This paper reviews energy-efficient wireless communications, providing an introduction to basic concepts, summarizing existing fundamental works and advanced techniques, and discussing open issues. Some techniques discussed include OFDMA networks, MIMO techniques, relay transmission, and resource allocation between signaling and data symbols. The paper concludes that while techniques like OFDMA, MIMO, and relaying can improve energy efficiency, joint optimization across techniques and consideration of practical factors is still needed, particularly for multi-user and multi-cell scenarios.

1. PAPER REVIEW
ENERGY-EFFICIENT WIRELESS COMMUNICATIONS
TUTORIAL, SURVEY, AND OPEN ISSUES
GROUP - 10
PROJECT MEMBERS
AADITYA SHAH, AYAM AJMERA, JAY SHAH, KAIVALYA SHAH, MAITREY MEHTA, MOHIT VACHHANI

2. ABOUT PAPER AND AUTHORS
• IEEE Wireless Communications, December 2011
• GEOFFREY YE LI – Professor, Georgia Institute of Technology
• ZHIKUN XU - Pursuing Ph.D. degree at Beihang University, Beijing, China
• CONG XIONG – Pursuing Ph.D. degree at Georgia Institute of Technology
• CHENYANG YANG – Professor, Beihang University, Beijing, China
• SHUNQING ZHANG – Huawei Technologies, Ph.D. from HKUST, Hong Kong
• YAN CHEN – Research engineer in the Green Radio project, Huawei
• SHUGONG XU – Principal Scientist, Huawei Corporate Research

4. ABSTRACT
In this paper, basic concepts of energy-efficient communications are
first introduced and then existing fundamental works and advanced
techniques for energy efficiency are summarized, including
information-theoretic analysis. Here, we review the given data &
propositions.
Some of the discussed technologies are:
• OFDMA networks
• MIMO techniques
• Relay transmission
• Resource allocation for signaling

5. INTRODUCTION
• Communication techniques have been exploited & optimized to
provide high Spectral Efficiency (SE) in wireless networks.
• Due to the rise in energy limited devices, pursuing high Energy
Efficiency (EE) is a trend for the design of future wireless
communications.
• Moreover, due to increasing data rates, energy consumption has
risen with time.
• It has been shown that reducing cell size can increase the number
of delivered information bits per unit energy for given user density
and total power in the service area.

6. FUNDAMENTAL CONCEPTS
Channel Capacity (C & R)
𝑹 =
𝟏
𝟐
𝒍𝒐𝒈 𝟐 𝟏 +
𝑷
𝑵 𝑶 𝑩
bits/degree of freedom
𝑪 = 𝟐𝑩𝑹 bits/sec
Where, P = Transmit Power
B = System Bandwidth
N0 = Noise Power Spectral Density
Energy Efficiency (𝜼 𝑬𝑬)
𝜼 𝑬𝑬 =
𝑪
𝑷
=
𝟐𝑹
𝑵 𝑶 𝟐 𝟐𝑹 − 𝟏
The result in the above equation is obtained by assuming an infinite size of information block
and infinite number of DOF. However, the system behavior is totally different in the finite case.

7. MATHEMATICAL PROPERTIES OF ENERGY
EFFICIENCY (𝜂 𝐸𝐸)
It is evident from the equation
as well as the plot that Energy
Efficiency decreases
monotonically with R.
𝜂 𝐸𝐸 𝑚𝑎𝑥 =
1
𝑁 𝑜 𝑙𝑛2
𝑎𝑠 𝑅 → 0
𝜂 𝐸𝐸 𝑚𝑖𝑛 = 0 𝑎𝑠 𝑅 → ∞

8. ORTHOGONAL FREQUENCY DIVISION MULTIPLE
ACCESS (OFDMA)
• Extensively studied for next generation wireless communication systems
like WiMAX, the Third Generation Partnership Project (3GPP), and LTE.
• While using OFDMA, system resources like subcarriers and transmit
power need to be properly allocated to different users to achieve high
performance.
• Popular dynamic resource allocation schemes include Rate Adaptation
(RA) & Margin Adaptation (MA). RA maximizes throughput while MA
minimizes total transmit power.
• While using these schemes along with OFDMA provides high throughput
and SE, they are not energy efficient.

