Relay Node And Multi Hop Cellular

Published Date: 02 Nov 2017

Research by 3GPP (2009) team and Gesbert et al (2003) suggest that Multiple Input Multiple Outputs (MIMO) is a sure mode that uses receiving antenna and multiple transmissions to enhance efficiency. MIMO has the qualities of enhancing efficiency in a manner to handle essential characteristics of quality, spectral efficiency and throughput.

With technology like OFDM radio access of the 3GPP LTE network; Alamouti (1998) identifies 3 essential types of MIMO, that is, Array gain, Diversity gain and spatial multiplexing. The Diversity gain is further divided into two; Single user characterized by many one UE antennae that receive data across several diverse paths and Multi user that enable information transfer and reception by various users over several diversity paths. Beh et al (2009) explains the two MIMO techniques as space frequency block and space-time block coding where a signal is sent over more than one antenna, however, this is at a different time and frequency. When the signals are received, a higher quality is obtained by combining the streams leading to a better Signal to Noise ratio and lower Bit Error Rate (BER). For maximum signal quality and strength at the receiver, the Array Gain MIMO is used to send the same signal from each transmitter, but this is at a set phase and gain (time and amplitude) shift (also called coding) over the transmission path. The receiver also constructively combines the various multipath environments by the array gain to increase signal strength. A constructive and dynamic Array Gain can be obtained in the pre-coding feedback process to transmit from the receiver. These give relevant data a significance signal quality for adjustment of transmitter and improve the "beam" in the users’ direction. This result finds support from Gesbert et al (2003), Beh et al (2009), Samdanis et al (2010) and Wu et al (2010). Spatial multiplexing is characterized by splitting of high bit rate streams to several lower bit rate streams and each bit is then transmitted via a different transmitter antenna but on the same frequency channel. In case multiple signals arrive at the receiver with a large spatial difference, it is orthogonally separated. The significance of spatial multiplexing as proposed by Gesbert et al (2003), Beh et al (2009), Samdanis et al (2010) and Wu et al (2010) is to have an increased channel capacity and improved signal to noise ratio for a greater spectral efficiency.

Self Optimizing/Organizing Networks (SON)

Holma and Toskala (2009) argue that LTE standard development, USE of SON method offers a technique to reduce energy consumption. The method entails intelligent and automatic technique to alter procedure characteristics of the eNodeB’s that are possible to reduce consumption of energy.

Relay Node and Multi-hop Cellular

The UE communicates like the traditional cellular networks, that is, links directly to eNodeB in regard to relay cellular networks and links directly with dominant eNodeB or through a transitional relay station. Videv, Haas and Grant (2011) say that relay stations between eNodeB and UE of significance in saving energy and enhancing performance. Liu et al (2009) also suggests that the Relaying method splits longer paths into shorter segments, thus reducing the sum of path loss that might occur as a result of non-linear association of path distance and path loss. Moreover, Using sophisticated shorter radio links instead of longer ones has an economic effect on power budget. For mechanical and human-based relaying method in wireless access network, there is a need to focus on ways to minimize overall energy used, a fact investigated by Kolios et al (2011). This form of scenario, mechanical relaying involves Store-and –Carry the received information before forwarding (SCF). This is perfect for an elastic service.

Power Amplifier (PA) and Radio Frequency (RF) Amplifier

A radio modem needs a high linearity from the RF (radio frequency) that will isolate the RF design and the digital signal processing design. Class AB amplifiers are useful in forming RF envelope waveforms towards peak power. However, most of the time power amplifiers operate in the less efficient range as a result of waveforms having a high peak to average. By this mean, ETSI TS (2009) notes that only15-25% of the waveform can be determined for modems using LTE, UMITS and WiMAX. Jada (2011) further clarifies that RF power increase is only to a certain threshold. It is also difficult to get a rise in power output when input power is also on the rise. In addition, the output gain compresses with input increase toward saturation point of an RF amplifier

