Coverage in Heterogeneous Coverage in Heterogeneous Networks Xiaoli Chu King ’ s College London UC4G Beijing Workshop August 2010
Outline • Introduction ▫ Heterogeneous networks Heterogeneous networks ▫ Challenges • Coverage in heterogeneous networks ▫ System model ▫ Outage probability analysis ▫ Maximum density of co-channel femtocell transmissions y ▫ Femtocell transmit power • Simulation results ▫ Analytical results verified by simulations A l ti l lt ifi d b i l ti ▫ Low attenuation vs. high attenuation environments • Conclusions • Future work UC4G Beijing Workshop August 2010 - 2 -
Introduction UC4G Beijing Workshop August 2010
Why Heterogeneous? Why Heterogeneous? • Needs for substantial improvements in cellular capacity and coverage and coverage ▫ Radio link improvement alone cannot meet increasing demands ▫ Traffic demand and channel condition vary with time and location • Improve spectral efficiency/area/cost ▫ Need to increase cell density cost-effectively ▫ Network topology provides gains beyond radio technology p gy p g y gy • Heterogeneous networks ▫ Deploy macro-, pico-, femto-cells, and relays in the same spectrum ▫ Improve coverage and capacity through better spatial reuse I d it th h b tt ti l ▫ Address hot-spot needs and coverage holes ▫ Lower traffic load on macrocells UC4G Beijing Workshop August 2010 - 4 -
Deployment Deployment • Initial coverage with macro base stations (MBS) • Add pico, femto and relay stations for capacity increase, hot spots, indoor coverage, etc. • Pico, femto and relay stations require lower power, offer flexible site acquisition, and add little or no backhaul expense. Source: Intel Labs UC4G Beijing Workshop August 2010 - 5 -
Heterogeneous Networks Heterogeneous Networks • Macrocells ▫ Operator-deployed BSs use dedicated backhaul p p y ▫ Open to public access ▫ P Tx ~ 43 dBm, G ~ 12-15 dBi • Picocells Picocells ▫ Operator-deployed BSs use dedicated backhaul ▫ Open to public access ▫ P T ~ 23-30 dBm G ~ 0-5 dBi 23 30 dBm, G 0 5 dBi P Tx • Femtocells ▫ User-deployed BSs use user’s broadband connection as backhaul ▫ Open access closed access or a hybrid access policy ▫ Open access, closed access, or a hybrid access policy ▫ P Tx ≤ 23 dBm, G ~ 0-2 dBi • Relays ▫ Operator-deployed BSs use over-the-air link to MBS as backhaul O t d l d BS th i li k t MBS b kh l ▫ P Tx ~ 23-30 dBm, G ~ 0-5 dBi UC4G Beijing Workshop August 2010 - 6 -
HN Characteristics HN Characteristics • Coexistence of potentially different access technologies • Different cell scales: macro > micro > pico > femto • BSs owned by operators, enterprises and consumers • Wired or wireless backhaul with guaranteed or best-effort QoS Wi d i l b kh l ith t d b t ff t Q S • Femtocells potentially offer coverage only to subscribed UEs Source: Qualcomm UC4G Beijing Workshop August 2010 - 7 -
Challenges Challenges • Maintain uniform user experience ▫ More cell-edges created ▫ Near-far effect ▫ Closed-access femtocells in co-channel deployments ▫ Relay stations may have different duplexing schedules • Handoff decisions have to consider backhaul capacity, H d ff d i i h t id b kh l it especially for relays and femtocells • Optimized use of radio resources Optimized use of radio resources • Inter-cell interference management ▫ Lack of coordination between cells ▫ Scalability, security and limited backhaul bandwidth UC4G Beijing Workshop August 2010 - 8 -
Femtocells Femtocells • Femtocells are low-power wireless access points that operate in licensed spectrum to connect standard mobile devices to a li d t t t t d d bil d i t mobile operator’s network using residential DSL or cable broadband connections [Source: Femto Forum]. UC4G Beijing Workshop August 2010 - 9 -
Coverage Coverage • Inter-cell interference creates dead spots where UE QoS cannot be guaranteed. t b t d ▫ Location w.r.t. MBS ▫ Path loss, shadowing, fading Path loss shadowing fading • Minimum distance of an FAP from the MBS s.t. a femto outage probability (OP) constraint outage probability (OP) constraint • Maximum density of simultaneously transmitting co- channel femtocells meeting a macro/femto OP constraint • Maximum allowed transmit power of FAPs s.t. a macro OP constraint • Minimum required transmit power of FAPs s.t. a femto OP Mi i i d t it f FAP t f t OP constraint UC4G Beijing Workshop August 2010 - 10 -
