tailieunhanh - Cellular Systems: Multiple Access and Interference Management

The total bandwidth is divided into many narrowband channels. (200 kHz in GSM) Users are given time slots in a narrowband channel (8 users) Multiple access is orthogonal: users within the cell never interfere with each other. Interference between users on the same channel in different cells is minimized by reusing the same channel only in cells far apart. | 4. Cellular Systems: Multiple Access and Interference Management Cellular Systems So far we have focused on point-to-point communication. In a cellular system, additional issues come into forefront: Multiple access Inter-cell interference management Some History Cellular concept (Bell Labs, early 70’s) AMPS (analog, early 80’s) GSM (digital, narrowband, late 80’s) IS-95 (digital, wideband, early 90’s) 3G/4G systems Four Systems Narrowband (GSM) Wideband system: CDMA (IS-95) Wideband system: OFDM (Flash OFDM) Opportunistic Communication (1x EV-DO) (later) Narrowband (GSM) The total bandwidth is divided into many narrowband channels. (200 kHz in GSM) Users are given time slots in a narrowband channel (8 users) Multiple access is orthogonal: users within the cell never interfere with each other. Interference between users on the same channel in different cells is minimized by reusing the same channel only in cells far apart. Users operate at high SINR regime The price | 4. Cellular Systems: Multiple Access and Interference Management Cellular Systems So far we have focused on point-to-point communication. In a cellular system, additional issues come into forefront: Multiple access Inter-cell interference management Some History Cellular concept (Bell Labs, early 70’s) AMPS (analog, early 80’s) GSM (digital, narrowband, late 80’s) IS-95 (digital, wideband, early 90’s) 3G/4G systems Four Systems Narrowband (GSM) Wideband system: CDMA (IS-95) Wideband system: OFDM (Flash OFDM) Opportunistic Communication (1x EV-DO) (later) Narrowband (GSM) The total bandwidth is divided into many narrowband channels. (200 kHz in GSM) Users are given time slots in a narrowband channel (8 users) Multiple access is orthogonal: users within the cell never interfere with each other. Interference between users on the same channel in different cells is minimized by reusing the same channel only in cells far apart. Users operate at high SINR regime The price to pay is in reducing the overall available degrees of freedom. Frequency Reuse Frequency reuse is poor in narrowband systems because of lack of interference averaging. Wideband System: CDMA Universal frequency reuse: all the users in all cells share the same bandwidth ( MHz in IS-95 and 1x) Main advantages: Maximizes the degrees of freedom usage Allows interference averaging across many users. Soft capacity limit Allows soft handoff Simplify frequency planning Challenges Very tight power control to solve the near-far problem. More sophisticated coding/signal processing to extract the information of each user in a very low SINR environment. Design Goals 1) make the interference look as much like a white Gaussian noise as possible: Spread each user’s signal using a pseudonoise noise sequence Tight power control for managing interference within the cell Averaging interference from outside the cell as well as fluctuating voice activities of users. 2) apply point-to-point .

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