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HSPA, which uses uplink macro-diversity and therefore defines both serving and non-serving cells (see Chapter 10), LTE only defines a serving cell as there is no uplink macro-diversity. The serving cell is the cell the mobile terminal is connected to and the cell that is responsible for scheduling and hybrid-ARQ operation.

15.2.1Logical channels and transport channels

The MAC offers services to the RLC in the form of logical channels. A logical channel is defined by the type of information it carries and are generally classified into control channels, used for transmission of control and configuration information necessary for operating an LTE system, and traffic channels, used for the user data. The set of logical-channel types specified for LTE includes:

•Broadcast Control Channel (BCCH), used for transmission of system control information from the network to all mobile terminals in a cell. Prior to accessing the system, a mobile terminal needs to read the information transmitted on the BCCH to find out how the system is configured, for example the bandwidth of the system.

•Paging Control Channel (PCCH), used for paging of mobile terminals whose location on cell level is not known to the network and the paging message therefore needs to be transmitted in multiple cells.

•Dedicated Control Channel (DCCH), used for transmission of control information to/from a mobile terminal. This channel is used for individual configuration of mobile terminals such as different handover messages.

•Multicast Control Channel (MCCH), used for transmission of control information required for reception of the MTCH, see below.

•Dedicated Traffic Channel (DTCH), used for transmission of user data to/from a mobile terminal. This is the logical channel type used for transmission of all uplink and non-MBMS downlink user data.

•Multicast Traffic Channel (MTCH), used for downlink transmission of MBMS services.

A similar logical-channel structure is used for WCDMA/HSPA. However, compared to WCDMA/HSPA, the LTE logical-channel structure is somewhat simplified, with a reduced number of logical-channel types.

From the physical layer, the MAC layer uses services in the form of Transport Channels. A transport channel is defined by how and with what characteristics the information is transmitted over the radio interface. Following the notation from HSPA, which has been inherited for LTE, data on a transport channel is organized into transport blocks. In each Transmission Time Interval (TTI), at most one transport block of a certain size is transmitted over the radio interface in absence

Figure 15.3 Example of mapping of logical channels to transport channels.

relation between the type of information and the way it should be transmitted, there are certain restrictions in the mapping of logical channels to transport channels. An example of mapping of logical channels to transport channels is given in Figure 15.3. Other mappings may also be envisioned.

15.2.2Downlink scheduling

One of the basic principles of the LTE radio access is shared-channel transmission on the DL-SCH and UL-SCH, that is, time-frequency resources are dynamically shared between users in both uplink and downlink. The scheduler is part of the MAC layer and controls the assignment of uplink and downlink resources. Uplink and downlink scheduling are separated in LTE and uplink and downlink scheduling decisions can be taken independently of each other (within the limits set by the UL/DL split in case of TDD operation). Uplink scheduling is discussed in Section 15.2.3, while the remainder of this section focuses on downlink scheduling.

The basic principle for the downlink scheduler is to dynamically determine, in each 1 ms interval, which terminal(s) that are supposed to receive DL-SCH transmission and on what resources. Multiple terminals can be scheduled in parallel, in which case there is one DL-SCH per scheduled terminal, each dynamically mapped to a (unique) set of frequency resources. The basic time-frequency unit in the scheduler is a so-called resource block. Resource blocks are described in more detail in Chapter 16 in conjunction with the mapping of data to physical resources, but in principle a resource block is a unit spanning 180 kHz in the frequency domain. In each 1 ms scheduling interval, the scheduler assigns resource blocks to a terminal for reception of DL-SCH transmission, an assignment used by the physical-layer processing as described in Chapter 16. The scheduler is also responsible for selecting the transport-block size, the modulation scheme, and the antenna mapping (in case of multi-antenna transmission). As a consequence of the scheduler controlling the data rate, the RLC segmentation and MAC multiplexing

will also be affected by the scheduling decision. The outputs from the downlink scheduler can be seen in Figure 15.1.

Although the scheduling strategy is implementation specific and not specified by 3GPP, the overall goal of most schedulers is to take advantage of the channel variations between mobile terminals and preferably schedule transmissions to a mobile terminal on resources with advantageous channel conditions. In this respect, operation of the LTE scheduler is in principle similar to the downlink scheduler in HSDPA. However, due to the use of OFDM as the downlink transmission scheme, LTE can exploit channel variations in both frequency and time domains, while HSDPA can only exploit time-domain variations. This was mentioned already in Chapter 14 and illustrated in Figure 14.1. For the larger bandwidths supported by LTE, where a significant amount of frequency-selective fading often will be experienced, the possibility for the scheduler to exploit also frequency-domain channel variations becomes increasingly important compared to exploiting time-domain variations only. Especially at low speeds, where the variations in the time domain are relatively slow compared to the delay requirements set by many services, the possibility to exploit also frequency-domain variations is beneficial.

