Cellular Systems and Infrastructure-Based Wireless Networks · · 2005-12-28Introduction WL9001", ... with unity channel gain within a cell and a ... CONCLUSION Well designed

Cellular Systems and Infrastructure-Based Wireless Networks

ABSTRACT

Infrastructure-based wireless networks have base stations (access points).Advantages over Ad-hoc networks :

- efficiently utilize network resources.- single-hop routes, results in :- lower delay and loss- higher data ratesExamples : Wireless LANs, paging systems, and

cellular phone systems.

References

[1] Andrea Goldsmith, "Wireless Communications", Cambridge University Press, 2005.

[2] David Tse; Pramod Viswanath, "Fundamentals of Wireless Communication", Cambridge University Press, 2004.

[3] Simon Haykin, "Communication Systems", John Willy & sons, 2001.

[4] William Stallings, "Data and Computer Communications", Pearson Education, 2004.

[5] Lucent Technologies; Bell Labs Innovations, "GSM Introduction WL9001", 1998.

OUTLINECellular System Fundamentals

Reuse Distance Ddiamond-shaped CellsHexagonally-shaped cells

SINR & User Capacity

Interference Reduction Techniques

Dynamic Resource AllocationSchedulingDCAPower Control Fundamental Rate limitsShannon’s CapacityArea Spectral EfficiencyCase Study

Cellular System Fundamentals:Channel Reuse

Different channel sets Ci are assigned to different cells.

Adjacent cells are assigned different channel sets to avoid interference.

Cells sufficiently spaced from each other can use the same channel set to in crease system capacity ( co-channels).

Cellular System Fundamentals:Handover

is the process of automatically switching a call in progress from one traffic channel to another to neutralize the adverse effects of user movements [5]

Cellular System Fundamentals:Access Techniques

Time Division Multiple Access (TDMA), the time axis is divided into non-overlapping time slots, and each user is

assigned a different cyclically-repeating timeslot to transmit during it.


Frequency Division Multiple Access (FDMA), the frequency axis is divided into non-ovelapping channels, and each user is assigned a different frequency channel to transmit over it. The channels usually have guard bands to overcome imperfect filters effects and adjacent channel interference


Code Division Multiple Access (CDMA),information of different users are modulated by orthogonal or non-orthogonal spreading codes. The resulting spread signals simultaneously occupy the same time and bandwidth.


Space Division Multiple Access (SDMA), user can access the channel at the same time and the same frequency by exploiting the spatial separation of the individual user. multibeam (directional) antennas are used to separate radio signals by pointing them along different directions

Cellular System Fundamentals:Cell Size

Shrinking the sizeof a cell increases the number of users as long as all aspects of the system scale so that SINR remains the same.

Cellular System Fundamentals:Cell Shape

Cellular System Fundamentals:Reuse Distance D

the distance between the centers of cells that use the same channels.

is a function of cell shape, cell size, and the number of intermediate cells between the two cells sharing the same channel.


For Diamond-shaped cells;

D = R + 2RNI + R= 2R (NI + 1)

NI, no, of intermediate cells any two between co-channel cells.


For hexagonally-shaped cells;

E. g. cell G is located at (0, 1), cell S is located at (1, 1), cell P is located at (−2, 2), and cell M is located at (−1,−1).

Cellular System Fundamentals:Cell Clustering

For diamond-shaped cellsa tesselating cell cluster forms another diamond, with K cells on each side (K = 4 in fig.)The number of cells per cluster is N = K², which is also called the reuse factor: since D = 2KR, we have N = .25(D/R),


For hexagonally-shaped cells:total BW is broken into N channel sets C1,..., CN, ,N =cluster sizeStrating from origin, assign channels as shown (move i cells and then move j cellsThen start from another arbitrary cell, and so on until cells assigned channels.


SINR & User CapacitySINR

Orthogonal Channelization (TDMA & FDMA):Intercell Interfernce (Co-channel interference)

Non-Orthogonal Channelization (CDMA):Intercell Interference + intracell Interfernce

SINR & User CapacitySINR

Pr is the received signal power, PI is the received power associated with both intracell and intercell interference, and N0B is noise power.

SINR & User CapacitySIR

Good cellular system designs are interference-limited [1]i. e. PI >> N0B

N0B ≈ zero

SIR = Pr / PI

SINR & User Capacityexample: SIR (Uplink TDMA)

the simplified path loss model

d=propagation distance; K is a unitless constant depends on antenna characteristics and average channel attenuation, do is a reference distance for the antenna far-field, and γ is the path loss exponent, the value of γ depends on the propagation environment: for free-space model 2 ≤ γ ≤ 4

SINR & User Capacityexample: SIR ( uplink TDMA)

Setting do = 1 m for our calculations yields

γI = pathloss exponent (in-cell propagation)γo= pathloss exponent intercell interference)M = no. of interferers

SINR & User Capacityexample: SIR ( uplink TDMA)

For simplicity, If d = R, and all interferes are located at reuse distance D, then

and if γI = γo = γ this yields

SINR & User CapacityUser Capacity (Cu)The user capacity Cu is defined as the total number of active users per cell that the system can support while meeting a common BER constraint for all users [1].

the user capacity Cu = Nc,N c = No. of channels assigned to any given cell.

