- Open Access
Link adaptation for energy-efficient uplink coordinated multi-point receptions
© Nam et al; licensee Springer. 2011
- Received: 29 June 2011
- Accepted: 23 November 2011
- Published: 23 November 2011
We investigate link adaptation methods for energy-efficient uplink coordinated multi-point receptions. A system model for practical cellular networks is introduced, in which only a subset of base stations participates in cooperative link adaptation and cooperative decoding for uplink transmissions. To cope with channel-state-information (CSI) mismatch incurred from the system model, link adaptation controllers implementing rate back-off from the maximum achievable rate calculated with the mismatched CSI is introduced. From analytical and simulation results, it is concluded that under a certain condition, the rate back-off does not help to improve energy efficiency, where, for example, the condition holds when the CSI errors are modeled as additive Gaussian random variables. Furthermore, energy efficiency of multi-user spatial-division-multiple-access uplink transmissions is studied in isolated cooperative cellular networks. In this scenario, an analytical expression for the optimal link adaptation achieving maximum energy efficiency is obtained.
- coordinated multipoint receptions
- energy efficiency
Spectral efficiency and energy efficiency are important metrics for wireless communication systems. While contemporary wireless telecommunications standards, e.g., LTE-Advanced mainly focuses on enhancing spectral efficiency (e.g., ), there is growing interest in improving energy efficiency, partly because the development of battery technology has not kept in pace with the demand of mobile communications [2, 3]. Energy-efficient communications also tend to reduce electromagnetic interference and lessen environmental impacts, for example, heat dissipation and electronic pollution. Therefore, recent research starts to focus on energy-efficient communication techniques [4–9].
Coordinated multi-point (CoMP) transmissions/reception is one example scheme studied in LTE-Advanced  primary targeting on increasing cell-edge user equipment's (UE's) spectral efficiency. In CoMP transmissions, multiple base stations coordinate their transmissions so that served UEs receive data streams with higher downlink (DL) spectral efficiency. In CoMP receptions, multiple base stations coordinate reception/decoding of packets from UEs to achieve higher uplink (UL) spectral efficiency.
While there have been lots of researches on throughput improvement of CoMP (for UL CoMP refer to  and references therein; for DL CoMP refer to [11, 12] and references therein), to the authors' best knowledge, little efforts have been put so far on energy-efficient CoMP communications. In this article, we investigate energy-efficient link adaptation for UL CoMP communications in cellular networks. For this purpose, we define an energy-efficiency metric, in a unit of nats/Joule,1 by extending the energy-efficiency metric introduced in . The energy-efficiency metric accounts for both transmit power and circuit power. The transmit power models all the power used for reliable data transmission. On the other hand, the circuit power represents the average power consumption of device electronics, e.g. filters, mixers, and digital-to-analog converters, and this portion of power consumption excludes that of the power amplifier and is independent of the transmission state. The newly introduced metric can measure energy efficiency of both the single-cell operations and the CoMP operations, despite fundamental differences of available channel state information (CSI).
In the first part of the article, we investigate the energy efficiency of a CoMP reception scheme of a single UE's uplink transmission in a cellular network, where only a subset of base stations participate in the cooperative decoding, and the other UEs' signals intended to the other base stations in the network may interfere with the single UE's uplink signals. In this scenario, the CSI experienced during the actual transmission may be different from the CSI used for link adaptation, owing to un-coordinated interference from the other UEs.
To cope with the CSI mismatch, we consider a link adaptation controller implementing a rate back-off from the maximum achievable rate calculated with the mismatched CSI. Then, we analyze the energy efficiency of the link adaptation controller and we show that if a certain condition is satisfied, then the link adaptation controller may still rely on the mismatched CSI in order to achieve the maximum energy efficiency.
In the second part of the article, we analyze the optimal energy efficiency of a CoMP reception scheme of multiple UEs' uplink transmission in a cellular network, where all the base stations participate in the cooperative decoding. In this interference-free scenario, we assume that perfect CSI is available at the link adaptation controller and analyzes conditions for achieving the optimal energy efficiency. In particular, we provide analytical expressions for the optimal power allocation for a two-UE two-base-station system.
The notations used in this article are summarized as in the following. Italic characters, e.g., K, h, are used for representing scalar variables. Boldface lowercase Roman characters, e.g., h, are used for representing vectors, and boldface uppercase Roman characters, e.g., H, are used for representing matrices. Boldface Italic lowercase characters, e.g., h, are used for representing either random variables or random vectors, while boldface Italic uppercase characters, e.g., H are used for representing random matrices. A H denotes the Hermitian transpose of matrix A, and h* denotes complex conjugate of a complex scalar h. |h| denotes the absolute value of a complex scalar h, and ||h|| denotes the L2 norm of a complex vector h. ℂ denotes the set of complex numbers.
