24.4.1 Multiple Access Schemes

3GPP LTE/LTE-Advanced uses asymmetric multiple access schemes in the downlink and uplink. The multiple access scheme for the 3GPP LTE/LTE-Advanced physical layer is based on Orthogonal Frequency Division Multiple Access (OFDMA) with a Cyclic Prefix (CP) in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) with CP in the uplink.

The OFDMA scheme is particularly suited for frequency-selective channels and high data rates. It transforms a wideband frequency selective channel into a set of parallel flat fading narrowband channels. This ideally, allows the receiver to perform a less complex equalization process in frequency domain; that is, single-tap frequency-domain equalization.

24.4.2 Operating Frequencies and Bandwidths

The E-UTRA is designed to operate in the frequency bands defined in Table 24.2. The requirements were defined for 1.4, 3, 5, 10, 15, and 20 MHz bandwidth with a specific configuration in terms of number of resource blocks (see Table 24.2). Using carrier aggregation, a number of contiguous and/or noncontiguous frequency bands can be aggregated to create a virtually larger bandwidth.

Figure 24.4 illustrates the relationship between the total channel bandwidth and the transmission bandwidth; that is, the number of resource blocks. The 3GPP LTE/LTE-Advanced channel raster is 100 kHz, which means the center frequency must be a multiple of 100 kHz [14]. To support transmission in paired and unpaired spectrum, two duplexing schemes are supported: Frequency Division Duplex (FDD), allowing both full and half-duplex terminal operation, as well as Time Division Duplex (TDD). Table 24.2 shows the band classes where the 3GPP LTE systems can be deployed.

24.4.3 Frame Structure

Downlink and uplink transmissions are organized in form of radio frames with 10 ms duration. 3GPP LTE/LTE-Advanced supports two radio frame structures, Type 1, applicable to FDD duplex scheme and Type 2, applicable to TDD duplex scheme. In frame structure Type 1, as illustrated in Figure 24.5, each 10 ms radio frame is divided into 10 equally sized subframes. Each subframe consists of two equally sized slots. For FDD, 10 subframes are available for downlink transmission and 10 subframes are available for uplink transmissions in each radio frame. Uplink and downlink transmissions are separated in the frequency domain. The transmission time interval (TTI) of the downlink/uplink is 1 ms [10,6].

Frame structure Type 2 is illustrated in Figure 24.6. Each 10 ms radio frame consists of two 5 ms half-frames. Each half-frame consists of eight slots of 0.5 ms length and three special fields: Downlink Pilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot (UpPTS). The length of DwPTS and UpPTS is configurable subject to the total length of DwPTS, GP, and UpPTS being equal to 1 ms (see [10] for more details). Both 5 ms and 10 ms switching-point periodicity are supported. The first subframe in all configurations and the sixth subframe in configuration with 5 ms switching-point periodicity consist of DwPTS, GP, and UpPTS. The sixth subframe in configuration with 10 ms switching-point periodicity consists of DwPTS only. All other subframes consist of two equally sized slots.

TABLE 24.2 3GPP LTE/LTE-Advanced Band Classes

For TDD systems, the GP is reserved for downlink to uplink transition. Other subframes/fields are assigned for either downlink or uplink transmission as shown in Table 24.3. Uplink and downlink transmissions are separated in the time domain.

24.4.4 Physical Resource Blocks

The smallest time–frequency resource unit used for downlink/uplink transmission is called a resource element, defined as one subcarrier over one symbol [10]. For both TDD and FDD duplex schemes as well as in both downlink and uplink, a group of 12 subcarriers contiguous in frequency over one slot in time form a Resource Block (RB) as shown in Figure 24.7 (corresponding to one slot in the time domain and 180 kHz in the frequency domain). Transmissions are allocated in units of RB. One downlink/uplink slot using the normal CP length contains 7 symbols. There are 6, 15, 25, 50, 75, and 100 RBs corresponding to 1.4, 3, 5, 10, 15, 20 MHz channel bandwidths, respectively [10]. Note that the physical resource block size is the same for all bandwidths.

