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Interference Analysis of Femtocells


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The project manages to derive the range of operation of a user in interference based scenarios between Femtocells and Macrocells, in terms of Signal to Noise and Interference ratios. The simulation was carried out for both the uplink and the downlink scenario. It could be successfully concluded that the environment that the user is in plays an important part in performance evaluation of the user.

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Interference Analysis of Femtocells

  2. 2. Chapter 1:Cellular Communications
  3. 3. 1.1 HistoryO Guglielmo Marconi demonstrated wireless abilities of radio in 1897.O Since then, this topic has been researched by many scientists and engineers.O Communication was done through Telegraph and Postal service before wireless communications was introduced in the market.
  4. 4. O Cellular communications came into existence with the introduction of ‘Cellular Concept’ by Bell Laboratories in 1960’sO This concept revolutionized the communication industry and people were eager to try this new technology.O One example of this was seen when there were 3700 people on the waiting list of Bell Mobile Phone Service, which had a capacity of 12 channels and 543 customers in 1976.
  5. 5. 1.2 Cellular GenerationsO We will give a brief introduction of cellular generations in the section. A figure showing the timeline of cellular generations is shown below.
  6. 6. 0-GO Very first mobile communication systems.O Also called as ‘Pre-Cellular systems’ or ‘Mobile Radio Telephony’.O Mostly worked on AM, FM modulation schemes.O Poor capacity, low efficiency, fewer customers.
  7. 7. 1-GO Called as First Generation systems.O Born after introduction of cellular concept in 1970’s.O Still poor capacity, low efficiency and analog systems.O AMPS, TACS, NMT were 1-G.
  8. 8. 2-GO First popular cellular mobile communication systems.O Called as Second-Generation systems.O Electronics field was experiencing revolution during 1-G period, hence 2-G was the First Digital Cellular Communication Systems.O Due to digitalization; efficiency increased, higher performance, higher capacity, roaming abilities and increased battery life.O GSM, D-AMPS, PDC, IS-95 were 2-G.
  9. 9. 3-GO Called as Third-Generation Systems.O 2.5G(GPRS), 2.75G(EDGE) had data capabilities and formed the basis of 3G systems.O Better modulation techniques and improvement in electronics helped in gaining higher speeds, better interference, video abilities and lower power requirements.O W-CDMA and cdma2000 were two main sections of 3G systems.O HSPA, EVDO were 3G.
  10. 10. 4-GO Next generation of cellular communications expected in a couple of years.O Some of the advanced features in communication industry like MIMO, AMC, UWB and Software Radio are expected to be part of 4G.O Broadband speeds, better coverage, high capacity, low interference are some aspects of 4G.O WiMAX, LTE are 4G.
  11. 11. 1.3 Cellular ArchitectureO As stated earlier, Bell Labs came up with an innovative solution of ‘Cellular Concept’ to address the ever-green capacity question.O The concept said that “the entire geography was divided in to smaller ‘cells’ in which each cell uses a different frequency in order to increase capacity”, an example of the same is shown in the next slide.
  12. 12. O We can see that one large cell is divided into 7 smaller cells.O Each cell with a different number uses different band of frequencies.O Each smaller cell is placed at a minimum re-use distance from its interfering cell.O This pattern is a 7- cell Reuse pattern
  13. 13. O Because of introduction of cellular concept, capacity increased as shown below.
  14. 14. 1.3.1 ProblemsO With invention of every new technology comes its problems.O Problem with cellular arrangement was that 1. Lower Re-use factor: Larger cells, High interference due to close interfering cells. 2. Higher Re-use factor: Smaller cells, Lower capacity, Complex hand-over abilities.
  15. 15. 1.3.2 Solutions1. Introduce new channels until saturation2. Cell splitting3. Cell sectoring4. Cellular hierarchy
  16. 16. Cellular HierarchyO Soon the other three solutions came to saturation and engineers had to come up with a new solution.O Engineers brought in cellular hierarchy in to the existing cells to reduce load on cellular base station and increase coverage.O This meant reducing the size of the cell into still smaller cells creating a cellular pattern into the present pattern.