Information about the downlink channel conditions, necessary for channeldependent scheduling, is fed back from the mobile terminal to the eNodeB via channel-quality reports. The channel-quality report, also known as ChannelQuality Indicator (CQI), includes information not only about the instantaneous channel quality in the frequency domain, but also information necessary to determine the appropriate antenna processing in case of spatial multiplexing. The basis for the CQI report is measurements on the downlink reference signals. However, additional sources of channel knowledge, for example channel reciprocity in case of TDD operation, can also be exploited by a particular scheduler implementation as a complement to the CQI reports.

In addition to the channel quality, a high-performance scheduler should also take buffer status and priorities into account in the scheduling decision. Both differences in the service type, as well as the subscription type, may affect the scheduling priority. For example, a voice-over-IP user with an expensive subscription should maintain its service quality even at high system loads, while a user downloading a file and having a low-cost subscription may have to be satisfied with the resources not required for supporting other users.

Interference coordination, which tries to control the inter-cell interference on a slow basis as mentioned in Chapter 14, is also part of the scheduler. As the scheduling strategy is not mandated by the specifications, the interference-coordination

scheme (if used) is vendor specific and may range from simple higher-order reuse deployments to more advanced schemes.

15.2.3Uplink scheduling

The basic function of the uplink scheduler is similar to the downlink, namely to dynamically determine, for each 1 ms interval, which mobile terminals are to transmit data on their UL-SCH and on which uplink resources. Uplink scheduling is used also for HSPA, but due to the different multiple-access schemes used, there are some significant differences between HSPA and LTE in this respect.

In HSPA, the shared uplink resource is primarily the acceptable interference at the base station as described in Chapter 10. The HSPA uplink scheduler only sets an upper limit of the amount of uplink interference the mobile terminal is allowed to generate. Based on this limit, the mobile terminal autonomously selects a suitable transport format. This strategy clearly makes sense for a non-orthogonal uplink as is the case for HSPA. A mobile terminal not utilizing all its granted resources will transmit at a lower power, thereby reducing the intra-cell interference. Hence, shared resources not utilized by one mobile terminal can be exploited by another mobile terminal through statistical multiplexing. Since the transport-format selection is located in the mobile terminal for the HSPA uplink, outband signaling is required to inform the NodeB about the selection made.

For LTE, the uplink is orthogonal and the shared resource controlled by the eNodeB scheduler is time-frequency resource units. An assigned resource not fully utilized by a mobile terminal cannot be partially utilized by another mobile terminal. Hence, due to the orthogonal uplink, there is significantly less gain in letting the mobile terminal select the transport format compared to HSPA. Consequently, in addition to assigning the time-frequency resources to the mobile terminal, the eNodeB scheduler is also responsible for controlling the transport format (payload size, modulation scheme) the mobile terminal shall use. As the scheduler knows the transport format the mobile terminal will use when it is transmitting, there is no need for outband control signaling from the mobile terminal to the eNodeB. This is beneficial from a coverage perspective taking into account that the cost per bit of transmitting outband control information can be significantly higher than the cost of data transmission as the control signaling needs to be received with a higher reliability.

Despite the fact that the eNodeB scheduler determines the transport format for the mobile terminal, it is important to point out that the uplink scheduling decision is taken per mobile terminal and not per radio bearer. Thus, although the eNodeB scheduler controls the payload of a scheduled mobile terminal, the terminal is

Figure 15.4 Transport format selection in downlink (left) and uplink (right).

still responsible for selecting from which radio bearer(s) the data is taken. Thus, the mobile terminal autonomously handles logical-channel multiplexing. This is illustrated in the right part of Figure 15.4, where the eNodeB scheduler controls the transport format and the mobile terminal controls the logical channel multiplexing. For comparison, the corresponding downlink situation, where the eNodeB controls both the transport format and the logical-channel multiplexing, is depicted to the left in the figure.

The radio-bearer multiplexing in the mobile terminal is done according to rules, the parameters of which can be configured by RRC signaling from the eNodeB. Each radio bearer is assigned a priority and a prioritized bit rate. The mobile terminal shall then perform the radio-bearer multiplexing such that the radio bearers are served in priority order up to their prioritized bit rate. Remaining resources, if any, after fulfilling the prioritized bit rate are given to the radio bearers in priority order.

To aid the uplink scheduler in its decisions, the mobile terminal can transmit scheduling information to the eNodeB using a MAC message. Obviously, this information can only be transmitted if the mobile terminal has been given a valid scheduling grant. For situations when this is not the case, an indicator that the mobile terminal needs uplink resources is provided as part of the uplink L1/L2 control-signaling structure, see further Chapter 16.

Channel-dependent scheduling is typically used for the downlink. In principle, this can be used also for the uplink. However, estimating the uplink channel quality is not as straightforward as is the case for the downlink. Downlink channel conditions can be measured by all mobile terminals in the cell by simply observing

the reference signals transmitted by the eNodeB and all mobile terminals can share the same reference signal for channel-quality-estimation purposes. Estimating the uplink channel quality, however, require a sounding reference signal transmitted from each mobile terminal for which the eNodeB wants to estimate the uplink channel quality. Such a sounding reference signal is supported by LTE and further described in Chapter 16, but comes at a cost in terms of overhead. Therefore, means to provide uplink diversity as a complement or alternative to uplink channeldependent scheduling are important.

