Energy efficient wireless technology

 Green technology is the development and application
of products, equipment and systems used to conserve
the natural environment and resources, which
minimizes and reduces the negative impact of human
activities.

 CRITERIA OF GREEN TECHNOLOGY:
 It minimizes the degradation of the environment.
 It has a zero or low green house gas (GHG) emission.
 It conserves the use of energy and natural resources.

Eisha Madhwal 5/30/2012 2

 Energy Efficiency: EE is defined as the number of bits
transmitted per Joule of energy.
 Unit: bit/J

 Spectral Efficiency: SE refers to the information rate that
can be transmitted over a given BW in a specific
communication system.
 Unit: bit/s/Hz

 Area Spectral Efficiency: ASE is defined as the max.
throughput-summed over all users in the system, divided by
the channel BW.
 Unit: bit/s/Hz per unit area





 According to their definitions, SE and EE can be expressed
as

----(2)

----(3)

 Putting the value of EE from (2) in (3) we get the SE-EE
relationship as

------(4)


Fig. 1: Sketch of the trade-off relations without and with practical
concerns [2]


 Impact of cell sizes on EE: Reducing the cell size can
increase the no. of delivered information bits per unit
energy for given user density and total power in the
service area.

 If a sleep mode is introduced, the EE can be further
enhanced.


Some advanced communication techniques fully
exploited in wireless networks to provide SE are:

 Orthogonal Frequency Division Multiple Access
(OFDMA)

 Multiple –Input, Multiple –Output (MIMO) techniques

 Relay Transmission


 OFDMA is a multi-user OFDM that allows multiple
access on the same channel (a group of evenly spaced
subcarriers).

 Input data gets divided into several parallel sub-
streams of reduced data-rate and each sub-stream is
modulated and transmitted on a separate Orthogonal
Subcarrier.


Fig. 2: Difference between OFDM & OFDMA [3]


 System resource, such as subcarriers & transmit power,
needs to be properly allocated to different users to
achieve high performance.

 Two schemes used for dynamic resource allocation are:
1. Rate Adaptation (RA): maximizes the throughput.
Aims at SE.
2. Margin Adaptation (MA): Minimizes total transmit
power. Aims on transmit power efficiency.


 Energy-efficient OFDM systems was first addressed
with consideration of circuit consumption for
frequency-selective fading channels.

 This scheme maximizes the overall EE by adjusting
both- the total transmit power and its distribution
among subcarriers.

 15% reduction in energy consumption is observed
when frequency diversity is exploited.


 Energy-efficient power distribution not only boosts
system EE but also refines the EE-SE trade-off.

 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.

 General EE-SE trade-off is not yet addressed yet.


 MIMO (Multiple Input, Multiple Output) is an antenna
technology for wireless communications in which multiple
antennas are used at both the source (Tx) and the destination
(Rx).
 The antennas at each end of the communication circuit are
combined to minimize errors and optimize data speed.

 The use of two or more antennas, along with the transmission
of multiple signals (one for each antenna) at the source and the
destination, eliminates the trouble caused by multipath wave
propagation, and can even take advantage of this effect.


 Some special cases of MIMO are:
I. Single Input, Single Output (SISO)
II. Single Input, Multiple Output (SIMO)
III. Multiple Input, Single Output (MISO)

 MIMO can also be used with single user or multiple
users to form:
i. Single-User MIMO (SU-MIMO)
ii. Multi-User MIMO (MU-MIMO)
iii. Coordinated Multipoint Transmission (CoMP)


Fig. 3: Diagram of MIMO schemes [1]


 MIMO techniques are effective in improving capacity
& SE of wireless systems, but on the cost of increased
energy consumption.

 Switching off some radio frequency (RF) amplifiers
units at night can save significant energy while
maintaining QoS of active users.

 Adaptive switching between MIMO and SIMO is
addressed to save energy at mobile terminals.


 For short range transmission, MISO decreases EE as
compared with single antenna transmission if they are
not combined with adaptive modulation.
 By adapting modulation order to balance transmit
energy & circuit energy consumption, MISO systems
outperform SISO systems.
 Channel State Information (CSI) helps to adapt the
following:
 Spatial Division Multiplexing
 Space Time Coding
 SISO Transmission


 Smart adaptation can achieve better EE-SE trade-off
compared with single MIMO mode & the improvement
of EE is up to 30% compared with non-adaptive
systems.


 Relay transmission technique involves forwarding of
information to the local base station (BS) via relay
stations installed at different locations to carry out this
process.
 Service coverage and the overall throughput can be
enhanced by using this technique.
 Data from the source (Tx) can be delivered through
multiple wireless links to the receiver (Rx).
 Due to the multiple transmissions, the time to transmit
a fixed amount of data reduces and so does the
consumed energy.


 In a typical relay system, a transmission period consist
of two phases:

1. Broadcasting Phase: Source node sends data to the
air, which may be received by the relay nodes, or
both the relay & the destination nodes.
2. Multi-Access Phase: The relay nodes or both the
source & relay nodes transmit data to the destination
nodes.


 There are basically two types of relay systems:

 Pure relay systems: The role of the relay nodes is only
to help the source node to transmit data.

 Cooperative relay system: All the nodes act as
information sources as well as relays.


Fig. 4: Two structures of Relay system [1]


 The performance of a pure relay system depends upon:
i. The transmission strategy of each node
ii. The locations of the relay nodes
iii. The data rate used by each node

 Although power allocation & the no. of locations of
nodes affect the EE significantly, such joint design is
very complex & may not be suitable for practical
scenarios.
 For simplifying the relay networks, only two hop
communications are set up between the source & the
destination nodes.


Fig. 5: EE v/s Data Rate in a multi-hop relay system
[1]


 Cooperation among users makes it more complex to
optimize resource management.
 It has been studied that the user cooperation can improve
user’s EE.

 Difficulties:
 Resource at each user should be split for transmitting data
both from itself & from other users.
 Finding an appropriate user as a relay node: It is very
complicated to find the optimal partner in a network with a
large no. of users (since the no. of possible pairings is
huge).


 Besides data streams, signaling symbols are widely
used to assist data transmission in wireless
communications.
 The representative are the signaling for synchronization
& channel estimation.
 By gaussian assumption of interference introduced by
channel estimation error, it is observed that the EE
decreases to zero as the SNR goes to zero, & the
maximum EE is achieved at a non-zero SNR value as
shown in Fig 6.

Fig 6: EE v/s SNR in the worst case scenario for block fading channels with ‘m’
symbol coherence duration & unit variance when the PSD of noise N0=-
174dBm/Hz [1]


[1]. Geoffrey Ye Li, Zhikun Xu, Cong Xiong, Chenyang
Yang, Shunqing Zhang, Yan Chen, and Shugong Xu,
“Energy-Efficient Wireless Communications:Tutorial,
Survey, and Open Issues”, 2011
[2]. Yan Chen, Shunqing Zhang, Shugong Xu, and
Geoffrey Ye Li, “Fundamental Tradeoffs on Green
Wireless Networks”, 2011
[3]. http://www.scribd.com/doc/49762490/7/Difference-
between-OFDM-and-OFDMA
[4]. Complex2real.com, http://www.complextoreal.com/


THANK YOU!


Energy efficient wireless technology

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