LTE-A radio interface uses techniques such as carrier aggregation, MIMO, millimeter waves, low-power nodes (e.g. pico cells eNB, relays) and device-to-device (D2D) communications. Combining various technologies seamlessly makes it necessary to support LTE-A heterogeneous network (HetNet) deployment. HetNet consists of various radio access technologies (e.g., LTE, HSPA, Wi-Fi, CDMA2000), multiple cell formats (multi-tier cells) with different cell sizes and power levels, different backhaul technologies etc.
D2D communication is a form of device relaying. Device refers to any user-owned mobile device with cellular connectivity. Device relaying makes it possible for devices in a network to function as transmission relays for each other and form a massive ad hoc mesh network.
D2D allows data to be routed directly between spatially close mobile user equipment (UE). Data between a UE pair do not traverse via the eNB or the core network. The short communication range improves energy efficiency, throughput, delay and spectrum efficiency. Possible D2D applications are content sharing, gaming, connectivity extension and traffic offloading.
The diversity of radio access available on UE (at the moment, cellular network, Bluetooth and Wi-Fi) provides D2D communications with flexibility in link establishment, resource allocation, energy efficiency, and applications and services. D2D pairs may be located in the same cell or in different cells. They may communicate as an underlay or overlay to the existing LTE-A network. They may use the licensed or unlicensed band.
D2D pairs may form clusters or multi-hop route. They may operate autonomously, or under partial or full control of an operator. D2D communication can be easily integrated with cooperative communication and cognitive radio to further enhance the potential in improving spectrum efficiency. If it is under the control of an operator, it is possible to achieve network-wide performance optimization by properly allocating frequency bands to D2D pairs and managing interference.
Interference management handles the interference control aspect of D2D communication. D2D communication should never affect or degrade the surrounding ongoing communication services (e.g., cellular communication or Bluetooth). The interference control may be between:
- D2D communications and cellular uplink/downlink communications when D2D UEs use the same frequency as cellular UEs,
- D2D communication and surrounding ISM band when D2D UEs use the ISM band.
For D2D-cellular interference (both UL and DL), interference is managed by using the power control mechanisms and strictly limiting the maximum D2D transmit power so that D2D transmissions can reuse the cellular spectrum without degrading the performance of cellular network. Among schemes that can be used to achieve this are fixed power scheme, fixed SNR target scheme, open loop fraction power control scheme and close loop power control scheme. D2D-to-cellular interference can also be managed by properly allocating channel resources to D2D transmitters.
Cellular-D2D interference should be addressed to maintain appropriate signal-to-interference-plus-noise Ratio (SINR) for D2D links. Because the eNB may obtain full channel state information (CSI) of all cellular links, the cellular-to-D2D interference in DL can be efficiently managed using the transmit beam forming technique. For D2D communication that reuses UL resources, the δD-interference limited area (ILA) based control scheme can be used to manage interference. Any cellular UE located within the δD-ILA is not allowed to use the same UL resources as a D2D transmitter.
D2D-D2D interference may occur among simultaneous D2D transmissions. For centralised interference management, the D2D-D2D interference can be effectively controlled using the interference-aware resource allocation, which allocates proper frequency resources to D2D pairs. In a distributed interference control, autonomous power adjustment at each D2D transmitter according to its required SINR target is used to manage interference among multiple D2D links.
Radio resource management and optimization. Depending on the air interfaces available at UEs, D2D pairs can use either licensed spectrum resources or other unlicensed or unused band for communication. The frequency resources should be allocated according the rule of network design. E.g., a D2D pair may select a resource pool for both upstream/downstream, or dedicated resources for upstream/downstream autonomously and individually. It is also possible that a D2D pair have to use the radio resources assigned by the eNB and is not allows to decide for itself.
There are three resource allocation modes for reusing licensed spectrum resources:
- Underlay Mode — D2D pairs and cellular UEs share the same spectrum, which helps achieve the best spectrum efficiency. The transmit power of both D2D links and cellular links are controlled to control interference.
- Overlay Mode — Dedicated frequency resources are allocated for D2D communications and the remaining is for cellular communications. In this case, there is no interference between D2D and cellular communication.
- Cellular Mode — Instead of communicating directly with each other, D2D UEs communicate with the eNB that acts as an intermediate relay, which is the same as the traditional cellular system.
Source
J. Liu, N. Kato, J Ma, N. Kadowaki, "Device-to-Device Communication in LTE-Advanced Networks: A Survey", IEEE Communication Surveys & Tutorials, Vol. 17, No. 4, 2015
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