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A Novel Spectrally Efficient Local Service Insertion Scheme With Universal Frequency Reuse

Local content insertion enabled single frequency networks (SFN) for TV broadcast is a topic of considerable research interest. The SFN broadcast system is not ideally suited for localized content, but it offers enormous spatial diversity and power gain advantages. Orthogonal Frequency Division Multiple Access (OFDMA) allows reserving a fraction of available subcarriers for broadcasting local content. However, a user in the transition region from one local service area (LSA) to another experiences high co-channel interference (CCI). This work explores the possibility of using a `buffer zone' based on universal frequency reuse schemes. In such systems, the power allocation to each content is varied to reduce the CCI-dominated region in the LSA boundary. Simulation results show that depending on the threshold signal-to-interference plus noise (SINR) ratio selected, the proposed buffer architectures increase the percentage of users served by the SFN compared to the scheme where such a buffer is absent. Moreover, the proposed method is spectrally efficient compared to an orthogonal allocation of local content across the LSAs.

Layered division multiplexing (LDM) [6], which was initially proposed under the name cloud transmission [7]- [10], is a technique that enables local content delivery. LDM employs power division multiplexing to deliver the highly robust core layer (CL) signal to all users and the less robust enhanced layer (EL) signal to users with a high signal-to-interference-plus-noise ratio (SINR). It employs an advanced receiver that requires successive interference cancellation (SIC) or joint detection. This approach has high spectral efficiency and has been recommended in the ATSC standards [11].
The DVB-NGH standards [12] recommend a local content insertion strategy known as hierarchical local service insertion (H-LSI) [13]. It employs hierarchical modulation [14], a technique where a low priority (LP) bit stream is combined with a high priority (HP) bit stream by imposing a secondary constellation on top of the basic primary constellation [15]. The local content coverage in H-LSI is limited in a manner similar to LDM. Another scheme known as bit-division multiplexing (BDM) [16] allocates the global and local contents to different bit positions within a constellation by taking advantage of unequal error protection. It requires continuous adaptation of transmitside parameters to jointly maximise local and global content coverage.
The study in [17] gives us insight into the co-channel frequency reassignment problem. It studies the best reuse distance for cells that employ LDM. In cases where inter-layer interference becomes an issue, [18] suggests using an optimal detector that also models the EL signal in the decoder for the CL signal. Although such joint detectors work well compared to the traditional SIC, the complexity of the receiver is higher and is advised mainly for systems employing robust signals like QPSK. Another approach that handles downlink broadcast interference is the linear precoding algorithm proposed in [19].
The orthogonal local content insertion (O-LSI) scheme [13] implements frequency reuse for the local contents with subbands of the orthogonal frequency division multiple access (OFDMA) spectrum. Both the global and local contents can have equal coverage in this scheme. It has a simple implementation at both the transmitter and the receiver. However, it has a lower spectral efficiency as a fraction of the subcarriers will not be used to transmit any information.
This literature survey indicates the scope for a local content insertion scheme that can leverage the advantages of the SFN framework while being spectrally efficient. While O-LSI meets the former criteria, the orthogonality requirement makes it inefficient. If this requirement is removed, O-LSI reduces to an OFDMA system that employs universal frequency reuse or reuse-1 [20], [21]. Such a system, however, usually requires careful planning to minimize the CCI. We propose an interference management scheme for the LSA boundary region of a reuse-1based broadcast system without resorting to joint decoding algorithms such as the one studied in [18]. Instead, different power control algorithms similar to those presented in [20] are devised by us to reduce the CCI-dominated region. A comparison of the proposed universal reuse scheme with O-LSI and LDM is presented in Table I. Note that H-LSI is similar to LDM in terms of the characteristics discussed in the table.
The key contribution of this paper is introducing a spectrally efficient universal frequency reuse/reuse-1-based broadcast system. This work studies power allocation strategies among the global and local contents and their effect on the percentage of users served. The spectral efficiency of O-LSI is compared with the proposed universal reuse schemes. These include a power scaling of the local content at the LSA boundary and generalizations of the  [20] schemes. The present work determines the SINR for each local content according to its spatial location. This expression is used to determine the percentage of users covered by different power allocation schemes. Based on simulation results, a comparison is made between the power control schemes to find the best choice for the LSA boundary.
The only similar study found in the literature is [22], which considers SFR and FFR-based frequency allocation schemes for multicast service. However, [22] proposes a system that carries out dynamic band allocation as and when required. It does not address the transition between LSAs as the study is not in a broadcast framework. In the broadcast scenario, the band allocation would be static, and the flexibility lies in power allotted to each band. Thus, the present study is unique and explores the possibility of a spectrally efficient non-orthogonal broadcast system that can leverage the diversity advantages of SFN. Universal reuse suffers from CCI in the regions where the coverage of LSAs overlap. In the absence of interference-aware receivers, CCI can make the reception of local content nearly impossible at the LSA boundary.
Thus, in between LSAs, outage zones due to interference occur naturally. This paper aims to reduce the interference- M −1 . The notation (O) makes it explicit that the O-LSI scheme [13] is used. The set of all subcarriers that are occupied by the local contents of LSA 1 shall be denoted by The subcarriers in which the local contents in LSA 2 are transmitted shall be denoted by I   The local contents in the O-LSI framework do not suffer from co-channel interference from the adjoining LSA.
Thus, the SINR expression for the m th content is simply the SNR. We denote SIN R(m, x 0 , y 0 ) as γ m where d l,(x0,y0) is the Euclidean distance from the tower in the cell l to the point (x 0 , y 0 ) and η is the path loss exponent. N 0 is the noise spectral density, and B m is the bandwidth of the m th content. The SINR expression (2) assumes that the guard interval is long enough to accommodate the multipath components from the various towers in the SFN. If this is not the case, a weighing function similar to the one proposed in [23] should be employed.
The effect of fading and shadowing can also be modelled into (2), but it is assumed here that these effects have been averaged out. The assumptions for the SINR expression used in this work are similar to that used in [24].

