3.5 GHz Spectrum Analysis and Challenges

Radio spectrum is the core resource for the mobile communications. 5G Spectra are identified as multiple 5G frequency bands with their own advantages and disadvantages. The world’s prime commercial 5G networks are deployed with the high frequency bands, including 3.5 GHz (3.3 – 3.8 GHz, band n78), 2.6 GHz (2.496 – 2.69 GHz, band n41) and millimeter-wave frequency band. Compared with the main FDD-LTE spectrum bands such as 1.8 GHz (band 3), 3.5Ghz band introduces some disadvantages including higher penetration loss and a lower uplink ratio based on TDD (Time Division Duplex) mode. So, to meet the 5G services requirement by deploying 5G networks over 3.5 GHz will face the challenges in the following fields: uplink bandwidth, uplink coverage and transmission latency.

  • Uplink Bandwidth

With TDD mode, the uplink and downlink are identified by different timeslots on the same frequency range. In China, the configuration of 3 uplink slots to 7 downlink timeslots over the 3.5 GHz band is recommended, that is, 30% of the time slots is used for uplink and 70% of the time slots is used for downlink. Taking the 100 MHz bandwidth as an example, the equivalent uplink bandwidth is only 30 MHz, which is only 1.5 times that of the 4G single carrier.

  • Uplink Coverage

The higher the frequency, the greater the space propagation loss and the shorter the coverage distance. For example, the propagation loss of the 3.5 GHz is 5 dB more than that of the 2.1 GHz.

In addition, the higher the frequency, the greater the penetration loss, and further shorter the coverage distance.

  • Transmission Latency

With TDD mode, terminals cannot send uplink data while receiving downlink data, which results in an extra latency for uplink. For the 3.5 GHz band with the uplink proportion of 30%, there will be an extra delay of 0 to 2 ms, with an average of 0.8 ms. Likewise, in the downlink direction, an extra delay of 0 to 1 ms and an average of 0.2 ms.

FDD Assisted Super TDD (FAST) to Improve 5G Network Capacity and Coverage

Based on the above spectrums analysis and the development progress of ecosystem, 5G uplink performance enhancement with low-frequency bands such as 2.1 GHz and 700 MHz is the main concern for many operators.

To address this requirement, ZTE proposes the 5G FAST (FDD Assisted Super TDD) solution to effectively improve 5G uplink and downlink performance with FDD and TDD deep cooperation. The FDD band, based on frequency division duplex mode, is characterized by low frequency range with small bandwidth, wide coverage, and no extra transmission latency. Contrastively, TDD band means the middle/high frequency range with large bandwidth, the MIMO technology adopted in uplink and downlink, and the coverage and latency of TDD are not as good as that of FDD. To effectively combine the advantages of FDD and TDD, FAST, as shown in Figure 1, deeply aggregates the spectrum of FDD and TDD both in uplink and downlink for terminals in the center of the cell (near point) to achieve large throughput and low latency; and for the terminals at the cell edge (Far point),  FAST enables them switch to FDD in uplink for better coverage, while still maintains FDD and TDD carrier aggregation in downlink with the higher data speed.

Figure 1 FAST Transmission Mechanism

Based on the standard carrier aggregation framework which is already widely applied in 4G network and with innovative uplink TDM scheduling, FAST can flexibly aggregate and coordinate FDD and TDD in time and frequency domain to enable spectrum to reach its full potential, thus to solve the challenges of uplink bandwidth, uplink coverage and transmission latency.

  • Enhancing 5G Capacity

With 5G network deployed over 3.5GHz band, and assisted by 2.1 GHz band of 20MHz bandwidth, FAST can improve the single user’s uplink throughput by 23% and the downlink throughput by 28%. In the future, if 50 MHz bandwidth of the 2.1 GHz band is acquired, FAST can enhance the uplink and downlink throughput by 58% and 71% respectively.

The 5G terminal generally has up to 2 uplink transmit channels. For the TDD frequency band, the 2 x 2 MIMO transmission can be used in uplink, and the equivalent throughput is doubled. However, if the terminal utilizes the conventional uplink carrier aggregation technology to aggregate the FDD and TDD carriers, FDD and TDD can only use one transmit channel respectively, and TDD uplink 2 x 2 MIMO will be disabled, which results in the capacity loss. To address this problem, 5G FAST solution uses the TDM (time division mode) scheduling to reserve the 2 x 2 MIMO uplink capability for TDD carrier. TDM scheduling means that, during the TDD uplink time slots, 2 uplink transmit channels work in the TDD 2 x 2 MIMO transmission mode, and during the TDD downlink time slots, they are switched immediately to work in FDD mode for uplink transmission. This scheduling mechanism not only make the use of the time slots to nearly 100% in uplink as well as maintain the TDD 2×2 MIMO capabilities.

Figure 2 TDM scheduling mechanism of FAST

  • Improving 5G Coverage

When 5G is deployed over the 3.5 GHz bands, the coverage bottleneck will first appear in the uplink direction while the downlink coverage is still acceptable. This ‘asymmetry’ of uplink and downlink restricts the coverage of 3.5 GHz and reduces the network spectrum efficiency. With FAST, terminals can connect simultaneously to both FDD and TDD carriers. On the cell edge, the terminal benefits continuously the large TDD capacity in downlink, and the uplink transmission will be switched to the FDD carriers for better 5G coverage and 5G services is expanded beyond the TDD uplink coverage.

With the deep harmonization of FDD and TDD, the coverage is enlarged compared with the single carrier of 3.5GHz and the downlink data speed is improved when compared to that of 2.1GHz. Taking the 2.1 GHz (FDD) and 3.5 GHz (TDD) as an example, the terminal can switch to 2.1 GHz in uplink when moving beyond the uplink coverage edge of 3.5 GHz, the uplink time slot resource is increased by 2.3 times compared with the network with the single carrier of 3.5 GHz; and the downlink bandwidth is 2.5 times more than that of single 2.1 GHz.

  • Reducing 5G latency

With FAST, the terminals can be flexibly scheduled to transmit data on FDD and/or TDD carriers. The downlink and uplink time slots are both 100% available without introducing extra waiting latency, which lessens the transmission latency. Taking uplink as an example, the average uplink transmission latency of 3.5 GHz on the TDD single carrier is about 2.2 ms, which can be reduced to 1.5 ms with FAST, up to 31% decrease.

  • Flexible networking and easy deployment

Based on the standard carrier aggregation framework which is already widely applied in 4G network, FAST is the mainstream solution of 5G NR to facilitate the 5G commercialization. First, the flexible coordination of FDD and TDD among inter-cell and inter-site are all supported. Unlike other uplink enhancement solutions, FAST does NOT have the mandatory FDD and TDD co-site requirement for 5G deployment. Second, there is no tight bind relationship between FDD and TDD carriers, that is, FDD carrier can deeply be aggregated and coordinated with multiple TDD carriers, and vice versa. The aggregation and coordination among TDD and FDD are established dynamically for the terminal.

In the scenario that FDD is NOT co-site with TDD or the coverage of FDD and TDD does not completely overlap, the flexible scheduling technologies, including the use of static codebook and Two PUCCH group, etc, can be enabled by FAST to match the deployment scenarios requirements.

In November 2019, with joint verification conducted by China Telecom and ZTE, it was proved that with 2.1GHz and 3.5GHz, FAST can improve uplink throughput by 40% compared with that of the 3.5GHz single carrier. FAST technology is in the standardization process of 3GPP and is expected to be released in R16.