Lessons and Guidelines

This section summarizes the lessons learned from the experiments, including those that are not described in Section 6.3, and provides guidelines for indoor mesh

deployment. The suggestions are itemized as follows:

1.Activate RTS/CTS in the indoor deployment

Unlike the conclusion of [46], [47] and [48], this study shows that RTS/CTS should be activated in indoor field deployment, especially when there are many interference sources and the STAs are not in the same collision domain. Nevertheless, if the WMN is deployed in the same collision domain, the RTS/CTS scheme is still not recommended as suggested in [46]. Based on our observation, RTS/CTS can effectively resolve the hidden node problem. Studies in [47] and [48] focus on long-ranged outdoor WMNs, the signal is highly interfered by the non-WiFi interference sources. Therefore, RTS/CTS cannot alleviate the interference.

2.802.11n is duggested for multi-flow, multi-hop

Although 802.11b/g performs no worse than 802.11n in one-hop transmissions in the field, the low-rate 802.11b/g frames occupy more channel resources for each hop. The first-hop node in an 802.11b/g chain topology generates almost 70% of the total throughput, producing a serious channel capture effect. Therefore, starvation may occur at STAs with a large hop count in an 802.11b/g WMN. Thus, 802.11n is still preferred for WMN deployment.

3.Beacon interval matters

The beacon interval can affect the frequency of updating the link state. However, transmitting beacons with 1 Mbps consumes much of the channel. It is a tradeoff between the link stability and the maximum throughput. We recommend setting the beacon interval at a longer value, e.g. 500 ms, in a small and dense deployment. A smaller beacon interval of 100 ms is preferred for a large and sparse deployment, especially when the channel condition is not good. This is because the link stability is also critical for the WMNs. If the link is unstable, it is possible that the MAP cannot link to any other MAPs or packets drop frequently due to link loss.

4.Fixed rate Does not help

The results from two studies on outdoor testbeds [49] and [50] imply that a fixed rate should be used to achieve better throughput. However, the channel quality in our indoor field experiments is much more variable than their outdoor, line-of-sight communication. An aggressive fixed rate might destabilize the transmissions, while a conservative fixed rate produces a poor throughput. An auto data rate mechanism allows the transmission rate to be adjusted dynamically according to the channel quality, but this adjustment mechanism must be sensitive enough to adapt to the

fluctuant channel conditions in the field.

5.Bottleneck is the channel condition

Results show that the total throughput of a multi-stream experiment is close to the single-stream, one-hop throughput. The throughput bottleneck seems to be the MSTA, and especially the MPP, where traffic streams aggregate. This result matches the findings of previous research [95]. Therefore, the MPP should be located in a position with a clear channel condition, leading to better link quality between the MPP and its neighboring STAs.

6.Hop-count should not exceed four hops

Considering the performance and the stability for the end-to-end traffic, we recommend that the hop-count should not exceed four hops.

7.Mesh size is determined by MPP’s capacity and maximum hop count

Based on the item 5 and 6, the throughput performance of a WMN is highly related to the channel quality of the MPP and the hop count of the traffic flow. Therefore, the size of a WMN is determined by the MPP’s capacity and maximum hop count.

8.Cross-floor links are frangible

Results show that the signal of a cross-floor link is quite unstable. Thus, the cross-floor links should be avoided.

9.Angle and direction of antenna pairs are critical

IEEE 802.11n adopts the MIMO technique to achieve higher data rates. Similar to previous findings [96], angle and direction of the antennas of the MAPs in our experiments directly affected the measured received signal strength (RSS). Because RSS is one of the criteria of data rate adjustment, antennas placement is a critical factor in networking performance.

10.Avoid links with asymmetric RSS values

By changing the antenna placement, the RSS values seen by the peers in a link may have huge difference. RSS could be a factor in WMN routing metrics. If the RSS measured by the peers is asymmetric, the routing path may not be symmetric, e.g., there is another MSTA with a better RSS value measured by only one end of the peers.

Previous research [97] indicates that asymmetric routing could lead to serious problems for Ad hoc On-Demand Distance Vector Routing (AODV) [98], from which

the 802.11s routing protocol is derived. Therefore, the links with asymmetric RSS values may cause asymmetric routing problems.

6.5 Summary

This study develops and evaluates an IEEE 802.11s wireless mesh network testbed for indoor environments. Based on observations, this study provides guidelines for tuning various parameters in indoor WMNs. Extensive experiments are conducted in both laboratory and field environments. Unlike previous studies, the experiment results of this study recommend activating RTS/CTS if the mesh nodes do not coexist in the same collision domain. Results also show that the 802.11b/g PHY performs no worse than 802.11n with respect to one-hop transmissions. However, 802.11b/g can cause serious unfairness because one-hop nodes constantly occupy the channel. Besides, the beacon interval should be set to a longer value, e.g., 500 ms, in a dense deployment, and set to a smaller value, e.g. 100 ms, to enhance link stability in a sparse deployment. Finally, the observations summarized in this article can provide guidance for small or medium scale indoor 802.11 WMNs.

Chapter 7 Conclusion

Based on applications of the IEEE 802.11 WLAN standards, this dissertation addresses the designs of a compact MIMO-OFDM modem and an IEEE 802.11-based WMN by two parts (Part I and Part II, respectively).

