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15EC81

Wireless Cellular and LTE 4G Broadband Question Bank and Its Solution

Module -1 1. Enumerate key enabling features of LTE 4G. LTE design incorporates several important enabling radio and core network technologies. Some of them are:  Orthogonal Frequency Division Multiplexing [OFDM] This technology is more applicable to high speed application it over comes the usage of large bandwidth as seen in CDMA. The applications such as Wi-Fi ad Wi-Max are the systems employing this as core technology  SC-FDE and SC FDMA In order to keep the cost down and battery life up LTE incorporates a power efficient transmission scheme for uplink .i.e. Single Carrier Frequency Division Equalization. SC-FDE is conceptually similar to OFDM but instead of transmitting the IFFT of actual data symbols, the data symbols are sent as sequence of QAM symbols with a cyclic prefix added, the IFFT is added at the end of the receiver. It also has multipath resistance and low complexity ,with PAR of 4-5 dB. The uplink of LTE uses a multi-user version of SD-FDE called as SC-FDMA, its DFT precoded OFDMA, but has increased complexity of transmitter and receiver.  Channel Dependent Multi-user Resource Scheduling OFDMA provides flexibility to allocate the channel resources by designing algorithms such that they meet the requirements of arbitrary throughput, delay and others. The standard supports dynamic channel dependent scheduling to enhance overall system capacity.Given that each user will be experiencing uncorrelated fading channels, it is possible to allocate subcarriers among users inn such a way that the overall capacity is increased. This technique is called Frequency selective multiuser scheduling. Call for focusing transmission power in each user’s best channel portion, thereby increasing overall capacity.  Multiantenna Techniques  Multiantenna Techniques 1. Transmit diversity: This technique to combat multipath fading in the wireless channel, to send copies of same signal which are coded differently over multiple transmit antennas. Its mainly intended for common downlink channels that cannot make use of channel dependent scheduling. 2. Beamforming: Beamforming is a type of Radio frequency management in which an access point uses multiple antennas to send out the same signal. It make possible by transmitters and Questions & Solutions

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15EC81 receivers that use of MIMO technology such that capacity, reliability,battery life ,throughput and coverage range is improved. 3. Spatial Multiplexing: In order to parallel transmit the multiple independent streams over multiple antennas and separated at the receiver appropriately by using signal processing techniques. This case is suitable for scattering rich environments. 4. Multi-user MIMO: In order to cater to uplink such that the complexity and cost is reduced considerably Multi-user MIMO (MU-MIMO), thus supports multiple users in uplink each with a single antenna to transmit using the same frequency and time resource.  IP Based Flat Network architecture The LTE requires flat radio and core network architecture. Flat radio implies fewer nodes and less hierarchical structure for the network, hence has lower cost and lower latency. It also means fewer interfaces, protocol processing and reduced interoperability testing, which lowers development and deployment cost.

2. What are the advantages and disadvantages of OFDM? Advantages of OFDM: 1. Elegant solution to multipath interferenceAt high data rates critical challenge is Inter Symbol interference due to multipaths, the shorter symbol time cause ISI a bigger challenge for broadband wireless systems. In order to resolve this OFSM a multi-carries modulation technique is used. OFDM works with an idea of divide a given high bit data stream into parallel streams of lower bit rate and modulate each stream/on separate carrier often referred as subcarriers/tones. This parallel data processing increases the symbol duration of each stream such that the multipath delay spread is only a small fraction of symbol duration. The subcarriers are selected such that they are orthogonal to each other over a symbol duration, due to which there is no interference between the carriers thus its spectrally efficient. It also avoids the need to have non-overlapping sub-carriers which eliminates inter carrier interference. ISI can be completely eliminated by using large guard intervals between OFDM symbols ,at the cost of power wastage and decrease in bandwidth efficiency. 2. Reduced computational complexity: It uses Fast Fourier transforms (FFT/IFFT),its computational complexity is lower than time domain equalizers i.e. about O(Blog2BTm) and O(B2Tm) where B is the bandwidth ;Tm is the delay spread. The reduced complexity in downlink simplifies receiver processing and reduces mobile device cost and power consumption. It’s important for wide bandwidth transmission of LTE coupled with multi-stream transmissions. 3. Graceful degradation of performance under excess delay:

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15EC81 It is well suited for adaptive modulation and coding which allows system to make the best of the available channel conditions. This contrasts with abrupt degradation of owing to error propagation that single carrier systems experience as delay spread exceeds the value for which the equalizer is designed. 4. Exploitation of frequency diversity: LTE can be deployed to variety of spectrum allocations and different channel bandwidths since the channel bandwidth is scalable without impacting the hardware design od BS and MS. OFDM facilitates coding and interleaving across subcarriers in frequency domain, which provides robustness against burst errors caused by portions of transmitted spectrum undergoing deep fades. 5. Enables efficient multi-access schemes: OFDM can be used as multi access scheme by partitioning different subcarriers among multiple users, thus referred as OFDMA. It offers ability to provide fine granularity in channel allocation to achieve significant capacity improvements, particularly in slow time varying channels. 6. Robust against narrowband interference: Its robust since the interference affects only a fraction of subcarriers. 7. Suitable for coherent demodulation: Its relatively easy to do pilot based channel estimation in OFDM systems, which renders them suitable for coherent demodulation schemes that are more power efficient. 8. Facilities use of MIMO: MIMO stands for multiple input multiple output and refers to a collection of signal processing techniques that use multiple antennas both at transmitter and receiver to improve the system performance. The effectiveness is seen if its used for narrowband flat fading channels, i.e. the OFDM subcarriers which are frequency selective. MIMO is not applicable for traditional broad band channels. 9. Efficient support of broadcast services: A Single Frequency Network (SFN) can be designed by synchronizing BS to timing errors well within the guard intervals. This allows the broadcast signals form different cells to combine over air to significantly enhance the received power, thereby enabling high data rate broadcast transmission for a given transmitted power. LTE design leverages the OFDM capability to improve efficient broadcast services. Disadvantages of OFDM: The OFDM signals have high peak average ratio (PAR) which causes the nonlinearities and clipping distortion when passed through an amplifier If the above problem is resolved then it is at the cost of increased cost of transmitter and wastage of power.

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15EC81

3. What are the multi antenna techniques incorporated to combat multipath fading? 1. Transmit diversity: This technique to combat multipath fading in the wireless channel, to send copies of same signal which are coded differently over multiple transmit antennas. Its mainly intended for common downlink channels that cannot make use of channel dependent scheduling. LTE transmit diversity is based on space frequency block coding (SFBC) techniques complimented with frequency shift time diversity (FSTD) when four transmit antennas are used. It increases system capacity and cell range. This is also applicable for low data rate VoIP, where the additional overhead of channel dependent scheduling may not be justified. 2. Beamforming: Beamforming is a type of Radio frequency management in which an access point uses multiple antennas to send out the same signal. It make possible by transmitters and receivers that use of MIMO technology such that capacity, reliability ,battery life ,throughput and coverage range is improved. Multiple antennas transmit same information appropriately weighted for each antenna element such that effect is to focus the transmitted beam in the direction of the receiver and away from the interference, thereby improving the signal to interference ratio. 3. Spatial Multiplexing: In order to parallelly transmit the multiple independent streams over multiple antennas and separated at the receiver appropriately by using signal processing techniques. This case is suitable for scattering rich environments. Theoretically this technique provides data rate and capacity gains proportionally to number of antennas used. It works better for good SNR and light load conditions, hence more pronounced effect for peak data rates than overall system capacity. LTE supports this for four transmitters and four receiver antennas. It applicable for downlink rather than uplink since its complex and costly 4. Multi-user MIMO: In order to cater to uplink such that the complexity and cost is reduced considerably Multi-user MIMO (MU-MIMO), thus supports multiple users in uplink each with a single antenna to transmit using the same frequency and time resource. The signals from different MU-MIMO users are separated at base station receiver using accurate channel state information of each user obtained through uplink reference signals that are orthogonal between users. LTE supports beamforming in downlink.