Under the system model considered in this section, we consider both single-user and multi-user transmission scenarios with uplink CoMP receptions. In Section 3, we consider energy-efficient uplink CoMP receptions of a single user in a partially cooperating cellular wireless network, in which only a subset of base stations performs cooperation. For single-user transmissions, the further refined system model in Section 3 is general enough to reflect some important aspects of the real-life cellular networks, and at the same time, it is possible to obtain some analytical results. However, for multi-user multi-cell link adaptations, the system model of Section 3 is difficult to analyze. To obtain some insights of multi-user transmission scenarios aided by CoMP reception, we make further simplifying assumption of fully cooperating cellular networks in Section 4.
In this article, we investigated the energy efficiency of CoMP link adaptations and CoMP reception schemes of uplink transmission in a cellular network. To model typical implementation of CoMP in the cellular network, we first considered a scenario where only a subset of base stations participate in the cooperative link adaptation and cooperative decoding for a single UE, in which case, the other UEs' signals intended to the other base stations in the network interfere with the single UE's uplink signals, and hence the CSI experienced during the actual transmission is different from the CSI used for link adaptation. To cope with the CSI mismatch, we consider a link adaptation controller implementing a rate back-off from the maximum achievable rate calculated with the mismatched CSI. According to the analysis, we found that the maximum energy efficiency of the link adaptation controller is achieved when no rate back-off is employed, when a certain condition is satisfied. We also showed by simulation that the condition holds when the CSI errors are modeled as additive Gaussian random variables. Furthermore, in order to see benefits of multi-user uplink transmissions for energy efficiency, we analyzed the optimal energy efficiency of a link adaptation method and a CoMP reception scheme of multiple UEs' uplink transmissions in a cellular network, where all the base stations participate in the cooperative decoding. In this interference-free or perfect-CSI scenario, we obtained analytical expressions for the optimal power control with arbitrary numbers of UEs and base stations. The optimal power allocation gives similar intuition as water-filling in parallel channels, that is, we need to assign more power to a better-quality channel, to achieve the maximum energy efficiency We also provided a simulation result showing that two-user spatial-division-multiple-access transmissions achieves larger maximum energy efficiency than single-user transmissions in the same cellular network.
Proof of Theorem 2
The authors declare that they have no competing interests.
1For better mathematical representations and for convenience of analysis, we use "nats" instead of "bits," for the unit of information. 1 nats = log2e bits.
2This assumption can be justified as in the following. As mentioned earlier in this paper, because of the flash-light effect, the channel state used for demodulation, h and the one reported to the link adaptation controller, , are not necessarily the same. However, the link adaptation controller can obtain h from the receiver once the demodulation is finished. Therefore the link adaptation controller can determine the distribution of w.
- 3GPP TR 36814, Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA Physical layer aspects: Physical channels and modulation v1.0.0 2009.Google Scholar
- Lahiri K, Raghunathan A, Dey S, Panigrahi D: Battery-driven system design: a new frontier in low power design. In Proc Intl Conf on VLSI Design. Bangalore, India; 2002:261-267.Google Scholar
- Miao GW, Himayat N, Y Li, Swami A: Cross-layer optimization for energy-efficient wireless communications: a survey. 2009, 9(4):529-542.Google Scholar
- Verdu S: Spectral efficiency in the wideband regime. IEEE Trans Inf Theory 2002, 48(6):1319-1343. 10.1109/TIT.2002.1003824MathSciNetView ArticleGoogle Scholar
- Meshkati F, Poor HV, Schwartz SC, Mandayam NB: An energy-efficient approach to power control and receiver design in wireless networks. IEEE Trans Commun 2006, 5(1):3306-3315.Google Scholar
- Cui S, Goldsmith AJ, Bahai A: Energy-constrained modulation optimization. IEEE Trans Wirel Commun 2005, 4(5):2349-2360.View ArticleGoogle Scholar
- Miao G, Himayat N, Y Li, Bormann D: Energy-efficient design in wireless OFDMA. In Proc IEEE Conf Commun. ICC'; 2008. 2008Google Scholar
- Miao GW, Himayat N, GY Li, Talwar S: Low-complexity energy-efficient OFDMA. In Proc IEEE Conf Commun. ICC'; 2009:1-5. 2009Google Scholar
- Miao G, Himayat N, Li Y: Energy-efficient link adaptation in frequency-selective channels. IEEE Trans Commun 2010, 58(2):545-554.View ArticleGoogle Scholar
- Marsch P, Fettweis G: Uplink comp under a constrained backhaul and imperfect channel knowledge. IEEE Trans Wirel Commun 2010. (Submitted)Google Scholar
- Liu L, Nam Y, Zhang J: Proportional fair scheduling for multi-cell multiuser mimo systems. In Proc CISS. Princeton, NJ; 2010.Google Scholar
- Liu L, Zhang J, Yu J-C, Lee J: Intercell Interference Coordination through Limited Feedback, Int. J Digit Multimedia Broadcasting 2010. Article ID 134919, 7 pages, 2010Google Scholar
- Cover TM, Thomas JA: Elements of Information Theory. Wiely; 1991.View ArticleGoogle Scholar
- Nam Y, Gopala PK, El Gamal H: Resolving collisions via incremental redundancy: Arq diversity. In Proc INFOCOM. Anchorage, AK; 2007.Google Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.