Wireless Cellular and LTE 4G Broadband

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

Mo M odul ule e -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. 

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Channel Dependent Multi-user Resource Scheduling OFDMA provides flexibility to allocate the channel resources by designing algorithms such such that they meet the requirements of arbitrary throughput, dela y 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. overall capacity. 

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Multiantenna Techniques  Mult M ultian ianttenna Tech Techniq nique uess

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), (MU-MIMO), thus supports multiple users in uplink ea ch 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 interferenceinterferenceAt 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 2 domain equalizers i.e. about O(Blog2BTm) and O(B Tm) 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 s ystems experience as delay spread exceeds exceed s 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 Th e 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 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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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 ran ge. 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), (MU-MIMO), thus supports multiple users in uplink ea ch 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 do wnlink.


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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. 

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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


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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 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 b ase stations. BS are restricted to maintain power levels within the b oundaries 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 c ell 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 I i interference th power of i 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 .if the mean and standard deviation of the lognormal distribution 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.


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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 app roach 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 b e lost due to intersector interference. 

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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 through macro diversity and efficient sharing of spectrum.

9. Explain tapped delay line model for broadband wireless channels. The overall model we use for describing the channel in discrete time is a simple simple tap-delay line(TDL):


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15EC81 The discrete time channel is time varying and has non-negligible values aver a span of v+1 channel taps. Assuming channel is sampled at a frequency f s = 1/T ,where T is the symbol period and hence duration of the channel is in this case about vT. The v+1 sampled values are general complex numbers. Also assume channel is static over a period of (v+1)T seconds, the output of the channel can then be described as

The channel can be represented as time varying vector:

The tapped -delay line model is general and accurate. In order to design the communication system we need to know the following attributes: 

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Number of different effects cause the received power to vary over along (causes path loss) o o medium(causes shadowing) short (fading) o The channel coherence time based on relative movement between transmitter and receiver. Based on propagation distance and environment delay spread is evaluated.

10. Explain Path loss in BWC. In wireless communication system the signals traverse in free space as the distance increases the loss increases, the wave front is spherical in nature ,the energy is inversely proportional to 2 spherical surface area, 4πd 4πd . The free space path loss formula i.e. Friis transmission formula is given as follows:

If directional antennas are used at the transmitter and receiver a gain of G t and /or Gr is achieved ,the receiver power is simply increased by gain of these antennas.


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15EC81 The receiver power falls falls off quadratically with higher carrier carrier frequency, since range decreases. decreases. Its implications are on high data rate systems. The terrestrial propagation environment is not free space i.e. reflection on earth or other objects would actually increase received power since more energy would reach receiver. These reflections cause 180 degree phase shift at relatively large distances , thus serves as destructive interference and a common 2-ray approximation for path loss is shown below:

Form the above relation we can say that the antenna height plays an important role in propagation. o Wavelength and hence carrier frequency dependency has disappeared from formula. -4 o The distance dependency has changed to d implies energy loss is more severe with distance in terrestrial system than in free space. In order to apply on to different propagation environments one of the simplest empirical path loss formula is considered o

The SIR expressions compute to:

Demonstrating that overall system performance can be substantially improved when path loss is in fact large. These are viewed as upper bound where the SINR is less than SIR, due to addition of noise. Thus microcell grow increasingly attractive as performance is much better, with lower path loss and same transmit power.

11.

Explain Shadowing in BWC.

The transmission is effected by factors other than distance which causes pathloss, like wise obstacles such as trees and buildings may be located between transmitter and receiver, causes temporary degradation of received signal strength, meanwhile the LOS communication of signal would cause abnormal increase in signal strength. The standard method of accounting for these variations is to introduce a random effect called shadowing. Its empirical formula can be represented as :


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Shadowing is a random process. It causes perturbations in received signal from expected value. Its caused by macroscopic objects, has correlation distance on the order of meters or tens of meters. Its also called as large scale fading.

Its modelled as lognormal random variable i.e.:

Shadowing is an important effect in wireless wireless networks because it causes: o The received SINR to vary dramatically over long time scales. o In some locations in a given cell, reliable high rate communication may be nearly impossible. o The system design and BS’s deployment must account for lognormal shadowing either through macro-diversity, variable transmit power and/or by simply accepting that some users will experience poor performance at certain % of locations. Its beneficiary if there is an object blocking interference ,its detrimental to system performance because it requires a several dB margin to be built into the system

12.

Explain considering an example the effects of fading.

