27.4.1 Mapping of Data onto (Logical) Subchannels
LTE provides logical channels (which are defined by the type of information that they carry), that are mapped totransport channels and from there tophysical channels(which are defined by their physical properties, i.e., time, subcarrier, etc.). The logical channels are similar to those in WCDMA, but repeated here in order to enable independent reading of the chapters:
• Traffic channels
◦ Dedicated Traffic CHannel (DTCH): it carries the user data for all ULs, as well as for those downlink data that are not multicast/broadcast.
◦ Multicast Traffic CHannel (MTCH): it carries the user data for multicast/broadcast downlink transmission.
• Control channels
3GPP Long-Term Evolution 685
◦ Broadcast Control CHannel (BCCH): it carries system information data that are broadcast to the MSs in a cell. Note the difference from the MTCH, which also broadcasts to MSs, but carries user data.
◦ Paging Control CHannel (PCCH): it pages MSs in multiple cells (i.e., when it is not known exactly in which cell the MS currently is located).
◦ Common Control CHannel (CCCH): it transmits control data for the Random Access (RA), i.e., when a connection is started.
◦ Dedicated Control CHannel (DCCH): it is used for the transmission of control information that relates to a specific MS (as opposed to the system information relevant for all MSs, which is broadcast in the BCCH).
◦ Multicast Control CHannel (MCCH): it carries the control information related to multicast/broadcast services.
These channels are mapped onto the following transport channels:
• Broadcast Channel (BCH): it carries part of the BCCH (the remainder is on the DL-SCH described below). It has a fixed format, so that any MS can listen to it easily.
• Paging Channel (PCH): it carries the PCCH.
• Multicast Channel (MCH): it is used to support broadcast/multicast transmission. It has a semistatic scheduling and transport format.
• DownLink Shared Channel (DL-SCH) and UpLink Shared Channel (UL-SCH): they carry the user data, as well as most of the control information (except the one already mentioned above).
The data on transport channels are organized into transport blocks; in each transmission time interval (usually a subframe), one transport block is transmitted. A transport format is associated with each transport block.
Finally, these transport channels are mapped onto physical channels; there are also physical channels that do not carry any transport channel, but are purely used for PHY functionality.
• Downlink
◦ Physical Broadcast CHannel (PBCH): it carries the BCH.
◦ Physical Downlink Shared CHannel (PDSCH): it carries the DL-SCH, i.e., user data, some control data for the downlink, as well as the PCH.
◦ Physical Multicast CHannel (PMCH): it carries the MCH, which contains the multicast pay- load, as well as some of the control information for multicast.
◦ Physical Downlink Control CHannel (PDCCH): it carries control information, such as scheduling that is required for reception of the PDSCH. This channel does not carry any transport channel.
◦ Physical Control Format Indicator CHannel (PCFICH): it carries control information about the PDCCH. This channel does not carry any transport channel.
◦ Physical HARQ Indicator CHannel (PHICH): it carries the feedback bits indicating whether a retransmission of transport blocks is necessary. This channel does not carry any transport channel.
◦ Synchronization Signal (SS).
• Uplink
◦ Physical Uplink Shared Channel (PUSCH): it is the uplink counterpart to the PDSCH.
◦ Physical Uplink Control Channel (PUCCH): it carries mainly three types of information: (i) channel state feedback, (ii) resource requests (remember that the BS performs the scheduling, i.e., assigns all the resources also for the uplink; thus the MS must request resources when it has data to transmit), and (iii) HARQ feedback bits.
PCCCH BCCH CCCH DCCH DTCH MCCH MTCH
CCCH DCCH DTCH
ULSCH RACH
PUSCH PRACH
PUCCH PCCH
BCH
DLSCH
MCH
PBCH
PDSCH
PMCH
PDCCH PHICH PCFICH Logical Transport Physical
Downlink Channels
Logical Transport Physical Uplink Channels
Figure 27.14 Mapping between logical, transport, and physical channels.