  17. 17. Thus creating the need for femtocells
  18. 18. 1.4 Interference perspective O There are basically two types of interference based on frequency spectrum that occurs: O Cross-technology interference: Interference happens when two technology work on same frequency. Example: Bluetooth and Wi-Fi work on same 2.4GHz ISM band O Cross-hierarchy interference : Interference happens when two hierarchies of the same technology work on same frequency. Example: Macrocells and Femtocells share spectrum allocated to the technology. Focus of the project
  19. 19. O The next type of interference occurs on the basis of channels used. O Co-Channel Interference : This type of interference occurs when the interferer is on the same channel as the one being interfered. O Adjacent Channel Interference: This type of interference occurs when the information on the adjacent channel seeps into passband of the channel being transmitted, due to which performance of the main channel is degraded. Focus of the project
  20. 20. Interference pattern incellular communications - 1
  21. 21. Chapter 2:Femtocells
  22. 22. 2.1 DefinitionAs given in the Femto forum;“ Femtocells are low-power wireless access points that operate in licensed spectrum to connect standard mobile devices to a mobile operator’s network using residential DSL or cable broadband connections ”According to the definition we can deduce that they are (similar to Wi-Fi) hotspots which will connect the subscriber to the cellular network using Broadband connection at subscribers home/office.
  23. 23. 2.2 Need for femtocells Coverage•90% data services and 2/3 calls are expectedto be indoors.If we try to address these figures using conventionalmacrocell arrangement, we will surely get BLINDSPOTS and DROPPED CALLS.Femtocells can prove to be a rational solution to thisproblem.Femtocells can provide “high coverage” to customersindoor and can address the issue above.
  24. 24. CapacityCurrently, there is a lot of load on macrocells becauseit provides services to both indoor and outdoorcustomers.Femtocells will reduce load on Macrocells by takingup all the indoor subscribers.As a result, capacity of both hierarchies, Femtocellsand Macrocells will increase.
  25. 25. Backward compatibilityPeople are reluctant to switch to new technologybecause of the cost factor in changing to a new mobiledevice.Femtocells will be working on WiMAX, LTEtechnologies; as a result they will be backwardcompatible to all the devices working on the abovetechnologies.A user can use any device like laptop, palmtop, smartphones when he/she is in femtocell coverage area.
  26. 26. Data ratesAs we can see from the figure below that data usage inmobile devices is going to increase almost three times inthe coming years.This would mean providing high data rates to users.Femtocells will be working on WiMAX, LTE whichwould mean that users will automatically be servicedwith high data rates thanks to the underlyingtechnologies.
  27. 27. 2.3 Facts/Predictions aboutFemtocellsThe operator members of Femto Forum currently serve 1.7 billion subscribers, across Wi-MAX, CDMA and UMTS technologies and represent a third of total mobile customers globally.It is projected that femtocell deployments will reach around 50 million by the end of 2014 as shown in the figure
  28. 28. Revenues generated by femtocells would be comparable to cellular networks by the end of 2013 and the value is estimated to be around $2.4bn.60% of the surveyed consumers are interested in trying out femtocells.Figures indicated above show that the world will experience mass deployment of femtocells in the coming years, hence it becomes a priority topic for researchers and engineers to successfully get it into the market.
  29. 29. 2.4 Physical Deployment
  30. 30. 2.5 FAP (Femtocell Access Point)
  31. 31. 2.6 WiMAX“Worldwide inter-operability for microwave access”4-G technologyFixed Wireless Broadband and Mobile Wireless BroadbandIEEE 802.16 is the group responsible for standardization of WiMAXSeveral standards/revisions have come up for WiMAX, as given in the next slide.
  32. 32. 2.6.1 WiMAX featuresWiMAX supports high data rates addressing several customers at the same time ( considered as rival to Wi-Fi 802.11n)WiMAX suffices MIMO specifications.WiMAX has IP capability, thus considered as forerunner for femtocell deployment.From an operator’s perspective, Wi-MAX helps to achieve high spectral efficiency with reduced cost per bit and flexible radio planning.Wi-MAX has an aptitude to satisfy all technical challenges that have been put forward for broadband wireless systems.