This is one of the intriguing effect in wireless transmission caused due to reception of multiple version of same signal, its due to multipath (reflections). A simple example of signal reflections due to local scattering and main multipaths are shown.

Depending on the phase difference between the arriving signals, the interference can be constructive or destructive which could cause large observed difference in amplitude of received signal even over shorter distance.


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15EC81 Consider the tapped delay line channel model in order to formalize the fading, the response can be thought of as having 2-D-delay dimension (τ (τ ) and a time dimension(t). As the channel varies so the time ,the channel response can be modelled statistically. One such method is 2-D 2-D autocorrelation function, A( τ,Δ τ). The autocorrelation function is over 2-D,it can use fully be thought of as two simpler functions At(Δt) and Aτ(Δτ),where Δt and Δτ have been set to zero. This autocorrection function can defined as:

Assuming channel is wide sense stationary. The channel responses are uncorrelated. The channel can be addressed as wide sense stationary uncorrelated scattering. Its more popular in wideband fading channels, has relatively accurate in many practical scenarios.


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13. Explain the correlation between Delay spread and coherence bandwidth also comment on its relation with coherence time. Delay Spread is very important property of a wireless channel, since it specifies the duration of the channel impulse response. It’s the amount of time elapsed between last arriving path and last arriving path. Its Its termed as multipath intensity profile.

The channel coherence bandwidth is the frequency domain dual of the channel delay spread. It’s a rough measure for the maximum separation between frequency f 1 and f 2 where channel frequency response is correlated.

Doppler power spectrum gives the statistical power distribution of channel over time for a signal transmitted for just an instant. Doppler spectrum is caused due to motion Where as power delay profile was caused due to multipath between transmitter and the receiver.

Over a range of bandwidth B<
If the transmitter and receiver are moving fast relative to each other and hence Doppler is large, the channel will change much muc h more quickly than if transmitter and receiver are stationary.


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15EC81 The mobility places severe constraint on the system design. At high frequency and mobility the channel may change upto 1000 times per second, placing large burden on overhead channel.

14. Explain statistical modeling used for broadband fading channels. Statistical models are: Rayleigh fading Ricean Distribution Nakagami-m Fading Step 1: Consider special case of Multipath Intensity profile Aτ(Δτ) ≈ 0 if Δτ ≠ 0, i.e. all received energy arrives at the receiver at the same time. Step 2: Correlating all the values in time, frequency and space. How wide band fading channels evolve in time, frequency and space. Step 3: relax all the assumptions and consider Rayleigh Fading : It assumes all arriving reflections have a mean zero. If number of scatters are large and the angles of arrival between them are uncorrelated from central limiting theorem it can be shown that the in phase and quadrature phase follow two independent time correlated gaussian random processes. The distribution envelope can be shown as Rayleigh distribution and its power is exponentially distributed. The pathloss and shadowing determine the mean received power and total received power fluctuates around the mean due to the fading.   

The phase of the signal received can be expressded as:

The Ricean distribution : As LOS is not applicable for Rayleigh fading channel, this distribution is considered if there is a dominant path. Its depicts more accurately for wireless broadband systems, having typically one or more dominant components, especially for fixed wireless channels.

Where K is quantified by:


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A more general model: Nakagami-m fading The PDF of this fading is ungainly as ricean distribution, dependence on x is simpler hence using this in many cases used for tractable analysis anal ysis of fading channel performance.

15. Explain the method in which the received signal is correlated. Statistical correlation of received signal:  Time correlation  Frequency correlation  The dispersion selectivity duality  Multidimensional correlation Time Correlation: In time domain, the channel can intuitively be thought of as consisting of approximately one new sample from Rayleigh distribution every Tc seconds with the values in between interpolated. Its frequency domain Doppler power spectrum provides a band limited description of the same correlation its simply its Fourier transform.

For uniform scattering doppler spectrum is expressed as:


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Frequency correlation : Similar to time domain a simple intuitive notion of fading in frequency is that channel is in frequency domain. A complex gaussian values in time domain can be converted to a correlated Rayleigh frequency envelope. The correlation function that maps from uncorrelated timed timed domain random variable to correlated frequency response is multipath intensity profile. If only one path is arriving then the correlation is there for all the frequencies, the scenario is referred as flat fading.