9. ENERGY EFFICIENT RESOURCE ALLOCATION
SCHEME FOR OFDMA
• In contrast to the traditional scheme that maximizes throughput under a
fixed transmit power constraint, the new scheme maximizes the overall
Energy Efficiency by adjusting both the total transmit power and its
distribution among subcarriers.
• In multicell interference-limited scenarios, increasing transmit power even
does not necessarily benefit SE due to the associated higher interference
to the network.
• The existing research on energy-efficient OFDMA has mainly focused on
uplink scenarios or mobile terminal sides. More effort should be put on
the downlink or BS sides for the green design target.

10. BEST PRACTICES FOR OFDMA SYSTEM DESIGN
• The role of traffic statistics is crucial in energy-efficient broadband
communications.
• Existing approaches should be modified to incorporate traffic statistics,
which may be acquired from queue status of each user. Depending on
the traffic, the lengths of the active and sleep periods can be
dynamically assigned, and the power, modulation order, and coding can
be adjusted jointly to achieve desirable EE.
• Since EE and SE are two important system performance indicators, the
trade-off between EE and SE for general OFDMA networks should be
exploited to guide system design.

11. MIMO
MIMO techniques have been widely adopted in wireless networks
nowadays.
• Single-Input Single-Output (SISO),
• Single-Input Multiple-Output (SIMO)
• Multiple-Input Single-Output (MISO)
• Multiple-Input Multiple-Output (MIMO
Above given are various cases of MIMO.
Although MIMO techniques have shown to be effective in improving
capacity and Spectral Efficiency (SE), the energy consumption
increases.

12. ENERGY CONSUMPTION IN MIMO SYSTEMS
The increase in energy consumption is due to following reasons:
• More consumption of circuit energy due to duplication of transmit or receive
antennas.
• Depending on the ratio of the extra capacity improvement and the extra energy
consumption, the EE of a multiple-antenna system is lower than that of a single-
antenna system.
• In most of MIMO schemes, Channel State Information (CSI) is required at the
receiver or at both the transmitter and the receiver to obtain good performance.
• According to statistics, the number of active users at night is much lower than that
in the day. Switching off some Radio Frequency (RF) amplifier units at night can
save energy significantly while maintaining Quality of Service (QoS) of active users.
• It is shown that for short-range transmission, MISO decreases EE compared with
single-antenna transmission if they are not combined with adaptive modulation.

13. SOME RESEARCH TOPICS TO IMPROVE
ENERGY EFFICIENCY
By adapting modulation order to balance transmit energy and circuit energy
consumption, MISO systems outperform SISO systems.
• Closed-loop MIMO schemes are shown to enhance Spectral Efficiency.
Whether closed-loop MIMO schemes are more helpful than open-loop ones
to save energy is still an open issue.
• In multi-user and multi-cell environments, the existence of inter-user and
inter-cell interference complicates the design of energy-efficient MIMO
systems.
• MIMO schemes are usually incorporated into OFDMA systems. The spatial
and frequency resources can be jointly allocated to improve Energy Efficiency.
However, the complexity of the joint design may be prohibitive.

14. RELAY TRANSMISSION
• Relay transmission is used to increase the performance of a wireless
communication system.
• There are multiple nodes between the source and destination nodes. These
nodes are connected to both source and destination and they can send data
through multiple links created by them and due to this multiple links data
packets can be transferred at a high rate.
• In a normal connection, the data packets are sent one by one and if the link
between the source and destination is not available due to some issues then the
speed is decreased. This is not a problem in relay transmission.
• As the fading channels are independent, spectral efficiency is improved and
signal-to-interference ratio is improved, also , transmission energy is decreased
due to relay transmission

15. TYPES OF TRANSMISSION PERIOD IN RELAY SYSTEM
There are 2 types of transmission period namely: Broadcasting phase, Multiple
access phase.
• During the broadcasting phase, the source node transmits the data over the
air. This data can be received by any node.
• A source node acts as a transmitter and destination node or relay nodes act as
receivers. Now if the data is received by the relay nodes, then this data is
transmitted to destination node. This phase is called multiple access phase. A
source node or relay nodes act as transmitter and the destination node acts
as a receiver.

16. In both the phases, the nodes which transmit and receive information are dependent
on protocols like Amplify-and-Forward (AF) and Detect-and-Forward (DF).
• Amplify-and-Forward – The signal which is received by a relay node is amplified
and then transmitted to the next node.
The delay is less compared to other protocols as there is no decoding or quantization
operation.
The disadvantage of this scheme is that the noise is also amplified with the data
signal.
• Detect-and-Forward – It is also known as Demodulation-and-Forward relay
scheme. The signal received is demodulated by the receiver but this signal is not
checked for any errors. So the error is propagated to the receiver which causes
wrong data decoding.

