Abstract
Overlapping basic service set (OBSS) interferences cause significant reductions in the network throughput as seen at the medium access control (MAC) layer. When the frequency bands of multiple OBSSs are partially overlapped, the reduction of throughput are worsened because in addition to the OBSS interference problem there arises the OBSS-induced spectrum underutilization problem which has not been addressed so far. In this paper, two complementary contention-based MAC layer schemes are proposed to solve the OBSS-induced spectrum underutilization problem by setting up a virtual-primary channel for contention. The proposed time-limited (TL) scheme is designed for transmitting long data packets, which is suitable for applications requiring high throughputs. In contrast, the multi-contention (MC) scheme is designed for transmitting short data packets, which is suitable for applications requiring short delays. By efficiently exploiting the OBSS-induced underutilized spectrum, simulation results verify that the proposed TL scheme can increase the throughput greatly and the proposed MC scheme can reduce the delay for short data packets significantly.
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This work was supported in part by the Important National Science & Technology Specific Projects (2013ZX03003013-005).
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Appendix
Appendix
1.1 Time–Frequency Resource Ratio Analysis
The time–frequency resource is defined as the product of occupied/available time and bandwidth. Then, the time–frequency resource ratio (TFRR) of a MAC protocol can be defined as the ratio between the time–frequency resource needed to transmit some data frames using a direct link and the total time–frequency resource used to transmit these data frames under the MAC protocol.
1.2 TFRR of 802.11ac Protocol
Assume there are NS STAs (including AP) contending for the channel access in a BSS and each STA has the same probability to access the channel. If only one data frame is transmitted for each channel access, then M frames are transmitted during a given time when there are M times of successful channel accesses for all STAs in the BSS. Let pn be the probability that an STA in the BSS is a 20n megahertz capable STA, where n (n = 1, 2, …, N) is the number of 20 MHz channels that the STA can operate on. In 802.11ac, when the available bandwidth in the BSS is 80 MHz, the total number of available channels N is 4. Then, in average, Mpn data frames are transmitted by the NSpn 20n megahertz capable STAs in the BSS during the given time duration.
Let LMPDU be the MAC protocol data unit length in bytes, TS be the orthogonal frequency-division multiplexing (OFDM) symbol duration, m be the modulation and coding scheme (MCS) index and Lm,n be the number of data bits that can be transmitted per OFDM symbol over n 20 MHz channels corresponding to an MCS with index m. If the number of bits in the service field plus tail field of the physical layer convergence protocol data unit is ntail, the transmission time for each data frame (without considering the transmission time of the preamble) is
where ⌈·⌉ represents round toward positive infinity. Then, the total time–frequency resource needed to transmit the M data frames (with MCS m) using a direct link is
However, the total time–frequency resource used to transmit these M data frames in 802.11ac is
where \({\text{T}}_{\text{p}}\) is the preamble duration, \({\text{T}}_{\text{DIFS}}\) is the distributed coordination function inter-framing spacing (DIFS) time and \({\text{T}}_{\text{SIFS}}\) is the short-inter-frame-spacing (SIFS) time. The average backoff time is assumed to be equal to \({\bar{\text{T}}}_{{\rm B}} = {\text{T}}_{{\rm slot}} {\text{CW}}_{{\rm min} } /2\), where CWmin is the minimum contention window size and Tslot is the slot duration. (Since we use CWmin to calculate the backoff time, the TFRR to be defined later will be denoted as maximum TFRR). The transmission times for RTS, CTS and ACK frames are given by TRTS = TP + Tm,n(LRTS), TCTS = TP + Tm,n (LCTS) and TACK,n = TP + Tm,n(LACK). Here, LRTS, LCTS and LACK, denoting the number of bytes for RTS, CTS and ACK frames respectively. Then
1.3 TFRR of the TL Scheme
Without loss of generality, we assume that MCS m2 is adopted over the first i 20 MHz channels and MCS m1 is adopted over the left N − i 20 MHz channel to transmit more data when the first i 20 MHz channels are occupied by the neighbor BSS in the TL scheme. Let LSNI denote the number of bytes for SNI, TSNI = TP + Tm,n(LSNI). Then, the total bytes transmitted over the left N − i 20 MHz channels is
It is illustrated in Fig. 11. Then the total time–frequency resource needed to transmit the data frames (with MCS m2) using a direct link is
where
The total time–frequency resource used to transmit these data frames using TL scheme is
Then
1.4 TFRR of the MC Scheme
Assuming that there are NMC times of successful channel accesses over the left N − i 20 MHz channels (with MCS m2) during the transmission time \({\text{T}}_{{{\text{m}}_{2} ,{\text{i}}}} \left( {{\text{L}}_{\text{MPDU}} } \right)\) of the first i 20 MHz channels (with MCS m1), the total bytes transmitted over the left N − i 20 MHz channels is
It is illustrated in Fig. 12. Then the total time–frequency resource needed to transmit the data frames (with MCS m2) using a direct link is
where
However, the total time–frequency resource used to transmit these data frames using MC scheme is
Then
where \({\hat{\text{T}}}_{{{\text{m}}_{1} ,{\text{m}}_{2} ,{\text{n}}}}^{\text{i}} \left( {{\text{L}}_{\text{MPDU}}^{\text{MC}} } \right)\) is given by (14).
For convenience and without loss of generality, consider that the AP and the STAs in BSS1 and BSS2 are 80 MHz and 20 MHz capable devices respectively. Assuming the probability that the primary channel occupied by BSS2 is p, then p1 = p, p2 = 0, p3 = 0 and p4 = 1 − p1 = 1 − p. The minimum and maximum contention window sizes are 15 and 1023, respectively. MCS index m1 = m2 = 1 is used for data transmission. TS = 4 μs, ntail = 22 bytes, \({\text{T}}_{\text{p}} = 40\;\upmu{\text{s}}\), \({\text{T}}_{\text{DIFS}} = 34\;\upmu{\text{s}}\), \({\text{T}}_{\text{SIFS}} = 16\;\upmu{\text{s}}\) and Tslot = 9 μs. We assume that the transmitted frame length is 3000 bytes when the channel is accessed over primary channel. Table 3 shows the TFRR of the proposed scheme and 802.11ac protocol. Obviously, the TFRR of TL scheme is highest which is in line with the intuition. Just considering the results for MC scheme, we observe that the TFRR decreases as NMC increases. This is due to the fact that the bigger the NMC is, the larger the proportion of the overhead is.
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Guo, Y., Lu, IT., Fang, J. et al. MAC Layer Approaches for Mitigating the Spectrum Underutilization Due to Overlapping BSS Problem in WLAN. Wireless Pers Commun 105, 293–311 (2019). https://doi.org/10.1007/s11277-018-6113-7
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DOI: https://doi.org/10.1007/s11277-018-6113-7