore, the response message is scheduled on the DL-SCH and indicated on a L1/L2 control channel using an identity reserved for random-access response. All terminals which have transmitted a preamble monitors the L1/L2 control channels for random-access response. The timing of the response message is not fixed in the specification in order to be able to respond to sufficiently many simultaneous accesses. It also provides some flexibility in the base-station implementation.

As long as the terminals that performed random access in the same resource used different preambles, no collision will occur and from the downlink signaling it is clear to which terminal(s) the information is related. However, there is a certain probability of contention, that is multiple terminals using the same random access preamble at the same time. In this case, multiple terminals will react upon the same downlink response message and a collision occurs. Resolving these collisions is part of the subsequent steps as discussed below. Contention is also one of the reasons why hybrid ARQ is not used for transmission of the random-access response. A terminal receiving a random-access response intended for another terminal will have incorrect uplink timing. If hybrid ARQ would be used, the

in the first step listen to the same response message in the second step and therefore have the same temporary identifier. Hence, in the fourth step, each terminal receiving the downlink message will compare the identity in the message with the identity they transmitted in the third step. Only a terminal which observes a match between the identity received in the fourth step and the identity transmitted as part of the third step will declare the random access procedure successful. If the terminal has not yet been assigned a C-RNTI, the temporary identity from the second step is promoted to the C-RNTI; otherwise the terminal keeps its already assigned C-RNTI.

The contention-resolution message is transmitted on the DL-SCH, using the temporary identity from the second step for addressing the terminal on the L1/L2 control channel. Since uplink synchronization already has been established, hybrid ARQ is applied to the downlink signaling in this step. Terminals with a match between the identity they transmitted in the third step and the message received in the fourth step will also transmit a hybrid-ARQ acknowledge in the uplink.

Terminals which do not find a match between the identity received in the fourth step and the respective identity transmitted as part of the third step are considered to have failed the random-access procedure and need to restart the random-access procedure from the first step. Obviously, no hybrid-ARQ feedback is transmitted from these terminals.

17.3Paging

Paging is used for network-initiated connection setup. An efficient paging procedure should allow the terminal to sleep with no receiver processing most of the time and to briefly wake up at predefined time intervals to monitor paging information from the network.

In WCDMA, a separate paging-indicator channel, monitored at predefined time instants, is used to indicate to the terminal that paging information is transmitted. As the paging indicator is significantly shorter than the duration of the paging information, this approach minimizes the time the terminal is awake.

In LTE, no separate paging-indicator channel is used as the potential power savings are very small due to the short duration of the L1/L2 control signaling, at most three OFDM symbols as described in Chapter 16. Instead, the same mechanism as for ‘normal’ downlink data transmission on the DL-SCH is used and the mobile-terminal monitors the L1/L2 control signaling for downlink scheduling assignments. A DRX cycle is defined, which allows the terminal to sleep most of the time and only briefly wake up to monitor the L1/L2 control signaling. If the

DRX cycle

Figure 17.8 Discontinous reception (DRX) for paging.

terminal detects a group identity used for paging when it wakes up, it will process the corresponding paging message transmitted in the downlink. The paging message includes the identity of the terminal(s) being paged and a terminal not finding its identity will discard the received information and sleep according to the DRX cycle. Obviously, as the uplink timing is unknown during the DRX cycles, no ACK/NAK signaling can take place and consequently hybrid ARQ with soft combining cannot be used for paging messages.

The DRX cycle for paging is illustrated in Figure 17.8.

18.1Functional split between radio access network and core network

In the process of specifying the WCDMA/HSPA and the LTE/SAE systems, the first task in both cases was to distribute functions to the Radio Access Network (RAN) and Core Network (CN), respectively. Although this may initially appear to be a simple task, it can often turn out to be relatively complicated. The vast majority of the functions can easily be located in either RAN or the core network, there are some functions requiring careful attention.

18.1.1Functional split between WCDMA/HSPA radio access network and core network

For WCDMA/HSPA, the philosophy behind the functional split is to keep the core network unaware of the radio access technology and its layout. This means that the RAN should be in control of all functionality optimizing the radio interface and that the cells should be hidden from the core network. As a consequence, the core network can be used for any radio access technology that adopts the same functional split.

To find the origin of the philosophy behind the WCDMA/HSPA functional split, it is necessary to go back to the architecture of the GSM system, designed during the 1980s. One of the problems with the GSM architecture was that the core network nodes have full visibility of the cells in the system. Thus, when adding a cell to the system, the core network nodes need to be updated. For WCDMA/HSPA, the core network does not know the cells. Instead, the core network knows about service areas and the RAN translates service areas into cells. Thus, when adding a new cell in a service area, the core network does not need to be updated.

The second major difference compared to GSM is the location of retransmission protocols and data buffers in the core network for GSM. Since the retransmission protocols were optimized for the GSM radio interface, those protocols were radio interface specific and hence were not suitable for the WCDMA/HSPA radio interface. This was considered as a weakness of the core network and hence all the buffers and the retransmission protocols were moved to the RAN for WCDMA. Thus, as long as the radio access network uses the same interface to the core network, the Iu interface, the core network can be connected to radio access networks based on different radio access technologies.

Still, there are functional splits in WCDMA/HSPA that cannot solely be explained with the philosophy of making the core network radio-access-technology

independent. The security functions are a particularly good example. Again, the background can be traced back to GSM, which has the security functions located at different positions for circuit-switched connections and packet-switched connections. For circuit-switched connections, the security functions are located in the GSM RAN, whereas for packet-switched connections, the security functions are located in the GSM core network. For WCDMA/HSPA, this was considered too complicated and a common security location was desired. The location was decided to be in the RAN as the radio resource management signaling and control needed to be secure.

Thus the RAN functions of WCDMA/HSPA are:

•coding, interleaving, modulation, and other typical physical layer functions;

•ARQ, header compression, and other typical link layer functions;

•radio resource management, handover and other typical radio resource control functions; and

•security functions (that is ciphering and integrity protection).

Functions necessary for any mobile system, but not specific to a radio access network and that do not boost performance, was placed in the core network. Such functions are:

•charging;

•subscriber management;

•mobility management (that is keeping track of users roaming around in the network and in other networks);

•bearer management and quality-of-service handling;

•policy control of user data flows; and

•interconnection to external networks.

The reader interested in more details about the functional split is referred to the relevant 3GPP documents [89, 90].

18.1.2Functional split between LTE RAN and core network

The functional split of the LTE RAN and core network is similar to the WCDMA/HSPA functional split. However, a key design philosophy of the LTE RAN was to minimize the number of nodes and find a solution where the RAN consists of only one type of node. At the same time, the philosophy behind the LTE core network is, to the extent possible, be as independent of the radio access technology as possible. The resulting functional split is that most of the functions