755

One subframe two slots (Tsubframe 1 ms, Tslot 0.5 ms)

Normal CP

Extended CP

TCP : 160 · Ts 5.2 s (first DFT block), 144 · Ts 4.7 s (remaining DFT blocks) TCP-e : 512 · Ts 16.7 s

Figure 16.26 LTE uplink subframe and slot structure. One subframe consisting of two equally sized slots. Each slot consisting of six or seven DFTS-OFDM blocks in case of normal and extended cyclic prefix, respectively.

User #1 User #2 User #3

Figure 16.27 LTE uplink resource allocation.

In contrast to the downlink, uplink resource blocks assigned to a mobile terminal must always be consecutive in the frequency domain, as illustrated in Figure 16.27. Note that, similar to the downlink, the uplink resource block is defined as 12 DFTSOFDM subcarriers during one 0.5 ms slot. At the same time, uplink scheduling is carried out on a subframe (1 ms) basis. Thus, similar to the downlink, the uplink resource assignment is carried out in terms of, in the time domain, consecutive pairs of resource blocks.

In Figure 16.27, the assigned uplink resource corresponds to the same set of subcarriers in the two slots. As an alternative, inter-slot frequency hopping may be applied for the LTE uplink. Inter-slot frequency hopping implies that the physical resources used for uplink transmission in the two slots of a subframe do not occupy

One set of sequences with the CAZAC property is the set of Zadoff–Chu sequences [123]. In the frequency domain, a Zadoff–Chu sequence of length MZC can be expressed as

where u is the index of the Zadoff–Chu sequence within the set of Zadoff–Chu sequences of length MZC.

The number of available Zadoff–Chu sequences given a certain sequence length, that is the number of possible values of the index u in (16.1), equals the number of integers that are relative prime to the sequence length MZC. This implies that, to maximize the number of Zadoff–Chu sequences and thus to, in the end, maximize the number of available uplink reference signals, Zadoff–Chu sequences of prime length are preferred. At the same time, the frequency-domain length MRS of the uplink reference signals should be equal to the assigned bandwidth, i.e. a multiple of 12 (the resource-block size) which is obviously not a prime number. Thus, prime-length Zadoff–Chu sequences cannot be directly used as LTE uplink reference signals. Instead the uplink reference signals are derived from prime-length Zadoff–Chu sequences.

Two methods for deriving uplink reference signals of length MRS from primelength Zadoff–Chu sequences have been defined in the LTE physical–layer specification:

1.Method 1 (truncation): Zadoff–Chu sequences of length MZC, where MZC is the smallest prime number larger than or equal to MRS, are truncated to length

MRS.

2.Method 2 (cyclic extension): Zadoff–Chu sequences of length MZC, where MZC is the largest prime number smaller than or equal to MRS, are cyclically extended to length MRS.

These two methods are illustrated in Figure 16.31. It should be noted that this figure assumes truncation or cyclic extension of a single symbol which may not always be the case. As an example, if a reference-sequence length MRS = 96, corresponding to eight resource blocks, is desired, method 1 could use a Zadoff–Chu sequence of length NZC = 97 (a prime number) as a starting point. However, the largest prime number smaller than or equal to 96 is 89, implying that method 2 would need to use a Zadoff–Chu sequence of length NZC = 89 as a starting point and apply a seven-symbol cyclic extension to arrive at the desired length-96 reference signal.

Clearly, both these methods will to some extent degrade the CAZAC property of the uplink reference signals. Which method is best in terms of best retaining

the same resource within the same cell which may, for example, happen in case of uplink SDMA.

16.3.2.2 Reference signals for channel sounding

Downlink channel-dependent scheduling, in both the time and frequency domain, is a key LTE technology. As discussed in the previous chapter, uplink channeldependent scheduling, that is assigning uplink resources to a mobile terminal depending on the instantaneous channel quality can also be used. Uplink channel-dependent scheduling can increase the achievable data rates and reduce the interference to other cells, by having the mobile terminal transmitting within frequency bands that are, instantaneously, of good quality.

To carry out channel-dependent scheduling in the time and frequency domain, estimates of the frequency-domain channel quality is needed. For the downlink, this can, for example, be accomplished by the mobile terminal measuring the quality of the downlink cell-specific reference signal, which is transmitted over the full cell bandwidth, and reporting the estimated channel quality to the network by means of a Channel Quality Indicator (CQI).