B. Universal Reuse Schemes
In the absence of orthogonal subcarriers, local contents from different LSAs occupy the same frequency resources.
This allocation is shown in the right half of Fig.3. The resulting CCI is severe in the cells nearest to the LSA boundary. For example, Fig. 4 shows the boundary of two LSAs with three local contents each. The co-channel subcarrier sets will be denoted by I in the second LSA. Here, the notation (U ) denotes that the subcarrier allocation is as per the universal reuse scheme. In this scenario, The region shaded in grey (columns 5 and 6 in the cell grid) shall be referred to as the buffer region because it indicates a transition between the LSAs. The schemes introduced in this paper vary the power of the local and global contents in this buffer zone to improve the coverage area.
1) Soft Frequency Reuse-based Buffer: The SFR scheme described in [20] can be employed in the cells near the LSA boundary to transition from one set of local content to another. This scheme gives a fraction α (with 0 ≤ α ≤ 1) of the total transmit power P t to one of the local contents. In traditional SFR, α is chosen such that the content has enough power to reach the cell edge. The remaining power (1 − α)P t is distributed among the other contents. These contents will not reach the cell-edge for large α values and are hence called cell-center contents.
For small values of α, the cell-center contents and the cell edge contents interchange. This scheme is illustrated in The cells in the SFR buffer project only one of the three local contents of the corresponding LSA towards the cell edge. The two columns are arranged so that the co-channel local contents are as far apart as possible. The content given the power αP t in the buffer zone is placed before the slash, and the other contents are listed after the slash. Thus, the SFR buffer employs reuse-3 in the cell-edge and is approximately reuse-1 in the cell-center.
The idea is to vary the factor α to ensure that the interference stays at manageable levels.
All K local service areas share the same spectrum in a general universal-reuse design. This effectively increases the available spectrum for the local content by a factor of K. However, for every local content in a local service where d The following equations describe the signal and interference powers for each content in the OFDMA SFR system.
Here, ρ S (U ) of power division is known as fractional frequency reuse [20]. However, the term FFR generally implies that the cell-center content will be reuse-1, and the cell-edge content will be reuse-M − 1. However, in our definition, the opposite is also allowed. This architecture is shown in Fig. 5. The notation in the buffer zone places the content with the power αP t before the '/' symbol and the other content after it. For this power allocation, the signal and interference powers become, Here, α = (1 − α). These powers can be substituted to the same SINR expression (3) derived for SFR to obtain the SINR for FFR.
3) Power-scaled local contents: The next approach to improving local content coverage follows from the fact that for fixed signal power, SINR improves with the reduction of interference power. If the powers of the local content in the buffer zone are scaled, the SINR improves in the SFN region outside the buffer zone. This improvement is because the cells that transmit the interfering local content in the buffer zone are further away from the SFN zone than those that transmit the desired local content. This results in an effective SINR improvement. The ratio of the transmit power in the SFN zone to that in the buffer zone is denoted by β.
As an example, consider a user situated in one of the cells in the fourth column of the cell grid shown in Fig. 6.
This user receives contents 1, 2, 3 and 4 from the different cells within its SFN zone and power-reduced versions of the same local contents from the first column of the buffer zone. The primary interference this user sees will be from the second column of the buffer zone. Due to path loss, the interference from the subsequent columns that belong to the second LSA has a lower impact on the SINR of that user. Thus, the dominant interferers seen by the user will be power reduced compared to a case without such scaling of local content power. In the SFN zone, the power scaling scheme reduces the dominant interference power while reducing the non-dominant signal powers. This leads to an effective improvement in SINR. In the buffer zone, the power reduction scheme reduces the dominant interference power while reducing the dominant signal powers. However, this does not cause significant changes in the SINR distribution for the buffer zone. For m > 0, the SINR expression is given by, When considering the SINR for the j th LSA, either the first term or the second term of the numerator will be dominant depending on whether (x 0 , y 0 ) is in the SFN zone or the buffer zone. However, the dominant term in the Fig. 6: Power-scaled local contents in buffer zone for the case of three local contents per local service area denominator is always the second term (the term with l ∈ Γ I m ) because it will be the closest interfering cell to the LSA. This ensures that the power scaling method improves the SINR of the local contents with increasing β, Since a total transmitted power of P t = M −1 m=0 S m is available, the power freed up by scaling S m to S m /β can be used to boost the global content (m = 0). Thus, the global content power within the buffer zone becomes . Thus, for m = 0, the SINR expression for the global content is given by, As the global content has no interference, the SINR and SNR expressions are equivalent. From (9), it is evident that as β increases, the SINR of the global content increases due to the power boosted global content in the buffer zone cells. Consequently, the coverage of the global content also increases. Thus, the power scaling method increases the SINR (and hence the coverage area) of both local and global contents.

