Cellular Radio Systems Department of Electronics and IT Media Engineering

Transcription

1 Mobile 미디어 IT 기술 Cellular Radio Systems Department of Electronics and IT Media Engineering 1

2 Contents 1. Cellular Network Systems Overview of cellular network system Pros and Cons Terminologies: Handover, Frequency Reuse 2. Mobile Network Architecture Mobile Network and Radio Access Network (RAN) Radio Infrastructures 3. Femtocell, Pico-cell etc.

3 Cellular Network Systems 3

4 Cellular Radio System Cellular Networks A land area is divided into regular shaped cells (hexagonal, square, circular or some other shapes) Each of these cells are assigned with multiple frequencies (f 1 f 6 ) which have corresponding radio base stations. The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent neighboring cells as that would cause co-channel interference. 출처 : 4

5 Cellular Networks Advantages of Cellular Radio System More capacity than a single large transmitter the same frequency can be reused for multiple links as long as they are in different cells Mobile devices use less power than with a single transmitter or satellite since the cell towers are closer Disadvantages of Cellular Radio System Handover or handoff mobile transceivers move from cell to cell while a call is in progress, the mobile station will search for a new channel to attach to in order not to drop the call Once a new channel is found, the network will command the mobile unit to switch to the new channel and at the same time switch the call onto the new channel. Co-channel inter-cell interference 출처 : 5

6 3-Sector Cellular Networks 3-Sector Cellular Networks with Directional Antennas Each RBS has three sets of sectors with directional antennas. aimed in three different directions with 120 degrees for each cell Receiving/transmitting into three different cells at different frequencies 3-sector cellular networks greatly increases the channels improve reception in higher traffic areas more frequent handover Conventional cellular network Omni-directional antenna Directional Antennas 출처 : 6

7 Hard handover Handover (Handoff) the channel in the source cell is released and only then the channel in the target cell is engaged. the connection to the source is broken before or 'as' the connection to the target is made. break-before-make Ping-ponging: When the mobile is between base stations, the base stations bounce the link with the mobile back and forth. Soft handover the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. Reliable connection with macro diversity in cell boundary (edge) the connection to the target is established before the connection to the source is broken. make-before-break the phone's hardware needs to be capable of receiving two or more channels in parallel, which makes it expensive and complex. 출처 : 7

8 Inter System Handoff Handover (Handoff) Handover from one cellular system to a different cellular system When a mobile signal becomes weak in a given cell and MTSO can not find other cell within its system, then it uses Inter system handoff. Before implementation of Inter System Handoff MTSO compatibility must be checked and in Inter System Handoff local call may become long distance call Intra System Handoff Handover from one cellular system to a adjacent cellular system When a mobile signal becomes weak in a given cell and MTSO finds other cell within its system, then it uses Intra system handoff. In Intra System Handoff local calls always remain local call only since after handoff also the call is handled by same MTSO. 출처 : 8

9 Frequency Reuse Channelization for multiple users The band of frequency allocated for cellular system use can be reused with different Clusters. configuration of cells over which the complete frequency band is divided this configuration of cells is repeater over and over. Cluster Size N=i 2 +ij+j 2 Frequency Reuse Factor (FRF) FRF = 1/N Example: N=3-cell cluster 출처 : 9

10 Mobile Network Architecture 10

11 Mobile Network Architecture GSM architecture Any phone connects to the network via an RBS (Radio Base Station) which in turn connects to the Mobile switching center (MSC). The MSC provides a connection to the public switched telephone network (PSTN). RAN Radio Access Network 출처 : 11

12 Radio Infrastructures The Radio Infrastructure Stand-alone traditional Macro-base station (BS) Micro- or Pico-base station Macro-BS and Micro/Pico BS are the integral and stand-alone network element, connected to the cellular system core through the standard interfaces Remote radio Head/Equipment (RRH/RRE) Distributed Antenna System (DAS) RRH/RRE and DAS are just the RF subsystems from the BS baseband processing 12

13 Stand-alone Radio Infrastructures Traditional Macro-BS Shelter containing 3 to 4 equipment racks One for power supply One or two for baseband processing One or two for synchronization On for the RF subsystem Site sectorization is usually a tri-sectorial one Tx antenna output power is usually in the range from 20 to 40W 13