3.2.6 Two-Tier Macro-Femto Technology.

Consider a two-tier network with Femtocell and Macrocell. With regard to the OFDMA (orthogonal frequency division multiple access), the network is a fundamental technology in the 4th generation of networks like LTE and WiMAX. In the orthogonal frequency division multiple access system, the bandwidth is separated into several orthogonal sub-channels. Thus, macro and Femtocell users via an exclusive sub-channel can evade intracellular interference. N and WHz are mostly used to describe the quantity of sub-channels and the total bandwidth of the system. With the definition as a short-range data access point, lower power, base stations using Femtocells are stationed in highly dense hotspot points supporting stationary and low mobile users.

Also Chandrasekhar et al (2008) notes that Macrocell coverage is larger compared to radio, it enables lower power transmission to base stations and better signal reception. In essence, a two-tier Femtocell has both Femtocells and Macrocells that enable high spectral efficiency and low power consumption. One of the networks has an NC hexagonal standard macro BSs that uniform distribution of macros MSs. The other overlaid tier network has Femto (base station) BSs redistributed in accordance with Homogeneous Spatial Possion Point Process (SPPP) on the intensity plane λf on each Macrocell. Moreover, all Femto mobile stations are uniformly distributed. Let’s also assume Rf and Rc to be the radius of Femto BS and macro BS, respectively, such that, (Rc>>Rf). Rd is the distance from Femto BS to macro BS. The mean number of Femtocells BSs on each Macrocell can be calculated as Nf = λfS(Rc), with S(Rc) being Macrocell area. Two access policies exist in deploying Femntocell network that is open access and closed access. In the first case, users including Femto MSs and macro MSs in the operator network have rights to access Femtocell. In the second scenario, macro MSs can’t access the Femto BS even if received macro MSs Signals from the Femto BS is stronger. It is therefore essential to use open access in public areas and hot spots. However, closed access can be implemented for small office and home environments. Overall, open access has an advantage over closed; closed access has problems of cross tier macros MSs in the downlink transmission if located near a Femto BS. Contrary to these, closed access Femtocell networks are used by majority of operators due to security issues.

The efficiency of High power Amplifiers is a result of reducing peak to average power ratio (PAR) of the transmitted signal. The peak and average values of a power amplifier should be as close as possible to maximize on the efficiency of the power amplifier. However, use of Envelope tracking (ET) can improve on efficiency of the power amplifier. An ET has a dynamic system to change supplied voltage to the final RF stage power transistor, which synchronizes with transmitted RF signals via the device. This enables the output to be kept in a saturation region that is very efficient in operation. In a contemporary study by Gesbert et al (2003), with respect to PA effectiveness for BS, 50% efficiency has been reported for WCDMA (Wideband Code Division Multiple Access) waveforms for ET based PA. In other research, accuracies close to 60% have been obtained. If power amplification efficiencies can be radically raised to between15-25%, then almost 60% of energy used on cooling fans can be reduced.

Typically, in multi-antenna (for example MIMO) systems, amplifiers are located on the antenna. When the system load is low, transmit antennas that switch the energy on and off are used. An example is a UMITS that supports 2 transmitting antennas. For a non_MIMO supported system terminal, the base station may switch off to the next regular pilot channel broadcasted over the next antenna to minimize on energy. A base band unit of the station is further utilized by the power save modes to subunits, like:, Digital signal processors (DSPs) and channel cards, Application specific integrated circuits (ASICS), Field programmable gate arrays (FPGA) and clocks that can be turned on and off with regard to the base load station. In addition, phase modulation techniques can reduce PAR. By doing so, also high efficiency can be obtained in the User Equipment (UE) or Mobile Station transmitter, which will as well increase efficiency in the network.

Other means for Energy consumption reduction in Base Stations

Hasan et al (2011) suggests various other methods to reduce energy consumption in base stations. The methods include utilizing to the maximum the use of renewable resource, reducing BTS energy use and reducing the number base station sites. It also encompasses the use of system level features, energy saving by intra base stations and site solution in base stations.