4) Global content buffer:
At the extreme case of β = ∞, the local content ceases to be transmitted in the buffer zone. Thus, the buffer zone will consist of cells which transmit the global content at power P t . The SINR expressions now become, for the local content, and, for the global content. The local content now has one column of closest interfering cells in the buffer zone removed at the cost of eliminating one column of signal transmitting cells. Numerical simulations shall show that this leads to an overall improvement in the coverage area of the LSA.

III. SPECTRAL EFFICIENCY OF UNIVERSAL REUSE SCHEMES
Claim: The universal reuse scheme is advantageous in terms of spectral efficiency if there exist more than one LSA with multiple local contents.
Proof: Let µ m denote the modulation order of the m th content. The OFDM symbol duration is denoted by T sym . Then, the spectral efficiency of O-LSI is given by, bits/s/Hz (12) while that of universal reuse is given by, bits/s/Hz (13) Here, B is the total bandwidth of the OFDM system. To compare the two schemes, we take the ratio of the spectral efficiencies. A closed-form expression for the spectral efficiency gain can be calculated for the case when all contents get an equal number of subcarriers and assuming that all contents use the same modulation order. Let | denote the total number of usable subcarriers. Then, It can be seen that for K = 1, the ratio is 1. This corresponds to the case when there are no interfering LSAs. For the particular case studied in this paper, namely K = 2 and M = 4, the spectral efficiency gain is 1.6. The gains approach min(K, M ) for larger K and M . Thus, the universal reuse scheme is advantageous in terms of spectral efficiency for K, M ≥ 2.