14 Stand-alone Radio Infrastructures Micro- and Pico-BS Complete and stand-alone base station Single outdoor cabinet containing all the functions (RF and Baseband) needed for an integral BS Micro-BS volume: about 30 to 40 liters Tx antenna total output power might range from 5 to 15W Pico-BS volume : less than 40 liters Tx antenna total output power is below about 1W 14

15 Stand-alone Radio Infrastructures Micro- and Pico-BS Micro-BSs are typically adopted to complete the outdoor coverage Highly shadowed areas, or black hole Pico-BSs are more suited for indoor coverage Commercial centers, airports, hospitals and similar In both cases, the cost is much lower as compared to Macro-BS But the lower Tx power and the reduced traffic capacity imply by far smaller cells and call for a more fragmented Consequently more expensive O&M supervisory system 15

16 RF-Subsystem Radio Infrastructures Remote Radio Head (RRH) Single outdoor unit in which only the RF front-end functionalities are implemented Connected to the remaining baseband processing part of the BS through a bidirectional link In particular the usual choice of the optical fiber for the interconnection cable imposes high-linearity and very expensive optical lasers and drivers 16

17 RF-Subsystem Radio Infrastructures Remote Radio Head (RRH) The physical characteristics of a RRH are similar to the Micro-BS Total Tx output power, radio capacity and heat dissipation requirements So the volume of a RRH cabinet is in the range from 30 to 40 litres The typical RRH interconnection is a single bidirectional point-to-point link 17

18 RF-Subsystem Radio Infrastructures Distributed Antenna System (DAS) The concept is very similar to the RRH one The main difference with respect to the RRH concept, is the greater flexibility and capacity that can be dynamically allocated and optimized for varying traffic needs A DAS enables the independent operation of multiple frequencybands and multiple protocols across a single access network Consequently multiple operator network sharing can be supported by a DAS system 18

19 RF-Subsystem Radio Infrastructures Comparing DAS and RRH system An example with a DAS architecture An example with a RRH-based architecture 19

20 Femtocell 20

21 Femtocell Overview of Femtocell System a small, low-power cellular base station, typically designed for use in a home or small business It connects to the service provider s network via broadband (such as DSL or cable) Extension of service coverage indoors or at the cell edge, especially where access would otherwise be limited or unavailable. improved coverage and potentially better voice quality and battery life Coverage Standard BS: 35km, microcell: less than 2 km, picocell: less than 200 m Femtocell: 10 m Femtocell BS 3GPP femtocell: Home Node B (HNB) LTE femtocell: Home enode B (HeNB) 출처 : 21

22 Femtocell Overview of Femtocell System Femtocells are sold by a mobile network operator (MNO) to its residential or enterprise customers. A femtocell is typically the size of a residential gateway or smaller, and connects to the user s broadband line. Once plugged in, the femtocell connects to the MNO s mobile network, and provides extra coverage. From a user s perspective, it is plug and play, there is no specific installation or technical knowledge required anyone can install a femtocell at home. SON (Self-Organized Network) ocell/ Handover When these mobile phones arrive under coverage of the femtocell, they switch over from the macrocell (outdoor) to the femtocell automatically. 출처 : 22

23 Standard Architecture of Femtocell Femtocell gateway Femtocell security gateway terminates large numbers of encrypted IP data connections from hundreds of thousands of femtocells, signalling gateway aggregates and validates the signalling traffic, authenticates each femtocell and interfaces with the mobile network core switches using standard protocols, such as Iu 출처 : 23

24 Femtocell Standard Architecture of Femtocell management and operational system allows software updates and diagnostic checks to be administered. These typically use the same TR-069 management protocol published by the Broadband Forum and also used for administration of residential modems. 출처 : 24

25 Technical Issues in a Femtocell System Interference Mitigation The placement of a femtocell has a critical effect on the performance of the macro cell network femtocells can use the same frequency bands as the conventional cellular network If the femto network is sharing the channel (co-channel) with the macro network, interference can occur. Interference mitigation techniques detecting macrocells, adjusting power and scrambling codes accordingly 출처 : 25