5G OFDM Technology

First-generation mobile telephony was based on analog technology, while 2G was the first digital communication system that was based in Time Division Multiple Access (TDMA).  3G introduced Code Division Multiple Access, while 4G used Orthogonal Frequency Division Multiple Access (OFDMA) for the Downlink and Digital Fourier Transformation – Spread – OFDMA (DFT-S-OFDMA) for the Uplink.  5G technology is also planning to use Orthogonal Frequency Division Multiple Access (OFDMA) for the Downlink transmission while OFDMA and/or Digital Fourier Transformation – Spread – OFDMA (DFT-S-OFDMA) for the Uplink.

5G vs LTE

The difference of 5G compared to LTE is that an extensive range of frequency bands will be used, ranging from 400 MHz up to the millimeter-wave region.  Moving towards the higher frequencies of the millimeter-wave spectrum, the cell coverage will be reduced, but higher data rates and a denser network will compensate for coverage loss.  LTE bands and LTE/5G spectrum carrier aggregation will also cater for adequate coverage.

With 5G, complexity is reduced by optimized DSP implementations and easily scalable bandwidths.  Spectrum efficiency is improved with frequency selective scheduling that interoperates with MIMO.  Extra flexibility is added by efficiently scaling of subcarriers and symbols and by grouping in specifically assigned resource blocks.  There is also coexistence with LTE and other OFDM based systems and reduced out-of-band emissions by windowing.  5G is expected to have low power consumption since a single carrier variation with a low peak-to-average power ratio will be used for efficient uplink transmission.  Gradually, this technology will help emerging new applications with stringent requirements, especially regarding reliability and latency to cover mainly the Ultra Reliability Ultra-Low Latency (URLLC) use cases.

OFDM principles

In conventional systems, the signal transmission was in the time domain. The complexity of these systems, however, increased with bandwidth since TDM requires equalizing, and although having a simple RAKE receiver, the interference was still intense.  With OFDM,  a discrete finite time-domain signal (block of symbols) is transformed into an OFDM signal according to the distance between the subcarriers and the number of complex modulated subcarriers.  OFDM uses orthogonality, which is the relation between the rectangle width (symbol duration) and the subcarrier spacing.  This is automatically accomplished by Digital Fourier Transformation (DFT).

In multipath propagation, the demodulator integrates over a whole symbol.  Orthogonality is lost in the overlapping part, and the last part of the symbol is copied to the beginning.  In OFDM, a cycle prefix is used, which should be longer than the delay spread.  The cycle prefix length depends on cell size, but it is typically 10-20% of the symbol duration. This has an impact on the symbol rate and the SNR, but there is no Inter Symbol Interference (ISI) present.

OFDM, has high spectral efficiency, especially with a simple receiver design. OFDM has strong sidelobes, where a guard band and filtering/windowing is needed to take care of orthogonality. The Bandwidth is divided into many narrow tones, where the tones are basically orthogonal to each other with no Inter-Symbol Interference (ISI) by Cyclic Prefix.  In OFDM, the bandwidth can be allocated more flexibly by scaling the subcarriers, and the high-rate data is distributed onto many low-rate channels.  This can be achieved by assigning specific blocks of subcarriers to different users and multiplexing while keeping the users orthogonal.  Furthermore, emission masks in OFDM may reduce out-of-band emissions.

OFDM Multiple Access

In OFDM all subcarriers of the symbol are used for providing data to a specific user, while in OFDM Multiple Access (OFDMA) the subcarriers of each symbol may be divided by multiple users, making more efficient use of radio resources.  OFDMA uses a two-dimensional resource allocation, both in time and in frequency.  The minimum resource allocation is called a resource block (RB).  I

The problem with Uplink OFDM is the Peak to Average Power Ratio (PAR).  For example, a multi-carrier signal has 3dBs more PAR, which means 3dB less in power amplifier, 20% less range, and 44% more BS‘s.  The only way to compensate is the use of more expensive power amplifiers in the UEs.   This will have a significant cost impact on the business case and the corresponding CapEx.

The only way to get back to a single carrier is to spread the modulation symbols across the subcarriers.  That is where DFT is used.  In DFT, a similar transmitter structure is used except from the DTF spreading. However, the receiver requires an equalizer in case of multi-path phenomena, and the transmission is limited to continuous resource allocation.  DFT-Spread OFDMA is also the basis for LTE.

DFT-S-OFDMA is also supported in 5G NR, but OFDMA provides a more flexible resource assignment and has additional gain by exploiting frequency diversity.  DFT-S-OFDMA requires an equalizer in case of multi-path and there is a loss of orthogonality impact, especially at higher-order modulations and MIMO.  The added flexibility is considered beneficial for 5G. If there are sufficient power headroom and proper channel conditions, the loss by DFT-S-OFDMA is larger than its PAR gain.

5G NR Uplink transmission

As mentioned above, in OFDMA, the data is sent in parallel across multiple subcarriers.  This means a high peak-to-average (PAP) ratio, multi-user diversity, and no inter-symbol interference with the insertion of the cycle prefix.  However, the system is still sensitive to frequency offsets that cause inter-carrier interference between subcarriers.

In DFT-S-OFDM (SC-FDMA), the data is sent successively on a single carrier, with Low PAP ratio, that restricts scheduling (no diversity).  This means that inter-symbol interference must be equalized with a minimum mean-squared error (MMSE) equalizer, which is not sensitive against frequency/doppler offset.

For 5G systems, flexible use of both DFT-S-OFDMA and OFDMA is recommended.  DFT-S-OFDMA provides a low PAP ratio, which means a more efficient power amplifier, and a higher average transmit power to provide better coverage and data rate, and lower battery consumption.  Nevertheless, equalization is required in the frequency domain, which means increased complexity and inferior performance in some cases.  Uplink OFDMA provides full flexibility and could be used when the user equipment is not power limited.  5G Uplink will provide a fixed per cell or dynamic configuration of the UE, where the thresholds are broadcasted, or there is per UE switching by the gNB.