TABLE 24.3 Uplink/Downlink Configurations in Frame Structure Type 2 where “S” denotes the Special Subframe

24.4.5 Modulation and Coding

The baseband modulation schemes supported in the downlink and uplink of 3GPP LTE/LTE-Advanced are QPSK, 16QAM and 64QAM. The channel coding scheme for transport blocks in 3GPP LTE is Turbo Coding similar to UTRA, with the minimum coding rate of R = 1/3, two 8-state constituent encoders and a contention-free Quadratic Permutation Polynomial (QPP) turbo internal interleaver [11]. Trellis termination is performed by taking the tail bits from the shift register feedback after all information bits are encoded. The tail bits are padded after the encoding of information bits. Before the turbo coding, transport blocks are segmented into octet-aligned segments with a maximum information block size of 6144 bits. Error detection is supported by the use of 24 bit CRC. The coding and modulation schemes for various physical channels in the downlink and uplink of 3GPP LTE are shown in Table 24.4 [15,16].

24.4.6 Physical Channel Processing

Figure 24.7 illustrates different stages of 3GPP LTE physical channel processing in the downlink and uplink. In the downlink, the coded bits in each of the codewords are scrambled for transmission on a physical channel. The scrambled bits are modulated to generate complex-valued modulation symbols that are later mapped to one or several transmission layers. The complex-valued modulation symbols on each layer are precoded for transmission and are further mapped to resource elements for each antenna port. The complex-valued time-domain OFDMA signal for each antenna port is then generated following these stages [10].

In the uplink, the baseband signal is processed by scrambling the input coded bits and then by modulation of scrambled bits to generate complex-valued symbols. The complex-valued modulation symbols are transform-precoded (DFT-based precoding) and are later mapped to resource elements. The complex-valued time-domain SC-FDMA signal for each antenna port is then generated.

24.4.7 Reference Signals

In 3GPP LTE Release 8/9, four types of downlink reference signals are defined, cell-specific reference signals, associated with non-MBSFN transmission, MBSFN reference signals, associated with MBSFN transmission, positioning reference signals, and demodulation or UE-specific reference signals. There is one cell-specific reference signal transmitted per downlink antenna port [10]. Note that antenna port is a logical entity and shall not be confused with the physical transmit antennas, although there is a mapping between the two entities during operation in various transmission modes (see Figure 24.8). The cell-specific downlink reference signals consist of predetermined reference symbols that are inserted in the time-frequency grid over each subframe and are used for downlink channel estimation [10,6] and physical layer identifier (cell identifier) detection. The exact sequence is derived from cell identifiers. The number of downlink antenna ports with cell-specific reference signals equals 1, 2, or 4. The reference signal sequence is derived from a pseudo-random sequence and results in a QPSK type constellation. Cell-specific frequency shifts are applied when mapping the reference signal sequence to the sub-carriers. The two-dimensional reference signal sequence is generated as the symbol-by-symbol product of a two-dimensional orthogonal sequence and a two-dimensional pseudo-random sequence. There are 3 different two-dimensional orthogonal sequences and 168 different two-dimensional pseudo-random sequences. Each cell identity corresponds to a unique combination of one orthogonal sequence and one pseudo-random sequence, thus allowing for 504 unique cell identities (i.e., 168 cell identity groups with 3 cell identities in each group).

TABLE 24.4 Physical Channels and Signals and Their Corresponding Modulation Schemes for the 3GPP LTE Downlink and Uplink

The downlink MBSFN reference signals consist of known reference symbols inserted every other sub-carrier in the 3^rd, 7^th, and 11^th OFDM symbols of subframe in case of 15 kHz subcarrier spacing and extended cyclic prefix. The MBSFN reference signals are transmitted on antenna port 4.

The UE-specific reference signals are supported for single-antenna-port transmission of PDSCH and transmitted on antenna port 5, 7, or 8. The UE-specific reference signals are also supported for spatial multiplexing on antenna ports 7 and 8. These reference signals are utilized for PDSCH coherent demodulation, if the PDSCH transmission is associated with the corresponding antenna port. They are transmitted only on the resource blocks to which the corresponding PDSCH is mapped. The UE-specific reference signals are not transmitted in resource elements where one of the physical channels or physical signals other than UE-specific reference signal are transmitted using resource elements with the same index pair regardless of their antenna port.