The Dispersion-Selectivity Duality : The selectivity refers to signal value received is changed by channel over time and frequency. The dispersion refers to channel is dispersed or spread out over time and frequency. These are time frequency duals of each other,example for this is represented pictorially below,

Multidimensional correlation :


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15EC81 Considering that signals are correlated in all three domains i.e. time frequency and spatial domains. A broadband wireless data system with mobility and multiple antennas is an example of a system of this kind. The concept of doubly(time and frequency) selective fading channels has received recent attention for OFDM. A highly frequency selective channel resulting from a long multipath channel is in a wide area wireless broadband network requires a large number of potentially closely spaced subcarriers to effectively combat the ISI and small coherence bandwidth. A highly mobile channel with large Doppler causes the channel to fluctuate over the resulting large symbol period which degrades subcarrier orthogonality. In frequency domain the doppler frequency shift can cause significant inter carrier interference as carrier becomes closely spaced. Although the mobility and multipath delay spread must reach fairly severe levels before this doubly selective effect becomes significant, this problem facing mobile LTE system does not have a comparable precedent. The scalable nature of the LTE physical layer notably variable number of subcarriers and guard intervals will allow optimization of the system for different environments and applications.

16. Explain empirical modeling used for broadband fading channels. In order to ensure that specific wireless propagation environment is considered empirical and semi-empirical wireless channel models are developed to accurately estimate pathloss, shadowing and small scale fast fading. These models are not analytically tractable ,they are very useful for simulations and to fairly compare competing designs. These models takes in to account the following: Extensive measurement of various propagation environments Parameters and methods for modelling typical propagation scenarios in different wireless systems. They consider AoA, AoD, antenna array fashion, AS and Antenna array gain pattern. Examples of models:  

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Urban macro Sub-urban macro Urban micro etc. LTE channel models for Path Loss: It’s a widely used modelling in outdoor macro and micro cell wireless environments. Its refer as 3GPP channel models. Firstly consider the environment suburban macro/urban macro/urban micro etc.   


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15EC81 If its macro BS to BS distance is larger than 3 km and if its micro its less than 1 km . The path loss expression for 3GPP macro cell environment is given by COST-Hata model, its easily computable.

Hata model for Urban areas is:

Hata model for both Suburban and open areas drives from Urban model is given as:

17. What are the different channel model considered for mitigation of multipath. Consider for multipath and scattering with N time delayed versions of transmitted signal is received at mobile receiver. N paths are characterised by powers and delays chosen according to prescribed channel generation procedures. Each multipath component further corresponds to a cluster of M sub path, where each sub path characterises the incoming signal from a scatterers. The M sub path have random phases and sub-path gains, specified by the given procedure in different stands, also it defines cluster of adjacent scatterers and therefore have same multipath delay. For 3GPP ,the phases are random variables uniformly distributed from 0-360 degrees, n multipath component from uth transmitter antenna to sth receiver and sub path gains are given by the following equations: AoD: A narrow range in outdoor application due to lack of scatterers around BS transmitter and assumed to be uniformly distributed in indoor applications. AoA: An uniformly distributed due to the abundance of local scattering around mobile receiver.


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By modelling such channels we can reduce time consumption and computational complexity encountered in empirical models. These models are accurate in considering practical parameters in a real wireless system and maintains simplicity of statistical channel models. 3GPP2 system Examples are Pedestrian A , Pedestrian B, Vehicular A, vehicular B models for low mobility pedestrian mobile users and high mobility vehicular mobile users. These models are referred w.r.t power and multipath delay of each component. Each multipath is modelled as independent Rayleigh fading with different power levels, correlation in time domain is created according to doppler spectrum corresponding to specified speed.

18. Comment on the effects of narrow band fading. How to mitigate these effects. Effects of Narrow band fading: The received signal is random in nature thus we need a method to overcome narrowband fading different techniques are followed which are referred as diversity. The signals are uncorrelated, out of these one would have a adequate power, without diversity a high data rate wireless communication is virtually impossible. In order to overcome these effects we consider the following steps: The effects of unmitigated fading are: Probability of BER is metric of interest for physical layer of communication systems. This error decreases very rapidly with the SNR. So decreasing the SNR linearly causes the BER to increase exponentially. Since the channel is constant , the BER is constant over time. In a fading channel the BER become a RV that depends on the instantaneous channel strength


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15EC81 and occasional instances when the channel; is in deep fade therefore dominate the average BER. If this BER is low virtually all errors are made while in deep fades. This average BER varies depending on the precise constellation used. Although BER is more analytically convenient measure since iti is directly related to the SINR, a more common and relevant measure in LTE is the PER or equivalently BLER or FER, all these are referred w.r.t block of L bits. This approach is considered since single error in a packet can be detected by b y CRC, through which packet is discarded or retransmitted. Diversity is the key k ey to overcome the potentially devastating performance loss from fading channels and improving PER and BER.

