Green House Gases- water vapor, CO2, nitrous oxide, methane, ozone.
EE is using less energy to provide the same level of performance, comfort & convenience.e.g.: Using a CFL in place of a conventional incandescent lamp saves 85% of the energy.SE is a measure of how efficiently a limited frequency spectrum is utilized by the service provider, as the main goal of the service provider is to accommodate max. no. of users in the limited BW.ASE: Defined as the max. throughput—summed over all users in the system, divided by the channel BW.
Considering EE: reducing the cell size is the most energy saving way in the aspect of base station power consumption. However in terms of handover, reducing the cell size does not guarantee the user’s QoS or mobility. Finally, these two different aspects have trade-off relationship.Sleep Mode: Part of the BS components can be turned off a/c to hourly traffic conditions. In off-peak hours, less processing capability is needed, and therefore parts of the unit can be transferred to sleep mode.
OFDMA distributes subcarriers among users so all users can transmit & receive at the same time within a single channel distributed as sub-channels.OFDMA is a multiplexing technique that subdivides the BW into multiple frequency subcarriers.
In OFDM only one user is allowed to access the entire spectrum.In OFDMA a number of users can exploit the BW at the same time.
Resource allocation may be decided by using computer programs applied to a specific domain to automatically and dynamically distribute resources to applicants. It may be considered as a specialized case of automatic scheduling.e.g.: Channel allocation in wireless communication may be decided by a BSusing an appropriate algorithm.Rate adaptation algorithm adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission.A link margin adaptation method using feedback information of a wireless communication system. For transmission and reception of data between two nodes of the wireless communication system, a transmission node requests a receiving node to transmit a link margin. The transmission node receives the link margin, and selects one of a white noise table or a delay spread table according to a delay spread value between the two nodes. Then, the transmission node adapts a link between the two nodes to a transmission mode and decides a transmission rate to transmit data at the decided transmission rate, thereby improvingtransmission capacity of the wireless communication system, obtaining the optimum link state, extending power utilization time of stations, and reducing interference between stations.
In frequency-selective fading, the coherence bandwidth of the channel is smaller than the bandwidth of the signal. Different frequency components of the signal therefore experience de-correlated fading.For Eent transmission both—The transmission data rate & ii) The energy efficiency ---- increases with channel power gain.For Eent transmission, the modulation order on each sub-channel decreases with the increase of the no. of sub-channels assigned to a user, while the EE increases with it.
Future Research Topics in OFDMA:Role of traffic statistics
MIMO employs Spatial Diversity:The main idea behind diversity is that when several copies of the same signals are passed through different channels then they experience independent fading of each other-there will be high probability that some signals will undergo deep fades while other may not. When these signals reach the receiver then there will be significant energy to make a decision that what was actual sent. The major types of diversities are:
Cost increases due to increase in the no. of antennas in the system.Adaptive Switching:
RS may serveto expand the broadcast range of a television or radio station beyond the primary signal's coverage area, or to improve service in a part of the main coverage area which receives a poor signal due to geographic constraints.
We observe that energy consumption can be significantly reduced by slightly increasing the number of relay nodes over the minimum number necessary to maintain connectivity. We also observe that further increasing the number of relay nodes will not make much improvement in EE.
Pure Relay System: RS only helps the Source node to transmit the data.Cooperative Relay System: All the nodes act as information sources as well as relays.
Impact of the hop no., node locations, & data rate on EE
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 Eisha Madhwal 5/30/2012 3
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) Eisha Madhwal 5/30/2012 5
Fig. 1: Sketch of the trade-off relations without and with practicalconcerns  Eisha Madhwal 5/30/2012 6
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. Eisha Madhwal 5/30/2012 7
Some advanced communication techniques fullyexploited in wireless networks to provide SE are: Orthogonal Frequency Division Multiple Access (OFDMA) Multiple –Input, Multiple –Output (MIMO) techniques Relay Transmission Eisha Madhwal 5/30/2012 8
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. Eisha Madhwal 5/30/2012 9
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. Eisha Madhwal 5/30/2012 11
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. Eisha Madhwal 5/30/2012 12
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. Eisha Madhwal 5/30/2012 13
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. Eisha Madhwal 5/30/2012 14
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) Eisha Madhwal 5/30/2012 15
Fig. 3: Diagram of MIMO schemes  Eisha Madhwal 5/30/2012 16
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. Eisha Madhwal 5/30/2012 17
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 Eisha Madhwal 5/30/2012 18
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. Eisha Madhwal 5/30/2012 19
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. Eisha Madhwal 5/30/2012 20
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. Eisha Madhwal 5/30/2012 21
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. Eisha Madhwal 5/30/2012 22
Fig. 4: Two structures of Relay system  Eisha Madhwal 5/30/2012 23
The performance of a pure relay system depends upon:i. The transmission strategy of each nodeii. The locations of the relay nodesiii. 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. Eisha Madhwal 5/30/2012 24
Fig. 5: EE v/s Data Rate in a multi-hop relay system Eisha Madhwal 5/30/2012 25
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). Eisha Madhwal 5/30/2012 26
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. Eisha Madhwal 5/30/2012 27
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  Eisha Madhwal 5/30/2012 28
. Geoffrey Ye Li, Zhikun Xu, Cong Xiong, ChenyangYang, Shunqing Zhang, Yan Chen, and Shugong Xu,“Energy-Efficient Wireless Communications:Tutorial,Survey, and Open Issues”, 2011. Yan Chen, Shunqing Zhang, Shugong Xu, andGeoffrey Ye Li, “Fundamental Tradeoffs on GreenWireless Networks”, 2011. http://www.scribd.com/doc/49762490/7/Difference-between-OFDM-and-OFDMA. Complex2real.com, http://www.complextoreal.com/ Eisha Madhwal 5/30/2012 29