Positioning reference signals are only transmitted in downlink subframes configured for positioning reference signal transmission. If both normal and MBSFN subframes are configured as positioning subframes within a cell, the OFDM symbols in an MBSFN subframe configured for positioning reference signal transmission use the same cyclic prefix as used for the first subframe. Otherwise, if only MBSFN subframes are configured as positioning subframes within a cell, the OFDM symbols configured for positioning reference signals in these subframes use the extended cyclic prefix. The positioning reference signals are transmitted on antenna port 6 and are not mapped to resource elements allocated to physical broadcast, primary and secondary synchronization channels, regardless of their antenna port.

In 3GPP Release 10, a new set of reference signals called Channel State Information-Reference Signals (CSI-RS) were added which are used to support reporting of channel-state information from the user terminals to the network. The CSI reference signals are sparse in the time and frequency and transmitted at a configurable periodicity. Figure 24.9 shows the cell-specific reference signals corresponding to antenna ports 0 through 3 in the downlink. The physical resources at the locations of the reference signals of another antenna port are not used for data transmission on the other antenna ports.

The uplink reference signals, used for channel estimation for coherent demodulation, are transmitted in the 4^th SC-FDMA symbol in each slot assuming normal CP. 3GPP LTE uses UE-specific reference signals in the uplink. The uplink reference signal sequence length equals the size (number of subcarriers) of the assigned resource. The uplink reference signals are based on Zadoff–Chu sequences that are either truncated or cyclically extended to the desired length. There are two types of uplink reference signals (1) the demodulation reference signal is used for channel estimation in the eNB receiver in order to demodulate control and data channels. It is located on the 4^th symbol in each slot (for normal cyclic prefix) and spans the same bandwidth as the allocated uplink data, and (2) the sounding reference signal provides uplink channel quality information as a basis for scheduling decisions in the base station (see Figure 24.10). The UE sends a sounding reference signal in different parts of the bandwidths where no uplink data transmission is available. The sounding reference signal is transmitted in the last symbol of the subframe. The configuration of the sounding signal; for example, bandwidth, duration, and periodicity are configured by higher layers.

24.4.8 Physical Control Channels

The Physical Downlink Control Channel (PDCCH) is primarily used to carry scheduling information to individual UEs; that is, resource assignments for uplink and downlink data and control information. As shown in Figure 24.11, the PDCCH is located in the first few OFDM symbols of a subframe. For frame structure Type 2, PDCCH can also be mapped to the first two OFDM symbols of the DwPTS field. An additional Physical Control Format Indicator Channel (PCFICH), located on specific resource elements in the first OFDM symbol of the subframe, is used to indicate the number of OFDM symbols occupied by the PDCCH (1, 2, 3, or 4 OFDM symbols may be consumed where 4-OFDM-symbol PDCCH is only used when operating in the minimum supported system bandwidth). Depending on the number of users in a cell and the signaling formats conveyed on PDCCH, the size of PDCCH may vary. The information carried in PDCCH is referred to as Downlink Control Information (DCI), where depending on the purpose of the control message, different DCI formats are defined [15,16]. This information enables the UE to identify the location and size of the resources in that subframe, as well as provide the UE with information on the modulation and coding scheme, and HARQ operation. As mentioned earlier, the information fields in the downlink scheduling grant are used to convey the information needed to demodulate the downlink shared channel. They include resource allocation information such as resource block size and duration of assignment, transmission format such as multiantenna mode, modulation scheme, payload size, and HARQ operational parameters such as process number, redundancy version, and new data indicator. Similar information is also included in the uplink scheduling grants.

Control channels are formed by aggregation of Control Channel Elements (CCE), each control channel element consists of a set of resource elements. Different code rates for the control channels are realized by aggregating different numbers of control channel elements. QPSK modulation is used for all control channels. There is an implicit relationship between the uplink resources used for dynamically scheduled data transmission, or the downlink control channel used for assignment, and the downlink ACK/NACK resource used for feedback.

The Physical Uplink Control Channel (PUCCH) is mapped to a control channel resource in the uplink as shown in Figure 24.12. A control channel resource is defined by a code and two resource blocks, consecutive in time, with hopping at the slot boundary. Depending on presence or absence of uplink timing synchronization, the uplink physical control signaling can differ. In the case of time synchronization being present, the out-of-band control signaling consists of CQI, ACK/NACK, and Scheduling Request (SR). Note that a UE only uses PUCCH when it does not have any data to transmit on PUSCH. If a UE has data to transmit on PUSCH, it would multiplex the control information with data on PUSCH.