  33. 33. According to various references,  WiMAX can always provide 100% coverage to indoor users which is an essential requirement for femtocells.  Areal capacity(throughput per unit area) gains of 300 can be achieved in WiMAX femtocell scenarios.  Various other features such as time synchronisation, mobility management, list of neighbouring macrocells, interference management and more can be offered by WiMAX femtocells.
  34. 34. 2.7 LTE“Long Term Evolution”4G TechnologyBasically a project under 3GPP, a team to determine technicalities of UMTS technology.Comprises of two basic components:  Evolved-UMTS Terrestrial Radio Access – that consists of air interface specifications along with UE.  Evolved-UMTS Terrestrial Radio Access Network – that consists of RNC and base station
  35. 35. 2.7.1 LTE specifications
  36. 36. We can see that LTE possess similar features as WiMAX except a couple of them.According to various references,  LTE can deliver high data rates even under extreme interference conditions in a femtocell implementation scenario.
  37. 37. 2.9 OFDMOne common aspect seen in both WiMAX and LTE is the use of OFDM as a physical layer technology.“Orthogonal Frequency Division Multiplexing”.A digital multi-carrier transmission technology that uses DFT for efficient modulation and demodulation of the baseband signal.
  38. 38. We can see that because of orthogonality feature of subcarriers, a lot of bandwidth is saved.
  39. 39. 2.9.1 OFDM – Cyclic prefixPrime reason for OFDM to be resistant to multi-path effects and ISI.It converts linear convolution to circular convolution which makes it easier to recover signal at receiver.OFDM has become popular because of the orthogonality amongst the subcarriers and this feature is given by Cyclic prefix.
  40. 40. Multi-path propagation delayed duplicates Misalignment of OFDM sinusoids Loosing its OrthogonalitySOLUTION :CyclicPrefix
  41. 41. 2.9.2 OFDM advantagesResistance to narrowband interference or frequency selective fading.With an OFDM system we omit the inclusion of an equalizer in the system, because we can easily implement it using FFT algorithms, which makes it easier for FAP’s to be designed by manufacturers.OFDM systems allow adaptive modulation and coding schemes, which is useful when users have different SNR values on their subcarrier.
  42. 42. 2.9.3 OFDM parameters
  43. 43. 2.10 Femtocell advantages High coverage Extremely low power operation (10mW – 100mW)  High UE battery life  ‘Green’ Technology  Safe to be deployed in homes with children around  Low electric bills High capacity Cheap installation (can be done by users) Femtocells work on IP, hence ‘IPSec’ protocol can be used as an extra security feature Backward compatibility Operators can implement femtocells in their currently allocated frequency spectrum of macrocells Work is under progress to bring in more applications in femtocell technology.
  44. 44. 2.11 Femtocell disadvantages Focus of the projectSince femtocells and macrocells work in the same frequency band, they are bound to experience INTERFERENCE from each other.IP section of femtocells is handled by ISP, air interface section of femtocells is handled by Cellular operators. Thus TROUBLESHOOTING can be a hassle
  45. 45. Chapter 3:Femtocells and Macrocells The game of Hide and Seek
  46. 46. As highlighted in the previous slide, the major disadvantage of femtocells is its co- existence with macrocells.We will thus evaluate the performance, in terms of SINR, of femtocell and macrocell in the same environment.This chapter will simulate all possible cases of CCI between femtocells and macrocells.
  47. 47. Pre-RequisitesDownlink: Transmission in the direction from the base station to cellular mobile user.Uplink: Transmission in the direction from the cellular mobile user to base station.
  48. 48. Path-Loss: ◦ Signal decays as it passes through free space. Macrocell Environments: • There are many models prescribed for path- loss in macrocell environments. • Okumura-Hata model is the most widely used model, which is also used it in our simulations.
  49. 49. where a(hmu) is given by equations,We fix certain parameters for oursimulation as shown in the MATLABfunction below
  50. 50. Femtocell Environments:Above mentioned path-loss model cannotbe implemented into femtocellenvironments, thus we introduce newmodel for the same.
