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This chapter is from the book

6.4 Uplink SC-FDMA Radio Resources

For the LTE uplink transmission, SC-FDMA with a CP is adopted. As discussed in Chapter 4, SC-FDMA possesses most of the merits of OFDM while enjoying a lower PAPR. A lower PAPR is highly desirable in the uplink as less expensive power amplifiers are needed at UEs and the coverage is improved. In LTE, the SC-FDMA signal is generated by the DFT-spread-OFDM. Compared to conventional OFDM, the SC-FDMA receiver has higher complexity, which, however, is not considered to be an issue in the uplink given the powerful computational capability at the base station.

An SC-FDMA transceiver has a similar structure as OFDM, so the parametrization of radio resource in the uplink enjoys similarities to that in the downlink described in Section 6.3. Nevertheless, the uplink transmission has its own properties. Different from the downlink, only localized resource allocation on consecutive subcarriers is allowed in the uplink. In addition, only limited MIMO modes are supported in the uplink. In this section, we focus on the differences in the uplink radio resource from that in the downlink.

6.4.1 Frame Structure

The uplink frame structure is similar to that for the downlink. The difference is that now we talk about SC-FDMA symbols and SC-FDMA subcarriers. In frame structure type 1, an uplink radio frame consists of 20 slots of 0.5 ms each, and one subframe consists of two slots, as in Figure 6.8. Frame structure type 2 consists of ten subframes, with one or two special subframes including DwPTS, GP, and UpPTS fields, as shown in Figure 6.9. A CP is inserted prior to each SC-FDMA symbol. Each slot carries seven SC-FDMA symbols in the case of normal CP, and six SC-FDMA symbols in the case of extended CP.

6.4.2 Physical Resource Blocks for SC-FDMA

As SC-FDMA can be regarded as conventional OFDM with a DFT-based precoder, the resource grid for the uplink is similar to the one for the downlink, illustrated in Figure 6.12, that is, it comprises a number of resource blocks in the time-frequency plane. The number of resource blocks in each resource grid, n-ul-byrb.jpg, depends on the uplink transmission bandwidth configured in the cell and should satisfy

n-min.jpg
Figure 6.12

Figure 6.12 The structure of the uplink resource grid.

where 252fig01.jpg and 252fig02.jpg correspond to the smallest and largest uplink bandwidth, respectively. There are 252fig04.jpg resource elements in each resource block. The values of n-rb-sc.jpg and n-ul-by-symp.jpg for normal and extended CP are given in Table 6.6. There is only one subcarrier spacing supported in the uplink, which is Df = 15kHz. Different from the downlink, the DC subcarrier is used in the uplink, as the DC interference is spread over the modulation symbols due to the DFT-based precoding.

Table 6.6. Physical Resource Block Parameters for Uplink

Configuration

n-rb-sc.jpg

n-ul-by-symp.jpg

Normal CP

12

7

Extended CP

12

6

As for the downlink, each resource element in the resource grid is uniquely defined by the index pair (k, l) in a slot, where k = 0,..., 252fig05.jpg and l = 0,..., 252fig06.jpg are the indices in the frequency and time domain, respectively. For the uplink, no antenna port is defined, as only single antenna transmission is supported in the current specifications.

A PRB in the uplink is defined as n-ul-by-symp.jpg consecutive SC-FDMA symbols in the time domain and n-rb-sc.jpg consecutive subcarriers in the frequency domain, corresponding to one slot in the time domain and 180kHz in the frequency domain. The relation between the PRB number nPRB in the frequency domain and resource elements (k, l) in a slot is given by:

nprb.jpg

6.4.3 Resource Allocation

Similar to the downlink, shared-channel transmission and channel-dependent scheduling are supported in the uplink. Resource allocation in the uplink is also performed at the eNode-B. Based on the channel quality measured on the uplink sounding reference signals and the scheduling requests sent from UEs, the eNode-B assigns a unique time-frequency resource to a scheduled UE, which achieves orthogonal intra-cell transmission. Such intra-cell orthogonality in the uplink is preserved between UEs by using timing advance such that the transport blocks of different UEs are received synchronously at the eNode-B. This provides significant coverage and capacity gain in the uplink over UMTS, which employs non-orthogonal transmission in the uplink and the performance is limited by inter-channel interference. In general, SC-FDMA is able to support both localized and distributed resource allocation. In the current specification, only localized resource allocation is supported in the uplink, which preserves the single-carrier property and can better exploit the multiuser diversity gain in the frequency domain. Compared to distributed resource allocation, localized resource allocation is less sensitive to frequency offset and also requires fewer reference symbols.

The resource assignment information for the uplink transmission is carried on the PDCCH with DCI format 0, indicating a set of contiguously allocated resource blocks. However, not all integer multiples of one resource block are allowed to be assigned to a UE, which is to simplify the DFT design for the SC-FDMA transceiver. Only factors 2, 3, and 5 are allowed. The frequency hopping is supported to provide frequency diversity, with which the UEs can hop between frequencies within or between the allocated subframes. The resource mapping for different uplink channels is discussed in Chapter 8, and the uplink channel sounding and scheduling signaling is described in Chapter 9.

6.4.4 Supported MIMO Modes

For the MIMO modes supported in the uplink, the terminal complexity and cost are among the major concerns. MU-MIMO is supported, which allocates the same time and frequency resource to two UEs with each transmitting on a single antenna. This is also called Spatial Division Multiple Access (SDMA). The advantage is that only one transmit antenna per UE is required. To separate streams for different UEs, channel state information is required at the eNode-B, which is obtained through uplink reference signals that are orthogonal between UEs. Uplink MU-MIMO also requires power control, as the near-far problem arises when multiple UEs are multiplexed on the same radio resource.

For UEs with two or more transmit antennas, closed-loop adaptive antenna selection transmit diversity shall be supported. For this scenario, each UE only needs one transmit chain and amplifier. The antenna that provides the best channel to the eNode-B is selected based on the feedback from the eNode-B. The details of MIMO transmission in the uplink are described in Chapter 8.

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