By use of uplink frequency hopping on PUSCH, frequency diversity effects can be exploited and interference can be averaged. The UE derives the uplink resource allocation as well as frequency hopping information from the uplink scheduling grant. The downlink control information format 0 is used on PDCCH to convey the uplink scheduling grant. 3GPP LTE supports both intrasubframe and intersubframe frequency hopping. The hopping pattern is configured per cell by higher layers. In intrasubframe hopping, the UE hops to another frequency allocation from one slot to another within one subframe. In intersubframe hopping, the frequency resource allocation changes from one subframe to another.

The CQI reports are provided to the scheduler by the UE, measuring the current channel conditions. If MIMO transmission is used, the CQI includes necessary MIMO-related feedback.

The HARQ feedback in response to downlink data transmission consists of a single ACK/NACK bit per HARQ process. The PUCCH resources for SR and CQI reporting are assigned and can be revoked through RRC signaling. An SR is not necessarily assigned to UEs acquiring synchronization through the RACH (i.e., synchronized UEs may or may not have a dedicated SR channel). The PUCCH resources for SR and CQI are lost when the UE is no longer synchronized.

In 3GPP LTE, uplink control signaling includes ACK/NACK, CQI, scheduling request indicator, and MIMO feedback. When users have simultaneous uplink data and control transmission, control signaling is multiplexed with data prior to the DFT to preserve the single-carrier property in uplink transmission. In the absence of uplink data transmission, this control signaling is transmitted in a reserved frequency region on the band edge as shown in Figure 24.12. Note that additional control regions may be defined as needed. Allocation of control channels with their small occupied bandwidth to band edge resource blocks reduces out-of-band emissions caused by data resource allocations on innerband resource blocks and maximizes the frequency diversity for control channel allocations while preserving the single-carrier property of the uplink waveform.

24.4.9 Physical Random Access Channel

The physical layer random access preamble, illustrated in Figure 24.13 consists of a cyclic prefix of length T_CP and a sequence part of length T_SEQ. There are five random access preamble formats specified by the standard [10] where the parameter values depend on the frame structure and the random access configuration. The preamble format is configured through higher-layer signaling. Each random access preamble occupies a bandwidth corresponding to 6 consecutive resource blocks for both frame structures. The transmission of a random access preamble, if triggered by the MAC sublayer, is restricted to certain time and frequency resources. These resources are enumerated in increasing order of the subframe number within the radio frame and the physical resource blocks in the frequency domain such that index zero corresponds to the lowest numbered physical resource block and subframe within the radio frame.

The random access preambles are generated from Zadoff–Chu sequences with zero correlation zone. The network configures the set of preamble sequences the UE is allowed to use. There are 64 preambles available in each cell. The set of 64 preamble sequences 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 index RACH_ROOT_SEQUENCE, where RACH_ROOT_SEQUENCE is broadcasted as part of the System Information [10,15,16].

The random access procedure in 3GPP LTE consists of 4 steps. In step 1, the preamble is sent by UE. The time/frequency resource where the preamble is sent is associated with a Random Access Radio Network Temporary Identifier (RA-RNTI). In step 2, a random access response is generated by eNB and is sent on the downlink shared channel. It is addressed to the UE using the temporary identifier and contains a timing advance value, an uplink grant, and a temporary identifier. Note that eNB may generate multiple random access responses for different UEs which can be concatenated inside one MAC protocol data unit. The preamble identifier is contained in the MAC subheader of each random access response so that the UE can detect whether a random access response for the used preamble exists. In Step 3, UE will send an RRC CONNECTION REQUEST message on the uplink common control channel, based on the uplink grant received in the previous step. In Step 4, the eNB sends back a MAC PDU containing the uplink CCCH service data unit that was received in Step 3. The message is sent on downlink shared channel and addressed to the UE via the temporary identifier. When the received message matches the one sent in Step 3, the contention resolution is considered successful [13,15,16]; otherwise, the procedure is restarted at Step 1.