17. PURE RELAY SYSTEMS
• Here, in the pure relay system, the relay nodes are useful only for the source
node to transmit the data, also the relay nodes should be used efficiently as
number of nodes required for delivery of data and its transmission is a big
problem in this systems.
• To maximize the energy efficiency of a relay node, the power allocation should
be optimal.
• The performance as well as energy efficiency is dependent on the transmission
strategy of each node, location of each node, number of nodes and data rate
used by each node.
• The design for this type of system is very complex and not suitable for some
practical scenarios.
• The energy efficiency may not increase with the increase in the number of relay
nodes. This is due to cooperation overhead.

18. COOPERATIVE RELAY SYSTEMS
• Unlike pure relay systems, this scheme makes it more complex to
optimize resource management due to the cooperation required
between users.
• Resources should be split for transmitting the data from itself and other
users.
• Also, it is difficult to find a partner which acts as a relay node which is
optimal. This is a significant problem if the number of users are
significantly high.
• In a system having two users, the power allocated is optimal. Thus, the
user cooperation can improve the energy efficiency of both users.

19. POTENTIAL RESEARCH TOPICS FOR BETTER
ENERGY EFFICIENCY
• Existing research results have shown that relay systems can improve Energy
Efficiency significantly. However, several important issues are still to be solved.
• Relay transmission considering the overhead: Additional time and power may
be used for resource allocation during relay transmission. Minimizing the total
energy consumption taking the extra overhead into account is still to be known.
• Energy-efficient bidirectional relay systems: Bidirectional relaying is a booming
technique and provides more opportunity to save energy. Design of bidirectional
relay systems has still not been developed and is in progress
• Relay transmission in multi-cell environments: Most of the work that exists only
focuses on single-point – to - single-point transmission, allocation of resources in
multipoint-to-single-point or multipoint-to-multipoint transmission is still a point
in which research is in progress.

20. RESOURCE ALLOCATION BETWEEN SIGNALING
AND DATA SYMBOLS
• Besides data streams, signaling symbols are widely used to assist data transmission in
wireless communications.
• Resource allocation for signaling symbols is independent of that for data symbols.
• However, the separation of signaling and data symbol designs does not optimize system
performance. Therefore, joint resource allocation between signaling and data symbols is
very important for energy-efficient design.
• Through Gaussian assumption of interference incurred by channel estimation error, it is
demonstrated that EE decreases to zero as the SNR goes to zero, and the maximum EE is
achieved at a nonzero SNR value.
• Also the relationship between EE and SE is no longer a monotonically decreasing function.

21. OPEN ISSUES ON RESOURCE ALLOCATION
BETWEEN SIGNALING AND DATA SYMBOLS
In general, study of resource allocation between signaling and data symbols
is only in the initial stage.
Some of the open issues that need to be investigated are: -
• Resource allocation between signaling and data symbols in multi-user
cases: The EE study in the existing literature is limited to the point-to-
point case. In the multi-user case, different users may suffer from different
channel fading, which results in different requirements of signaling
symbols.
• Signaling design considering CSI feedback: Although CSI at the transmitter
can help to improve system capacity, the additional energy consumption
on the overhead of feedback may slow down the increase of EE.

22. CONCLUSION
• In this article, the authors have comprehensively surveyed energy-
efficient wireless communications from the information-theoretic and
technique oriented perspectives.
• As for the information theoretic aspect, most literature about EE mainly
focused on point-to-point scenarios and the impact of practical issues
on EE is not fully exploited. Thus, research on EE needs to be extended
to multi-user and/or multi-cell cases as well as considering the practical
issues such as transmission associated circuit energy consumption,
which is of great significance to practical system design.
• As for the advanced techniques that will be used in future wireless
systems, such as OFDMA, MIMO, and relay, existing research has proved
that larger EE will be achieved through energy-efficient design.

23. REFERENCES
• ENERGY-EFFICIENT WIRELESS COMMUNICATIONS: TUTORIAL, SURVEY, AND OPEN ISSUES
GEOFFREY YE LI, ZHIKUN XU, CONG XIONG, CHENYANG YANG, SHUNQING ZHANG,
YAN CHEN, AND SHUGONG XU

24. TRANSMITTER IS KING BUT
THE KING NEEDS TO BE
ENERGY EFFICIENT 

25. THANK YOU