CHARACTERISTICS OF 5G HANDOVERS

Motivated by our findings in §4, we systematically investigate the key characteristics of handovers (HOs) in 5G using our large dataset. We focus on three key aspects that affect the UE performance: HO frequency, HO duration, and HO energy consumption by UE.

Handover Frequency

We use our drive test data to quantify the frequency of HOs across radio access technologies (4G vs. 5G), architectures (SA vs. NSA), and bands (low-band vs. mid-band vs. mmWave). Our findings suggest that compared to 4G, HOs become more frequent in NSA 5G. Specifically, in our freeway drive tests (Table 1), NSA 5G HOs are triggered every 0.4 km on average, in contrast to every 0.6 km for 4G HOs. As NSA uses 4G as control plane and 5G as data plane, both NSA-4C and 5G-NR HOs are triggered on the UE. This leads to more frequent HOs in NSA 5G when compared to 4G. On the other hand, SA 5G experiences an HO every 0.9 km. This suggests that SA realizes the performance benefits promised by 5G and reduces HO overheads [61]. For different bands within NSA, mmWave 5G sees a HO every 0.13 km, mid-band every 0.35 km, and low-band every 0.4 km. The frequency of HOs in NSA mmWave is particularly high due to the small coverage of mmWave 5G cells (§6.1). This leads to high energy inefficiency as will be measured in §5.3.

We also compare HO-related signaling overheads across all radio access technologies (LTE vs. NSA vs. SA) and bands (low-band vs. mmWave). Specifically, we include three message types for RRC Layer (Measurement Reports, RRC Reconfiguration, and RRC Reconfiguration Complete [10]). We also consider Random Access (RACH) procedure on MAC layer [5] and SSR measurements (defined in §2) on PHY layer. We find that SA 5G reduces HO-related signaling messages by a factor of ∼3.8× when compared to LTE because of lower HO frequency. Additionally, HO-related signaling, especially PHYlayer procedures, increases significantly (over a 5-fold increase) in NSA mmWave compared to low-band, again due to the small mmWave cell coverage and beam management procedures [2, 53].

Handover Duration

Our application-layer study in §4 identified long 5G HOs to be a leading cause of application performance degradation during user mobility. This is also confirmed by previous studies in LTE [63, 65]. We now conduct an in-depth investigation of 5G HO duration. Overall, we find that HO duration increases significantly in NSA 5G. The average HO duration in NSA 5G is 167 ms, a 119% increase compared to 76 ms for 4G/LTE HOs. SA 5G HOs, on the other hand, are significantly shorter (110 ms) on average despite its high variation. To explain the above results, we split the HO into two stages based on the radio resource control (RRC) processes involved: (i) preparation stage (𝑇 1 ) during which the carrier decides a new cell for HO, and (ii) execution stage (𝑇 2 ) in which the actual HO is performed and the UE connects to a new cell.

[𝑇 1 ] HO Preparation Stage.𝑇 1 is key when deciding and preparing a new cell for HO, and it accounts for 41% of the overall HO duration in NSA 5G. Once the primary cell is notified about a measurement event via MR, it uses the carrier-specific HO logic to decide whether to perform a HO. If yes, the source cell requests the target cell to allocate radio resources for the incoming UE [11]. As HO is performed when UE’s signal strength is bad, a long 𝑇 1 duration causes the UE to stay in worse network conditions for a prolonged time. Fig. 8 shows the time consumed by OpY in𝑇 1 stage across their deployments: LTE vs. NSA vs. SA. We clearly notice that NSA 5G takes on average 92 ms (which is almost 48%) more time than LTE. This delay in NSA 5G is very likely due to additional signaling overheads. For instance, HOs in NSA 5G involve communication between distributed identities (eNB and gNB) that may or may not be co-located [4, 60]. On the other hand, the median time spent on 𝑇 1 phase by SA 5G is comparable and to some extent slightly better than LTE. But, SA 5G still experiences large variance in the time spent on 𝑇 1 . We suspect that SA 5G is still in rudimentary stages leading to high variations in HO duration. However, due to limited visibility into carrier’s network, we are unable to confirm this. Later, we also explore how carriers can reduce 𝑇 1 by intelligently configuring their HO decision logic (§6.3).

[𝑇 2 ] HO Execution Stage. Compared to 𝑇 1 , 𝑇 2 has more direct impacts on upper-layer performance, and it accounts for ∼59% of the overall HO duration in NSA 5G. During this phase, the HO from a source cell to a target cell is performed. Additionally, the data plane operations are completely halted 1 , hence the duration spent on 𝑇 2 is critical to upper-layer application performance and user QoE. The HO ends with a successful completion of RACH procedure. Due to additional signaling overheads [10, 65], NSA 5G leads to a higher 𝑇 2 that is 1.4-5.4× compared to LTE. Within NSA 5G, mmWave band incurs 42∼45 % larger 𝑇 2 time than low-band, despite the fact that the RACH procedure (part of 𝑇 2 ) takes less time in mmWave when compared to low-band due to shorter PRACH formats [7]. We suspect that beam management procedures involved in performing the complex beam tracking, searching, selection, etc. result in higher 𝑇 2 in mmWave 5G [2, 53].

Overall, the above decomposition highlights the complexities involved in 5G HOs. In particular, in NSA, the dependency of 5G on 4G’s control plane results in the exchange of additional signaling messages between eNB and gNB that leads to longer HO duration.