Paging is used for setting up network-initiated connection. A power-saving efficient paging procedure should allow the UE to sleep without having to process any information and only to briefly wake up at predefined intervals to monitor paging information (as part of control signaling in the beginning of each subframe) from the network [6,9].

24.4.10 Cell Selection

Cell search is a procedure performed by a UE to acquire timing and frequency synchronization with a cell and to detect the cell ID of that cell. 3GPP LTE uses a hierarchical cell search scheme similar to WCDMA. As shown in Figure 24.14, the E-UTRA cell search is based on successful acquisition of the following downlink signals: (1) the primary and secondary synchronization signals that are transmitted twice per radio frame, and (2) the downlink reference signals as well as based on the broadcast channel that carries system information such as system bandwidth, number of transmit antennas, and system frame number. The 504 available physical layer cell identities are grouped into 168 physical layer cell identity groups, each group containing 3 unique identities. The secondary synchronization signal carries the physical layer cell identity group, and the primary synchronization signal carries the physical layer identity 0, 1, or 2 [6].

As shown in Figure 24.11, the primary synchronization signal is transmitted on the 6^th symbol of slots 0 and 10 of each Type 1 radio frame; it occupies 62 subcarriers, centered on the DC subcarrier. The primary synchronization signal is generated from a frequency-domain Zadoff–Chu sequence. The secondary synchronization signal is transmitted on the 5^th symbol of slots 0 and 10 of each radio frame. It occupies 62 subcarriers centered on the DC sub-carrier. The second synchronization signal is an interleaved concatenation of two length-31 M-sequences. The concatenated sequence is scrambled with a scrambling sequence given by the primary synchronization signal [15,16].

As shown in Figure 24.11, PBCH is transmitted on OFDM symbols 0–3 of slot 1 and occupies 72 subcarriers centered on the DC subcarrier. The coded BCH transport block is mapped to 4 subframes within a 40 ms interval. The 40 ms timing is blindly detected; that is, there is no explicit signaling to indicate 40 ms timing [6,15,16]. These channels are contained within the central 1.08 MHz (corresponding to 6 resource blocks or 72 subcarriers) frequency band so that the system operation can be independent of the channel bandwidth.

24.4.11 Link Adaptation

Uplink link adaptation is used in order to improve data throughput in a fading channel. This technique varies the downlink modulation and coding scheme based on the channel conditions of each user. Different types of link adaptation are performed according to the channel conditions, the UE capability (such as the maximum transmission power, maximum transmission bandwidth, etc.), and the required QoS (such as the data rate, latency, and packet error rate, etc.). These link adaptation methods are as follows: (1) adaptive transmission bandwidth, (2) transmission power control, and (3) adaptive modulation and coding [6].

24.4.12 Multi-Antenna Schemes in 3GPP LTE/LTE-Advanced

For the 3GPP LTE Release 8/9 downlink, the 2 × 2 MIMO configuration is assumed to be the baseline; that is, two transmit antennas at the base station and two receive antennas at the terminal side [10,6,15,16]. Configurations with 4 transmit/receive antennas are also supported in the specification. Different downlink MIMO modes are supported in 3GPP LTE, which can be adapted based on channel condition, traffic requirements, and the UE capability. Those include Transmit diversity, Open-loop spatial multiplexing (no UE feedback), Closed-loop spatial multiplexing (with UE feedback), Multiuser MIMO (more than one UE is assigned to the same resource block), and Closed-loop rank-1 precoding.

In 3GPP LTE spatial multiplexing, up to two codewords can be mapped to different layers. Each codeword represents an output from the channel coder. The number of layers available for transmission is equal to the rank of the channel matrix. Precoding in transmitter side is used to support spatial multiplexing. This is achieved by multiplying the signal with a precoding matrix prior to transmission. The optimum precoding matrix is selected from a predefined codebook which is known to both eNB and UE. The optimum precoding matrix is the one which maximizes the capacity.

