OFDM is selected as the physical layer technique in the LTE release 8 [1] together with
MIMO, which can either improve spectral efficiency (space multiplexing) or enhance
signal robustness (space diversity). The work in this thesis is focused on multicarrier
transmission, thus, only OFDM is described here and MIMO will not be considered in
this section.
LTE frame structure is shown in Fig. 5.3 where one frame of length 10 ms consists
of 10 subframes. Ts=32.55 ns indicates sample time period which in frequency domain
corresponds to 30.72 MHz. Furthermore, each subframe can be divided into 2 time
slots with each accounting for 0.5 ms. Each time slot is composed of either seven
(normal CP) or six (extended CP) OFDM symbols. In this section, only normal one is
presented.
In Fig. 5.3, the useful symbol time is 66.7 us, which covers 2048 samples. In
a normal CP mode, the first symbol has a CP of length 5.2 us=160 × Ts and the
remaining six symbols have a CP of length 4.7 us=144 × Ts. The reason for this is to
make sure one time slot has 15360 samples. In an extended version, the CP is extended
to 512 samples, which is longer than a normal one. The normal one is used in the
is for rural areas, large cells and low data rate applications.
Figure 5.3: Frame structure type 1 (FDD frame).
Figure 5.4: OFDM resource block and resource element definition for a one antenna LTE system with normal CP mode.
symbols in time-domain while the y-axis means sub-carriers in frequency-domain. One
resource element indicates one M-QAM symbol modulated on one sub-carrier. Resource
block is the smallest component can be operated in LTE. Each resource block consists of
12 sub-carriers (1.e. 180 kHz) and 7 OFDM symbols. Therefore, with different number
of sub-carriers, the number of resource blocks are different, which can be noticed in
Table. 5.1.
Table 5.1 presents physical layer specifications for LTE downlink FDD mode. It
should be noted that DC sub-carrier is not used and 10% of sub-carriers are not used
on both sides of the channel. In other words, the occupied data bandwidth is roughly
90% of the channel bandwidth.
Table 5.1: LTE Downlink Physical Layer Specifications
Channel Bandwidth (MHz) 1.25 2.5 5 10 15 20
Sampling Frequency (MHz) 1.92 3.84 7.68 15.36 23.04 30.72
FFT Size 128 256 512 1024 1536 2048
Occupied Sub-carriers (inc. DC) 76 151 301 601 901 1201
Guard Sub-carriers 52 105 211 423 635 847
Number of Resource Blocks 6 12 25 50 75 100
Occupied Channel Bandwidth (MHz) 1.140 2.265 4.515 9.015 13.515 18.015
1st CP Length 10 20 40 80 120 160
Remaining CP Length 9 18 36 72 108 144
5.4.1 Time-Frequency Requirements of the LTE Resource Block The pilot locations are studied detailedly in [173]. The pilot location in time-domain is
configured based on coherence time (it is calculated according to the Doppler spread).
Its location in frequency-domain is based on the coherence bandwidth (it is calculated
according to the time delay spread).
5.4.1.1 Resource Block Size
Coherence bandwidth Bc is calculated according to (5.2) with η=0.5 in the typical
bandwidth is approximated to be Bc= 5D1 ≈187kHz. Therefore, the bandwidth of one
resource block should be within 187 kHz to make sure a flat channel response. In LTE,
this value is rounded to be 180 kHz. Considering the 15 kHz sub-carrier spacing, the
number of sub-carriers within one resource block is 180/15 = 12.
In terms of coherence time, taking into account the peak mobility speed associated
with a high speed train (roughly 300-350 km/h), together with a 2.6 GHz RF carrier,
the Doppler frequency is approximated to be 843 kHz, which is calculated according to
(5.3). Then based on (5.9), the coherence time is 0.423843 ≈ 0.5ms. To make the channel
time invariant over one resource block, the time period of one resource block should be
within 0.5 ms.
5.4.1.2 Sub-Carrier Spacing
The size of one resource block is configured and fixed based on multiplath fading con-
ditions. It is evident that CP consumes additional radio resources. A solution is to
aggregate more sub-carriers with smaller sub-carrier spacing ∆f in a given bandwidth.
Thus, the time period of one OFDM symbol is increased. However, it increases the sen-
sitivity of OFDM to frequency offset and fast fading. In LTE, the sub-carrier spacing
is 15 kHz since it may simplify the implementation of WCDMA/HSPA/LTE multi-
mode terminals [175]. Typically, the FFT size is a power of two, multiplying with 15
kHz sub-carrier spacing, the sampling rate is fs = ∆f · NF F T leading to a multiple
or sub-multiple of the WCDMA/HSPA chip rate fcr=3.84 MHz. Then, multi-mode
WCDMA/HSPA/LTE terminals can straightforwardly be implemented with a single
clock circuitry.
5.4.1.3 Carrier Raster
The location of LTE center carriers in frequency domain could be positioned anywhere
as explained in [176]. Mobile terminals have to search for a carrier within the supported
search, a set of frequencies are limited to be searched within the supported frequency
bands. Therefore, in LTE supported frequency bands, carriers exist on a 100 kHz
carrier raster, which is expressed as m × 100 kHz where m are integers.
In the carrier aggregation scenario, multiple CCs are put together. In order to be
compatible with legacy terminals (e.g. support below LTE-Advanced standard), each
CC should be centered on the 100 kHz carrier raster. Additionally, the sub-carrier
spacing is defined as 15 kHz. Therefore, the gap between adjacent CCs should be a
multiple of 15 kHz in order to preserve orthogonally of the sub-carriers. Overall, the
center carrier spacing between different CCs should be a multiple of 300 kHz [176],
which is the smallest carrier spacing being a multiple of both 100 kHz (the carrier
raster) and 15 kHz (the sub-carrier spacing).