As can be seen from Figure 16.29, the reference signals used for uplink coherent demodulation are only transmitted over the bandwidth dynamically assigned to each mobile terminal. These reference signals can thus not be used by the network to estimate the uplink channel quality for any other frequencies than those currently assigned to the mobile terminal and can thus not provide the information needed for uplink channel-dependent scheduling in the frequency domain. To support uplink frequency-domain channel-dependent scheduling, additional more wideband reference signals can therefore also be transmitted on the LTE uplink. These reference symbols are referred to as channel-sounding reference signals, to distinguish them from the (demodulation) reference symbols discussed above and illustrated in Figure 16.29.

The basic principles for the channel-sounding reference signals are similar to those of the demodulation reference signals. More specifically, also the channelsounding reference signals are based on prime-length Zadoff–Chu sequences and are transmitted within a complete DFTS-OFDM block. However, there are some key differences between the channel-sounding reference signals and the demodulation reference signals:

•As already mentioned, channel-sounding reference signals typically have a bandwidth that is larger, potentially much larger, than the uplink resource assigned to a mobile terminal. Channel-sounding reference signals may even need to be transmitted by mobile terminals that have not been assigned any uplink resource for UL-SCH transmission.

Transport block of dynamic size delivered from MAC layer

To DFTS-OFDM modulation including mapping to assigned frequency resource

Figure 16.33 LTE uplink transport-channel processing.

16.3.3Uplink transport-channel processing

The LTE uplink transport-channel processing can be outlined according to Figure 16.33. As there is no LTE uplink spatial multiplexing, only a single transport block, of dynamic size, is transmitted for each TTI.

•CRC insertion: Similar to the downlink, a CRC is calculated and appended to each uplink transport block.

•Channel coding: Uplink channel coding relies on the same Turbo code, including the same QPP-based internal interleaver, as is used for the downlink.

•Physical-layer hybrid-ARQ functionality: The uplink physical layer aspects of the LTE uplink hybrid ARQ are basically the same as the corresponding downlink functionality, i.e. the hybrid-ARQ physical-layer functionality extracts, from the block of code bits delivered by the channel encoder, the exact set of bits to be transmitted at each transmission/retransmission instant. Note that, in some aspects, there are some clear differences between the downlink and uplink hybrid-ARQ protocols, such as asynchronous vs. synchronous operation. However, these differences are not reflected in the physical-layer aspects of hybrid ARQ.

•Bit-level scrambling: Similar to the downlink, bit-level scrambling can also be applied to the code bits on the LTE uplink. The aim of uplink scrambling is the

same as for the downlink, that is to randomize the interference and thus ensure that the processing gain provided by the channel code can be fully utilized. To achieve this, the uplink scrambling is mobile-terminal specific, i.e. different mobile terminals use different scrambling sequences.

•Data modulation: Similar to the downlink, the uplink data modulation transforms a block of coded/scrambled bits into a block of complex modulation symbols. The set of modulation schemes supported for the LTE uplink are the same as for the downlink, i.e. QPSK, 16QAM, and 64QAM, corresponding to two, four, and six bits per modulation symbol, respectively.

The block of modulation symbols are then applied to the DFTS-OFDM processing as outlined in Figure 16.24, which also maps the signal to the assigned frequency band.

16.3.4Uplink L1/L2 control signaling

Similar to the LTE downlink, there is also for the uplink need for certain associated control signaling (uplink L1/L2 control) to support the transmission of downlink and uplink transport-channels (DL-SCH and UL-SCH). The uplink L1/L2 control signaling includes:

•Hybrid-ARQ acknowledgments for received DL-SCH transport blocks.

•CQI (Channel-Quality Indicator), indicating the downlink channel quality as estimated by the mobile terminal. Similar to HSPA, the CQI reports can be used by the network for downlink channel-dependent scheduling and rate control. However, in contrast to HSPA and due to the fact that LTE downlink scheduling can be carried out in both the time and frequency domain, the LTE CQI reports indicate the channel quality in both the time and frequency domain.

•Scheduling requests, indicating that a mobile terminal needs uplink resources for UL-SCH transmissions.

In contrast to the downlink, there is no uplink signaling indicating the UL-SCH transport format to the network. This is based on an assumption that the mobile terminal always follows the scheduling grants received from the network, including the UL-SCH transport format specified in those grants, as described in Section 15.2.3. With this assumption, the network will know the transport format used for the UL-SCH transmission in advance and there is no reason to explicitly signal it on the uplink. For similar reasons, there is also no explicit uplink signaling of information related to UL-SCH hybrid ARQ.

The uplink L1/L2 control signaling defined above needs to be transmitted on the uplink regardless of whether or not the mobile terminal has any uplink