◦ Physical Random Access CHannel (PRACH): it is used for the random access, i.e., MS com- municating to the BS before a connection with scheduling has been established.
Figure 27.14 summarizes the mapping between the channels.
27.4.2 Synchronization Signals
TheSScarries information about the timing of the cell, as well as thecell ID. LTE actually provides two SSs, the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). In contrast to other systems, these signals are not called “channels,” but perform similar functions as, e.g., the synchronization channels in WCDMA. To understand the functionality of the SS, keep in mind that there are 504 cell IDs defined for LTE, which are divided into 168 ID groups.
The PSS is transmitted in the last symbol of the first slot of subframes 0 and 5 of every frame, extending over 72 subcarriers. The waveform transmitted during that slot is one of three allowed Zadoff–Chu sequences of length 63, extended with 5 zeroes at the lower and 5 zeroes at the upper band edge. Which of the three sequences is transmitted depends on the cell ID within a group (but note that the PSS does not provide the group ID; that is transmitted in the SSS). The MS can obtain the fine-resolution timing from the PSS (within one symbol, because the signal lasts only one symbol, and even within one OFDM sample, by correlation with the Zadoff–Chu sequence).
However, there is still a timing ambiguity by multiples of 5 ms, i.e., the periodicity of the PSS signal; therefore, frame timing is not available.
The SSS signal is transmitted in the symbol directly before every PSS signal. The SSS carries information about the cell ID group: the signal also extends over 72 subcarriers. It is an interleaved combination of twom-sequences (see Section 18.2.6) of length 31. The resulting length-62 sequence is extended with zeroes at the band edges, just like for the PSS. Only 168 sequences are valid, and signify the cell ID group. In contrast to PSS, the signal transmitted in the first slot of a frame is different from the signal in the second slot: while it contains the samem-sequences, the interleaving in the second slot is different from that in the first slot. This allows the RX to acquire frame timing (i.e., ambiguity of timing is multiples of 10 ms), as well as the cell ID, from a single observation of an SSS.
3GPP Long-Term Evolution 687
The reason that both PSS and SSS use 72 subcarriers is that this is the smallest bandwidth permissible for an LTE system. At the time of the reception of those signals the MS does not know yet the actual system bandwidth in the considered cell; the SSs thus use the same bandwidth for all cells, namely the smallest possible.
Once a signal has acquired cell ID and frame timing, it can receive the pilot (remember downlink pilots depend on the cell ID), which are needed for the reception and decoding of the BCH.
27.4.3 Broadcast Channel
The BCH contains theMaster Information Block (MIB), which contains critical information about the cell system information:
• System bandwidth in the cell.
• PHICH configuration: some bits describing the specific configuration of the PHICH channel (for details see the standard). Knowledge of this configuration is necessary to receive the control information.
• System frame number.
The baseband processing and transmission of the MIB bits on the BCH proceed in the follow- ing steps:
• Addition of a 16-bit CRC.
• Encoding with tail-biting rate 1/3 convolutional code.
• Scrambling.
• QPSK modulation.
• Antenna mapping: this is trivial if only a single antenna is used. If the BS has two antennas, then space-frequency block codes must be used. If the BS has four antennas, then the combination of antenna hopping and space-frequency block codes (see Section 27.3.7)must be used. This is done so that the MS can learn from the BCH how many antennas the BS has.
• Demultiplexing: the signal is mapped onto four consecutive frames; particularly onto the first subframe of each of those four frames. Within each such subframe, the signal is transmitted on the first 4 symbols of the second slot, over 72 subcarriers (the choice of 72 subcarriers is motivated the same way as for the PSS and SSS). Repetition coding is used to “fill up” this available resource, which is much larger than the number of bits that need to be transmitted.
Note that the BCH extends over 40 ms, while the timing acquired from the PSS and SSS has an ambiguity of multiples of 10 ms. The MS must therefore try to decode the BCH with four different timing shifts, and determine from the CRC which is the correct one. This decoding also provides the two least significant bits of the system frame number; for this reason, those two bits are not included in the MIB.
