Plagiarism

1. Research Challenges

3.1 Licensed plus Unlicensed Using Small Cells

In this scenario, a heterogeneous network operator deploys small cells in unlicensed bands that use the modified version of the same technology (e.g., LTE-A or its future versions) that is used in the macrocell (which operates on a licensed band) to coexist with WiFi systems [10]. In principle, LTE-A offers a good opportunity for utilizing CA in both 2.4 GHz and 5 GHz bands due to its ability to utilize flexible bandwidths of different sizes for component carriers (CCs) (e.g., 1.4, 3, 5, 10, 15, or 20 MHz). The main idea behind this scheme is that when suitable carrier(s) are available in the unlicensed band, a user’s primary component carrier (PCC) in the licensed band is aggregated with these secondary component carriers (SCCs) in the unlicensed band. In addition, the unlicensed spectrum is used on an “on demand” basis, meaning that only the small cells that have active users are able to transmit in the unlicensed band, and do not transmit at all at other times. This is possible because of a mandatory “anchor” in the licensed band. There are two main deployment modes for aggregating unlicensed spectrum: supplemental downlink (SDL) mode and time-division duplex (TDD) mode.

SDL Mode — In the SDL mode, unlicensed spectrum will only be aggregated for the downlink, so that the data rates and capacity are greatly increased in the downlink, keeping the uplink as is. This can be beneficial, especially to address the typically heavy traffic in the downlink.

TDD Mode — In the TDD mode, the unlicensed spectrum will be aggregated for both downlink and uplink, just like a typical LTE TDD system, and it works like a typical LTE TDD carrier aggregation. The advantage here is the flexibility to adjust the amount of resources between the uplink and downlink. When CA is utilized in the 5 GHz band, to protect radars using the 5 GHz spectrum, it is also a mandatory regulatory requirement to use dynamic frequency selection and transmit power control (DFS/TPC).

3.2 Licensed Plus Unlicensed Using WiFi

Currently, the way WiFi is utilized is suboptimal in the sense that the decision to utilize WiFi for communication and also the decision whether or not to utilize Channel Bonding (CB) is made independent of real-time information about all the available resources (e.g., available licensed and unlicensed channels). To overcome this problem and achieve maximum gain from the efficient utilization of both licensed and unlicensed spectrum bands, CA across the licensed and unlicensed bands (using WiFi) is controlled by the network operator. In this approach, when CA is utilized across an unlicensed band, a user’s PCC is always in the licensed band. When the operator finds a good radio link using WiFi, it is the network operator that decides on channel selection/bonding decision for secondary carriers. In the following, it describe the existing CB methods for secondary carriers.

3.3 AP Discovery and Connection Configuration

The first needed step in an LTE-WiFi CA system is for the eNodeB to be aware of the available WiFi access points (AP) in its proximity. After AP discovery, proper connection establishment protocol between the LTE and WiFi systems is needed. In WiFi infrastructure-based systems, the WiFi stations use Beacon request/report pairs for collecting information about available nearby WiFi APs [17]. It follows that an LTE eNodeB that needs to borrow spectrum from the WiFi system should simply behave as another WiFi STA by using similar beacon requests to discover nearby WiFi APs to interact with. Another alternative is for the eNodeB of a small cell and the WiFi AP are wired, or collocated within an integrated physical element. In this case discovery may not be necessary as the two channel access units can communicate through a dedicated interface. After detecting available WiFi APs, the eNodeB needs to establish a connection with each AP it wishes to coordinate with.

3.4 Interference Coordination

Interference is another critical challenge for the application of the aggregation scheme. The eNodeB will borrow spectrum from the WiFi bands which may already be occupied by WiFi STAs in nearby cells. Thus, interference detection (i.e., sensing) and mitigation is needed between the two systems to prevent the LTE-A UE transmissions from interfering with the WiFi STAs, and this can be a function within the X2-like interface. In particular, two X2 functions, referred to as elementary procedures, are used for interference coordination; namely, the resource status seporting and the load indication procedures. Resource status seporting is used by the eNodeB to request the reporting of load measurements from another eNodeB. The corresponding measurement elements include radio-resource status and load information at each cell managed by the concerned eNodeB. The Load Indication procedure on the other hand, is used to transfer load and interference co-ordination information between eNodeBs controlling intra-frequency neighbouring cells. The main difference between this procedure and the sesource status reporting procedure is that the latter is initiated by an eNodeB that needs the measurement information and requires a response from another eNodeB, whereas the load indication procedure is just a load information message sent by an eNodeB, and requires no response meassages. Both procedures can be re-used or modified for LTE-WiFi interference coordination. Further details on X2 procedures can be found in [20].

3.5 MAC Layer Time Synchronization

The architecture of the WiFi MAC sublayer includes two basic functions: the Distributed Coordination Function DCF and the Point Coordination Function PCF. The DCF is used for contention-based services, where stations contend for accessing the radio channel based on the CA access scheme. A station must verify that the medium is idle for a period of time, after which it uses a pseudo-random back-off period before it accesses the channel, given that the channel remains idle. On the other hand, the PCF is collocated with the AP and provides contention-free services, where the AP acts as a polling master for granting permissions to stations in the range to transmit. The WiFi MAC divides time into constant-length superframes, each containing a CFP (Contention-free Period) followed by a CP (Contention Period). Hence, the PCF and DCF access methods alternate in time, where the CFP stretches or shrinks based on demand for transmissions given there is enough time to transmit at least one MAC frame [18]. The proposed CA scheme in [ this paper ] , the LTE eNodeB can only borrow spectrum during contention-free periods, obviously because contention-based access does not provide guaranteed access to the medium on the borrowed WiFi frequency channels to LTE-A UEs that are employing carrier aggregation, or else it will require making changes to the WiFi standard. Hence, proper LTE-WiFi time synchronization is required so that the beginning of the WiFi CFP coincides with the start time of the LTE subframe, and that its length is a multiple of 1ms so that the contention period (CP) starts with the beginning of another LTE-A subframe during which carrier aggregation is “paused” until the following CFP starts. This requirement is feasible since the WiFi stations synchronize their times with the AP through the beacons that the AP sends at the start of each superframe.