RIS-PP: RIS-aided Ping-Pong Avoidance

• The paper reports that RIS-PP effectively mitigates ping-pong handovers by dynamically reconfiguring the RIS-assisted path when the BER nears the hard handover threshold.
• It introduces a BER-triggered decision rule that selects a RIS-aided propagation path to maintain the serving BS link without initiating a full handover.
• Evaluations show that RIS-PP reduces measurement reporting and handover events while enhancing spectrum efficiency, energy efficiency, and overall QoS in dense deployments.

Searching arXiv for the specified paper and closely related RIS handover research. RIS-aided ping-pong avoidance (RIS-PP) is a mobility-management strategy in RIS-assisted cellular networks that addresses repeated handovers back and forth between two cells when link quality hovers around a handover decision threshold. In the cited formulation, RIS-PP does not hand the user equipment (UE) over to a neighboring base station (BS) when the direct serving-link quality oscillates near the handover margin; instead, it reconfigures the serving BS $\rightarrow$ RIS $\rightarrow$ UE path so that the same serving BS is retained. The mechanism is embedded in a broader handover framework for near-field (NF) and far-field (FF) RIS operation, where bit error rate (BER) is used as the key decision variable for minimizing unnecessary handovers, restricting measurement reports and handover requests, and improving spectrum efficiency (SE), energy efficiency (EE), and quality of service (QoS) in crowded cellular networks (Mondal et al., 29 Jul 2025).

1. Conceptual definition and distinction from conventional ping-pong avoidance

“Ping-pong” refers to repeated handovers (HOs) back and forth between two cells when the link quality hovers around a decision threshold, causing excessive signaling and interruption. RIS-PP is defined as a strategy that dynamically steers RIS reflection so that the instantaneous BER of the composite serving link remains just inside the acceptable QoS region, namely the margin around the hard-handover threshold. In this way, unnecessary hard handovers (HHO) and soft handovers (SHO) are forestalled while the serving BS remains unchanged (Mondal et al., 29 Jul 2025).

The distinction from conventional methods is explicit. Conventional ping-pong avoidance typically uses hysteresis margins or time-to-trigger counters at the BS or UE to delay handover decisions between two neighboring BSs. These methods do not exploit an additional surface node to reshape the propagation path. RIS-PP, by contrast, leverages a programmable metasurface to “refresh” or “boost” the serving-BS link on demand, thereby eliminating the need to switch BSs as long as the RIS-augmented link can be maintained within a small BER margin.

This difference is substantive rather than terminological. Conventional methods primarily delay a cell change; RIS-PP instead modifies the propagation conditions so that the original serving relation can continue. A plausible implication is that RIS-PP should be understood not merely as a mobility-policy refinement, but as a joint propagation-control and mobility-management mechanism.

2. BER-triggered decision rule

The mathematical trigger for RIS-PP is formulated around the instantaneous BER on the serving path, denoted $BER_{p_s}$, the BER threshold for hard handover $T_{hh}$, and a small BER margin $\epsilon$. RIS-PP is invoked when the serving-link BER drifts into the margin band around $T_{hh}$, while a RIS-aided non-direct path remains below the hard-handover threshold. The decision event is

$|BER_{p_s} - T_{hh}| \le \epsilon$

and there exists a RIS-aided path $p_j$ such that

$BER_{p_j} < T_{hh}.$

The corresponding handover probability expression for RIS-PP on link $j$ is given through inclusion-exclusion as

$\rightarrow$0

This formulation makes BER the central mobility variable. The key point is not simply that the RIS offers an alternative path, but that the RIS-augmented path is used specifically when the serving path enters a narrow threshold neighborhood. That threshold-centric design differentiates RIS-PP from generic RIS-assisted coverage enhancement, because the reconfiguration is tied directly to handover suppression rather than only to average-link improvement (Mondal et al., 29 Jul 2025).