The UE estimates the channel and selects the optimum precoding matrix. This feedback is provided to the eNB. Depending on the available bandwidth, this information is made available per resource block or group of resource blocks, since the optimum precoding matrix may vary between resource blocks. The network may configure a subset of the codebook that the UE is able to select from. In the case of UEs with high velocity, the quality of the feedback may deteriorate. Thus, an open-loop spatial multiplexing mode is also supported which is based on predefined settings for spatial multiplexing and precoding. In the case of 4 antenna ports, different precoders are cyclically assigned to the resource elements. The eNB will select the optimum MIMO mode and precoding configuration. The information is conveyed to the UE as part of the downlink control information on PDCCH.

In order for MIMO schemes to work properly, each UE has to report information about the channel to the base station. Several measurement and reporting schemes are available which are selected according to MIMO mode and network preference. The reporting may include wideband or narrowband CQI which is an indication of the downlink radio channel quality as experienced by this UE, Precoding Matrix Index (PMI) which is an indication of the optimum precoding matrix to be used in the base station for a given radio condition, and Rank Indication (RI) which is the number of useful transmission layers when spatial multiplexing is used [10,15,16].

In the case of transmit diversity mode, only one codeword can be transmitted. Each antenna transmits the same information stream, but with different coding. 3GPP LTE employs SFBC as transmit diversity scheme. A special precoding matrix is applied at the transmitter side in the precoding stage in Figure 24.9.

Cyclic Delay Diversity (CDD) is an additional type of diversity which can be used in conjunction with spatial multiplexing in 3GPP LTE. An antenna-specific delay is applied to the signals transmitted from each antenna port. This effectively introduces artificial multipath to the signal as seen by the receiver. As a special method of delay diversity, cyclic delay diversity applies a cyclic shift to the signals transmitted from each antenna port [10].

For the 3GPP LTE Release 8/9 uplink, MU-MIMO can be used. Multiple user terminals may transmit simultaneously on the same resource block. The scheme requires only one transmit antenna at UE side. The UEs sharing the same resource block have to apply mutually orthogonal pilot patterns. To take advantage of two or more transmit antennas, transmit antenna selection can be used. In this case, the UE has two transmit antennas but only one transmission chain. A switch will then choose the antenna that provides the best channel to the eNB [15,16].

3GPP LTE-Advanced extends the MIMO capabilities of 3GPP LTE Release 8/9 by supporting up to 8 downlink transmit-antennas and up to 4 uplink transmit-antennas. In addition to legacy MIMO transmission schemes, transmit diversity is possible in both downlink and uplink directions. In the downlink, using single-user spatial multiplexing scenario of 3GPP LTE-Advanced, up to two transport blocks can be transmitted to a scheduled UE in one subframe per downlink component carrier. Each transport block is assigned its own modulation and coding scheme. For HARQ ACK/NACK feedback on uplink, one bit is used for each transport block [10]. In addition to the spatial multiplexing scheme, the 3GPP LTE-Advanced downlink reference signal structure has been modified to allow an increased number of antennas and PDSCH demodulation as well as channel-state information estimation for the purpose of CQI/PMI/RI reporting when needed.

The reference signals for PDSCH demodulation are UE specific; that is, the PDSCH and the demodulation reference signals intended for a specific UE are subject to the same precoding operation. Therefore, these reference signals are mutually orthogonal between the layers at the eNB.

The design principle for the reference signals targeting PDSCH modulation is an extension of the concept of Release 8 UE-specific reference signals to multiple layers. Reference signals targeting CSI estimation are cell specific, are sparse in the frequency and time domain and are punctured into the data region of normal subframes.

With 3GPP LTE-Advanced, a scheduled UE may transmit up to two transport blocks. Each transport block has its own modulation and coding scheme. Depending on the number of transmission layers, the modulation symbols associated with each of the transport blocks are mapped to one or two layers. The transmission rank can be dynamically adapted. Different codebooks are defined depending on the number of layers that are used. Furthermore, different precoding is used depending on whether 2 or 4 transmit antennas are available. Also the number of bits used for the codebook index is different depending on the 2 and 4 transmit antenna case, respectively.

For uplink spatial multiplexing with two transmit-antennas, a 3-bit precoding method is defined. In contrast to the Release 8 downlink scheme, where several matrices for full-rank transmission are available, only the identity precoding matrix is supported in 3GPP LTE-Advanced uplink direction. 3GPP LTE-Advanced further supports transmit diversity in the uplink. However, for those UEs with multiple transmit antennas, an uplink single-antenna-port mode is defined. In this mode, the Release 10 UE behavior is the same as the one with a single antenna from eNB's point of view and it is always used before the eNB is aware of the UE antenna configuration. In the transmit diversity scheme, the same modulation symbol from the uplink channel is transmitted from two antenna ports, on two separate orthogonal resources.