IV. SIMULATION RESULTS
This section shows that when co-channel LSAs are present next to each other, using a universal reuse buffer can give superior performance compared to when such a buffer is absent. For the simulations, three local and one global content (M = 4) are considered, with the contents occupying equal bandwidths. Two co-channel LSAs (K = 2) as shown in Fig. 4 are deployed next to each other, which may or may not be separated by a buffer zone. When the buffer is absent, the 5 th and 6 th columns of Fig. 4 will be similar to the first four and last four columns, respectively. The simulation parameters are described in Table II and correspond to a low-power, low-tower (LPLT) deployment [25].  The percentage of users for whom the local content SINR is above the SINR threshold for various values of α are plotted in Fig. 7. It is seen that α = 1, i.e., reuse-M − 1 buffer, is worse than having no buffer at all. When the SINR threshold is low, α = 0.25 or the 'No buffer' is dominant. When the threshold is high, α = 0 is seen to dominate. This coupled with the fact that α = 1 gives poor performance, implies that it would be better to reduce the power of the local content in the buffer zone rather than increase it.
For the global content (Fig. 8), α = 1 is again the worst performer. This is logical as the global content is not transmitted in the buffer zone for this value of α. Following the same logic, α = 0 is found to be the best for global content coverage. Thus, among the SFR buffers, α = 0 seems to be better for both local and global content.

B. FFR-based scheme
In this scheme, α = 0 is equivalent to the reuse-M − 1 scheme of local contents, and α = 1 is equivalent to the case when no local contents are transmitted. It can be seen from Fig. 9 that for SIN R 0 ≤ 15, there is no need for a buffer zone. However, if the threshold is at, say, 20 dB, the LSA coverage area would be 53.1% without a buffer compared to 64.29% with an α = 1 FFR buffer. This corresponds to more than 11% improvement. Note that this is the same as a buffer with just the global content. Thus, when only high quality of service (QoS) users are expected to receive local content, it is better to separate the LSAs with a buffer zone that serves global content alone. As shown in Fig. 10, this also improves the global content coverage as the entire power is now being used to serve global content in the buffer zone. On the other hand, when the aim is to serve more users at the cost of a lower QoS, the buffer zone can be omitted, and the local contents can be broadcast in the transition region. The flexibility of this FFR scheme allows such decisions to be made by the broadcast transmitters as and when required.

C. Universal reuse with power scaled local contents
The results shown in Fig. 11 are for local contents when they are power scaled by a factor of β. It is seen that up to approximately 13 dB, there is no need for a buffer zone. At SIN R 0 = 14 dB, β = 2 is dominant. Then, the β required increases to 4, then 8, and then reaches ∞ as SIN R 0 increases from 17 dB to 20 dB. Higher values yield better results for the global content because an increased β implies a greater power is allotted to it. The results for the global content are shown in Fig. 12. It may be noted that β = ∞ for the power scaled local content and α = 1 for the FFR-based scheme are equivalent since both correspond to a buffer zone of purely global content. The area where users are served with the local content when there is no buffer is shown in Fig. 13 and in the presence of a power scaled buffer with β = ∞ is shown in Fig. 14. A visual comparison reveals that the coverage in columns 3 and 4 of the cell grid in LSA 1 improves when a power-scaled buffer is used. This improvement comes at the expense of not receiving local contents in the buffer zone.

V. CONCLUSION
This paper compared the popular O-LSI scheme and a proposed scheme based on local content power scaling with universal reuse. The SFR and FFR schemes for universal reuse were compared with a simple power scaling scheme for the local contents. It is found that the power scaling scheme is superior to the existing universal reuse approaches, as different values of β were found to yield the best performance at various threshold values SIN R 0 .
It is also more spectrally efficient than O-LSI as shown in (14). Thus, depending on the QoS requirement set for the users, the value of the power scaling factor β can be changed to maximize the number of users covered. This method also has the advantage that higher values of β improve the global content SINR in the buffer zone. This is unlike existing schemes such as [16] in which local content coverage is improved at the cost of reducing global content coverage. Hence, our proposed scheme improves the percentage of users receiving the local contents by as much as 11% for universal frequency reuse without incurring any additional complexity at the transmitter or receiver.