1. 1 Lecture 1 - V. Chandrasekhar and J. Andrews, “Femtocell Networks: A Survey” 09-09-2008 Instructor: Yong-Hoon Choi Assistant Professor Kwangwoon University Email: yhchoi@kw.ac.kr

2. 2 Introduction To increase the system capacity of a wireless link –Getting transmitter and receiver closer to each other Benefit –Higher link quality –More spatial reuse Require –More infrastructure: micro/pico cells, hotspot, relay, femtocells (home base stations) Recent wireless technologies to support higher data rates –WiMAX (802.16e) –3GPP HSPA, LTE, and LTE-Advanced –3GPP2’s EV-DO, UMB (Ultra Mobile Broadband) –Wi-Fi mesh network  low level of mobility

3. 3 Introduction Wireless capacity –Martin Cooper of Arraycomm Wireless capacity has doubled every 30 months –25x improvement from wider spectrum –5x improvement from dividing spectrum into smaller slices –5x improvement by designing better modulation schemes –1600x improvement through reduced cell sizes »Expensive!! Femtocell –short range, low cost and low power BS installed by the customer for better voice and data reception. –Backhaul: DSL, Cable modem, and etc  very little upfront cost to the ISP.

4. 4 Introduction Femto is a win-win deal –Studies More than 50% of all voice calls and more than 70% of data traffic originates indoors. –Data traffic requires higher signal quality (to support multi-Mbps data rate) –Solution Femtocells –The subscriber is happy with the higher data rates and reliability –The service provider reduces the amount on traffic on their expensive macrocell network.

5. 5 Introduction Femtocell Benefits –Better coverage and capacity Prolong handset batter life, higher SINR, higher capacity –Improved macrocell reliability Since the traffic originating indoors can be absorbed into the femtocell network over IP backbone, the macrocell BS can redirect its resources into providing better reception for mobile users. –Cost benefit Low OPEX and CAPEX –Macrocell: site lease cost ($1K/month), backhaul lease cost. –Reduced subscriber turnover WiBro vs LTE

6. 6 Technical Aspects of Femtocells Path losses causes the transmitted signal to decay as Ad -c, –where A is a fixed loses –d is the distance between the Tx and Rx –c is the path-loss exponent. Capacity benefit –Reduced distance btw femto and the users  higher SINR –Lowered transmit power –Mitigation of interference from neighboring macrocell and femtocell users. –Can devote a larger portion of their resources to each subscribers

7. 7 Business Aspects of Femtocells

8. 8 Technical Challenges Physical and Medium Access Layers: Broadband Femtocells –Interference mitigation Macro to femto Femto to femto (relatively small) Femto to macro –Challenge 1: spectrum allocation among femtocells and macrocell. Frequency planning is difficult because of ad hoc location of femtocells. Decentralized spectrum allocation between macrocell and femtocell is an open research problem. –Find an optimal splitting policy!!

9. 9 Technical Challenges Physical and Medium Access Layers: Broadband Femtocells –Challenge 2: Timing and Synchronization To align received signals To handoff users from a macrocell to a femtocell or vice versa –Difficult due to the lack of centralized coordination between them. Required timing –1 us Solutions –IEEE1588 (Precision Timing Protocol over IP – accuracy 100ns) –GPS »Requires stable indoor satellite reception

10. 10 Technical Challenges Physical and Medium Access Layers: Broadband Femtocells –Challenge 3: Backhaul provide acceptable QoS Current macrocell backhaul guarantees within 15 ms Current IP backhauls do not guarantee delay bound Market trials (in Spain) reveals that femtocells experienced difficulty transfering data, owing to the insufficient QoS.

11. 11 Technical Challenges Physical and Medium Access Layers: Voice Femtocells –Operator has two choices Different frequency bands to macrocell and femtocell. –Reduce cross-tier interference Same frequency bands to macrocell and femtocell –Maximize spectral efficiency –Challenge 4: Cross-tier interference handling Cell edge femtocell experience Significantly higher interference Compared to interior femtocells

12. 12 Technical Challenges Physical and Medium Access Layers: Voice Femtocells –Challenge 5: Open or closed access Open –privacy and security Closed –reduces the macrocell load –Strain the backhaul to provide sufficient capacity –Signaling overhead? –Challenge 6: Handoff Femto to macro: easy Macro to femto: difficult –Dwell time expectation (open femto)

13. 13 Technical Challenges Physical and Medium Access Layers: Voice Femtocells –Challenge 7: femtocell movement Can subscribers carry their femtocells for use outside the home area? –Of course, Wi-Fi is easy –Femtocell may cause some problems »Interference (licensed band) –Challenge 8: Providing location tracking and Macrocell user support with poor coverage. Should support location tracking (Government low) Emergency call support? Unsubscribed user support? –Open femto is possible.

14. 14 Network Infrastructure Mission –To provide equal parity service to femtocells over the internet. Ensuring QoS, security, and reliability as 3G backbone –3 possible interfaces Iu-b over IP: Iu-CS, Iu-PS (UTRAN to CS or PS) –CAPEX: low (using existing RNCs) IMS (Internet Media Sub-system)/SIP –CAPEX and OPEX: high –Scalability: high RAN Gateway based UMA (Universal Mobile Access) –Aggregating traffic from thousands of femtocells –Secure IP tunneling –2G GSM/GPRS networks use UMA for transporting data from a Wi-Fi network for home coverage, and 3GPP infrastructure for outdoor access