27.4.4 General Aspects of Control Channels Associated with a DL-SCH
There are three physical channels, namely PCFICH, PDCCH, and PHICH, associated with the DL- SCH. The control information contained by these channels is also called L1/L2 signaling, because it is relevant for both the PHY and the MAC layer. It is transmitted in thecontrol regionlocated at the beginning of each subframe; this control region occupies all the subcarriers (with the exception of the pilots) of the signal, and either (i) the first, (ii) the first two, or (iii) the first three OFDM
symbols (or four in the case of very narrow bandwidth); the amount of occupied OFDM symbols can be changed from one subframe to the next. The reason for transmitting control information at the beginning of a subframe is that information contained in it is required for the decoding of the user data; in particular, if an MS sees from the scheduling information that none of the subsequent user data are intended for it, the MS can power down part of the receive circuitry.
The mapping of the control channels to particular REs is done in units of RE groups which consist of four REs each. This is done because the BS has up to 4 antenna elements that can be used for transmit diversity (note that spatial multiplexing is not used for control information).
27.4.5 Physical Control Format Indicator CHannel
The PCFICH carries the information of how many OFDM symbols are dedicated to the control region. Since the control regions can comprise up to three symbols, this information requires 2 bits.
The reason for having a dynamical choice of the control region size is that the usage in a cell can vary dramatically. Sometimes, resources are taken up by a few high-data-rate users (in which case the control region, whose size is mainly determined by the resource allocation information, can be shorter). At other times, there are a lot of users (e.g., when most users voice users), which requires a longer control region.
If the PCFICH information is decoded incorrectly, all the remainder of a subframe is interpreted incorrectly. Therefore, the information uses a strong error correction, namely a rate 1/16 block code.
The encoded bits are then scrambled with a scrambling sequence that depends on the subframe number and the cell ID, and thus reduces the effect of interference. Since the information is needed for the interpretation of the remainder of the subframe, it is always at the same location and modulation format (QPSK). In order to enhance frequency diversity, the 16 QPSK symbols are mapped onto 4 RE groups that are each spaced 1/4 of the available bandwidth apart; the starting subcarrier depends on the cell ID.
27.4.6 Physical HARQ Indicator CHannel
The PHICH carries the feedback information for the HARQ. Groups of eight feedback bits are formed in the following way: first, each feedback bit is repeated three times (repetition coding), and Binary Phase Shift Keying (BPSK) modulated. The resulting waveforms are multiplexed through a combination of In-phase – Quadrature phase (IQ) multiplexing and code multiplexing with length-4 orthogonal spreading sequences. The scrambled output of the multiplexer is mapped to three RE groups. The PHICH group number, IQ branch, and spreading sequence are related to the index of the first RB on which the data block that is to be acknowledged was originally transmitted.
27.4.7 Physical Downlink Control CHannel
The PDCCH occupies the largest part of the control zone. For each of the users, it carries the DCI, i.e., information that is required for decoding the payload information. In particular, it car- ries information about the resource allocation, i.e., which RBs in the data region are assigned to which MS (see also Section 27.3.3). The PDCCH also carries, for each user, the transport format, i.e., the modulation and coding scheme, power control information, and control information for spatial multiplexing.
There are seven different formats for encoding the DCI, reflecting the tradeoff between size of the control information and performance. They are as follows:
3GPP Long-Term Evolution 689
• Format 1: the fundamental DCI format, using Type 0 or 1 resource allocation notification. It does not assume multiple antennas at the BS.
• Format 1A: similar to Format 1, but more compact, since it employs Type 2 resource allocation notification.
• Format 1B: like Format 1A, but intended when the BS uses precoding for multiple antenna elements.
• Format 1C: extremely compact format with fixed modulation (QPSK); to be used only for special system messages.
• Format 1D: like format 1B, but with an additional power offset message.
• Format 2: using Type 0 or 1 resource allocation notification, this format is intended for systems with closed-loop spatial multiplexing.
• Format 2A: like Format 2, but for systems with open-loop spatial multiplexing.
