4.2 Downlink MU-MIMO LTE-LAA for Coexistence with Asymmetric Hidden
4.2.6 Discussion
Table 12: Fairness in terms of Throughput and 99%-Guaranteed Delay
Fairness in WLAN
FS FD
without IC with IC without IC with IC 1 eNB and 2 APs 0.998 0.979 0.987 0.972 1 eNB and 5 APs 0.649 0.732 0.718 0.641 1 eNB and 10 APs 0.467 0.640 0.694 0.729
decreases D99% of some APs while increasing D99% of the other APs. Overall, it is confirmed that the proposed scheme enhances maximum possible delay of WLAN and LTE-LAA.
In summary, the proposed scheme improves the throughput and delay performance of LTE- LAA as well as WLAN even in the presence of many APs.
4.2.5.5 Fairness in WLAN Table 12 presents the Jain’s fairness index in terms of through-
Figure 63: Communicatable area around an eNB.
at UEs larger. Hence, the larger the asymmetric hidden area, the more serious the asymmetric hidden terminal problem.
In the practical exemplary indoor scenario, it turns out that the ratio is 0.68 [57], which implies that the asymmetric hidden area occupies68percent of the eNB’s service area. Therefore, the asymmetric hidden terminal problem is indeed an inevitable critical problem.
Is the asymmetric hidden terminal problem really serious?: In [57], to show that the asymmetric hidden terminal problem can really affect the performance of LTE-LAA and Wi-Fi, we investigated the ratio of the communicatable area of the eNB (or the AP) to its coverage area. In Fig. 63, the communicatable area of the eNB (or the AP) is defined as the area within the coverage where the SINR of a UE (or a STA) is larger than the minimum SNR required for the lowest modulation and coding scheme (MCS) in the standard. In other words, a UE (or a STA) within its communicatable area can retain reliable communication even if the eNB and the AP transmit concurrently. Based on the area, we calculated the ratio of the communicatable area to the coverage area of an eNB (or an AP). If the ratio is small, the UE (or the STA) is vulnerable to collision incurred by the AP (or the eNB). In the practical exemplary indoor scenario [57], we figured out that the communicatable area occupies a very small portion of the total coverage. This means that once an AP is located within the asymmetric hidden area, both UEs and STAs become vulnerable to collision in most parts of the coverage.
In addition, the average per-UE (or per-STA) downlink throughput within the communi- catable area was compared in two scenarios. In Scenario 1, only one of the eNB and the AP
(a)XED=−72dBm (b)XED=−62dBm
Figure 64: The change of the relation between an eNB and APs with different XED values.
transmits, whereas in Scenario 2 both of them transmit concurrently. The analysis shows that the per-UE (or per-STA) downlink throughput in Scenario 2 is 94% (or 87%) smaller than in Scenario 1 due to the interference from the AP (or the eNB). The analysis also confirms that neither LTE-LAA nor Wi-Fi can maintain reliable and effective downlink communication to their intended receivers if they are in the asymmetric hidden relation.
As a result, the asymmetric hidden terminal problem is indeed an inevitable problem in LAA- WLAN coexistence, and can severely degrade the performance at receivers of an LTE-LAA eNB and Wi-Fi APs.
4.2.6.2 Changing the energy detection threshold or the transmission power can be a simple solution? Transmit Power: According to the standard [34], the maximum transmit power is limited to 23 dBm in the 5 GHz unlicensed spectrum. In our work, the transmit power of an eNB and APs is set to the maximum transmit power, which is the most typical case in practical scenarios. Therefore, it is not possible to further increase the transmit power of an eNB to resolve the asymmetric hidden terminal problem.
Energy Detection Threshold: According to [6], the Wi-Fi’s energy detection threshold should be set to−62dBm. In addition, numerous conventional Wi-Fi’s APs have been deployed everywhere in reality, and thus it is difficult to change the energy detection threshold of them.
The current LTE-LAA specification [46] regulates the energy detection threshold at LTE-LAA devices, denoted by XED, as XED ≤ −72 dBm for 20MHz bandwidth and transmit power of 23 dBm. Thus, XED cannot be set to −62 dBm as Wi-Fi for simply resolving the asymmetric hidden terminal problem.
One may argue that XED should be set to −62 dBm by amending the LTE-LAA standard.
However, this proposal cannot be a method to solve the asymmetric hidden terminal problem, which can be explained as follows. If XED is set to−62dBm, the asymmetric hidden area (red region in Fig. 64(a)) between the−62dBm and −72 dBm lines around an eNB will be changed to the symmetric hidden area (yellow region in Fig. 64(b)). In such a case, not only are APs in
the symmetric hidden area unable to detect the eNB’s signal, but the eNB also cannot detect the signal from the APs. Hence, the eNB with XED =−62 dBm can initiate its new transmission during transmissions by the APs within the donut-like region between the −62 dBm and −72 dBm lines while an eNB withXED =−72dBm guarantees the APs’ transmissions by deferring its new transmission.
As a result, if an eNB setsXED =−62dBm, the eNB and APs cannot detect each other’s sig- nals, and thus will undergo more collisions, leading to severer degradation in their performance.
In fact, a similar discussion was provided in [57], where it is confirmed that the symmetric hid- den terminal problem degrades the performance of both LAA and Wi-Fi even further than the asymmetric hidden terminal problem.
4.2.6.3 Tradeoff between WLAN’s throughput and LTE-LAA’s throughput with 10 APs In Figs. 61(a) and 61(b), the WLAN’s throughput slightly decreases while UEs’ through- put significantly increases, yielding a profitable trade-off. In addition, both the 99%-delay and fairness performance of WLAN considerably improves by using the proposed scheme even with 10 APs. It is always possible to sacrifice the throughput of LTE-LAA to further improve the performance of WLAN by performing the proposed beamforming and power allocation scheme not on every MCOT.