Strongest Cell Handover Decision Algorithm

where the numerator corresponds to the receive signal strength for user in the serving cell , the first and the second terms of the denominator to the interference caused by cells and users operating in-band, respectively, and the third term to the noise power at cell . By using (4.1) and taking into account the requirement for sustaining the prescribed mean SINR target , the mean UE transmit power of user for a candidate cell ∈ can be estimated as follows:

→ =

∙ ∑ _{∈ { }} ∙ _→ ∑ _{∈ { }} ∙ _→

→

(4.2) Note that the positive impact of handing over to cell ∈ , in terms of lower interference, is incorporated in (4.2) by omitting the interference caused to cell by the ongoing user connection with the current serving cell , i.e., ∙ ℎ _→ . Eq. (4.2) can also be used to estimate the mean UE power consumption of the tagged user in cell , owing to transmit power.

Table 7: Signal quality measurements for the LTE-A system [16]

Measurement Measured

by Notation

Reference Signal Received Power (RSRP) UE _→

E-UTRAN Carrier Received Signal Strength Indicator

(RSSI) UE →

Reference Signal Received Quality (RSRQ) UE _→

Downlink Reference Signal Transmitted Power (DL RS

Tx) E-UTRAN ,

Received Interference Power (RIP) over the RB set E-UTRAN

The LTE-A standard describes a wide set of signal quality measurements for the LTE-A access network and the UEs in [16], which can be utilized to accurately estimate the mean UL SINR in (4.1) and the mean UE transmit power in Eq. (4.2). The LTE-A measurements used in this paper, along with the respective notation for a tagged user , cell , and time interval , are summarized in Table I. Note that the RIP measurement in Table I, denoted by , corresponds to the linear average of the RIP measurements performed over the utilized RB of cell . To the remainder of this paper, it is assumed that for all the UEs connected to it, each serving cell has a consistent list of candidate cells and signal quality measurements describing their status. Even though the acquisition of these measurements is described in section 4.5, the network discovery phase is outside the scope of this paper.

4.1.2 Strongest Cell Handover Decision Algorithm

In the context of LTE-A, the SC HO decision algorithm consists of handing over to the candidate cell with the highest RSRP status, which also exceeds over the RSRP status of the serving cell plus a policy-defined HHM for a time period namely the Time To Trigger ( ) [51]. The HHM is typically introduced to

mitigate frequency-related propagation divergences, and the negative impact of the ping-pong effect. Based on the system model description in Section 3.1, the SC HO decision algorithm can be summarized as follows:

arg max ∈ → ,( ) ≔ | → ,( ) > → ,( )+ ,( ) (4.3) where _,( ) corresponds to the HHM for cell c ∈ , and ( ) to the value of

X in decibels (dB). Taking into account the definition of the RSRP measurement

[16], it follows that:

→ = , ∙ ℎ → (4.4) By substituting (4.4) to (4.3), it can be shown that the SC algorithm facilitates mobility towards candidate cells with higher RS transmit power, and/or improved channel gain. However, in order for the SC algorithm to improve the channel gain for the tagged LTE-A link (4.4), comparable RS transmit powers should be radiated among the candidate cells. However, this is not in effect in the two-tier LTE-A network provided that a) eNBs typically radiate higher RS transmit power compared to HeNBs, and b) femtocell self-optimization can result in different RS transmit powers between the HeNBs. In addition, the SC algorithm does not necessarily improve the SINR performance ((4.1) and (4.2)) given that divergent interference levels are expected at the LTE-A cell sites owing to the unplanned deployment. The SC algorithm’s unawareness on the actual RS transmit power and the interference level at the cell sites, is also expected to increase the UE transmit power, which in turn rises the interference level network-wide and exhausts the UE battery lifetime. The value of the HHM parameter is another open issue for the SC algorithm.

4.2 The Proposed Handover Decision Policy

In this section we propose the UE Transmit Power Reduction (UTPR) policy in the following, which relies on handing over to the cell with the minimum required UE transmit power, while maintaining the mean SINR target.

The following analysis is pursued to derive the HHM required for minimizing the UE power transmissions, based on the available set of standard LTE-Advanced measurements in Table 13. It is assumed that user receives service from cell , which has consistent LTE-Advanced measurements describing the status of every candidate cell c ∈ for user , for the time interval = . Using (4.4) under the assumption of a symmetric channel gain, the following estimation can be made:

ℎ _→ ≅ ℎ _→ = →

,

(4.5) By the RIP measurement definition in [16], it follows that:

= ∑ _∈ { } ∙ ℎ _→ + ∑ ∈ ∙ ℎ → + (4.6) Using (4.3), (4.5), and (4.6), it can be shown that the UE power transmission on the serving cell is given by (4.7).

≜ → =

∙ , ∙

→ (4.7)

Following a similar approach, the UE transmit power on the candidate cell c can be estimated as follows:

→ =

∙ , ∙ ∙ →

→ (4.8)

where the term ∙ ℎ → is introduced to include the positive impact of handing over to cell ∈ , if cells and operate in the same LTE-Advanced band (if not, it is omitted), i.e, if , ∈ . Accordingly, handing over to the candidate cell c, is expected to result in reduced UE transmit power compared to the one used in the current serving cell , if the following are in effect:

→ > → (4.9) ∙ , ∙ → > ∙ , ∙ ∙ → → (4.10) → > → ∙ , ∙ ∙ → , ∙ (4.11) where (4.10) is derived by using (4.7), and (4.8), and (4.11) by rearranging (4.10). Note that the parameter is given by (4.7). By taking the respective parameter values in dB, (4.11) can be rearranged as follows:

→ ,( ) > → ,( )+ ,( ) (4.12) where the parameter _{,( )}is given by (4.13).

,( ) = ⎩ ⎪ ⎨ ⎪ ⎧10 log , ∙ ∙ → , ∙ , ∈ 10 log , ∙ , ∙ ℎ (4.13)

It can be seen that (4.12) can be utilized as a HO decision criterion for minimizing the UE power transmissions in the two-tier LTE-Advanced network. To achieve this, (4.13) can be incorporated in the standard LTE-Advanced HO procedure, as an adaptive HHM. Given that a HHM for mitigating the side-effects of user mobility is still required, the _{,( )} parameter should be incorporated as an additional HHM in the strongest cell HO decision policy. Taking this into account, the proposed UTPR HO decision policy can be described as follows:

arg max ∈ → ,( )≔ | → ,( )> → ,( )+ ,( )+ ,( )

(4.14) Summarizing, the proposed UTPR policy is based on standard LTE-Advanced measurements, while it is employed by introducing an adaptive HHM to the standard LTE-Advanced HO procedure. The employment of the UTPR policy does not require any enhancements for the LTE-Advanced UEs. An enhanced network signaling procedure is necessitated, however, to convey the E-UTRAN measurements amongst the cells. This signaling procedure can be based on directly exchanging the required measurement information through the standard X2 – interface [26]. Alternatively, a core network entity can be deployed for gathering, maintaining, and disseminating the required E-UTRAN measurements on demand. This CN entity can also control the E-UTRAN measurement signaling load, i.e., LTE-Advanced measurement requests and reports, depending on the current CN load, the LTE-Advanced cells’ status, and other network-related parameters.

4.3 Numerical Results

This section includes selected numerical results to evaluate the performance of the proposed HO decision policy in the two-tier LTE-Advanced network. The simulation scenario is based on the evaluation methodology described in [123], while the proposed UTPR policy is compared against the strongest cell HO decision policy, referred to as SC policy in the following.