  51. 51. In this case, • q = 1 ( number of walls ) • W = 5dB ( wall loss = 5 dB ) • Ignore Window loss here ( Low = 0 )
  52. 52. SINR: ◦ SINR = Signal power (Interfering power + Noise power)Link Budgeting: ◦ Calculation of received power considering all losses and gains in the system
  53. 53. Noise: ◦ A common degradation that all signals are subjected to. ◦ We cannot completely make a signal noise- free but we can remove noise from the signal. ◦ Most common type of noise considered is the AWGN, which has also been considered in the simulations.From here on, we will follow a defined pattern to explain every situation.
  54. 54. 3.1 Femtocell downlink withMacrocellDescription: ◦ Interfered: Femtocell User ◦ Interferer: Macrocell User ◦ State: Downlink ◦ Femtocell User receives stronger Macrocell signal that affects the performance of the femtocell user
  55. 55. Design: ◦ PT_macroBS: Macrocell Base station power = 10W (40dBm) ◦ PT_femtoBS: Femtocell Base station power = 0.1W (20dBm) ◦ Gain_macroBS: Macrocell Base station antenna gain = 15dBi ◦ Gain_femtoBS: Femtocell Base station antenna gain = 2dBi ◦ Gain_femtoMS: Femtocell User mobile antenna gain = 0dBi ◦ Bandwidth = 10MHz ◦ Thermal noise amplitude = -174+10*log10(Bandwidth) ◦ Channel : Rayleigh channel over 100 iterations
  56. 56. Architecture: ◦ Macrocell Radius = 1.5km ◦ Femtocell Radius = 250m
  57. 57. Results:PERFORMANCE: POOR
  58. 58. 3.2 Femtocell uplink with MacrocellDescription: ◦ Interfered: Femtocell User ◦ Interferer: Macrocell User ◦ State: Uplink ◦ Femtocell User receives stronger Macrocell signal that affects the performance of the femtocell user
  59. 59. Design: ◦ PT_femtoMS: Femtocell User mobile power = 0.5W (27dBm) ◦ Gain_macroBS: Macrocell Base station antenna gain = 15dBi ◦ Gain_femtoBS: Femtocell Base station antenna gain = 2dBi ◦ Gain_femtoMS: Femtocell User mobile antenna gain = 0dBi ◦ Bandwidth = 10MHz ◦ Thermal noise amplitude = -174+10*log10(Bandwidth) ◦ Channel : Rayleigh channel over 100 iterations
  60. 60. Architecture: ◦ Macrocell Radius = 1.5km ◦ Femtocell Radius = 250m
  61. 61. Results:PERFORMANCE: POOR
  62. 62. 3.3 Macrocell downlink withFemtocellDescription: ◦ Interfered: Macrocell User ◦ Interferer: Femtocell User ◦ State: Downlink ◦ Macrocell User receives Femtocell signal that affects the performance of the macrocell user
  63. 63. Design: ◦ PT_macroBS: Macrocell Base station power = 10W (40dBm) ◦ PT_femtoBS: Femtocell Base station power = 0.1 (20dBm) ◦ Gain_macroBS: Macrocell Base station antenna gain = 15dBi ◦ Gain_femtoBS: Femtocell Base station antenna gain = 2dBi ◦ Gain_femtoMS: Femtocell User mobile antenna gain = 0dBi ◦ Bandwidth = 10MHz ◦ Thermal noise amplitude = -174+10*log10(Bandwidth) ◦ Channel : Rayleigh channel over 100 iterations
  64. 64. Architecture: ◦ Macrocell Radius = 1.5km ◦ Femtocell Radius = 250m
  66. 66. 3.4 Macrocell uplink with FemtocellDescription: ◦ Interfered: Macrocell User ◦ Interferer: Femtocell User ◦ State: Uplink ◦ Macrocell User receives Femtocell signal that affects the performance of the macrocell user
  67. 67. Design: ◦ PT_macroMS: Macrocell User mobile power = 0.5W (27dBm) ◦ Gain_macroBS: Macrocell Base station antenna gain = 15dBi ◦ Gain_femtoBS: Femtocell Base station antenna gain = 2dBi ◦ Gain_femtoMS: Femtocell User mobile antenna gain = 0dBi ◦ Bandwidth = 10MHz ◦ Thermal noise amplitude = -174+10*log10(Bandwidth) ◦ Channel : Rayleigh channel over 100 iterations
