The Electronic Communication Committee (ECC) in Europe proposed a location-based transmission power allocation rule for secondary devices operating in the TV white space (TVWS). The further the secondary device is located from the TV cell border the higher transmission power level it can utilize. The Federal Communication Committee (FCC) in the US proposed a fixed transmission power allocation rule for all secondary transmitters. Both rules do not consider the secondary system’s self-interference while setting the transmission power levels. In this paper, we propose a power allocation scheme for a cellular secondary system. Unlike the ECC and the FCC proposals we do the power allocation by considering the self-interference. We define the power allocation scheme as an optimization problem. The sum cell border data rate of the secondary network is selected to be the optimization objective. We observe that the optimal transmission power levels become approximately constant over the secondary deployment area. The FCC rule captures the general trend for cellular deployment in the TVWS, since it suggests the use of constant power. However, the transmission power should not be set equal to 4 W but according to the allowable generated interference at the borders of the TV and secondary cells.

Recently, cellular networks have transitioned from providing mobile telephony with limited data to supporting diverse types of applications with high capacity requirements. The increasing capacity demands for the next-generation cellular systems cannot be accommodated within the currently allocated spectrum resources. To some extent, the cellular spectrum deficit can be overcome by enabling cellular access to the unused portions of spectrum in the TV bands, also known as TV white spaces (TVWSs). The operation of cellular networks in the TVWS has been already recognized as a scenario with clear business and economic impact [

The main requirement for secondary operation in the TVWS is to maintain the QoS at the TV receivers. The QoS can be maintained if the interference level at the TV receivers is controlled. In the absence of secondary transmissions the TV receivers experience only the TV system’s self-interference. The difference between the TV self-interference level and the maximum interference level not violating their QoS is called interference margin [

The standardization bodies in the USA and Europe have so far proposed different approaches for transmission power allocation to secondary spectrum users. The Electronic Communication Committee (ECC) in Europe [

The existing aggregate interference control methods adopted by the two standards can lead to unacceptable interference increase at the TV receivers as demonstrated in [

In this paper, we propose a scheme for setting the transmission power level in a cellular secondary network without violating the protection criteria of the TV receivers. While planning the coverage of a cellular network the system designer must guarantee a minimum signal to interference and noise ratio (SINR) at the cell edge. Unlike the ECC and the FCC proposals we take into consideration the secondary system self-interference constraints while setting the transmission power levels. Our scheme is formulated as an optimization problem. The sum cell border data rate is selected to be the optimization objective. Our scheme can be viewed as a method to divide the available interference margin among the cellular base stations.

We observe that the optimal transmission power becomes approximately constant over the secondary deployment area. The FCC rule appears to capture the general trend for cellular secondary deployment in the TVWS, since it suggests the use of constant power. However, the transmission power level should not be arbitrarily set equal to

The outline is as follows. Section

Figure

System illustration for single TV transmitter case. The cellular network operates co-channel to the TV transmitter and it is deployed outside of the TV protection area. The aggregate interference has to be controlled at the TV test points and the cellular cell borders.

In TV network planning the location probability describes the percentage of locations within a square area of

For successful TV operation a target SINR,

Similarly, while planning the coverage of a cellular network, a minimum data rate should be guaranteed at the cell edge. The impact of fast fading to the achievable data rate is ignored which is a valid assumption for a low mobility scenario. In the presence of slow fading the minimum data rate is achieved if a target SINR

The SINR,

Data traffic in cellular systems has an asymmetric behaviour with more traffic generated in the downlink than in the uplink [

The generated interference at the

Since the locations of the secondary transmitters and the TV receivers are known, the parameters

In the absence of secondary transmissions condition (

The interference margin at the TV coverage cell border has been calculated in [

In general, the interference margin depends on the locations of secondary transmitters. However, one has to notice that the secondary generated interference is usually an order of magnitude less than the TV signal level. This fact provides the approximation tightness for the lower bound of the interference margin illustrated in [

By using a similar approach as in [

Unlike the TV test points, the generated interference at the cellular cell borders is in the same order with the useful signal level. Nevertheless, Figure

(a) Distribution of the SINR at the cellular cell borders. The simulations are compared to the calculations. In our calculations the aggregate interference is modelled by the log-normal distribution. Different secondary cell radiuses are tested. The standard deviation is taken equal to