Handover Energy Consumption

We quantify the energy overhead for NSA 5G HOs and compare our results with 4G HOs. We use 5G Tracker, XCAL, and Monsoon Power Monitor (MPM) introduced in §2 to conduct drive tests in areas with OpX NSA 5G (low-band and mmWave) and LTE coverage. Here, we focus on NSA HOs that bear higher HO frequency and in general smaller cell coverage compared to SA HOs.

Data Collection Methodology. To precisely calculate the energy consumption of HOs, ideally we need two pieces of data: (i) lowerlayer measurement events, reports, and HOs information that can be precisely obtained from XCAL, and (ii) the actual power readings during HOs. We use MPM to profile the power consumption of a high-end smartphone (Samsung Galaxy S20 Ultra 5G). A practical challenge is that XCAL and MPM cannot be used simultaneously as the smartphone will draw current from the tethered XCAL laptop, making the MPM’s power reading meaningless. To address this challenge, we first survey 42 km+ using XCAL to identify spots where a HO is triggered repeatedly by a single measurement event. Then, we drive 6 loops around identified spots with 5G Tracker and XCAL to establish the ground-truth of HOs. Specifically, we verify that the HO, radio technology, and band information reported by 5G Tracker’s Android APIs is exactly same as XCAL data. Finally, we drive 10 loops with 5G Tracker (which does not require laptop tethering) and MPM to collect HO power measurements. To keep the UE in RRC connected state [10], we send a 32-byte ping packet every 5 seconds 2 . To exclude the ping transmission power, we take a +1s window starting from the time when a ping packet was transmitted and remove the corresponding measurements. We set the phone brightness to 25% for consistency and subtract baseline power from the total when presenting results. The baseline power is calculated when there is no HO and the UE is stationary. The transmission power of PING packets is also subtracted.

HO Energy Results. We calculate the battery drain for a typical smartphone using NSA 5G low-band. We find that a smartphone traveling at 130 km/h for 1 hour can witness on average 553 5G HOs. This will result in ∼34.7 mAh energy drain. 4G HOs, on the other hand, only consume ∼3.4 mAh energy. Similarly, NSA mmWave can experience 998 HOs and drain ∼81.7 mAh energy using the same settings.

Intuition suggests that when the device is in RRC connected state [10] (transmitting or receiving data), the data-plane energy consumption overwhelms the control-plane (HO) energy, but our experiments tell a different story for commercial 5G. We compare the HO energy consumption with the data-plane energy consumption. Narayanan et al. [54] present the power consumption per byte for the same smartphone model as ours i.e., S20U. In particular, we use the slopes of Throughput-Power curves presented in Table 8 of Narayanan et al.’s [54] work. We find that S20U using NSA low-band can download ∼4.3 GB data (or upload ∼2.0 GB data) with 34.7 mAh worth of battery capacity. Likewise, NSA mmWave can download ∼75.4 GB data (or upload ∼14.5 GB data) using 81.7 mAh energy. These results indicate the non-trivial energy footprint for 5G HOs, in particular small form-factor devices such embedded IoT devices that relatively have lesser and limited power resources.

Fig. 10 provides further details of our HO energy experiments. The figure shows two metrics: (i) the power consumption of a single HO (left y-axis), and (ii) the energy consumption per unit distance (right y-axis). To compute energy per-unit distance, we take into account the frequency of HOs measured in §5.1. We separately plot the HO power consumption of 4G/LTE mid-band (left), NSA low-band (middle), and NSA mmWave (right). As shown in Fig. 10, HOs in NSA 5G consume 1.2-2.3× more energy when compared to HOs in 4G/LTE. The HO energy consumption is higher for NSA 5G HOs because both 4G and 5G radio are involved in the HO process. Surprisingly, a single mmWave HO in NSA 5G is 54% more energy efficient than a single low-band HO. This is likely because the improved RACH procedure in mmWave [7] results in lower HO energy consumption. Despite this, since HOs are highly frequent in NSA mmWave bands (§5.1), they cumulatively incur a greater energy footprint. For instance, we find that NSA mmWave HOs result in 1.9-2.4× more energy consumption per-unit distance compared to low-band HOs.

TL;DR

5G切换的关键特征:

为了深入了解5G切换的特点，我们从三个方面进行了分析：切换频率、切换持续时间和能耗。

1. 切换频率：我们的研究表明，NSA 5G的切换频率高于4G。具体来说，NSA 5G在高速公路上平均每0.4公里发生一次切换，而4G则是每0.6公里。SA 5G的切换频率较低，每0.9公里发生一次切换。NSA 5G中的毫米波频段切换频率最高，每0.13公里发生一次切换，这是由于毫米波小细胞覆盖范围较小所致[1]。

2. 切换持续时间：NSA 5G的切换平均持续时间为167毫秒，相比4G的76毫秒增加了119%。SA 5G的切换持续时间较短，平均为110毫秒，但存在较大变异。我们将切换过程分为准备阶段和执行阶段。准备阶段占NSA 5G切换总时间的41%，主要是由于NSA 5G依赖4G的控制平面，导致额外的信令开销[1]。

3. 能耗：我们量化了NSA 5G切换的能耗，并与4G进行了比较。结果显示，NSA 5G切换的能耗显著高于4G。例如，一部智能手机在1小时内经历553次NSA 5G低频段切换，能耗约34.7毫安时，而4G切换仅消耗约3.4毫安时。尽管NSA毫米波单次切换的能耗较低，但由于切换频率高，累积能耗更大[1]。