In recent years, requirement of wireless data rate has continuously been increased due to vigorous development of new applications, e.g., multimedia transmission. However, it is difficult to provide users high data rate for these new applications in conventional cellu-lar networks owing to longer transmission distance. In order to support higher through-put, more and more research focuses on short-range communications over the past decade.
Among different standards, IEEE 802.11 and femtocell techniques are considered the two most well-adopted suites due to their popularization in the deployment. Various amend-ments are contained in the IEEE 802.11 suite, mainly including IEEE 802.11a/b/g [1–3]
and IEEE 802.11e [4] for quality-of-service (QoS) support. Moreover, with increasing de-mands to support multimedia applications, the new amendment IEEE 802.11n [5,6] has been proposed for achieving much higher throughput performance. The medium access control (MAC) protocol within the IEEE 802.11 standard supports the distributed coordi-nation function (DCF) to regulate the random and complex medium accessing behaviors among the wireless stations (WSs) within the same wireless local area networks (WLANs).
Furthermore, the point coordination function (PCF) initiated by the access point (AP) pro-vides centralized polling-based schemes to support time-constrained traffic for the WSs.
On the other hand, according to the statistical data in [7], it is expected that there will be nearly 90% of data services and 60% of phone calls taken place in indoor envi-ronments. Hence, for long-term evolution-advanced (LTE-A), 3rd generation partnership project (3GPP) suggests that femtocell base stations (fBSs) with the properties of short-range, low-power, low-cost, and plug-and-play can be designed to connect into the end
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Figure 1.1: Wireless environment that contains macrocell, femtocell and IEEE 802.11 networks
user’s broadband line in order to provide high throughput and QoS for the user equip-ments (UEs). Moreover, installation of fBSs can share the traffic load of its coexisting macrocell BSs (mBSs) [8]. Note that in order to distinguish IEEE 802.11 and LTE-A systems,
“WS” and “AP” are employed in the IEEE 802.11 networks; while “UE” and “BS” are ap-plied in the LTE-A system. Fig. 1.1shows a wireless environment that contains macrocell, femtocell and IEEE 802.11 networks. It can be observed that WSs/UEs can benefit from better link qualities in IEEE 802.11 and femtocell networks than in conventional macrocell network due to shorter distance transmission.
The DCF is utilized as the basic access mechanism in the IEEE 802.11 MAC proto-col. Based on the carrier sensing multiple access with collision avoidance (CSMA/CA) scheme, a WS will first perform carrier sensing to verify if the channel is available before data transmission. As the channel is idle for the time interval of DCF interframe space (DIFS), the random backoff process will be started for the purpose of decreasing the prob-ability of data collision. Moreover, both the request-to-send (RTS) and clear-to-send (CTS) packets exchanged before the data transmission is exploited to resolve the potential hid-den terminal problem. Moreover, in order to support QoS requirements, the enhanced distributed channel access (EDCA) mechanism is proposed in the IEEE 802.11e standard [4]. The EDCA protocol inherits conventional DCF’s CSMA/CA scheme with the
en-hanced RTS/CTS handshaking process. Furthermore, four prioritized access categories (ACs) are defined in EDCA in order to support different types of network traffic. The QoS requirements for each AC is defined by selecting feasible values of the contention win-dow (CW) size and arbitration interframe space (AIFS) length. It is intuitive to observe that higher priority AC should possess smaller values of CW and AIFS sizes. Each AC will wait for its AIFS length and independently select its own backoff number. Until the backoff number for a specific AC has been decremented to zero, the corresponding AC can initiate an RTS frame for channel contention. Each AC within a WS is considered as a standalone entity to contend with the ACs both in the same WS and the other WSs for channel access in the network. Distributed DCF/EDCA algorithms can be well-adapted for the irregular WLANs, nevertheless the performance cannot be further improved due to their low efficiency. Both RTS packet collisions and backoff delays will significantly reduce the network throughput.
Furthermore, the IEEE 802.11 task group N (TGn) [5] enhances the physical (PHY) layer data rate up to 600 Mbps by adopting advanced communication techniques, such as multi-input multi-output (MIMO) technology [9]. The MIMO technique utilizes spatial diversity to improve both the range and spatial multiplexing for achieving higher data rate. How-ever, it has been investigated in [10] that simply improves the PHY data rate will not be sufficient for enhancing the system throughput from the MAC perspective. Accordingly, the IEEE 802.11 TGn further exploits frame aggregation (FA) and block acknowledgement (BA) techniques [6, 11] to moderate the drawbacks that are originated from the MAC/
PHY overheads. Owing to adoptions of FA and BA, high-efficiency automatic repeat re-Quest (ARQ) mechanisms are necessary to further improve system performance for IEEE 802.11n.
With the properties of easy implementation and low cost, DCF/EDCA schemes are popularly employed for the short-range wireless communication. However, it is difficult to execute seamless handover from IEEE 802.11 system to conventional macrocell due to definitely different core designs. On the other hand, since being proposed by telecommu-nication operator, femtocell techniques are much compatible with conventional macrocell.
The seamless handover from femtocell to macrocell can therefore be achieved. Moreover, with adoption of centralized scheduling scheme, the efficiency of MAC layer in the fem-tocell is higher than that in the IEEE 802.11 network. That is why femfem-tocell plays more and more important role in the current wireless networks. Nevertheless, there still exist several critical issues in the femtocell techniques, especially for the interference
coordina-tion between the macrocell/femtocell coexisting heterogeneous networks (HetNets). In order to enhance the spectrum efficiency, femtocell can be operated in the same band as the macrocell to serve its UEs. However, co-channel interference to the macrocell UEs may therefore be produced. The level of interference mainly depends on the access strategies of fBSs. Two major access policies are considered in femtocell network, including the closed access mode and open access mode. The closed access mode only permits authorized sub-scribers to utilize the fBS; while all users are allowed to connect to the fBS by adopting the open access mode. Closed access will intuitively be advantageous to the femtocell sub-scribers, however interference from the fBS to mBS’s UEs can become severe in the closed access mode than in open access mode. On the other hand, system performance of the entire HetNet can be improved if fBS is operated in the open access mode. Nevertheless, open access mode is an obvious barrier to promote the popularization of femtocells since subscribers will not be interested in installing fBSs but accessed by other users [12].