Why OFDM Transmission Scheme In 5G NR technology uplink & downlink
Orthogonal Frequency Division Multiplexing (OFDM) has been chosen for 5G NR waveform. It was a suitable waveform for NR because of it’s characteristics like robustness to time dispersion and it could be easily exploited both in time and frequency domains. Cycling prefix is a important feature of OFDM. It overcomes the inter symbol interference (ISI) and inter channel interference (ICI). ISI and ICI are introduce due to multi path propagation. It is the basic transmission scheme for both uplink and downlink in 5g NR. We have divided bandwidth into sub carriers, So, OFDM symbol rate is low because a data stream is divided into several parallel streams before transmission. Because of this, fading becomes slow and the channel remains constant during one OFDM symbol interval.
What is the difference between LTE OFDM and 5G NR OFDM?
OFDM in the down link direction is same for LTE and NR. The difference is in the uplink direction. In case of LTE DFT precoded OFDM is the only transmission scheme in the UL while NR will use OFDM as a baseline for UL. NR can also have DFT-precoded OFDM in the UL based on need.
Advantage of OFDM Transmission Scheme In 5G NR technology
- Robustness against multi path propagation
- Resistance to time dispersion
- Low fading
- High transmission bit rates
- Narrow band transmission is possible
Dis advantage of OFDM
- Highly PAPR
- Requires higher synchronization accuracy
What are the Advantages of DFT Precoded OFDM?
- Higher power amplifier efficiency.
- Low peak to average power (PAPR) ratio.
What are the dis-advantages and drawbacks of DFT Precoded OFDM?
- 5G supports uplink spatial multiplexing (MIMO) unlike LTE. So, spatial multiplexing receivers becomes too much complex.
- Difficult to maintain symmetry between uplink and downlink transmission scheme.
- Scheduling restriction: Only contiguous allocations in the frequency domain are possible. This is not ideal to obtain frequency diversity where noncontinuous allocations required.
There are some advantages and dis-advantages with DFT precoded OFDM. That’s why 5G NR has opted for OFDM in the UL directions with complementary support for DFT-precoded OFDM. Please note that when DFT precoded is used UL transmissions are restricted to only single layer while four layers are possible with OFDM.
External source 3GPP Release 15.
Initial access and cell selection call flows in 5g nr network.
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