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15EC81

4. Explain IP based flat network architecture? The LTE requires flat radio and core network architecture. Flat radio implies fewer nodes and less hierarchical structure for the network, hence has lower cost and lower latency. It also means fewer interfaces, protocol processing and reduced interoperability testing, which lowers development and deployment cost. It has an advantage of better optimization of radio interface, merging of some control plane protocols and short session start-up time. 3GPP evolution towards flat LTE SAE architecture can be shown as follows:

Considering few releases i.e.  In release 6 (2G/3G) architecture similar to its predecessors has four network elements, they are BS/Node B, Radio Network Controller, serving GPRS serving Node ,Gateway GPRS serving node.  In release 7 (3G/HSPA) architecture it has direct tunnel option form RNC to GGSN ,which bypasses SGSN from data path. similarly in release 7 (3G LTE) the RNC and Node-B are combined functionally and GGSN is modified as Architecture Evolution Gateway (SAE-GW) and enhanced Node B(e-Node B).  In release 8 (3G LTE) architecture merges BS and RNC unit together into single functional system. The control path includes functionality v=called as Mobility Management Entity (MME),it provides functions such as subscriber, mobility and session management.  The MME and SAE-GW could be collocated in a single entity called access gateway (ASW).  This architecture supports all services including Voice. It has a single evolved packet switched network for all services which would provide huge operational and Questions & Solutions

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15EC81 infrastructure cost savings. In order to provide backward compatibility non IP aspects of 3GPP architecture such as GPRS tunneling protocol and PDCP still exist within the LTE network architecture.

5. Briefly explain the LTE network architecture.

Core network design for 3GPP Release 8 LTE is called Evolved Packet Core (EPC). It provides a high capacity, all IP, reduced latency, flat architecture which dramatically reduce cost and supports advanced real-time and media rich service with enhanced quality of experience. It works with new radio access networks such as LTE, interworks with legacy 2G GERAN and 3G UTRAN connected via SGSN. EPC provides functions such as packet routing and transfer, access control ,mobility management and network management. It has four elements: i. Serving Gateway (SWG) which terminates the interface toward the 3GPP radio access networks. ii. Packet Data Network Gateway (PGW) which controls IP data services, does routing, allocates IP address, enforces policy and provides access for non-3GPP access network. iii. Mobility Management Entity (MME) supports user equipment context and identity as well as authenticates and authorizes. iv. Policy and charging rules function (PCRF) manages QoS aspects.  Serving Gateway (SGW): It acts as demarcation point between the RAN and core network and manages user plane utility.

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15EC81 It serves as mobility anchor when terminals move across areas served by different eNode B elements in E-UTRAN as well as across other 3GPP radio networks such as GERAN and UTRAN. It functions are to downlink packet buffering and initiation of network triggered service request procedures; lawful interception; packet routing and forwarding; transport level packet marking in uplink and downlink accounting support for per user and inter-operator charging.  Packet Data Network Gateway (PGW): It’s a termination of EPC towards other PDN/IMS network providing end users. It serves as an anchor point for sessions toward external PDN and provides functions such as IP address allocation, policy enforcement(operator defined rules for resource allocation to control data rate, QoS and usage), packet filtering( deep packet inspection for application detection) and charging support.  Mobility Management Entity (MME): It performs signalling and control functions to manage user terminal access to network connections, assignment of network resources and mobility management functions such as idle mode location tracking, paging, roaming and session management. It provides security functions such as providing temporary identities for user terminals, interacting with Home Subscriber Server (HSS) for authentication and negotiation of ciphering and integrity protection algorithms. It also selects appropriate serving and PDN gateways and selecting legacy gateways for handovers to other GERAN and UTRAN networks. It manages thousands of eNode-B elements which differentiates with 2G/3G services which use RNC and SGSN Platforms.  Policy and Charging rules function (PCRF): It concatenates Policy decision function and Charging rules function. This feature is deployed in release 7 and enhances in release 8 which supports non 3GPP also. It acts as interface with PDN gateway and supports service data flow detection ,policy enforcement and flow based charging.