3. Propagation and channel models supporting RIS-PP

RIS-PP is developed in a framework that explicitly accounts for both the radiative near-field Fresnel region and the far-field region of the RIS. In the NF region, if the UE position relative to the RIS is uniformly distributed over the circular aperture, the RIS–UE distance $\rightarrow$1 has the probability density function

$\rightarrow$2

When the UE travels along a linear trajectory through the NF band $\rightarrow$3, the alternative model is

$\rightarrow$4

The FF reflection-gain approximation assumes an incident BS$\rightarrow$5RIS plane wave of wavelength $\rightarrow$6, RIS aperture diameter $\rightarrow$7, and $\rightarrow$8 elements with per-element reflection amplitude $\rightarrow$9. Neglecting small-scale fading, the aggregate two-hop path gain is

$BER_{p_s}$0

In the NF regime, the RIS phase shifters can form a true focusing lens. The normalized NF reflection gain is given as

$BER_{p_s}$1

where $BER_{p_s}$2 is the extended Fraunhofer distance, $BER_{p_s}$3, $BER_{p_s}$4 is the focal-point deviation, and $BER_{p_s}$5 and $BER_{p_s}$6 are the Fresnel cosine and sine integrals.

The instantaneous composite SNR model incorporates small-scale variations by assigning each element $BER_{p_s}$7 independent gains $BER_{p_s}$8 and $BER_{p_s}$9. Under perfect phase alignment,

$T_{hh}$0

By the Central Limit Theorem for large $T_{hh}$1, $T_{hh}$2, yielding a closed-form PDF $T_{hh}$3 and the average BER

$T_{hh}$4

These models provide the analytical basis for a mobility decision that depends on how RIS reflection gains vary along the UE trajectory in both NF and FF regions (Mondal et al., 29 Jul 2025).

4. Placement of RIS-PP within the handover algorithm

RIS-PP is one branch of a handover-management algorithm that also includes HHO, SHO, and RIS-aided cell breathing (RIS-CB). The inputs are the UE position $T_{hh}$5 and trajectory model; the set of direct line-of-sight links $T_{hh}$6; the set of non-direct RIS-aided links $T_{hh}$7; BER measurements for each link; the HHO threshold $T_{hh}$8; the SHO threshold $T_{hh}$9; the BER margin $\epsilon$0 around $\epsilon$1; and the serving-BS load threshold $\epsilon$2.

The decision flow begins by measuring $\epsilon$3 for the current serving link $\epsilon$4. If $\epsilon$5, the algorithm attempts HHO by scanning direct candidates $\epsilon$6 and triggering HHO to the best $\epsilon$7 satisfying $\epsilon$8. If instead $\epsilon$9, the algorithm attempts SHO, again over direct candidates, and triggers SHO to the best $T_{hh}$0 satisfying $T_{hh}$1.

RIS-PP appears in the next decision region. If $T_{hh}$2, the algorithm invokes RIS-PP: for each RIS path $T_{hh}$3 with $T_{hh}$4, the RIS is reconfigured and propagation is switched to $T_{hh}$5 while the serving BS remains unchanged. If the current serving load satisfies $T_{hh}$6, the algorithm instead invokes RIS-CB and similarly selects the best RIS-aided path satisfying $T_{hh}$7, again without changing BS (Mondal et al., 29 Jul 2025).

The dynamic operation in the RIS-PP step updates only the RIS phase-shift matrix $T_{hh}$8 so as to steer the focus toward the UE and boost link quality back below $T_{hh}$9. Because the serving BS is retained, no new BS search or core signaling is needed. The cited formulation states that this greatly reduces measurement-reporting overhead and authentication procedures. In operational terms, RIS-PP is therefore a selective reconfiguration policy that sits between ordinary link monitoring and full inter-cell handover.

5. Performance metrics and reported numerical behavior

The evaluation framework uses average HO rate, signaling load, SE, EE, and ping-pong event count as the primary metrics. Ping-pong event count is defined as handovers back to the original BS within Time-to-Trigger. These metrics reflect the paper’s emphasis on reducing unnecessary mobility events while preserving seamless connectivity (Mondal et al., 29 Jul 2025).

The representative numerical trends reported in the cited study are specific. As the number of RIS elements $|BER_{p_s} - T_{hh}| \le \epsilon$0 grows from $|BER_{p_s} - T_{hh}| \le \epsilon$1 to $|BER_{p_s} - T_{hh}| \le \epsilon$2, the average distance traveled before an HO trigger increases by up to $|BER_{p_s} - T_{hh}| \le \epsilon$3, which is associated with delayed HO under NF focusing. Over the same range, the hard HO probability $|BER_{p_s} - T_{hh}| \le \epsilon$4 rises up to $|BER_{p_s} - T_{hh}| \le \epsilon$5, and the soft HO probability $|BER_{p_s} - T_{hh}| \le \epsilon$6 falls by up to $|BER_{p_s} - T_{hh}| \le \epsilon$7. The paper also reports that restricting to RIS-PP eliminates BS discovery and association for a large fraction of borderline events, yielding up to $|BER_{p_s} - T_{hh}| \le \epsilon$8 fewer measurement-report messages in dense deployments.