  68. 68. Architecture: ◦ Macrocell Radius = 1.5km ◦ Femtocell Radius = 250m
  70. 70. 3.5 Femtocell downlink withFemtocellDescription: ◦ Interfered: Femtocell User 1 ◦ Interferer: Femtocell User 2 ◦ State: Downlink ◦ Femtocell User 1 receives Femtocell User 2 signal that affects the performance of the Femtocell user 1
  71. 71. Design: ◦ PT_femto1BS: Femtocell Base station 1 power = 0.1 (20dBm) ◦ PT_femto2BS: Femtocell Base station 2 power = 0.1 (20dBm) ◦ Gain_femto1BS: Femtocell Base station 1 antenna gain = 2dBi ◦ Gain_femto2BS: Femtocell Base station 2 antenna gain = 2dBi ◦ Gain_femto1MS: Femtocell User 1 mobile antenna gain = 0dBi ◦ Bandwidth = 10MHz ◦ Thermal noise amplitude = -174+10*log10(Bandwidth) ◦ Channel : Rayleigh channel over 100 iterations
  72. 72. Architecture: ◦ Femtocell Radius = 250m
  73. 73. Results:PERFORMANCE: GOOD
  74. 74. 3.6 Femtocell uplink with FemtocellDescription: ◦ Interfered: Femtocell User 2 ◦ Interferer: Femtocell User 1 ◦ State: Uplink ◦ Femtocell User 2 receives Femtocell User 1 signal that affects the performance of the Femtocell user 2
  75. 75. Design: ◦ PT_femto2MS: Femtocell User mobile 2 power = 0.5 (27dBm) ◦ Gain_femto1BS: Femtocell Base station 1 antenna gain = 2dBi ◦ Gain_femto2BS: Femtocell Base station 2 antenna gain = 2dBi ◦ Gain_femto2MS: Femtocell User 2 mobile antenna gain = 0dBi ◦ Bandwidth = 10MHz ◦ Thermal noise amplitude = -174+10*log10(Bandwidth) ◦ Channel : Rayleigh channel over 100 iterations
  76. 76. Architecture: ◦ Femtocell Radius = 250m
  77. 77. Results:PERFORMANCE: GOOD
  78. 78. 3.7 Motivation Scenarios that have been simulated are quasi-practical and will not occur in actual world, but they will help us form the basis of the main project and the motivation to undertake this research work. We see from the above simulations that the performance analysis of first and second scenarios are the worst amongst the rest of them and thus it forms our cue to the main research work. Metaphorically, in the game of ‘hide and seek’ between femtocells and macrocells; it is usually the femtocell user that loses against other players in the game. Thus, increase the femtocell user’s chances of winning in this game, we have to simulate and analyse the user’s gameplay to give him a fair chance to play and win the game.
  79. 79. Chapter 4: A FemtocellDownlink Scenario
  80. 80. 4.1 Description Femtocell user experiences interference from other femtocells and macrocells in downlink state. Scenario takes inspiration from Section 3.1, where a femtocell user was interfered with a macrocell user. We have made the simulation more practical by introducing variables instead of fixed values. The parameters will have the same definition by will be more precise in order to emulate the outside world conditions.
  81. 81.  Macrocell Path-Loss:
  82. 82.  Femtocell Path-Loss:
  83. 83.  Channel considerations:  In the previous simulations, we have considered Rayleigh channel.  In this case, we will consider Rayleigh fading channel.  Rayleigh fading channel will help us to consider multi-path situations.  The number of multi-path reflections are called as ‘taps’.  This could be due to reflection, refraction and scattering.  In our case, we consider a 10-tap Rayleigh fading channel.