7.1 Summary

For the design of a compact MIMO-OFDM modem, a FD receiver adopted the FD-ADC technology is proposed to support both of SC and OFDM modes over the frequency domain. In order to realize this FD receiver, two key modules are essential to built. The first one is the FD symbol synchronizer which needs to find the symbol boundary with the frequency domain signal vectors generated by FD ADC. The FD symbol synchronizer is designed with a symbol-rate sequential search algorithm and

implemented by TSMC 65-nm silicon technology. In comparisons with other existing literatures, this FD symbol synchronizer shows almost the best performance at low SNR range (i.e. 0≤SNR≤10) and the lowest implementation cost in terms of (normalized) area, cell and gate count. The main cost of the proposed synchronizer is to require one additional preamble. The second key module is the SC-FDE which needs to handles the channel estimation and data equalization/decoding in the frequency domain which receiving IEEE 802.11b (SCBT) data packets. Compared to the other approaches, the proposed SC-FDE only requires single-FFT architecture and provides good performance by ML detection. Moreover, the ML detection can adopt the sphere decoding algorithm to reduce the search complexity. The features of single-FFT architecture and SD implementation make the proposed SC-FDE integrate with the MIMO-OFDM modem efficiently. The third key module is the MIMO detection used in MIMO-OFDM modem. A pre-pruning scheme is proposed to reduce the search space of K-best algorithm. Using ZF detection result, the pre-pruning scheme performs a cluster-based search in the multilevel structure of Nq-QAM constellation to find the reliable constellation points. Compared to the conventional K-best algorithm with the same K value, the proposed pre-pruning scheme achieves the same performance with fewer search nodes. The scheme provides a further complexity reduction way not only for a low complexity K-best algorithm but also for

the receiver equipping with both K-best and ZF detectors.

For the IEEE 802.11-based WMN, complete system phases from design, implementation, and deployment are investigated in the part II of this dissertation.

The WMN is developed based on the D2.03 of IEEE 802.11s and the related mesh functions of IEEE 802.11s software package are integrated to the commercial off-the-shelf WLAN chipsets. To improve the portability of the IEEE 802.11s software package, a modularized design is proposed to separate platform-independent functions (i.e. HWMP routing function) from the kernel and implement them as a Linux daemon program. To improve the transmission reliability of broadcast-type control frames, several broadcasting strategies are evaluated with a testbed in the laboratory. Experimental results show that the multiple-unicast scheme can provide the best balance between routing construction ratio, acceptable latency, and channel utilization. Then, a deployment testbed with 3-by-3 grid topology is constructed in both laboratory and field crossing three floors of the building. Disagreeing with previous research, the results of this study indicate that RTS/CTS can improve throughput by up to 87.5%. Moreover, compared with the IEEE 802.11b/g, 802.11n achieves better fairness for multi-stream or multi-hop communications. Experimental results also suggest that a longer beacon interval, e.g. 500 ms, can improve channel efficiency for a denser deployment. On the other hand, sparser deployments should

use a shorter beacon interval, e.g. 100 ms, to enhance link stability. Finally, the observations summarized in Chap 7 can provide guidance for small or medium scale indoor 802.11 WMNs.

7.2 Future Work

At the end of this dissertation, several analyses are suggested to be further study to make current research results become more complete and stronger. The first one is the analysis of VLSI architecture of FD symbol synchronizer. Since the proposed architecture uses higher clock speed to achieve more hardware sharing, the tradeoff between silicon area and power consumption should be further analyzed. The second one is the computation complexity of the proposed pre-pruning K-best scheme can be analyzed deeply and compared with other advance K-best algorithms. To extend the related research topics, a number of open issues are also summarized as follows:

1. For the evolution of IEEE 802.11 WLAN standards, the next generation WLAN communication systems are defined in the amendments of IEEE 802.11ac and IEEE 802.11ad. IEEE 802.11ac can be viewed as the BW extension version from IEEE 802.11n. Therefore, most IEEE 802.11n receiver modules can still be reused with slight modifications to the IEEE 802.11ac receiver architecture. However, IEEE 802.11ad enables the multi-Gbps Wi-Fi via a new 60G PHY. The longer

preamble length (i.e. Golay codes) and symbol block size (i.e. Reed Solomon code in SC mode) significantly increase the complexity of the proposed frequency-domain synchronization and equalization. Moreover, high sampling rate also leads critical VLSI implementations. To realize the FD receiver in IEEE 802.11ad will be therefore a very challenge topic in the future research efforts.

2. In this dissertation, we design a single-channel WMNs. The robustness of routing protocol is what we consider firstly. Therefore, the broadcasting strategies for broadcast-type control frames are evaluated. However, the co-channel interference avoidance is another important issue which significantly degrades the network capacity. The multi-channel WMNs may suppress the effect of co-channel interference, but it introduces the adjacent channel interference and makes protocol complex. Recently, the MIMO spatial multiplexing scheme has been applied to reduce the con-channel interference. Moreover, the new coming standard, IEEE 802.11ac, invites the scheme of multi-user (MU)-MIMO, which allows multiple receivers transfer data to single transmitter in up-link way or single transmitter transfers data to multiple receivers in down-link way. This scheme can significantly reduce the co-channel interference and increase the system capacity in single-channel WMN. Therefore, the integration of MU-MIMO scheme into our WMN becomes an emergent and meaningful topic. It also needs

to highlight that this topic includes interference mulling, MU-MIMO scheduling and fairness problem, which should be designed with cross-layer over physical, media access control (MAC), and even application layers.

3. Mobile communication systems have also evolved rapidly in the last decade. The 3^rd Generation Partnership Project (3GPP) has submitted the Long Term Evolution-Advance (LTE-A) to International Telecommunication Union Telecommunication Standardization Sector (ITU-T) as a candidate 4G system in late 2009. One of the new concept in LTE-A is the coordinated multipoint (CoMP) transmission, which enables Multiple base stations cooperate to determine the scheduling, transmission parameters, and transmit antenna weights for a particular user equipment. Unlike the conventional synchronization algorithm focus on point-to-point transmission, the synchronization in CoMP becomes an interesting topic since the received data may come form multiple cooperative stations.

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