24.5.1 Logical and Transport Channels

The logical and transport channels in 3GPP LTE/LTE-Advanced are illustrated in Figures 24.17 and 24.18, respectively. Each logical channel type is defined by what type of information is transferred. The logical channels are generally classified into two groups: (1) Control Channels (for the transfer of control-plane information) and (2) Traffic Channels (for the transfer of user-plane information) as shown in Figure 24.17 [6].

The control channels are exclusively used for transfer of control-plane information. The control channels supported by MAC can be classified as follows (see Figure 24.17):

• Broadcast Control Channel (BCCH): A downlink channel for broadcasting system control information.

• Paging Control Channel (PCCH): A downlink channel that transfers paging information and system information change notifications. This channel is used for paging when the network does not know the location of the UE.

• Common Control Channel (CCCH): Channel for transmitting control information between UEs and eNBs. This channel is used for UEs having no RRC connection with the network.

• Multicast Control Channel (MCCH): A point-to-multipoint downlink channel used for transmitting MBMS control information from the network to the UE, for one or several MTCHs. This channel is only used by UEs that receive MBMS.

• Dedicated Control Channel (DCCH): A point-to-point bidirectional channel that transmits dedicated control information between a UE and the network. It is used by UEs that have an RRC connection.

The traffic channels are exclusively used for the transfer of user-plane information. The traffic channels supported by MAC can be classified as follows (as shown in Figure 24.18):

• Dedicated Traffic Channel (DTCH): A point-to-point bidirectional channel dedicated to a single UE for the transfer of user information.

• Multicast Traffic Channel (MTCH): A point-to-multipoint downlink channel for transmitting traffic data from the network to the UE. This channel is only used by UEs that receive MBMS.

The physical layer provides information transfer services to MAC and higher layers. The physical layer transport services are described by how and with what characteristics data are transferred over the radio interface. This should be clearly separated from the classification of what is transported, which relates to the concept of logical channels at MAC sublayer. As shown in Figure 24.18, downlink transport channels can be classified as follows:

• Broadcast Channel (BCH) that is characterized by fixed, predefined transport format and is required to be broadcasted in the entire coverage area of the cell.

• Downlink Shared Channel (DL-SCH) that is characterized by support for HARQ, support for dynamic link adaptation by varying the modulation, coding and transmit power, possibility for broadcast in the entire cell, possibility to use beamforming, support for both dynamic and semi-static resource allocation, support for UE discontinuous reception to enable power saving, and support for MBMS transmission.

• Paging Channel (PCH) that is characterized by support for UE discontinuous reception in order to enable power saving, requirement for broadcast in the entire coverage area of the cell, mapped to physical resources which can be used dynamically also for traffic or other control channels.

• Multicast Channel (MCH) which is characterized by requirement to be broadcast in the entire coverage area of the cell, support for macro-diversity combining of MBMS transmission on multiple cells, support for semi-static resource allocation.

The uplink transport channels are classified as follows (see Figure 24.18):

• Uplink Shared Channel (UL-SCH) which is characterized by possibility to use beamforming, support for dynamic link adaptation by varying the transmit power and modulation and coding schemes, support for HARQ, support for both dynamic and semi-static resource allocation.

• Random Access Channel (RACH) that is characterized by limited control information and collision risk.

The mapping of the logical channels to the transport channels in the downlink and uplink is shown in Figure 24.19.

As shown in Figures 24.15 and 24.16, the main services and functions provided by the RLC sublayer include Transfer of upper layer PDUs supporting AM or UM, TM data transfer, Error correction through ARQ (since CRC check is provided by the physical layer, no CRC is needed at RLC level), Segmentation according to the size of the transport block, Re-segmentation of PDUs that need to be retransmitted, Concatenation of SDUs for the same radio bearer, In-sequence delivery of upper layer PDUs except during handover, Duplicate detection, and Protocol error detection and recovery.