In addition to the resource allocation information, the formats also contain some additional information (see also Table 27.4):
• Modulation/coding: those 5 bits indicate the modulation scheme and code rate used in the block. Of the possible 32 combinations, 29 are used to actually indicate combinations of mod- ulation format and code rate; the remaining 3 can be used to indicate modulation format during retransmission.
• HARQ process number: this 3-bit message is the index of the HARQ process associated with the DCI. As explained in Section 27.5.2, several HARQ processes are active simultaneously.
• New data indicator: it indicates whether the data are a retransmission and thus need to be combined with previously received soft information for HARQ.
• Redundancy version: it indicates the type of redundancy used in the HARQ transmission.
• Flag for 1A/0 differentiation: this is a single bit indicating whether Format 1A or Format 0 (which is used for uplink scheduling grants, see below) is used.
• Localized/distributed VRB: flag indicating whether localized or distributed VRBs are used.
• Gap value for VRB: the spacing between the two parts of the VRB.
• Transport blocksize index: only used in the 1C Format, it indicates the size of the data block to be transmitted. Note that in the other formats, the number of transport blocks follows Table 27.4 DCI in various DCI message formats
Field 1 1A 1B 1C 1D 2 2A
Modulation/coding (2) (2)
HARQ process number
New data indicator (2) (2)
Redundancy version (2) (2)
TPC command for PUCCH
Flag for 1A/0 differentiation
Localized/distributed VRB
Gap value for VRB
Transport blocksize index
TPMI for precoding
Downlink power offset
Downlink assignment index
Transport block to codeword swap flag
Open-loop precoding info
TPMI, Transmitted Precoding Matrix Indicator.
implicitly from the number of allocated RBs and the code rate (the standard contains a table providing this mapping, since there are minor deviations from the nominal rates obtained by closed-form computations).
• Precoding Matrix Indicator (PMI) for precoding: the bits in this field contain the index of the precoding codebook for closed-loop multiple-antenna systems (see Section 27.3.7.).
• DL assignment index: it is related to the HARQ operation in the TDD mode; for details see the standard.
• Transport block to codeword swap flag: it indicates how the two transport blocks (in spatial multiplexing system) are mapped to the codewords.
• Open-loop precoding info: this field indicates the type of open-loop precoding (e.g., details of the cyclic delay diversity) used in the transmission with multiple-antenna systems.
Uplink Control Information
Also, control information related to the uplink is carried in the PDCCH. Remember that the control of all transmission parameters rests with the BS, while the MS can only make requests. Thus, the PDCCH carries uplink scheduling grants, which tell the MS when, and with which format, to transmit. Just like for the downlink, that includes information on which RBs to transmit, the transport format, power control, and spatial multiplexing information. This information for the uplink is transported in message of Format 0, which employs Type 2 resource allocation format.6 Finally, Format 3 is used for the transmission of power control commands for PUCCH and PUSCH with 2-bit power adjustments.
Once all the control information is assembled, the actual transmit signal is obtained in the following way:
1. A CRC is computed. The CRC depends on the identification number of the MS for which the DCI is intended. Thus, each MS determines whether information is intended for it by checking the CRC: if the CRC checks, then the MS concludes that (i) the DCI is intended for it and (ii) was decoded correctly.
2. The CRC-protected DCI is encoded with a rate 1/3 tail-biting convolutional code (i.e., a convo- lutional code that ensures that starting state and end state in the trellis are identical).
3. After rate matching, multiple PDCCHs are multiplexed.
4. The resulting data are scrambled by a PN sequence (Gold sequence, see Section 18.2.6), and QPSK modulated. The QPSK symbols are grouped into groups of four symbols each. In contrast to the PCFICH, these symbols are not directly mapped onto RE groups, but first interleaved, and then subjected to a cell-specific cyclic shift. The purpose of these measures is to ensure (i) full exploitation of the frequency diversity, and (ii) avoiding consistent collisions between the same signals in neighboring cells.