19. What are the roles of spatial diversity, coding and interleaving on narrowband fading effects? Spatial Diversity: Its most powerful form of diversity and particularly desirable since it does not necessitates redundancy in time or frequency. It’s usually achieved by having two or more antennas at the receiver and /or the transmitter. The simplest one is having two antennas at the receiver, strongest of the two would be selected. based on the distance between the antennas received signal undergo approximately uncorrelated fading. This type of diversity is called as selection diversity. The more sophisticated form of these are receive antenna with arrays of antennas Coding and interleaving: An ubiquitous form of diversity in nearly all contemporary digital communication systems is the natural pair of coding and interleaving, interleaving, it’s a traditional time diversity. If its multicarrier system then its frequency diversity. Error correction codes are to be considered which sometimes are forward error correction codes, these techniques introduces redundancy so as to avoid error introduction due to noise and other interferences.

The techniques are categorised by their coding rate r <=1. Consider a 1/3 conventional encoder defined by LTE which should be used in Broadcast Channel (BCH). It’s a rate ⅓ coder which has 1 input and 3 outputs. outputs. The constraint length code is 7, 6 delay elements or 64 possible states.


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15EC81 Each of the outputs are defined by a generator polynomial denoted in octal notations and modulo -2 operators. The codes so generated are transmitted and received at the destinations, the received code is matched for its correctness, and decoded using reduced-state sliding window maximum likelihood sequence estimator or or Viterbi decoder. Consider a 1/3 parallel concatenated turbo encoder defined by LTE which should be used in Uplink and downlink shared channels among others.

It provides increased resilience to errors through iterative decoding. This type of coding has 8 state rate ½ systematic encoder and a 8 state rate 1 systematic encoder that operates on an interleaved input sequence, for a net coding rate of 1/3. In some of the LTE codes are punctured codes, where the output is coded in form of bits are simply dropped in order to lower the transmission rate. At the decoder random or fixed coded bit is inserted in the decoding process, which has 50% chance of being incorrect. The decoded sequence is therefore less reliable than in the case of a lower rate unpunctured code, but if received SINR was relatively good, the decoded sequence would be correct. The punctured codes are simple and complex, puncturing the same code to achieve different coding rates allows the decoder structure to remain the same regardless of the code rate. In convolutional codes Interleaving is used to shuffle coded bits to provide robustness to burst errors that would be due to bursty noise and interference/sustained fade over time. It seeks to spread out coded bits evenly roughly over frame or block, after deinterleaving. In turbo codes interleaving is to provide statistical independence between two encoder outputs. At decoder the iteration continues back and forth until the symbol estimate converges. Interleaving is effective in allowing ECC’s designed for constant, time invariant additive noise channels to also work well on fading time variant noisy channels.


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20. What are the techniques used for improving modern wireless communication technologies. Automatic Repeat Request (ARQ) Its another often used techniques for modern wireless communication systems including LTE. Types are: ARQ and Hybrid ARQ ARQ is a simple MAC layer retransmission protocol that allows erroneous packets to be retransmitted, it is in conjunction with PHY layer ECC’s and parity checks to ensure reliable links even in hostile channels. H-ARQ combines the two concepts of ARQ and FEC to avoid such a waste by combining received packets, by extracting additional time diversity in a fading channel. It used by convolution encoder and turbo encoder to generate additional redundancy to the information bits. Only a fractional number of bits are transmitted by using punctured codes to create effective code rate. After transmission once the packet is received an acknowledgement is sent by receiver, if the information is incorrect/erroneous then ARQ is raised for retransmission of encoded bits. In type I H-ARQ its termed as Chase combining during a retransmission. As the transmitter sends copy of encoded stream which is identical the receiver soft combines the received bits with previous transmission. In type II H-ARQ its referred as incremental redundancy in this transmitter changes the bits that are punctured during a retransmission, as the retransmissions increases the code rate decreases, and reduces error probability. Adaptive Modulation and Coding (AMC) LTE employs AMC in order to take advantage of fluctuations in the channel over time and frequency. For Achieving lower data rates small constellations such as QPSK is used and low rate error correcting codes such as 1/3 turbo codes are used. For achieving higher data rates larger constellations such as 64 QAM and less robust error correcting codes are used. Example punctured turbo codes. An AMC system is as shown below it has a transmitter , receiver, channel and feedback channel. The channel is featured to have variable SINR due to fading. The transmitter is to transmit data from its queue as rapidly as possible, based on the data being demodulated and decoded reliably at the receiver. Feedback is critical for adaptive modulation and coding.


