755

To OFDM modulation

Figure 16.16 Processing chain for downlink L1/L2 control signaling.

blocks) and transport format, and information related to the DL-SCH hybrid ARQ. This signaling is thus similar to the HS-SCCH defined for HSPA (see Chapter 9).

•UL-SCH-related scheduling messages, more specifically scheduling grants informing a scheduled mobile terminal what uplink resources and transport format to use for UL-SCH transmission. This signaling is thus similar to the E-AGCH defined for HSPA (see Chapter 10).

As multiple mobile terminals may be scheduled simultaneously there must be a possibility to transmit multiple scheduling messages for each TTI. Each such message is transmitted as one downlink L1/L2 control channel. As illustrated in Figure 16.16, each control channel, corresponding to a single scheduling message, is first separately processed, including CRC insertion, channel coding, bit-level scrambling, and QPSK modulation. The modulation symbols are then mapped to the downlink physical resource, i.e. to the OFDM time-frequency grid. The physical resource to which the L1/L2 control signaling is mapped is, in the LTE specification, referred to as the Physical Downlink Control Channel (PDCCH).

Figure 16.17 LTE time/frequency grid with certain resource elements occupied by downlink reference symbols and L1/L2 control signaling.

As illustrated in Figure 16.17, the L1/L2 control channels are mapped to the first (up to three) OFDM symbols within each subframe. By mapping the L1/L2 control channels to the beginning of the subframe, the L1/L2 control information, including the DL-SCH resource allocation and transport format, can be retrieved before the end of the subframe. Thus decoding of the DL-SCH can begin directly after the end of the subframe without having to wait for the decoding of the L1/L2 control information. This minimizes the delay in the DL-SCH decoding and thus the overall downlink transmission delay.

Furthermore, by transmitting the L1/L2 control channel at the beginning of the subframe, that is by allowing for early decoding of the L1/L2 control information, mobile terminals that are not scheduled may turn off their receiver circuitry for a large part of the subframe, with a reduced terminal power consumption as a consequence.

In more details, the physical resource to which the L1/L2 control signaling is mapped consists of a number of control-channel elements, where each controlchannel element consists of a predefined number of resource elements. The modulated symbols of each L1/L2 control channel is then mapped to one or several control-channel elements depending on the size, in terms of number of modulation symbols, of each L1/L2 control channel. Note that this size may be different for different L1/L2 control channels.

The network explicitly signals the number of control-channel elements within a subframe. As the control-channel elements are of predefined size and located at the beginning of the subframe, this implies that a scheduled mobile terminal will know what resource elements are occupied by L1/L2 control channels and thus to what resource elements DL-SCH is mapped (the remaining resource elements).

However, mobile terminals are not explicitly informed about the more detailed L1/L2 control structure, including the exact number of L1/L2 control channels and the exact number of control-channel elements to which each L1/L2 control channel is mapped. Rather, the mobile terminal has to blindly try to decode multiple control-channel candidates in order to, potentially, find an L1/L2 control channel carrying scheduling information to this specific mobile terminal. As an example,

Control-channel element 2

Control-channel element 3

Control-channel element 4

Control-channel element 5

Control-channel element 6

Control-channel candidates on which the UE attempts to decode the information

(10 decoding attempts in this example)

Figure 16.18 Control channel elements and control channel candidates.

v1 v2

Figure 16.19 LTE antenna mapping consisting of layer mapping followed by pre-coding. Each square corresponds to one modulation symbol.

Figure 16.18 illustrates the case of six control-channel elements and a possibility to map L1/L2 control channels to one, two, or four control-channel elements. As can be seen, in this specific case there are 10 different control-channel candidates. The mobile terminal decodes each of these candidates and then check the CRC for a valid control channel.

16.2.5Downlink multi-antenna transmission

The transport-channel processing described in Section 16.2.3 included the Antenna Mapping, at that time simply described as the processing of blocks of modulation symbols from, in the general case, two coded transport blocks and mapping to the (up to four) transmit antennas. As illustrated in Figure 16.19, the LTE antenna mapping actually consists of two separate steps, Layer mapping and Pre-coding.

Figure 16.21 Beam-forming within the LTE multi-antenna framework.

Figure 16.22 Spatial multiplexing within the LTE multi-antenna framework (NL = 3, NA = 4).

