Table 5.8 shows modulations used for different physical channels and physical signals, which were discussed in this chapter.
Physical channel
or physical signal Modulation
P-SS Zadoff-Chu sequence
S-SS Interleaved concatenation of two length-31 binary se- quences
RS Gold sequence (pseudo random) of QPSK symbols
PBCH QPSK
PCFICH QPSK
PDCCH QPSK
PDSCH QPSK, 16-QAM, 64-QAM
PHICH BPSK
6 LTE uplink physical channels
During the LTE development phase different alternatives for the optimum uplink transmission scheme were investigated. While OFDMA is seen optimum to fulfil the LTE requirements in DL, OFDMA properties are less favourable for the UL. This is mainly due to worse Peak-to-Average Power Ratio (PAPR) properties of an OFDMA signal, resulting in worse UL coverage. Thus, the LTE UL transmission scheme for FDD and TDD mode is based on Single Carrier Frequency Division Multiple Access (SC-FDMA) with cyclic prefix. SC-FDMA signals have better PAPR properties compared to an OFDMA signal, see Figure 6.1. This was one of the main reasons for selecting SC-FDMA as LTE UL access scheme. The PAPR characteristics are important for cost-effective design of UE power amplifiers. Still, SC-FDMA signal processing has some similarities with OFDMA signal processing, so parametrisation of downlink and uplink can be harmonised.
Figure 6.1: SC-FDMA versus OFDMA spectral power distribution. There are different possibilities of SC-FDMA signal generation. Discrete Fourier Transform spread-OFDM (DFT-s-OFDM) has been selected for LTE.
The principles of the DFT-s-OFDM are illustrated in Figure 6.2. A size-M DFT is first applied to a block of M modulation symbols (i.e. complex numbers). QPSK, 16QAM or 64QAM may be used as uplink modulation schemes, the latter being optional for the UE. The DFT transforms the M modulation symbols into another
M modulation symbols in the frequency domain. The result is mapped onto the M available UL subcarriers, that is inputs of the size-N IDFT. Unused inputs of
the IDFT are set to zero. In UL, only localised transmission on consecutive M subcarriers is allowed. An size-N IDFT, where N > M , is then performed as in OFDM (see Figure 1.20), followed by addition of the cyclic prefix and parallel to serial conversion.
M is the number of transmitted subcarriers and changes during UL transmission.
Figure 6.2: Block diagram of the UL DFT-s-OFDM transmitter.
then M = 6· 12 = 72. N is the size of the IDFT build in the UE microprocessor and in LTE is equal to N = 211= 2048, the same as in DL.
If the DFT size M would equal the IDFT size N , the cascaded DF T and IDF T blocks of Figure 6.2 would completely cancel out each other. However, if M is smaller than N and the remaining inputs to the IDF T are set to zero, the output of the IDF T will be a signal with ’single-carrier’ properties, i.e. a signal with low power variations, and with a bandwidth that depends on M .
6.1
PUSCH
The UL SC-FDMA subcarrier spacing equals ∆f = 15 kHz and RBs, consisting of 12 subcarriers in the frequency domain, are defined also for the UL. However, in contrast to the DL, no unused DC subcarrier is defined for the UL as this would destroy the ’single-carrier’ property of the UL transmission (single-carrier charac- teristics require the transmission of consecutive subcarriers).
Similar to the DL, the SC-FDMA used for the UL, also allows for a very high degree of flexibility in terms of transmission bandwidth by allowing for, in essence, any number of UL subcarriers. However, from a DFT implementation point of view, the DFT size M should preferably be constrained to a power of 2 (M = 2n). On the other hand, such constraint is in direct conflict with a desired flexibility of UL bandwidth allocation to different 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 (M = 2α· 3β· 5γ, where α, β, γ = 0, 1, 2, ...). Thus, as an example, DFT of
6.1 PUSCH
sizes 84 is not allowed, because 84 = 2· 2 · 3 · 7. Observe, that M = 84 = 12 · 7 correspond to 7 RBs, therefore 7 RBs allocation is not allowed. As a consequence, the number of UL RBs allocation is also limited to products of the integers two, three and five.
Figure 6.3: UL resource allocation.
Also in terms of the more detailed time-domain structure the LTE UL is very similar to the DL. Each 1 ms UL subframe consists of two slots of length Tslot = 0.5 ms,
see Figure 6.4. Each slot consists of seven or six DFT-s-OFDM blocks including the cyclic prefix. Also similar to the downlink, two cyclic prefix lengths, a normal cyclic prefix (for seven DFT-s-OFDM blocks symbol) and an extended cyclic prefix (for six DFT-s-OFDM blocks symbol) are defined for the UL.
Figure 6.4: UL subframe structure for normal cyclic prefix.
In Figure 6.3, UEs gets radio resources on the same subcarriers in the two slots. As an alternative, inter slot frequency hopping may be applied for the LTE uplink. In this case different frequencies are used for transmission in the two slots of a subframe as presented in Figure 6.5. There are two potential benefits with UL frequency hopping if the hopping pattern are different in neighbouring cells.
• Frequency diversity. • Interference averaging.
Figure 6.5: UL frequency hopping.