  84. 84. 4.2 Design Parameters:  noFperM: number of Femtocells per Macrocell (10-40)  PTx_macro: Macrocell Base station power = 20W (43dBm)  PTx_femto: Femtocell Base station power = 0.1W (20dBm)  GTx_macro: Macrocell Base station antenna gain = 15dBi  GTx_femto: Femtocell Base station antenna gain = 2dBi  GRx_user: Femtocell User mobile antenna gain = 0dBi
  85. 85.  We consider femtocell user in a random femtocell in a random macrocell for every channel simulation. We will then take the average of these simulations to get a precise figure of SINR value for that particular case. Considering the user in a random position will help us get the range of operation of user will all the possible interference. FFT-1024 subcarrier arrangement.
  86. 86.  Noise considerations, keeping SNR levels at 20dB
  87. 87. 4.3 Architecture Macrocell Radius = 866m Femtocell Radius = 50m
  88. 88. Case 1 : 10 femtocells / macrocell
  89. 89. Case 2 : 20 femtocells / macrocell
  90. 90. Case 3 : 30 femtocells / macrocell
  91. 91. Case 4 : 40 femtocells / macrocell
  92. 92. 4.4 Algorithm
  93. 93. 4.5 Results
  95. 95. 4.6 Extensions Once we have the SINR we can find  Shannon limit data rate of the system  Best Modulation and coding scheme of the system
  96. 96.  M can only take values in powers of 2. Hence we can find the best available MCS and data rate of the system using the table below.
  97. 97.  We can add the below extension to our main program in order to find the data rate of the system as shown below:
  98. 98. Chapter 5: A FemtocellUplink Scenario
  99. 99.  Femtocell user experiences interference from other femtocell users and macrocell users in uplink state. This situation takes inspiration from section 3.2, where a femtocell user was interfered by a macrocell user. More practical approach  We consider only those interferers who share the same subcarrier index as the user in consideration.  This scenario will randomly allocate subcarriers to users and then check for interferers.
  100. 100.  Random allocation of spectrum:  We will randomly allocate subcarriers to all the users in that femtocell/macrocell  Spectrum will be shared among 720 subcarriers upon the number of users present in the femtocell/macrocell  A MATLAB function was defined to do the same
  101. 101.  Power-control for Macrocell users  In order to avoid blinding the femtocell completely due to macrocell user uplink power, we have introduced power-control for macrocell users.  The function will take into consideration the distance of the macrocell user from Macrocell base station and allocate appropriate uplink power to the user.
  102. 102.  Step 1: We first define the necessary parameters required for simulation nooffemtocells: Number of femtocells in the macrocell (50-200) PTx: Femtocell User mobile power = 0.2 (23dBm) GTx: Femtocell User mobile antenna gain = 0dBi GRx_macro: Macrocell Base station antenna gain = 15dBi GRx_femto: Femtocell Base station antenna gain = 2dBi
  103. 103.  Step 2: Architecture  Macrocell Radius = 1km  Femtocell Radius = 50m1. Generate a macrocell of radius 1km in order to accommodate femtocells and macrocell users.2. Generate ‘nooffemtocells’ number of femtocells randomly.3. Generate macrocell users; make sure they are placed out of the femtocells to distinguish them from femtocell users. Get final count of macrocell users.4. Place 1 to 4 random number of users in random femtocells. Get final count of femtocell users in each femtocells and overall.5. Place the user under consideration in a random femtocell. Get the position of the user in terms of total femtocell users.