16.2.5.3 Spatial multiplexing

In case of spatial multiplexing (Figure 16.22) there is, in the general case, two codewords, NL layers, and NA antennas, with NL ≥ 2 and NA ≥ NL. More specifically, Figure 16.22 illustrates the case of three layers (NL = 3) and four transmit antennas (NA = 4). The layer mapping de-multiplexes the modulation symbols of the two codewords onto the NL layers. As can be seen, in the case of three layers, the first codeword is mapped to the first layer while the second codeword is mapped to the second and third layer. Thus, the number of modulation symbols of the second codeword should be twice that of the first codeword to ensure the same number of symbols on each layer. The pre-coding then applies the pre-coding matrix W of size NA × NL to the each layer vector v¯i.

In general, LTE spatial multiplexing relies on codebook-based pre-coding, implying that for each combination of number of antennas NA and number of layers NL, a set of pre-coder matrices are defined by the specification. Based on measurements on the downlink reference signals of the different antennas, the mobile decides on a suitable rank (number of layers) and corresponding pre-coder matrix. This is then reported to the network. While a single rank, valid for the entire system bandwidth, is reported, multiple pre-coder matrices, valid for different parts of the system bandwidth, may be reported. The network takes this information into

account, but does not have to follow it, when deciding on what rank and set of pre-coder matrices to actually use for the downlink transmission. As the network can decide on a different set of pre-coder matrices than what has been reported by the mobile terminal, the network must explicitly signal what pre-coder matrices are used by means of the downlink L1/L2 control signaling.

A similar approach is used for downlink multi-antenna beam-forming, i.e. based on measurements on the downlink reference signals of the different antennas, the mobile decides on a suitable pre-coder (beam-forming) vector and reports this to the network. The network takes this information into account, but does not have to follow it, when deciding on what pre-coder vector to actually use for the downlink transmission. Similar to the case of spatial multiplexing, the network must thus explicitly signal what beam-forming vector is used to the mobile terminal. As a consequence, pre-coding can only be used for the DL-SCH transmission but not for the L1/L2 control signaling.

16.2.6Multicast/broadcast using MBSFN

As discussed in Chapter 4, OFDM transmission offers some specific benefits in terms of the provisioning of multi-cell multicast/broadcast services, more specifically the possibility to make synchronous multi-cell multicast/broadcast transmissions appear as a single transmission over a multi-path channel. As already mentioned in Chapter 14, for LTE this kind of transmission is referred to as

Multicast/Broadcast over Single Frequency Network (MBSFN).

LTE supports MBSFN-based multicast/broadcast transmission by means of the MCH (Multicast Channel) transport channel. The transport-channel processing for MCH is, in many respects, the same as that for DL-SCH (Figure 16.9), with some exceptions:

•In case of MBSFN transmission, the same data is to be transmitted with the same transport format using the same physical resource from multiple cells typically belonging to different eNodeB. Thus, the MCH transport format and resource allocation cannot be dynamically selected by the eNodeB.

•As the MCH transmission is simultaneously targeting multiple mobile terminals, hybrid ARQ is not directly applicable in case of MCH transmission.

Furthermore, as also mentioned in Section 16.2.3, the MCH scrambling should be identical for all cells involved in the MBSFN transmission (cell-common scrambling).

Channel estimation for coherent demodulation of an MBSFN transmission cannot directly rely on the ‘normal’ cell-specific reference signals described in Section 16.2.2 as these reference signals are not transmitted by means of MBSFN and thus

340 3G Evolution: HSPA and LTE for Mobile Broadband

Figure 16.23 Cell-common and cell-specific reference symbols in MBSFN subframes. Note that the figure assumes an extended cyclic prefix corresponding to 12 OFDM symbols per subframe.

do not reflect the aggregated MBSFN channel. Instead, additional reference symbols are inserted within MBSFN subframes, as illustrated in Figure 16.23. These reference symbols are transmitted by means of MBSFN, i.e. identical reference symbols (same complex value within the same resource element) are transmitted by all cells involved in the MBSFN transmission. The corresponding received reference signal can thus directly be used for estimation of the aggregated MBSFN channel, enabling coherent demodulation of the MBSFN transmission.

The transmission of MCH using MBSFN is not allowed to be multiplexed with the transmission of other transport channels such as DL-SCH within the same subframe. Thus, there is also no transmission of downlink L1/L2 control signaling related to DL-SCH transmission (transport-format, resource indication, and hybrid-ARQ related information) in MBSFN subframes. However, there may be other downlink L1/L2 control signaling to be transmitted in MBSFN subframes, e.g. scheduling grants for UL-SCH transmission. As a consequence, the normal ‘cell-specific’ reference signals, as described in Section 16.2.2, also need to be transmitted within the MBSFN subframes, in parallel to the MBSFN-based reference signal. However, as the L1/L2 control signaling is confined to the first part of the subframe, only the cell-specific reference symbols within the first OFDM symbol of the subframe (as well as the second OFDM symbol of the subframe in case of four transmit antennas) are transmitted within MBSFN subframes, see Figure 16.23.

