LTE technology basics
LTE key parameters
Introduction to OFDMA and downlink frame structure:
What is OFDM?Single Carrier Transmission (e.g. WCDMA)
Orthogonal Frequency Division Multiplexing
OFDM signal generation chain
OFDM signal generation is based on Inverse Fast Fourier Transform (IFFT) operation on transmitter side:
On receiver side, an FFT operation will be used.
Difference between OFDM and OFDMA
*OFDM allocates users in time domain only
*OFDMA allocates users in time and frequency domain
LTE downlink :conventional OFDMA
LTE provides QPSK, 16QAM, 64QAM as downlink modulation schemes
- Cyclic prefix is used as guard interval, different configurations possible:
- Normal cyclic prefix with 5.2 Os (first symbol) / 4.7 Os (other symbols)
- Extended cyclic prefix with 16.7 Os
- 15 kHz subcarrier spacing
- Scalable bandwidth
OFDMA time-frequency multiplexing
How to generate SC-FDMA?
-DFT “pre-coding” is performed on modulated data symbols to transform them
into frequency domain,
-Sub-carrier mapping allows flexible allocation of signal to available sub-carriers,
-IFFT and cyclic prefix (CP) insertion as in OFDM,
-Each subcarrier carries a portion of superposed DFT spread data symbols,
-therefore SC-FDMA is also referred to as DFT-spread-OFDM (DFT-s-OFDM).
How does a SC-FDMA signal look like?
-in OFDMA, each sub-carrier only carries information related to one specific symbol,
-in SC-FDMA, each sub-carrier contains information of ALL transmitted symbols.
SC-FDMA signal generation
-We have seen that DFT will distribute the time signal over the frequency domain
SC-FDMA parameterization (FDD and TDD)
LTE FDD
l Same as in downlink,
TD-LTE
-Usage of UL depends on the selected UL-DL configuration (1 to 8), each
configuration offers a different number of subframes (1ms) for uplink
transmission,
-Parameterization for those subframes, means number of SC-FDMA symbols
same as for FDD and depending on CP,
- Cyclic prefix is used as guard interval, different configurations possible:
- Normal cyclic prefix with 5.2 Os (first symbol) / 4.7 Os (other symbols)
- Extended cyclic prefix with 16.7 Os
- 15 kHz subcarrier spacing
- Scalable bandwidth
-DFT “pre-coding” is performed on modulated data symbols to transform them
into frequency domain,
-Sub-carrier mapping allows flexible allocation of signal to available sub-carriers,
-IFFT and cyclic prefix (CP) insertion as in OFDM,
-Each subcarrier carries a portion of superposed DFT spread data symbols,
-therefore SC-FDMA is also referred to as DFT-spread-OFDM (DFT-s-OFDM).
-in OFDMA, each sub-carrier only carries information related to one specific symbol,
-in SC-FDMA, each sub-carrier contains information of ALL transmitted symbols.
-We have seen that DFT will distribute the time signal over the frequency domain
LTE FDD
l Same as in downlink,
-Usage of UL depends on the selected UL-DL configuration (1 to 8), each
configuration offers a different number of subframes (1ms) for uplink
transmission,
-Parameterization for those subframes, means number of SC-FDMA symbols
same as for FDD and depending on CP,