27.4.8 Physical Random Access CHannel
The Random Access CHannel (RACH) is intended for signals from MSs that do not yet have resources assigned to them. Even though the MS knows the cell bandwidth by the time it uses the PRACH, it still eases the implementation to have the same PRACH in all systems. Therefore, the PRACH is transmitted on 72 subcarriers (just like the SS). In the time domain, the BS usually
6In addition to that allocation, Format 0 also contains the Flag for 1A/0 differentiation, a Hopping Flag (indicating whether frequency hopping is to be used), modulation/coding and redundancy version, new data indicator, Transmit Power Control (TPC) commands for the scheduled PUSCH, cyclic Shift for the demodulation pilot, Channel Quality Indicator (CQI) request (i.e., a request from the BS to the MS to provide it with the channel quality information), and a UL index employed only in the TDD mode.
3GPP Long-Term Evolution 691
reserves 1-ms long blocks for the PRACH during which no uplink payload data are scheduled, so that there is no possibility of collision. The periodicity of the PRACH intervals can be configured, e.g., depending on how many possible users are in the cell, and what the latency requirements for random access are.
The most common configuration, a 1-ms PRACH, consists of: (i) 0.1-ms cyclic prefix, (ii) 0.8-ms long Zadoff–Chu sequence, and (iii) 0.1-ms guard interval. Note the requirement for long guard intervals because when the PRACH is used, the necessary timing advance (see Section 24.5.3) has not yet been established. There are 64 different sequences that can be used in the “main part” of the PRACH: either distinguished by phase shifts or sequences with different indexu(see Section 27.3.5). The advantage of using different phase shifts is that those sequences are truly orthogonal; however, the amount of phase shift is lower bounded by the runtime and delay dispersion of the signal in the cell. If the cell size is below 1.5 km, all 64 sequences can be produced by phase shifting of one basic sequence; otherwise, sequences with different index have to be used. Different MSs pick, at random, the sequence that they want to use for the PRACH.
Due to the large number of sequences, it is unlikely (though not impossible) that two MSs pick the same sequence and try to use it in the same timeslot. If that occurs, later stages of the random access procedure resolve this collision (see Section 27.5.1).
The PRACH channel also foresees power ramping. In other words, if the first attempt at reaching the BS is not successful, the transmit power of the random access preamble is increased, and transmission is repeated. This approach is similar to the one in WCDMA, though it is not as important: usually a random access preamble is orthogonal to other random access preambles (due to the different sequences being used) and payload data (due to the time/frequency resource being dedicated exclusively to the PRACH).
27.4.9 General Aspects of Control Signals Associated with PUSCH
In LTE, there is little information that needs to be signaled in the uplink, because the BS has central control of almost all transmission parameters. The few pieces of information that need to be sent in uplink direction are as follows:
• Scheduling requests: indications that the MS wants to send data; the actual resource assignment is then sent by the BS in the DCI.
• Channel state information: this is essential for the BS to determine the appropriate transmission format and scheduling. The channel state information contains the following pieces:
◦ Rank Indicator (RI): this is the rank of the channel matrix, which in turn determines the maximum number of layers that can be transmitted in a meaningful way (see Chapter 20).
Relevant for multiantenna systems only.
◦ PMI: this is the index of the codebook entry that should be used for precoding by the BS (see Section 27.3.7). Due to the frequency selectivity of the channel a different setting could be optimal for different RBs. The standard allows to transmit either a different setting for each RB or a setting for a group of RBs; in the extreme case of “wideband” feedback, there is one setting for the whole available bandwidth. Clearly, the smaller the groups for which the settings are transmitted, the better the performance, but also the larger the overhead. The PMI is relevant for multiantenna systems only.
◦ CQI: this index actually represents the modulation and coding scheme that should be used.
This is information about the downlink direction, which is intended to help the BS in its task.
While the BS can choose to ignore the recommendation and use a different precoder setting, it then has to signal explicitly to the MS which setting it is using.
• HARQ ACKnowledgements (ACKs).