16.3Uplink transmission scheme

16.3.1 The uplink physical resource

As already mentioned in the overview provided in Chapter 14, LTE uplink transmission is based on so-called DFTS-OFDM transmission. As described in Chapter 5, DFTS-OFDM is a low-PAR ‘single-carrier’ transmission scheme that allows for flexible bandwidth assignment and orthogonal multiple access not only in the time domain but also in the frequency domain. Thus, the LTE uplink transmission scheme is also referred to as Single-Carrier FDMA (SC-FDMA).

Figure 16.24 Basic structure of DFTS-OFDM transmission.

Figure 16.24 recapitulates the basic structure of DFTS-OFDM transmission with a size-M DFT being applied to a block of M modulation symbols. The output of the DFT is then mapped to selective inputs of a size-N IFFT. The DFT size determines the instantaneous bandwidth of the transmitted signal while the frequency mapping determines the position of the transmitted signal within the overall available uplink spectrum. Finally, a cyclic prefix is inserted for each processing block. As discussed in Chapter 5, the use of a cyclic prefix in case of single-carrier transmission allows for straightforward application of low-complexity high-performance frequency-domain equalization at the receiver side.

As discussed in Chapter 5, in the general case both localized and distributed DFTSOFDM transmission is possible. However, LTE uplink transmission is limited to localized transmission, i.e. the frequency mapping of Figure 16.24 maps the output of the DFT to consecutive inputs of the IFFT.

From a DFT-implementation point-of-view, the DFT size M should preferably be constrained to a power of two. However, such a constraint is in direct conflict with a desire to have a high degree of flexibility in terms of the amount of resources (the instantaneous transmission bandwidth) that can be dynamically assigned to different mobile terminals. From a flexibility point-of-view, all possible values of M should rather be allowed. For LTE, a middle-way has been adopted where the DFT size is limited to products of the integers two, three, and five. Thus, as an example, DFT sizes of 15, 16, and 18 are allowed but M = 17 is not allowed. In this way, the DFT can be implemented as a combination of relatively low-complex radix-2, radix-3, and radix-5 FFT.

As mentioned in Chapter 5, and as should be obvious from Figure 16.24, DFTSOFDM can also be seen as conventional OFDM transmission combined with DFTbased pre-coding. Thus, one can very well speak about a subcarrier spacing also in case of DFTS-OFDM transmission. Furthermore, similar to OFDM, the DFTSOFDM physical resource can be seen as a time–frequency grid with the additional

NRB resource blocks (12.NRB ‘subcarriers’)

Figure 16.25 LTE uplink frequency-domain structure.

constraint that the overall time–frequency resource assigned to a mobile terminal must always consist of consecutive subcarriers.

The basic parameters of the LTE uplink transmission scheme have been chosen to be aligned, as much as possible, with the corresponding parameters of the OFDMbased LTE downlink. Thus, as illustrated in Figure 16.25, the uplink DFTS-OFDM subcarrier spacing equals f = 15 kHz and resource blocks, consisting of 12 subcarriers, are defined also for the LTE uplink. However, in contrast to the downlink, no unused DC-subcarrier is defined for the uplink. The reason is that the presence of a DC-carrier in the center of the spectrum would have made it impossible to allocate the entire system bandwidth to a single mobile terminal and still keep the low-PAR single-carrier property of the uplink transmission. Also, due to the DFT-based pre-coding, the impact of any DC interference will be spread over the block of M modulation symbols and will therefore be less harmful compared to normal OFDM transmission.

Thus, the total number of uplink subcarriers equals Nsc = 12 · NRB. Similar to the downlink, also for the uplink the LTE physical-layer specification allows for a very high degree of flexibility in terms of overall system bandwidth by allowing for, in essence, any number of uplink resource blocks ranging from six resource blocks and upwards However, also similar to the downlink, there will be restrictions in the sense that radio-frequency requirements will, at least initially, only be specified for a limited set of uplink bandwidths.

Also in terms of the more detailed time-domain structure, the LTE uplink is very similar to the downlink, as can be seen from Figure 16.26. Each 1 ms uplink subframe consists of two equally sized slots of length Tslot = 0.5 ms. Each slot then consists of a number of DFT blocks including cyclic prefix. Also similar to the downlink, two cyclic-prefix lengths are defined for the uplink, the normal cyclic prefix and an extended cyclic prefix.