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21. What are the methods employed to overcome effects of broadband fading? In broadband transmission system the major hindrance is ISI which has to be addressed some of the methods followed are: o Spread spectrum RAKE receivers o Equalization o o Multicarrier modulation Spread Spectrum: Spread spectrum technology has blossomed from a military technology into one of the fundamental building blocks in current and next-generation wireless systems. From cellular to cordless to wireless LAN (WLAN) systems, spectrum is a vital component in the system design process. Since spread-spectrum is such an integral ingredient, it's vital for designers to have an understanding of how this technology. In this tutorial, we'll take on that task, addressing the basic operating characteristics of a spread-spectrum system. We'll also examine the key differentiators between frequency-hop (FHSS) and direct-sequence spread spectrum (DSSS) implementations. This method supports CDMA but not EVDO which is high speed network system, rather its considered spread spectrum is not natural choice for large broadband wireless system, rather equalizers are chosen. Later for LAN s OFDM was preferred. RAKE receivers The rake receiver consists of multiple correlators, in which the receive signal is multiplied by time-shifted versions of a locally generated code sequence. The intention is to separate signals such that each finger only sees signals coming in over a single (resolvable) path. The spreading code is chosen to have a very small autocorrelation value for any nonzero time offset. This avoids crosstalk between fingers. In practice, the situation is less ideal. It is not the full periodic autocorrelation periodic autocorrelation that determines the crosstalk between signals in different fingers, but rather two partial two partial correlations, with correlations, with contributions from two consecutive bits or symbols. It has


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15EC81 been attempted to find sequences that have satisfactory partial correlation values, but the crosstalk due to partial (non-periodic) correlations remains substantially more difficult to reduce than the effects of periodic correlations. The rake receiver is designed to optimally detected a DS-CDMA a DS-CDMA signal transmitted over a dispersive multipath channel. It is an extension of the concept of the matched the matched filter. A spread spectrum receiver with rake outperforms rake outperforms a simple receiver with a single correlator. Equalization: Time-dispersive channels can cause inter symbol interference (ISI), which is a form of distortion that causes symbols to overlap and become indistinguishable by the receiver. In a multipath scattering environment, the receiver sees delayed versions of a symbol transmission, which can interfere with other symbol transmissions. Equalizers attempt to mitigate ISI and improve the receiver performance. A linear equalizer is a filter that can undo the channel effect .Ideally, the output of an equalizer is a delayed delayed version of the transmitted signal • A fixed equalizer measures the time-invariant time-invariant channel and compensates the frequency selectivity during the entire transmission of data. An adaptive equalizer adjusts its coefficients to track a slowly time-varying ch annel A non linear equalizer uses previous symbol decisions made by the receiver to cancel out their subsequent interference and are often called as decision feedback equalizer. The drawbacks are error propagation , increased computational complexity, but improved performance. MSLD is optimized mathematical algorithm to extract useful data out of a noisy data stream. For an optimized detector for digital signals the priority is not to reconstruct the transmitter signal, but it should do a b est estimation of the transmitted data with the least p ossible number of errors. The receiver emulates the distorted channel. All possible transmitted data streams are fed into this distorted channel model. The receiver compares the time response with the actual received signal and determines the most likely signal. In cases that are most computationally straightforward, root straightforward, root mean square deviation can be used as the decision criterion for the lowest error probability. In practice suboptimal decision systems are used such as delayed decision feedback sequence estimator and reduced state sequence estimator. Multicarrier modulation: OFDM It utilizes diversity through large subcarriers to mitigate ISI, based on the ratio of bandwidth to no of carrier signals w.r.t channel bandwidth, flat Fading occurs and time dispersion is negligible. Single carrier modulation with frequency domain equalization is performed in order to overcome the draw backs of OFDM such as High PAR relative to single carrier signal. The dynamic range of power is too large and causes clipping and distortion. This approach follows FFT to transform signal to frequency domain and 1-tap frequency equalizer and the IFFT to convert back to time domain at receiver for decoding and detection. In LTE this technique is termed as SC-FDMA.


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Wireless Cellular and LTE 4G Broadband

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