A is initially discussed; the best way for energy efficiency is by use of power amplifiers in transmitter. This efficiency is too dependent on the required modulation, frequency and operating environment. Various linearization techniques like feed forward and digital pre-distortion Cartesian feedback are available to develop a power efficient energy model. It is not advisable to only study power saving modes in hardware, but also on BTS solutions that makes energy saving mode possible. Three site solutions are; the Modular BTS and RF head, in which case the RF transmitter and antenna are close to each other, and significant powerless equipments are engaged in improving performance. The second is the solution in the indoor sites, where energy is saved by evading air-conditioning. The third is the outdoor site involving the use of BTS over a wide temperature range resulting to less requirement of heating or/and cooling.

Typically the allocation of radio resource rather than taking care of channel condition is a result of manufactured base station. NEC (2013) proposes that to reduce power use, data scheduler can queue the information in a packet buffer and only transmit when the channels are able to handle. Hence as the amount of energy to send a packet increases, channel gain also increases. A close scrutiny at the base station resource allocation should show the relationship between the various base stations which are considered appropriate, and are able to coordinate base stations to optimize saving energy by identifying coverage and capacity demands. Coordination is in the form of coverage, sharing loads, collective decision and interference. Two key factors to be put under consideration to reduce energy consumption are the number of base station sites and energy consumed by a single base station. A number of features also exist to balance base station capacity and base station size. Samdanis et al (2010) identifies them as smart antenna, extended cell, modular BTS, RF head4-way diversity (K., D., M.) and 6-sector site and finally 2-way.

ETSI TS (2009) urges to use of renewable resources to increase efficiency of a base station. This energy includes wind, solar, which as well reduces CO2 emissions, address the issue of unreliable power and long distances to the power grid.

With a rapid growth in mobile communication technology demand, cellular base station numbers are on the rise worldwide. Thanks to the new data intensive by Deloitte (2010). This is cellular standards on power consumptions; it can increase each base station to 1.4 kW with an annual base station cost to $3200 and a carbon footprint of 11 energy consumption. Three units in a base station identified by Bianzino et al (2010) consume most energy are the feeder network, the radio and the baseband unit. However, the radio consumes most power energy of the BS

Energy consumption Minimization in a base station

BS hardware improvement is one way to reduce energy consumption. This is achieved by addressing the part that consumes more power in a base station, that is, the power amplifier. The radio consumes about 80% of base station power consumption. Deloite (2010) studies reveal that the PA consumes 50% of which about 80-90% goes to waste due to the PA requiring operational cost and additional energy. Raab (1987) gives the solution to this by using a switch mode power amplifier for higher operating frequencies on wireless mobile systems. He also suggests the use of laterally diffused metal oxide semiconductor (LDMOS) technology and Multi Storage Doherty Power amplifier that a power efficiency of about 70% under the Rayleigh envelope signal analysis.

Another significant technique to be improved on is the increase of output power to maintain signal quality as constant as signal fluctuations result is more power consumption. However, the process of low traffic load leads to a lot of energy losses. It is therefore essential to obtain a flexible PA architectural design capabilities to adapt to desired outputs. In recent architecture, mobile terminals and base stations continuously transmit pilot signals while newer ones like LTE and LTE-Advanced transmit data at higher rates. When there is no signal for transmission, then switching off the transceiver is a way to save power. The LTE standards utilized this by a power saving protocol like discontinuous transmission (DTX) and discontinuous reception (DRX) modes in mobile handsets. DTX and DRX save power by temporarily powering off the device while maintaining connection with little throughput. Base stations with fixed cellular networks, there are lots of activities linked to reducing energy wastages

Edler and Lundberg (2004) proposes Ericson’s’ idea of innovative methodologies for environmental energy saving. They also describe on how to improve RBS efficiency and reduce site cooling costs. Electrical power consumption of radio access network has a direct impact on cost and the environment. There are suggestions to study and understand network performance in order to reduce operational costs. This includes determining power consumed by various parts of the network. Also the knowledge of the relationship between environment and climate is essential to assist in cooling of the base station.

