V aloración global

2.3.1

Transmission Scheme

In the LTE E-UTRAN, the multiple access schemes are based on OFDM. OFDMA scheme is used in the downlink and SC-FDMA scheme is used in the uplink. The reason why OFDMA is applied in the LTE downlink is its robust features against frequency selective channels. In order to provide lower peak-to-average ratio between the transmitted signals, SC-FDMA is used in the uplink transmission, which is beneficial for saving batter life of mobile devices.

OFDM

OFDM is actually a frequency division multiplexing scheme used as a digital multi- carrier modulation method [19]. The long history of the OFDM technique has witnessed its growing popularity in wireless and wired networking. Due to recent

enhancements in digital processing technology, OFDM has been accepted for wireless high-speed communications in many standards.

The principle of OFDM is to divide a high-rate serial data stream into a set of low-rate parallel data streams, each of which is modulated on a sub-carrier [20]. Therefore, the bandwidth of sub-carriers is smaller than the coherence bandwidth of multipath fading, and hence each sub-carrier can experience flat fading with simple equalisation. As it is shown in Figure2.3 from [20], these sub-carriers are computed by an inverse Fast Fourier Transformation (iFFT) to generate the baseband OFDM signal. When the signal is received by the receiver, the parallel data streams can be obtained by a forward FFT and a symbol detector. Through the iFFT and FFT, the sub-carriers overlap with each other and guarantee the orthogonality. High spectral efficiency can be obtained, because the interference between the sub-carriers is eliminated through the orthogonality. In order to ensure the orthogonality of the sub-carriers in the mobile wireless environment, a Cyclic Prefix (CP) is employed before each OFDM symbol.

Figure 2.3: A Simple point-to-point transmission using OFDM [20]

The basic OFDM parameters include sub-carrier spacing, number of sub-carriers and CP length. The LTE uses a basic sub-carrier spacing of 15 kHz. The number of sub carriers depends on the transmission bandwidth. As an example, 600 sub- carriers are considered over a 10 MHz system bandwidth.

OFDMA

The Orthogonal Frequency Division Multiple Access (OFDMA) scheme is achieved by assigning different sub-carriers to different users. Thus, the interference in OFDMA systems is mainly generated between the users in different cells reusing the same sub-carriers. The mitigation of Inter-Cell Interference (ICI) is a hot re- search issue of RRM in LTE and LTE-Advanced. Differentiated QoS levels are supported by scheduling the sub-carriers between the users.

One of the major drawbacks of OFDMA is the high Peak-to-Average Power Ratio (PAPR) of the transmitted signals [21]. The signals with a high PAPR require highly linear power amplifiers, which bring a heavy burden to portable wireless devices with low power efficiency. Meanwhile, another problem of OFDMA in the uplink transmission is the inevitable frequency offset of the reference signals. This frequency offset will destroy the orthogonality of uplink transmissions, and thus introduce inter-channel interference.

SC-FDMA

Single Carrier Frequency Division Multiple Access (SC-FDMA) is introduced in LTE as an uplink multiple access scheme due to its distinguishing feature of lower PAPR. The SC-FDMA transmitters use different orthogonal sub-carriers to transmit symbols, which are first preprocessed by a Discrete Fourier Transform (DFT) block. Therefore, the SC-FDMA sub-carriers are transmitted sequentially rather than in parallel, resulting in lower PAPR of the signals. Besides, it also has a low sensitivity to carrier frequency offset. However, the main drawbacks of SC-FDMA are lower performance than OFDMA and complex signal processing at the BS side.

2.3.2

Resource Grid

The transmitted signal in each time slot consists of N_RBU L× NRB

sc sub-carriers and NU L

symb SC-FDMA symbols in the uplink, and NRBDL × NscRB sub-carriers and NsymbDL OFDM symbols in the downlink. NU L

RB and NRBDL represent the numbers of resource blocks in the uplink and in the downlink respectively. In each resource block, there are NRB

sc sub-carriers. NsymbU L and NsymbDL denote the numbers of SC-FDMA symbols in the uplink and OFDM symbols in the downlink separately. Uplink resource gird and downlink resource grid are illustrated in Fig. 2.4, which is redrawn from [22].

SC-FDMA symbols subcarriers subcarriers A time slot UL symb N UL R B RB sc NN  RB sc N Resource element Resource block UL RB symb sc N N OFDMA symbols subcarriers subcarriers A time slot DL symb N DL RB RB sc NN  RB sc N DL RB symb sc N N

Table 2.1: Resource block parameters [22]

Configuration N_scRB N_symbU L N_symbDL

Normal cyclic prefix 12 7 7

Extended cyclic prefix (∆f =15kHz) 12 6 6 Extended cyclic prefix (∆f =7.5kHz) 24 / 3

Each element in the resource grid is called a resource element. A Physical Re- source Block (PRB) in the uplink is defined as N_symbU L consecutive SC-FDMA symbols in the time domain and NRB

sc consecutive sub-carriers in the frequency domain. Sim- ilarly, a PRB in the downlink is defined as NDL

symb consecutive OFDM symbols in the time domain and NRB

sc consecutive sub-carriers in the frequency domain. NsymbU L , NDL

symb and NscRB in different configurations are given in Table 2.1.

2.3.3

Frame Structure

To support transmission in paired and unpaired spectrum, two duplex modes are defined in the LTE standards [22], i.e. Type 1 Frequency Division Duplex (FDD) supporting full duplex and half duplex operation, and Type 2 Time Division Duplex (TDD).

Frame structure Type 1 is illustrated in Figure 2.5. Each 10 ms radio frame is divided into ten equally sized sub-frames. Each sub-frame consists of two equally sized time slots. There are 10 sub-frames available for downlink transmission and 10 sub-frames available for uplink transmissions in each 10 ms radio frame. Uplink and downlink transmissions are separated by the guard frequency band in the frequency domain.

Time

slot 1 2 3 ... 19 20

Sub-frame

One radio frame = 10 ms Time slot 1 2 3 ... 19 20 UL DL Guard frequency band

Figure 2.5: Frame structure type 1 [22]

consists of two half-frames of 5 ms each. Each half-frame consists of eight time slots of length 0.5 ms and three special fields: Downlink Pilot Timeslot (DwPTS), Guard Period (GP) and Uplink Pilot Timeslot (UpPTS). The length of DwPTS and UpPTS is configurable subject to the total length of DwPTS, GP and UpPTS being equal to 1ms. Both 5ms and 10ms switch-point periodicity are supported. Subframe 2 in all configurations and subframe 7 in the configuration with 5ms switch-point periodicity consist of DwPTS, GP and UpPTS. Subframe 6 in the configuration with 10 ms switch-point periodicity consists of DwPTS only. All other subframes consist of two equally sized time slots. GP is reserved for the downlink to uplink transition. Other subframes/fields are assigned for either downlink or uplink transmission. Uplink and downlink transmissions are separated in the time domain.

Sub-frame

1 3 4 5 6 8 9 10

DwPTS GP UpPTS DwPTS GP UpPTS A time slot

One radio frame = 10 ms UL

DL DL DL DL UL DL DL

Half-frame = 5 ms

Figure 2.6: Frame structure type 2 [22]