We are looking for the power allocation maximizing the sum cell border data rate of the cellular system while not violating the protection criteria of TV and cellular systems. We assume one-by-one scheduling in each cell. In order to evaluate the sum rate optimization function we compute for each cell the average cell border data rate over the test points of that cell. Then, we sum the calculated values over all the cells. Our optimization function has the form of (

The interference margins

Note that the optimization problem (

The data rates of secondary cells are coupled due to mutual interference. Optimal power allocation is known to be difficult to achieve due to this complicated coupling among the SINR of different links [

Possible methods for solving this nonconvex optimization problem are approximation, relaxation, and transformation. For instance, in the high SINR regime the individual rate can be approximated by

Both terms are increasing concave functions in

Feasibility means there exists a set of positive transmit power levels

Minimum protection distance satisfying TV and secondary system cell coverage constraints with respect to different cellular SINR target

It is computationally difficult to solve (

The optimal common power level that maximizes the cellular cell border data rate under the TV protection and cellular coverage constraints can be obtained by solving the following concave problem:

Thanks to its low complexity, the uniform power allocation rule gives an opportunity to get quickly insight on the impact of various parameters on the cellular data rate and the TV protection criteria. For instance, the system designer can identify the minimum possible protection distance that allows the TV and the cellular network to coexist without violating their own protection constraints. Also, the low complexity makes the uniform power allocation rule attractive for cellular network planning in the TVWS over a country-wide level.

In this section we study the problem of allocating the transmission power level in a cellular network deployed outside the protection area of a TV transmitter. When the cell size is fixed, it is illustrated that all the cellular base stations transmit approximately at the same transmission power level. When the cell size can vary based on the population density it is illustrated that cells of the same size tend to use approximately equal transmission power levels. We use this approximation to study a country-wide cellular deployment in the TVWS.

Different models are used to estimate the field attenuation in the propagation path for the TV transmitters and the cellular base stations. The propagation prediction for DVB-T signal over land path is obtained by using the Recommendation ITU-R P.1546 [

First, we carry out the simulation in a single TV cell scenario and ignore the self-interference in the TV network,

The optimization constraint (

In Figure

Spatial power density emitted from the secondary deployment area obtained by solving the optimization problem (

In Figure

Distribution of average cell data rate for different protection distances. The cellular transmission power levels are obtained by solving the optimization problem (

One can also notice that the uniform power allocation rule becomes more accurate for larger protection distance. When the secondary network is deployed far from the TV cell border, it can reach the interference-limited mode. In interference-limited mode and provided that all cells have the same radius, the data rate is maximized when all cells utilize the same maximum transmission power level. In our computations the maximum allowable transmission power is set equal to

Note also that for

Next, we consider a cellular layout with non-uniform cell size (Figure

System illustration for different cell size coexistence case.

Distribution of average cell data rate by optimizing the transmission power for each base station and also by assuming equal transmission power for cells of the same size. (a) Upper part of secondary deployment area with cell radius 1 km. (b) Lower part of secondary deployment area with cell radius 2 km.

For the Finland case study the TV transmission power levels are not arbitrarily set as for the single TV cell study. The actual transmission power levels have been taken from Finnish Communications Regulatory Authority (FICORA). The DVB-T coverage is calculated according to [

In order to simplify the cellular deployment we cover the country with square cells. We consider three different cell types, urban, suburban, and rural. The distance

We assume that the transmission power level is common for cells of the same type. In that case, the solution of the optimization problem (

(a) Cellular layout in Finland based on the population density. The black space corresponds to the area where the secondary transmissions are not allowed. (b) Color-coded map of average data rate for a cellular network operating in TV channel

The power density can be calculated as the ratio of allocated transmission power divided by the cellular cell size scaled by the reuse distance. The power density values allocated to urban, suburban, and rural cells are equal to

Figure

(a) SINR distribution at the TV test points and the cellular cell borders. (b) Distribution of the outage probability at the TV test points and the cellular cell borders.

Figure

(a) SINR distribution at the TV test points. (b) Distribution of the outage probability at the TV test points.

In this paper we proposed a method to set the transmission power level in a cellular network operating in the TVWS without violating the protection criteria of TV receivers. Unlike the existing ECC and FCC rules we consider the cellular self-interference constraint while setting the transmission power levels. The paper shows that allocating same power to cells of the same size results in approximately maximum sum cell border data rate. However, the common transmission power level should not be set equal to

This work was partially supported by the TEKES-funded Project IMANET+ and by the EU FP7 Project INFSO-ICT-248303 QUASAR.