For a number of years, there has been interest in an efficient design of wireless network with prolonged battery lifespan for mobile terminals and sensor nodes. Environmental impacts due to emissions of warming have changed the focus of energy efficient wireless networks. Energy reduction also has an economic effect on revenue. It is estimated that in the recent past, over $10B of electricity were spent on wireless network operation, which is a huge fraction of the expenditure. There is a 15-20% rise of energy consumption for IT sectors and is expected to double every 5 years. These results have forced major operators to to begin finding ways to improve energy efficiency in their operation.

Alamouti (1998) says that the BSs consumes about 60-80% of the total power. This section aims to reduce power consumption in the BSs unit. This has been spurred by the interest by a cellular industry desire to increase efficiency in wireless communication. The cellular industry accounts for most power consumption in the communication industry. This is a result of enhancing data rates for broadband access to 3td generation mobile systems. Data from other companies and cellular operators studied by Tuttlebee, Fletcher et al (2010), Khalife et al (2008), George (2006) and Deutsche Telekom, (2008) ranks the cellular operators high in energy consumption in networks. With growth predictions, there are fears of increase costs to other relevant players in the same field. Therefore reducing base station costs is of primary interest.

The green cellular architecture, at no radiation, aims to reduce mobile station emission. The design utilizes the transceiver base station to receive only devices that are deployed within the proximity of the mobile station. For ETSI TS (2009) study, in this architecture, both 3G and 4G (modern cellular and wireless networks) have a classic power control system able to control transmission (TX) power of macros BS and BSs. With a focus on the uplink, the distance from BS determines received MS Tx power. Majority of cellular and wireless networks use transceiver BSs architecture, however, the green cellular design proposes argumentation of the BSs transceiver and receive-only-device.

To have reduced cellular radiation exposure, the green antenna is connected to the network infrastructure via direct point-to-point microwave or wire connection. Currently, green antenna can be set on a sufficient grid and decreasing the mean Tx power for MS to any desired value supported by MS, with no additional source of radiation. Keeping in mind the importance of cellular indoor use, an ordinary embodiment of green cellular is defined as strengthening outdoor BS with regard to indoor antenna, denoted as Green Spots (GSs). As a result of moderate MS Tx indoor power, the GSs are of important significance in this architecture. GSs can be installed in school buildings where radiation is a key issue. An outdoor BS can be positioned anywhere outdoors and still allow access. On achieving Green antenna, MSs transmits low TX power (and vice versa), reducing user interference to both the neighboring BSs and the same BS. This is because majority of communication systems is restricted by interference, resulting in decrease in need Tx power of the other MSs. For this discussion, it is assumed that the Green antenna decodes uplink (UL) transmission and is linked to all leading neighboring BSs. The serving BS respective to each MS is determined by the green antenna and forwards the MS UL information to the serving BS. On the other hand, this configuration can be utilized in IP based networks between the neighboring BSs and the Green Antenna as suggested by ETSI TS, (2009). In other terms, consumed power is in two parts; energy to run the system and energy to be created and installed in the system. In this case, embodied energy is not directly linked to the communication itself. Embodied energy is also a factor to be considered in the design of a new generation of wireless network systems.

In accordance to statistical data, the BS is a key source of energy consumption for mobile operations, hence a key factor to deal on. BS looses most of its power during conversion by power amplifiers that consume most of the power. Other energy loses as a result of cabling, AC/DC current converting, and energy for cooling can also be reduced. For example, for a a global mobile communication BS system to supply 1.2kW to an antenna, Karl (2003) calculates a 3802Watts input with a 3.1% overall efficiency. Other studies are underway on improving energy efficiency of the BS at the various perspectives like using energy efficient backhaul solutions, Increasing PA efficiency, using non-active cooling techniques, improving protocols for energy efficiency, applying energy efficient deployment strategies and using the masthead PA to reduce feeder loss