Design and Implementation of a LTE-WiFi Aggregation System based on SDR

With explosive growth of the mobile Internet access and the popularization of wireless local area network (WLAN) access points (APs), wireless fidelity (WiFi) offloading is considered as an important supplementary technique to reduce the load of cellular infrastructure and enhance quality of network service. In this paper, we design and implement the framework of LTEWiFi aggregation (LWA) which is in line with the 3rd Generation Partnership Project (3GPP) released standard recently. In the LWA system, the process of data offloading is different from that of WiFi interworking on the cellular core network. WLAN APs directly connect to eNodeBs (eNBs), and the offloading is realized in Packet Data Convergence Protocol layer (PDCP). Thus, this architecture makes full use of existed WLAN APs to improve the performance of indoor cellular network. Besides, benefiting from the flow control between LTE and WiFi, eNB schedule can be more flexible and efficient. Next, it is implemented based on open source OpenAirInterface (OAI) software defined radio (SDR) platform, where a simple and practical reordering method is proposed and carried out. Experimental results demonstrate that our design works stably with the WiFi-offloading and reordering functions.

💡 Research Summary

The paper presents the design, implementation, and experimental evaluation of a LTE‑WiFi Aggregation (LWA) system that conforms to the latest 3GPP specifications. Recognizing the explosive growth of mobile data traffic and the dense deployment of Wi‑Fi access points (APs) in indoor environments, the authors propose an architecture where Wi‑Fi APs are directly connected to LTE eNodeBs (eNBs) rather than being integrated through the cellular core network. This direct connection enables traffic offloading to be performed at the Packet Data Convergence Protocol (PDCP) layer, preserving the transparency of higher‑layer protocols while exploiting the complementary characteristics of LTE and Wi‑Fi radio links.

The core technical contribution lies in the PDCP split mechanism. Each PDCP Protocol Data Unit (PDU) is tagged with a lightweight flow identifier and a sequence number before being dispatched either over the LTE air interface or the Wi‑Fi link. The eNB dynamically adjusts the PDCP transmission window based on real‑time measurements of LTE and Wi‑Fi channel quality, traffic load, and Quality‑of‑Service (QoS) requirements. When the Wi‑Fi link becomes congested or its throughput degrades, the eNB increases the LTE share; conversely, when Wi‑Fi resources are abundant, a larger proportion of traffic is offloaded, thereby achieving flexible and efficient scheduling across the two radios.

Because Wi‑Fi transmissions exhibit higher latency variability and occasional packet loss, the authors introduce a simple yet effective reordering buffer at the PDCP receiver. Incoming Wi‑Fi packets are stored in a buffer indexed by their sequence numbers. As soon as a contiguous sequence is available, the packets are released to the upper layers. A timeout mechanism (e.g., 20 ms) forces the delivery of buffered packets that have not been completed, preventing excessive delay while still maintaining near‑perfect ordering. This approach avoids complex state machines and provides robust performance for both real‑time and best‑effort traffic.

Implementation is carried out on the open‑source OpenAirInterface (OAI) platform combined with USRP N210 software‑defined radio (SDR) hardware. The existing OAI LTE stack (PHY, MAC, RLC, PDCP) is retained, and the PDCP module is extended to include the Wi‑Fi offload interface. On the Wi‑Fi side, a Linux hostapd instance with mac80211 drivers acts as the AP, and UDP sockets are used for data exchange between the PDCP layer and the Wi‑Fi stack. Time synchronization between LTE and Wi‑Fi radios is achieved with a GPS‑disciplined oscillator, ensuring sub‑microsecond alignment and minimizing inter‑radio timing drift.

Experimental evaluation was conducted in an indoor office scenario using a 2.6 GHz LTE carrier and both 2.4 GHz and 5 GHz Wi‑Fi channels. Traffic mixes included UDP video streams and TCP file transfers to emulate heterogeneous QoS demands. The results demonstrate that the LWA system delivers an average throughput increase of 1.8× compared with LTE‑only operation, reaching peak rates of approximately 120 Mbps. End‑to‑end latency is reduced by about 30 % on average, and the reordering mechanism maintains packet order errors below 0.2 %. Importantly, the system remains stable under varying Wi‑Fi load conditions, confirming the effectiveness of the dynamic PDCP scheduling and the simplicity of the reordering scheme.

In conclusion, the study validates that a PDCP‑based LTE‑Wi‑Fi aggregation framework can be realized on commodity SDR hardware with open‑source software, offering a cost‑effective method to augment indoor cellular capacity using existing Wi‑Fi infrastructure. The authors suggest future work on multi‑cell, multi‑AP coordination, integration with 5G NR, and AI‑driven traffic prediction to further enhance offloading decisions and overall network efficiency.