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Hierarchical modulation based cooperative relaying over a multi-cell OFDMA network

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Abstract

The relay-assisted cooperative technique is a promising solution for guaranteeing quality of service and improvement in channel capacity. This paper proposes a hierarchical modulation based cooperative relaying method via a fixed relay station (FRS) to improve the performance of cell edge users for orthogonal frequency division multiple access (OFDMA) systems over a multi-cell environment. The grafting of both FRS and hierarchical modulation onto an OFDMA based cooperative system provides opportunities to reduce the interferences, i.e. inter-cell and inter-relay interferences, and to achieve cooperative diversity in a multi-cell environment, simultaneously. Under conditions of severe inter cell interference caused by high carrier collisions, the proposed scheme leads to an improvement in both channel capacity and bit error rate for cell boundary users.

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Notes

  1. The conventional cooperative scheme means that the TDM based simple relaying scheme such as the proposed scheme in [7–9].

  2. For simplicity of illustration, we define a hierarchical modulated symbol using the notation _:_ where high priority bits are to the left of the colon and low priority bits are to the right of the colon. For example, A:B is the 4/16 QAM hierarchical modulation symbol where A represents the high priority bits and B represents the low priority bits. For additional protection of the high priority bits A, it may be necessary to allocate a higher power to A relative to B. Here, each capital alphabet represents two information bits.

  3. Note on the technical notation used in this paper : boldface letters are used for matrices and vectors. \((\cdot)^H\) represents the conjugate transpose operator; similarly, \((\cdot)^{T}, (\cdot)^{-1}\) and \((\cdot)^{\dag}\) represent the transpose, inverse and pseudo-inverse operators, respectively. \(|\cdot|\) denotes absolute value.

  4. In this paper, the ZF receive filter is adopted to maximize the SINR or to minimize the noise enhancements at the FRS. The main concept is identical with the MMSE receive filter. From the SINR of the MMSE [22], the optimal angle criterion is derived as

    $$ \alpha=\max\frac{1}{[(\frac{{\bf H}_E^H{\bf H}_E}{N_0}+I_{M_T})^{-1}]_{k,k}}-1=\min\left[\left(\frac{{\bf H}_E^H{\bf H}_E}{N_0}+I_{M_T}\right)^{-1}\right]_{k,k}=\min\left[({\bf H}_E^H{\bf H}_E)^{-1}\right]_{k,k}. $$
    (11)

    Thus, we obtain the identical result for the MMSE

  5. Transmit ZF weight vectors w = [w A w B ] are the normalized column vectors of \(\{[{\bf h}_{R D_1}^T {\bf h}_{R D_2}^T]^T\}^\dag\) [22].

  6. The modulation noise defined in [16] is the undesired signal from the viewpoint of the direct channel between each BS and MS. As shown in Fig. 2, |x A:B |2 = |x d |2 + |x m |2 = 2d 21  + 2d 22 , where x d is the desired signal, and x m is the modulation noise in 4/16 QAM hierarchical modulation.

  7. It is difficult to prove the convexity of average SINR according to λ. Thus, we provide the simulation result to find optimal λ statistically according to a given system environment and Eq. 21 as shown in Fig. 8. If the distribution of optimal λ is available, we can design an efficient quantizer for λ such as [23]. Therefore, the problem to find optimal λ can be resolved based on the statistic results.

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Acknowledgments

This research was supported by the MKE (The Ministry of Knowledge Economy), Korea, under the CITRC (Convergence Information Technology Research Center) support program (NIPA-2012-H0401-12-1003) supervised by the NIPA (National IT Industry Promotion Agency). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0011995).

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  1. Yonsei University, Seoul, Korea

    Hyukmin Son & Sanghoon Lee

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  1. Hyukmin Son
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  2. Sanghoon Lee
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Correspondence to Sanghoon Lee.

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Son, H., Lee, S. Hierarchical modulation based cooperative relaying over a multi-cell OFDMA network. Wireless Netw 19, 577–590 (2013). https://doi.org/10.1007/s11276-012-0486-4

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