The same numerical study reports SE increases of $|BER_{p_s} - T_{hh}| \le \epsilon$9–$p_j$0 as the RIS boosts average link SNR in the NF region, and EE rises of $p_j$1 because of fewer BS associations and lower transmit power per bit under RIS focusing. Ping-pong events are reduced by $p_j$2–$p_j$3 in typical mobility scenarios when RIS-PP is active, relative to a baseline handover scheme with only hysteresis margins. Numerical results are also described as showing significant reductions in HO rates and signaling load while ensuring seamless connectivity and improved QoS.

Taken together, these trends locate RIS-PP at the boundary between radio enhancement and mobility optimization. The factual result is that the same mechanism used to reshape propagation is also reported to alter the frequency and type of handover events.

6. Configuration, complexity, and deployment considerations

The practical recommendations for RIS-PP are framed around threshold design, RIS geometry, and control overhead. RIS reconfiguration complexity is stated as $p_j$4 per update, covering phase-shift computation and the over-the-air control link. However, updates occur only when BER drifts into the $p_j$5-margin, which is described as typically infrequent under moderate mobility. Signaling between the serving BS and the RIS is characterized as a lightweight control plane, with tens of bits per element, and no core-network messages or target-BS coordination required during RIS-PP phases. In dense small-cell deployments, the overall reduction in total HO-related signaling is reported as at least $p_j$6 (Mondal et al., 29 Jul 2025).

Threshold selection is also specified. The hard-handover threshold $p_j$7 should be chosen at the QoS knee point, with $p_j$8 given as an example for QPSK or 16-QAM. The soft-handover threshold is recommended as $p_j$9 so that SHO is admitted only when link quality remains comfortably above the HHO margin. The BER margin is recommended as $BER_{p_j} < T_{hh}.$0–$BER_{p_j} < T_{hh}.$1 so that RIS-PP can engage before full HO is required while preventing oscillation within the margin band. The RIS aperture $BER_{p_j} < T_{hh}.$2 and element spacing not exceeding $BER_{p_j} < T_{hh}.$3 should be selected so that the virtual Fresnel radius $BER_{p_j} < T_{hh}.$4 matches the typical cell-edge distance, maximizing NF focusing and the range over which RIS-PP can defer HO.

A common misconception would be to regard RIS-PP as a variant of hysteresis tuning. The cited framework rejects that equivalence. Its summary states that RIS-PP uses the RIS as a quasi-beamformer to “buy time” for the serving BS link whenever BER crosses a small hysteresis band around the HO threshold, and that it differs fundamentally from conventional ping-pong avoidance, which only delays decisions through timers or hysteresis, by actually reshaping the radio path. This suggests that the main deployment question is not whether hysteresis values are conservative enough, but whether RIS control can be executed with sufficient timeliness and fidelity to preserve the serving-BS link in the threshold band.

7. Position within RIS-enabled mobility management

RIS-PP is not an isolated procedure but one of several handover categories within a unified RIS-enabled mobility-management scheme that spans HHO, SHO, RIS-CB, and RIS-PP. Its operational niche is the borderline regime in which the serving link is not yet decisively unacceptable, but its BER has entered a margin around the hard-handover threshold. In that regime, RIS-PP suppresses unnecessary transitions by exploiting a non-direct RIS-aided path instead of changing BS (Mondal et al., 29 Jul 2025).

Its dependence on both NF and FF modeling is also significant. In FF operation, the RIS contributes through the two-hop aperture-based gain that scales with $BER_{p_j} < T_{hh}.$5 under the stated approximation. In NF operation, the RIS acts as a focusing surface whose gain depends on focal-point deviation and the Fresnel integrals. Because the broader handover framework derives the RIS–UE distance distribution in the NF region and ties BER decisions to these propagation gains, RIS-PP is explicitly designed for trajectory-dependent mobility control rather than static coverage enhancement.

Within that formulation, the role of RIS-PP is precise: it minimizes unnecessary handovers by maintaining connectivity with the serving BS through RIS-aided pathways, reducing the requirement for frequent target-BS searching, measurement reporting, and handover requests. Its encyclopedic significance lies in making programmable propagation itself part of the handover-decision apparatus.