How to Maximize Spectral Efficiency with DSS
Dynamic spectrum sharing (DSS) enables mobile network operators to increase 5G coverage without having to permanently refarm Long Term Evolution (LTE) spectrum or buy 5G spectrum. One of the challenges from DSS though is its potential impact on network capacity, which comes from the combined control channels of the two cellular technologies sharing the spectrum – LTE and New Radio (NR).
The overhead from the control channels can reduce network capacity anywhere between 10 and 40% depending on the DSS implementation technique. The scheduler needs to manage the always-on periodic channel or channel/signal resources efficiently to limit the overhead on LTE and NR capacity.
Estimating the impact of the DSS overhead is crucial for any network operator considering implementing DSS. An end-to-end test setup that includes a network, a base station, and real devices can help you achieve that goal.
First, you’ll need:
- Two or more devices including LTE-only and NR-DSS devices
- A test network with no live traffic
- A software measurement platform
Your setup should include at least two devices, an LTE device and a 5G device that supports DSS. The devices should connect to a software measurement platform like Keysight’s Nemo Outdoor or Nemo Walker Air. The software platforms control the test from the device. They also record the radio key performance indicators (KPIs) from the model diagnostic interface and the quality of service (QoS) ones from the application protocol stack. The devices should also connect to the cell. Nemo Outdoor create data transactions on the downlink or the uplink depending on the direction being tested.
Then comes the evaluation part of the process. The metrics are relatively simple:
- L1 or media access control (MAC) throughput for each device
- Physical resource block (PRB) allocations for each device
- Total PRB allocation
- SS reference signal received power (SS-RSRP), RSRP
Figure 1 provides example test results for a 5G non-standalone (NSA) network with two devices, a Qualcomm Snapdragon X55-based 5G device and a legacy LTE device. The test network has no live traffic in LTE or the 5G cell and uses the multimedia broadcast multicast service over single frequency network (MBSFN) subframe technique to implement DSS. The LTE anchor carrier for 5G is on the 800 MHz LTE band 20 while the LTE<>5G shared carrier is on 1800 MHz (5G n3 FDD band and LTE band 3).
Figure 1. Throughput measurements with DSS enabled for a 5G NSA test network
The time series at the top of the chart shows the throughput of the 5G device. At the beginning of the test, the data rate is at 140 Mbps but there is no other traffic on the 5G cell or the overlapping DSS 4G cell. The 4G device then starts a download with a 80 Mbps downlink throughput. At that time, the 5G device throughput drops from 140 to 80 Mbps. The dark red line at the bottom shows the total throughput from the DSS channel. It is at 160 Mbps when both transfers are ongoing.
You can observe the same phenomena for PRB allocations (Figure 2). At first, the 5G device receives 100 PRBs, but when the 4G device starts its download, the PRB allocation drops to around 57 PRBs. The PRB allocation for the LTE device is only around 40-44 PRBs. The uneven split is due to the 5G device needing slightly more spectrum than the LTE device to achieve the same throughput probably because DSS generates a higher overhead for the 5G device.
Figure 2. PRB allocation measurements with DSS enabled for a 5G NSA test network
With this information, you can then calculate the DSS overhead. Without DSS enabled, the available 20 MHz bandwidth with 256 quadrature amplitude modulation (QAM) and 2x2 multiple-input multiple-output (MIMO) used in the test can deliver a maximum capacity of 195 Mbps for LTE and 226 Mbps for 5G. The test realized a maximum total throughput of 160 Mbps for both LTE and 5G. This translates to a 21% overhead compared to using LTE without enabling DSS and 41% for 5G as per the following formulas:
- 1-(195 Mbps/160 Mbps)= 21%
- 1-(226 Mbps/160 Mbps)= 41%
While DSS requires functional testing and a range of mobility and handover test cases, spectral efficiency is a key aspect in evaluating DSS performance for your network. Measuring the DSS overhead enables you to estimate the impact from implementing DSS on network capacity, a crucial aspect to understand before moving forward with deploying the technology in your network.
For more information on DSS, visit Keysight’s 5G Dynamic Spectrum Sharing webpage where you can download the application note Dynamic Spectrum Sharing (DSS) Functional and Performance Verification with Keysight Nemo Tools, learn about test solutions, and view demonstration videos.