6. Briefly explain the features of cellular communication system? A cellular communication system define the architecture for deploying the LTE systems. In cellular systems the service area is subdivided into smaller geographical area called cells, they are served by their own base stations. BS are restricted to maintain power levels within the boundaries such that they avoid interference with neighboring cells.

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15EC81 The propagation path loss allows the spatial isolation of different cells operating on same frequency channel at the same time, thus same frequency can be reassigned to different cells as long as they are spatially isolated. Considering the example model of cell and cell cluster given below it shows that frequency reuse factor is 1/7 (one cluster with seven cell in each). The cells are hexagonal in shape and they are allocated intelligently in order to maximize the geographical distance between co-channels. In the cellular system the, BS transmit power decreases as the area of cell decreases correspondingly. The only disadvantage is need of more BS installations and frequent handoffs. Hand off process would provide a means of the seamless transfer of connection from one BS to another BS. A smooth handoff is a challenging aspect.

7. What are the parameters considered during the analysis of efficacy cellular networks? The performance of wireless cellular network is significantly limited by co channel interference which comes from other users in the same cell or from other cells. The other cell interference (OCI)is the deceasing function of radius of the cell (R) and distance to the center of neighbouring co channel cell and an increasing function of transmit power which determines the performance in terms of capacity ,reliability i.e. SIR. If in case all the base station increase /decrease the power simultaneously there is no change in its performance it termed as interference-limited system. The spatial isolation between co-channel cells can be measured by defining the parameter Z called cochannel reuse ratio ,its defined as ratio of distance to centre of the nearest co channel cell to radius of the cell. Z= D / R =√(3/f) Where 1/f is size of a cluster and the inverse of frequency reuse factor.

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15EC81 Lower the value of f reduction in co channel interference so that it improves the quality of communication link and capacity. It should be chosen such that SINR is above acceptable levels, else Overall spectral efficiency decreases with the size of a cluster. SIR can be used as background noise is negligible. If NI interfering number of cells for S as received power of desired signal and Ii interference power of ith co channel BS are define then signal to interference ratio (SIR) for mobile station is given as S/I =S/ΣIi i = 1 to NI If the empirical path loss formula and universal frequency reuse are considered, the received SIR for worst case is given by

The outage probability that the received SIR falls below a threshold can be derived from the distribution .if the mean and standard deviation of the lognormal distribution are µ and σ in dB, the outage probability is derived from Q function.

8. What is the meaning of sectoring with reference to cellular technology? In order to effectively use the spectrum and also have better frequency reuse method a technique called as sectoring of cells are performed, by using directional antennas instead of an omni directional antennas at base stations. Questions & Solutions

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15EC81 This technique also reduces co-channel interference significantly. Each sector and reuse time and code slots, hence capacity of cell remains unaffected, rather its higher than that of the non-sectored cellular systems because interference is only due to the sector at their frequency.

If each sector 1 points the same direction in each cell, then the interference caused due to neighbouring cells will be dramatically reduced. An alternative to use sector is to reuse frequency in each sector. In this scenario all of the time/code/frequency slots can be reused in each sector but no reduction in interference. This method is a effective and practical approach to address OCI problem, but at the cost of:  Increased number of antennas at each base stations and reduces trunking efficiency due to channel sectoring at BS.  It also increases the overhead due to increased number of intersector handoffs. In heavy scattering channels desired power might be lost due to intersector interference. New approaches to OCI: Advanced signal processing techniques at the receiver and /or transmitter as a means of reducing or cancelling perceived interference. Network level approaches such as co-operative scheduling or encoding across BSs, multicell power control and distributed antennas can be considered, since the require relatively little knowledge and effectively reduce OCI...