Slot Jamming Effect and Mitigation Between LTE-LAA and WLAN Systems With Heterogenous Slot Durations

Abstract

To improve spectrum sharing between long-term evolution (LTE) license assisted access (LAA) and incumbent systems such as wireless local area networks (WLANs) in unlicensed spectrum, listen before talk (LBT) has been proposed as a candidate for LAA channel access. To allow for a robust spectrum sensing performance, LBT may use a backoff-slot duration that is substantially larger than its WLAN counterpart. There is potential for an unknown backoff slot-jamming (SJ) effect, which may significantly decrease channel access probability (CAP) and throughput of LAA-LBT links. In this paper, we study the SJ effect and propose an effective anti-SJ (ASJ) LBT scheme. To gain theoretical insight, we develop a new performance analysis approach on coexisting systems with different slot durations. We model the LAA backoff process with super-counters, provide an in-depth analysis of the backoff process, and derive key performance indicator (KPI) statistics. These KPIs include backoff hold time, successful transmission probability, CAP, and throughput. Simulation results thoroughly validate our analytical results, and show that the ASJ-LBT scheme is effective in mitigating the SJ effect. These results fill a major technical gap in spectrum sharing research and may be extended to support system optimization and coexistence analysis of other heterogeneous systems.

Keywords: CSMA/CA; LTE-LAA; MAC-layer Performance Analysis; WLAN; Wireless Spectrum Sharing.

Fig. 1:

Flow diagram of LTE downlink LAA LBT Category-4 procedure (aka, an original LBT), adopted from [4], [5] with minor revisions. We mark the backoff SJ effect assuming that the LTE-LAA system has a backoff-slot duration larger than that of the WLAN system.

Fig. 2:

Backoff counter reduction and transmission process of one LAA link (with original-LBT) and one WLAN link, when the LAA has backoff-slot duration three times as large as that of the WLAN system (N_s = 3).

Fig. 3:

Backoff counter reduction and transmission process of one LAA link (with the proposed ASJ-LBT) and one WLAN link, when N_s = 3.

Fig. 4:

Markov model for the LTE-LAA category-4 LBT procedure in coexistence with WLAN, when overall state transition has K + 1 stages, and backoff stage k (state R_k) is illustrated (with N_s > 1).

Fig. 5:

Proposed super-counter model for the original LTE-LAA LBT procedure in coexistence with WLAN (when N_s > 1).

Fig. 6:

Proposed backoff model on probability paths for the LAA with ASJ-LBT.

Fig. 7:

Throughput of two CSMA/CA links, when N_s = 1 ~ 5. CW = 16 for both links, without retransmission.

Fig. 8:

Average successful transmission probability per link of the LAA and WLAN systems vs. n_W, when N_s = 2, K = 1, M = 3, Z₀ = W₀ = 16, n_L = n_W, and with RTS/CTS schemes. (a) Original LBT and (b) ASJ-LBT.

Fig. 9:

Backoff counter hold time per reduction of LAA and WLAN systems vs. n_W, when N_s = 3, K = 1, M = 3, Z₀ = W₀ = 16, n_L = n_W, and with RTS/CTS schemes.

Fig. 10:

System throughput of LAA and WLAN vs. PER (P_{E P,L} = P_{E P,W}), when N_s = 2, K = M = 3, W₀ = Z₀ = 16, n_L = n_W = 10, and with basic access schemes.

Fig. 11:

System throughput of LAA and WLAN systems vs. PER (P_{E P,L} = P_{E P,W}), when N_s = 2, K = M = 3, W₀ = Z₀ = 16, n_L = n_W = 10, and with RTS/CTS access schemes.

Fig. 12:

System throughput of LAA and WLAN vs. n_W, when N_s = 3, K = 1, M = 3, Z₀ = W₀ = 16, n_L = 4, and with RTS/CTS schemes.

Fig. 13:

Throughput per link in the LAA and WLAN systems vs. n_L, when N_s = 2, K = M = 3, W₀ = Z₀ = 16, and n_W + n_L = 20. (a) WLAN basic access and LAA RTS/CTS access; (b) Both WLAN and LAA use RTS/CTS.

Fig. 14:

System throughput of LAA and WLAN vs. Z₀, when N_s = 2, K = M = 3, W₀ = 16, n_L = n_W = 10, and with RTS/CTS access schemes.

Fig. 15:

Illustration of Markov model for the LAA CR when N_s = 1.