Picocell Range Expansion

4.2.1 Cell Association Schemes

In a traditional macro-only (homogeneous) network, typically the cell selection (or cell association) is based on the criterion of maximal downlink (average) received signal strength (RSS). In other words, the UE is typically associated to the cell with the strongest downlink RSS (max-RSS) and it can be further expressed as

 

 

arg max_{i i}

Serving Cell RSS , (4-1)

where the index i corresponds to the candidate cell index. This cell selection scheme is commonly adopted and identical to the existing cell section scheme used in LTE and WCDMA systems. Note that in this chapter “RSS” denotes a long-term average measurement taking into account of transmit power, distance-dependent path loss, shadowing and antenna gain. In practical, RSS can be acquired by observing the power of a received cell-specific reference signal (or pilot signal). In a homogeneous deployment, since all the macrocells typically have similar transmission configurations (such as transmit power level, antenna patterns, etc.) and load conditions, the UE is typically best served by the cell which provides the largest downlink RSS.

However, in the deployments of macro-pico HetNets, under the conventional cell association rule of selecting the cell with the highest downlink received power, the number of UEs associated with picocells is very small. As a consequence, very few UEs benefit from the presence of the picocells and this may further leads to the case where the picocells serve only a few users while at the same time in the macrocells the competition for the available resources would remain high. The limited coverage of picocells is a result of lower transmit power, lower antenna gain and worse propagation conditions compared with macrocells. It is therefore beneficial to have an UE connect to a picocell even if it is not the cell which provides the strongest received power. Generally, such a cell selection scheme is referred as cell range expansion (CRE) of low power nodes.

Very recently, biased-RSS cell selection has been proposed and considered in 3GPP as a promising scheme for realizing CRE of picocells [23][53, 54]. This scheme causes users to select a picocell by adding a cell selection bias to the RSS from picocells and it can be given as

 

 

arg max_{i i i}

Serving Cell RSS bias , (4-2)

in which the biasi (in dB) is chosen to be a positive, non-zero value whenever the candidate cell i corresponds to a picocell and is set to zero for all macrocells. Note that the CRE bias values for picocells can have different settings, but they usually have the same setting in a region. As illustrated in Fig. 4-1, such a cell selection strategy would extend the area in which the picocell is selected. In this work, the CRE concept is fulfilled by using biased-RSS cell selection, and we further assume the same CRE bias setting for all picocells in the evaluation system. Unless otherwise stated, the CRE technique spells biased-RSS cell selection scheme in this study.

Macro ^Pico Pico_RSS > Macro_RSS

Picocell range expansion: Pico_RSS + bias > Macro_RSS

Fig. 4-1 An illustration of picocell range expansion with biased-RSS cell selection

4.2.2 Benefits and Challenges of using CRE

Picocell range expansion may be beneficial in several aspects as follows:

Traffic Offloading: As more and more users are associated to picocells, the loading of those cells will increase while the loading of macrocells will decrease. Obviously, CRE technique potentially provides greater offloading of UEs from the macro-layer onto the pico-layer.

Data rate fairness: To ensure that users remain satisfied, it is very important to deliver a consistent user experience throughout the network. Since CRE technique results in more balance of user distribution between macro-layer and pico-layer, a more uniform user data rate throughput experience across cells (including macrocells and picocells) can be expected.

Uplink Interference: On the uplink, all the UEs have the same maximum transmit power. From uplink point of view, the optimal serving cell choice is determined by the lowest path loss rather than the highest downlink received power. If an UE is associated with the macrocell with the strongest downlink signal (but not with the cell with the minimum path loss), it may cause significant uplink interference to a picocell that is closer

to the UE. With the use of CRE technique, the terminal transmit power, and thus such uplink interference occurrences would be reduced since many more UEs now are able to connect to the picocells that are with lower path loss even if the received signal power from macrocell is significantly higher.

Even so, HetNet deployments using CRE give rise to strong and varied interference conditions across layers. As mentioned above, CRE forces a number of users to connect to picocells even when the picocell is not their strongest serving cell. It should be noted that a large bias value will result in a low experienced SINR (signal to interference plus noise power ratio) values for UEs connected to picocells; and further, it increases the risks of introducing higher user outage rate (in terms of user SINR) problems in the system. As a consequence, inter-layer interference management is critical in order to ensure robust communications in a macro-pico HetNet that realizes picocell range extension.