15.2.4Hybrid ARQ

LTE hybrid ARQ with soft combining serves a similar purpose as the hybrid-ARQ mechanism for HSPA – to provide robustness against transmission errors. It is also a tool for enhanced capacity as discussed in Chapter 11. As hybrid-ARQ retransmissions are fast, many services allow for one or multiple retransmissions, thereby forming an implicit (closed loop) rate-control mechanism. In the same way as for HSPA, the hybrid-ARQ protocol is part of the MAC layer, while the soft-combining operation is handled by the physical layer.

Clearly, hybrid ARQ is not applicable for all types of traffic. For example, broadcast transmissions, where the same information is intended for multiple users, do typically not rely on hybrid ARQ. Hence, hybrid ARQ is only supported for the DL-SCH and the UL-SCH.

The LTE hybrid-ARQ protocol is similar to the corresponding protocol used for HSPA, that is multiple parallel stop-and-wait processes are used. Upon reception of a transport block, the receiver makes an attempt to decode the transport block and informs the transmitter about the outcome of the decoding operation through a single ACK/NAK bit indicating whether the decoding was successful or if a retransmission of the transport block is required. Further details on ACK/NAK transmission in uplink and downlink are found in Chapter 16. To minimize the overhead, a single ACK/NAK bit is used. Clearly, the receiver must know to which hybrid-ARQ process a received ACK/NAK bit is associated. Again, this is solved using the same approach as in HSPA as the timing of the ACK/NAK is used to associate the ACK/NAK with a certain hybrid-ARQ process. This is illustrated in Figure 15.6. Note that, in case of TDD operation, the time relation between the reception of data in a certain hybrid-ARQ process and the transmission of the ACK/NAK is also affected by the uplink/downlink allocation.

Similarly to HSPA, an asynchronous protocol is the basis for downlink hybridARQ operation. Hence, downlink retransmissions may occur at any time after the initial transmission and an explicit hybrid-ARQ process number is used to indicate

is de-multiplexed into the appropriate logical channels and reordering is done per logical channel using the sequence numbers. In contrast, HSPA uses a separate MAC sequence number for reordering. The reason for this is that HSPA is an add-on to WCDMA and for backwards compatibility reasons, the RLC or MAC architecture was kept unchanged when introducing HSPA as discussed in Chapter 9. For LTE, on the other hand, the protocol layers are all designed jointly, implying fewer restrictions in the design. The principle behind reordering is, however, similar for both systems, only the sequence number used differs.

The hybrid-ARQ mechanism will correct transmission errors due to noise or unpredictable channel variations. As discussed above, the RLC is also capable of requesting retransmissions, which at first sight may seem unnecessary. However, although RLC retransmissions seldom are necessary as the MAC-based hybrid ARQ mechanism is capable of correcting most transmission errors, the hybridARQ may occasionally fail to deliver error-free data blocks to the RLC, causing a gap in the sequence of error-free data blocks delivered to the RLC. This typically happens due to erroneous feedback signaling, for example, a NAK is incorrectly interpreted as an ACK by the transmitter, causing loss of data. The probability of this to happen can be in the order of 1%; an error probability far too high for TCP-based services requiring virtually error-free delivery of TCP packets. More specifically, for sustainable data rates exceeding 100 Mbit/s, a packet-loss probability lower than 10−5 is required [76]. Basically, TCP views all packet errors as being due to congestion. Packet errors therefore triggers the TCP congestion– avoidance mechanism, with a corresponding decrease in data rate, and to maintain good performance at high data rates, RLC-AM serves the important purpose of ensuring (almost) error-free data delivery to TCP.

Hence, from the above discussion, the reason for having two retransmission mechanisms on top of each other can be seen in the feedback signaling. As the hybrid-ARQ mechanism targets very fast retransmissions, it is necessary to send the one-bit ACK/NAK status report to the transmitter as fast as possible–once per TTI. Although it is in principle possible to attain an arbitrarily low error probability of the ACK/NAK feedback, very low error probabilities come at a relatively high cost in terms of ACK/NAK transmission power. Keeping this cost reasonable typically results in a feedback error rate of around 1% which thus determines the hybrid-ARQ residual error rate. As the RLC status reports are transmitted significantly less frequent than the hybrid-ARQ ACK/NAK, the cost of obtaining a reliability of 10−5 or lower is relatively small. Hence, the combination of hybrid ARQ and RLC attains a good combination of small roundtrip time and a modest feedback overhead where the two components complement each other.

Since the RLC and hybrid ARQ are located in the same node, tight interaction between the two is possible. For example, if the hybrid-ARQ mechanism detects