The LTE downlink employs OFDMA as the multiple access technique because of its efficiency in managing allocated resources and low latency in sending packets, OFDMA single user equipment communicates on all the sub carriers at any time. UEs are allocated specific number of sub carriers for a predetermined amount of time by the base station called the
39 eNodeB in LTE, so that the multiple UEs can be scheduled for data transmission simultaneously. The scheduling resource unit is known as the physical Resource block (PRB).
A PRB is the smallest unit of resource that can be allocated to a user. Also PRB has both time and frequency dimension. The OFDMA signal used in LTE downlink consist of 2048 different sub carriers at maximum while operating 20MHz bandwidth with sub carrier spacing of 15 KHz. One resource block (RB) consists of 12 sub carriers irrespective of the overall LTE bandwidth
In OFDMA, the RB is 180 kHz wide in frequency and 0.5ms long in time domain. In frequency, the standard number of subcarriers used per RB is 12 for most channels. The minimum unit of the time domain is a Symbol, which amounts to 66.7 μs. Regardless of bandwidth, the symbol length does not change. The largest unit in time domain is a frame, each of which is 10 ms in length. Each of the frame consists of 10 sub frames, each of which is 1 ms in length. Each of sub frames consists of 2 slots, each of which is 0.5 ms in length. Each of slots consists of 7 symbols or 6 symbols for normal and extended cyclic prefix respectively, each of which is 66.7 μs.
Standard LTE radio frame format is shown in Figure 2.15 has time duration of 10 ms, consisting of 20 slots of each 0.5 ms, two adjacent slots form a sub-frame of 1 ms duration, which is also one Transmission Time Interval (TTI). Each slot consists of seven OFDM symbols with short/normal Cyclic Prefix (CP) or six OFDM symbols with long/extended CP.
CP is the process of extending each symbol to avoid inter-symbol-interference by duplicating a portion of the signal at the symbol ends, which is removed at the receiver.
40 Figure 2.15: Standard LTE Radio Frame (Telesystem, 2010)
In the time domain, different time intervals within LTE are expressed as multiples of a basic time unit Ts = 1/30720000. The radio frame has a length of 10 ms (Tframe = 307200 ・Ts).
Each frame is divided into ten equally sized sub-frames of 1 ms in length (Tsub-frame = 30720
・Ts). Scheduling is done on a frame basis for both the downlink and uplink. Each
sub-frame consists of two equally sized slots of 0.5 ms in length (Tslot = 15360 ・Ts). Each slot in turn consists of a number of OFDM symbols which can be either seven (normal cyclic prefix) or six (extended cyclic prefix).
41 Figure 2.16: LTE FDD Mode Frame Structure (Wrey, 2011)
Figure 2.16 represents FDD radio frame. Recall, the frequency channel is common for both uplink and downlink. One FDD frame is 10 ms and is subdivided into 20 slots; each slot is 0.5ms duration. For most information transmission, two slots are combined to form a sub-frame which gives 1ms and corresponds to one TTI (Transmission time interval). Scheduling occurs during one TTI, up to the first three symbols in the sub frame can be used for carrying control and scheduling messages. Then the remaining symbols of the first slot and the second slot of the sub-frame are made available be used for data traffic by users.
The useful symbol time is Tu = 2048 × Ts ≈ 66.7 μs. For the normal mode, the first symbol has a cyclic prefix of length TCP = 160 × Ts ≈ 5.2 μs. The remaining six symbols have a cyclic prefix of length TCP = 144 × Ts ≈ 4.7 μs. The reason for different CP length of the first symbol is to make the overall slot length in terms of time units divisible by 15360. For the extended mode, the cyclic prefix is TCP-e = 512 × Ts ≈ 16.7 μs. The CP is longer than the typical delay spread of a few microseconds typically encountered in practice. The normal cyclic prefix is used in urban cells and high data rate applications while the extended cyclic prefix is used in special
42 cases like multi-cell broadcast and in very large cells (e.g. rural areas, low data rate applications).
Figure 2.17: LTE Symbol Structure (Telesystem, 2010)
The CP uses up part of the physical layer capacity: 7.5% in the case of normal cyclic prefix.
One way to reduce the relative overhead due to cyclic-prefix insertion is to reduce the sub-carrier spacing Df, with a corresponding increase in the symbol time Tu as a consequence.
However, this will increase the sensitivity of the OFDM transmission to frequency instability resulting from fast channel variations (i.e. high Doppler spread) as well as different types of frequency errors due to electronics.
In the frequency domain, the number of sub-carriers N, ranges from 128 to 2048, depending on channel bandwidth with 512, 1024 and 2048 for 5MHz, 10MHz and 20 MHz, respectively.
The sub-carrier spacing is Df = 1/Tu = 15 kHz. The sampling rate is fs = Df・N = 15000 N. This result in a sampling rate that’s multiple or sub-multiple of the Wideband Code Division Multiple Access (WCDMA) chip rate of 3.84 Mcps: LTE parameters have been chosen such that Fast Fourier Transform (FFT) lengths and sampling rates are easily obtained for all operational modes while at the same time ensuring the easy implementation of dual-mode devices with a common clock reference. In a macro cell, the coherence bandwidth of the
43 signal is in the order of 1 MHz. LTE carrier bandwidth of up to 20 MHz, there are some sub-carriers that are faded and other are not faded. Transmission is done using those frequencies that are not faded. The transmission can be scheduled by Resource Blocks (RB) each of which consists of 12 consecutive sub-carriers, or 180 kHz, for the duration of one slot (0.5 ms). This granularity is selected to limit signalling overhead. A Resource Element (RE) is the smallest defined unit which consists of one OFDM sub-carrier during one OFDM symbol interval.
Each Resource Block consists of 12 x 7 = 84 Resource Elements in case of normal cyclic prefix (72 for extended CP)
Figure 2.18: LTE TDD Mode Frame Structure (Telesystem, 2010)
Figure 2.18 depicts the frame used for LTE TDD configured systems, they share 10 ms frame structure and 1 ms sub frame like in FDD but with a demarcation known as a half frame. Each of the half frames carries 5 sub frames and each of them can be used for TDD downlink to Uplink switching. The switching point in the first half is mandatory while in the second is
44 optional. In the configuration parameter, it is flexible to set or allocate more slots for uplink or downlink