  104. 104. Case 1: 50 femtocells/macrocell
  105. 105. Case 2: 100 femtocells/macrocell
  106. 106. Case 3: 150 femtocells/macrocell
  107. 107. Case 4: 200 femtocells/macrocell
  108. 108.  Step 3: Path-loss is calculated as the previous example Step 4: Channel considerations with a 10-tap Rayleigh fading channel Step 5: Allocating subcarrier to femtocell users  Afterallocating thee subcarriers we will store the user subcarrier index for further processing
  109. 109.  Step 6: Finding femtocell interferers  This step will check for any femtocell interferers comparing the subcarrier index of the user under consideration  Once we have found the interferers, we will find the exact position of the interferers to find their stats like path-loss, distance from BS, channel conditions and others  A unique list of interferers is produced after checking for duplicates Step 7: Allocating subcarrier indices to Macrocell users and finding interferers
  110. 110.  Step 8: Assigning transmit power conditions on interfering macrocell users Step 9: SINR calculation  After calculating all necessary requirements for SINR calculation, we find SINR  We will then average the SINR over channel iterations Step 10: Plotting the results
  112. 112.  We will modify the above program by comparing the performance of the femtocell user with regards to number of macrocell users. We will consider a random number of macrocell users first. After eliminating the macrocell users in the femtocells, we will get the final count of macrocell users. We will fix the number of femtocells to 50 with random number of femtocell users (1-4) in each femtocell.
  113. 113. Chapter 6: The End
  114. 114. n this research work, we have simulated the effect ofother users on a femtocell user in both uplink anddownlink situationimulations in chapter 3 were only for understandingand no conclusions will be derived from theme will be deducing reasonable conclusions for chapter4 and chapter 5 since they were the focus of ourresearch worklthough we have got the predicted results, it is not thesame every time because the user maybe subjected to
  115. 115. 6.1 Conclusions from chapter 4he performance evaluation of a femtocell user who is communicatingwith its base station and being interfered by the other femtocells andmacrocellse have increased the number of femtocells in each case from 10 to 40he effective SINR value of the user in first case is around 43dBmwhich reduces to 20dBm for the last case which is a dip of around 50%in performance the user is expected to achieve SINR levels in the range of 20dBm to40dBm in a femtocell downlink scenario and the effect of number offemtocells over SINR is highly dependent on surrounding location of the
  116. 116. ecommendations • Well-defined macrocell range considerations: This would be helpful so that macrocell power will not overpower the femtocell power • Proper distribution of femtocells across the geography: Femtocells can be implemented in areas where macrocells cannot service its customers, like blind spots. • Implementing appropriate methods of channel estimation be applied in femtocells in order to enhance the user performance in downlink scenario.
  117. 117. 6.2 Conclusions from chapter 5he interference impact of number of femtocells and number ofmacrocell users over a femtocell user in uplink state, who is randomlylocated in any of the femtocell in the geographye have analysed the uplink case in a way that there will beinterference only if the other user is on the same subcarrier index asthe user under microscopehe user achieves a SINR level of 15dBm with the least number offemtocells in the macrocell and the level deteriorates to -10dBm withmore number of femtocellsthe user is able to achieve SINR in the range of 20dBm to -20dBm in theuplink scenario and the performance degradation is more severe due topresence of macrocell user interferers rather than the femtocell user
  118. 118. ecommendations • Proper power-control for macrocell users This should be done so that power levels of femtocells and macrocells can be easily differentiated • Common subcarrier allotment system for the geography This will eliminate any chances of interference based on subcarriers. • Appropriate channel estimation techniques
  119. 119. 6.3 Factor responsible for varied behavior incellular systems1. User position: We do not expect a user to be at a single place all the time and hence userposition plays a pivotal role in interference analysis.2. Path Loss: The most important factor that determines path loss is the distance betweenthe user and base station. Other factors include: • number of walls between user and femtocell base station • height of macrocell base station • height of mobile user from the ground • operating frequency.3. Allocation of bandwidth and subcarriers to user: Interference happens when there isoverlap between same subcarrier of two users. Thus, allocation of subcarrier indices andhence the bandwidth to femtocell users and macrocell users will play an important role ininterference analysis.4. Channel conditions: This is the most fragile of all the conditions because of its high levelof randomness.5. Noise levels: Noise levels affect the signals upto a certain level after which they can beneglected.
  120. 120. 6.4 Future Worko investigate effects of interference over a user in a constant positionin all the cases • This analysis would also have helped to understand the effect of user position on the performance, which was one of the factors that affect the performance of the user in both the transmission scenarios.FDMA based system simulation • The analysis performed takes into consideration an OFDM system with 10MHz bandwidth. Implementing OFDMA system will help to simulate the system on a link-layer basis