Coverage Analysis g y UC4G Beijing Workshop August 2010
S S ystem Model ystem Model • OFDMA downlink of collocated spectrum-sharing macrocell and closed-access femtocells ▫ A central MBS covers a disc area with radius r M ▫ Femtocells of radius r F are randomly distributed on R 2 as a are randoml distrib ted on R 2 as a Femtocells of radi s spatial Poisson point process (SPPP) with a density of λ F . ▫ N F femtocells per cell site on average p g F ▫ U F indoor UEs per femtocell, each located on cell edge ▫ MBS transmit power is P M,Tx per RB ▫ FAP transmit power is P F,Tx per RB ▫ Each FAP transmits with a probability ρ within each RB. ▫ Spatial intensity of simultaneously transmitting co-channel S ti l i t it f i lt l t itti h l FAPs is u F = λ F ρ . UC4G Beijing Workshop August 2010 - 12 -
Channel Model Channel Model • Each subchannel sees Rayleigh flat fading and lognormal shadowing • Path loss follows the IMT-2000 channel model ▫ MBS to outdoor UE α α = φ φ = -7.1 3 PL PL 10 10 D D M f f D D M M M M M M M c M M ▫ MBS to indoor UE α α α = φ = φ ξ = -7.1 3 ξ PL 10 FM FM FM D D f D FM FM FM M FM c FM ▫ Home FAP to indoor UE α α = φ = 3 . 7 PL 10 F F D D F F F F ▫ FAP to outdoor UE FAP to outdoor UE α α = φ = φ ξ PL D MF D MF MF MF MF F MF ▫ Interfering FAP to indoor UE α α = φ = φ ξ 2 2 PL FF FF D D FF FF FF F FF ▫ ξ denotes wall-penetration loss UC4G Beijing Workshop August 2010 - 13 -
Femtocell DL S Femtocell DL S IR IR • For a given RB, the received SIR at an FUE is − − α 1 1 φ P H Q r F = F F F F F SIR ∑ F − − α − − α 1 1 φ + φ P H Q D FM P H Q D FF M FM FM FM FM F FF FF FF FF i i i ∈ Φ i i ▫ P F = P F,Tx G FAP G UE , P M = P M,Tx G MBS G UE ; ▫ D FM is the distance from the MBS to the FUE, D FF i is the distance f from interfering FAP i to the FUE; i t f i FAP t th FUE ▫ α F , α FM and α FF are path loss exponents from the home FAP, the MBS and an interfering FAP to the FUE, respectively; g p y ▫ H F , H FM and H FF i are unit-mean exponential channel power gains; ▫ Q F ~ LN( ζμ F , ζ 2 σ F 2 ), Q FM ~ LN( ζμ FM , ζ 2 σ FM 2 ) and Q FF i ~ LN( ζμ FF , ζ 2 σ FF 2 ) denote lognormal shadowing ζ denote lognormal shadowing, ζ = 0.1ln10; 0 1l 10 ▫ Φ is the set of FAPs transmitting in the given RB, with intensity u F . UC4G Beijing Workshop August 2010 - 14 -
Macrocell DL S Macrocell DL S IR IR • Co-channel interference from neighboring macrocells is ignored. • For a given RB, the received SIR at an MUE is S − α − φ 1 M P H Q D = M M M M M SIR ∑ ∑ M − − α 1 F φ φ MF MF P P H H Q Q D D MF MF MF MF i i i ∈ Φ i ▫ D M is the distance from the MBS to the MUE, D MF i is the distance from FAP i to the MUE; i to the MUE; ▫ α M and α MF are path loss exponents from the MBS and an FAP to the MUE; ▫ H M and H MF i denote unit-mean exponential channel power gains; ▫ Q M ~ LN( ζμ M , ζ 2 σ M 2 ) and Q MF i ~ LN( ζμ MF , ζ 2 σ MF 2 ) denote lognormal shadowing. UC4G Beijing Workshop August 2010 - 15 -
Femto Outage Probability Femto Outage Probability • Outage probability of an FUE w.r.t. the target SIR γ F ⎛ ⎛ ⎞ ⎞ ⎜ ⎟ ( ) S < γ = < γ F P SIR P ⎜ ⎟ ∑ F F − α F − + φ 1 ⎜ ⎟ FF I P H Q D FM F FF FF FF FF i i i ⎝ ⎝ ⎠ ⎠ ∈ Φ Φ i i ⎛ ⎞ ⎛ ⎞ S S ⎜ ⎟ ⎜ ⎟ = < γ + < γ ≥ γ F F P P SIR , ⎜ ⎟ ⎜ ⎟ F F F F ⎝ ⎝ ⎠ ⎠ ⎝ ⎝ ⎠ ⎠ I I I I FM FM FM FM • Based on the stochastic geometry theory and for an FUE at a distance d FM from the MBS, ⎛ ⎛ α ⎞ ⎞ φ γ ( ) P r F ~ ~ ~ ~ ⎜ ⎟ < γ = ≈ μ − μ σ 2 + σ 2 + M F F F P SIR ; , D d F ⎜ ⎟ ϑ F F FM FM F FM F FM α φ ⎝ ⎠ P d FM F FM FM ( ) ⎧ ⎧ ⎫ ⎫ ⎡ ⎤ 2 ⎪ ⎪ − ~ ( ) ( ~ ( ~ + χ σ + μ 2 2 σ + μ 2 α − ⎢ − κ a b − γ ⎥ b 1 exp ⎨ ⎬ F F w v u e n m e FM FM m FF F F F n m ⎢ ⎥ ⎪ ⎪ ⎣ ⎦ N M ⎩ ⎭ ∑∑ ∑∑ ( ( ) ) ~ ~ ( ( ) ) χ π + χ 2 b b a b e m = = 1 1 n m n m UC4G Beijing Workshop August 2010 - 16 -
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