Frequencies (or time slots or codes) are reused at spatially-separated locations  exploit power falloff with distance. Best efficiency obtained with minimum reuse distance  system capacity is interference-limited. Mainly designed for circuit-switched communications Base stations perform centralized control functions. (call setup, handoff, routing, etc.) 8C Cimini-7/98 Cellular Systems

Reuse Distance (D) – distance between cells using the same frequency, time slot, or code – smaller reuse distance packs more users into a given area, but also increases co-channel interference Cell Radius – decreasing the cell size increases system capacity, but complicates the network functions of handoff and routing 8C Cimini-7/98 DESIGN CONSIDERATIONS Access Technique: CD, TD, FD, or hybrid – Efficiency within a cell – Interference to other cells – Other considerations: - Frequency planning - Synchronization requirements - Soft handoff - Need for power control - Frequency reuse requirements – Cellular system “capacity”

TD Access FDD separates uplink and downlink. Timeslots allocated between different cells. –FDD separates uplink and downlink. One of the US standards for digital cellular – IS-54 in 900 MHz (cellular) band. – IS-136 in 2 GHz (PCS) band. IS-54 compatible with US analog system. –same frequencies and reuse plan.

GSM Access FDD separates uplink and downlink. Access is combination of FD,TD, and slow FH –Total BW divided into 200Khz channels. –Channels reused in cells based on signal and interference measurements. –All signals modulated with a FH code. FH codes within a cell are orthogonal. FH codes in different cells are semi- orthgonal –FH mitigates frequency-selective fading via coding. –FH averages interference via the pseudorandom hop pattern

Access in IS-95 (CDMA) Each user assigned a unique DS spreading code Code is reused in every cell –No frequency planning needed –Allows for soft handoff is code not in use in neighboring cell Power control required due to near-far problem –Increases interference power of boundary mobiles.

Capacity Comparison Shannon Capacity –Shannon capacity does no incorporate reuse distance. –Some results for TDMA systems with joint base station processing (Wyner, Wyner and Shamai). User Capacity –Calculates how many users can be supported for a given performance specification. –Results highly dependent on traffic, voice activity, and propagation models. –Can be improved through interference reduction techniques. Area Spectral Efficiency

8C Cimini-7/98 Area Spectral Efficiency Defines as the total throughput per unit area. Captures the design tradeoffs for reuse distance as well as other parameters General performance metric that can be applied to any system Rates can be computed based on analytical model or simulation ASE for equal rate users K R b (S/I) (.25 D 2  ) A e = bps/Hz/km 2 -K is the number of users per cell -S/I is a (time-varying) function of the access method, reuse distance, and propagation. - R b is the data rate per user - D is the reuse distance

ASE vs. Cell Radius Cell Radius R [Km] Average Area Spectral Efficiency [Bps/Hz/Km 2 ] D=4R D=6R D=8R f c =2 GHz

Interference Averaging (CDMA, FH) Interference Reduction (power adaption, sectorization) Interference Cancellation (smart antennas, multiuser detection) Interference Avoidance (dynamic resource/channel allocation) METHODS TO IMPROVE SPECTRUM UTILIZATION 8C Cimini-7/98

SECTORIZATION 120° sectoring reduces interference from co-channel cells. Out of the 6 co-channel cells in the first tier, only 2 of them interfere with the center cell. If omni-directional antennas were used at each base station, all 6 co-channel cells would interfere with the center cell. 8C Cimini-7/

Multiple antenna elements at the receiver and/or the transmitter form an antenna array. Space-time processing of the received signal at the array reduces interference, and also compensates for flat-fading and delay spread. Methods – switched beam – adaptive array SMART ANTENNAS 8C Cimini-7/98

Goal: decode the interfering signals to remove them from the desired signal Interference cancellation – decode strongest signal first, and subtract it from the remaining signals – repeat the cancellation process on the remaining signals – works best when signals are received at very different power levels Optimal multiuser detector (Verdu Algorithm) – cancels interference between users in parallel – complexity increases exponentially with the number of users Other techniques tradeoff performance and complexity – decorrelating detector – decision-feedback detector – multistage detector multiuser detection often requires knowledge of the channel parameters, which is difficult to obtain in a rapidly-changing environment MULTIUSER DETECTION 7C Cimini-9/97 