SINR & User CapacityUser Capacity (Cu)For orthogonal multiple access,

B=BWBs= channel BWNT= total No. Of orthogonal ChannelsN=reuse factorG=Ratio of total BW to individual user BW

Interference Reduction Techniques

Multipath can cause fading due to phase cancellation between different propagation paths which reduce signal power against noise.Delay Spread results from differences in propagation delays among multiple propagation delay, this delay spread can lead to a significant intersymbol interference in the received signal.Co-channel interference.

Interference Reduction TechniquesAntenna Sectorization

directional antennas to divide up a base station's 360 omnidirectional antenna to N sectorsinterference to a given mobile comes primarily from its sectorreducing interference power by roughly a factor of N

Interference Reduction TechniquesSmart Antennas

consists of an antenna array combined with signal processing in both time and space. form narrow beams to provide high gain to the desired user's signal and can provide spatial nulls in the direction of interference

Interference Reduction Techniquesother techniques

Interference Averaging.

Multiuser Detection.

Interference Precancellation.

Dynamic Resource Allocation

Cellular systems are dynamic in the number of users in any given cell and in their time-varying channel gains.As voice applications are migrated with Multimedia data, user no longer have uniform data rate requirements.

Dynamic Resource Allocation

a flexible resource allocation is required to dynamically assign

channels, data rates, and power levels

relative to the current system conditions and user needs.

Dynamic Resource AllocationScheduling

dynamically allocate resources to mobile users according to their required data rates and delay constraints. exploits multiuser diversity to allocate resources to the user with the best channel.unfair to users with inferior channels.

Dynamic Resource AllocationDynamic Channel Allocation

Falls into two categories,Dynamic assignment of multiple channels within a cell (intracell DCA).Assignment of channels between cells (intercell DCA) for orthogonal channelization.

Dynamic Resource AllocationDynamic Channel AllocationIntracell DCA allows dynamic assignment

of multiple channels within a cell to a given user. In TDMA systems this is done by assigning a user multiple timeslots, and in CDMA by assigning a user multiple codes and/or spreading factors.

Dynamic Resource AllocationDynamic Channel Allocation

In intercell DCA, every channel is available in every cell, i.e. no fixed channel reuse pattern exists. Each channel can be used in every cell as long as SIR requirements of each user are met. Thus, channels are assigned to users as needed.

Dynamic Resource AllocationPower Control

The goal of power control is to adjust the transmit powers of all users such that the SINR of each user meets a given threshold required for acceptable performance.


power control on the uplink results in more reduction of intercell interference since the transmission can come from cell boundaries which cause intercell interference to neighbors cells.


In an uplink with K interfering users we

denote the SIR for the kth user as

Dynamic Resource AllocationPower ControlAssume SIR requirement of the Kth user is

By substituting in the previous equation and representing it in matrix form we get

Where,

is the transmitted powers vector,


u is the column vector of noise powers scaled by the SIR constraints and channel gain, and F is an irreducible matrix with non-negative elements given by


The SIR requirements of all users are satisfied with

which meets the SIR requirements with the minimum transmitted power of the users.


Iterative Algorithm:

requires only SIR information at each transmitter.

Fundamental Rate LimitsShannon's Capacity of Cellular Systems

C = B LOG2 ( 1 + SNR ) bps

a bound on the rate of data transmission and a measure for the efficiency of the communication system

η = R / Ctrade-off between channel bandwidth and received SNR.a framework for comparing noise performance between the different modulation schemes.


Under full base stations cooperation assumption :base stations can be viewed as a single base station with multiple geographically-dispersed antennas and treated. The system then can be treated as MIMO ( in uplink an in down link)


and by characterizing the propagation between mobiles and the multiple-antenna base station (uplink) using AWGN model with unity channel gain within a cell and a channel gain of α, 0 ≤ α ≤ 1, between cells


Then, The per-user capacity which defined as the maximum possible rate that all user can maintain simultaneously is [1]

where B is the total system bandwidth, NoB is the noise PSD, K is the number of mobiles per cell, and P is the average transmit power of each mobile.


Under no base station cooperation Assumption,So, receivers in each cell treat signal from other cell as interference reflects the practical design of cellular systemsUnfortunately, Shannon’s theory in channels with interference is mostly unsolved [1].

Fundamental Rate LimitsArea Spectral Efficiency (ASE)

ASE is a capacity measure that allows the the reuse distance, to be optimized relative to fundamental capacity limits.

Since the reuse distance D, is the distance between any two cells use the same channel, then the area cover by each channel is approximately the area of a circle with radius 0.5D, i. e.

A = π (.5D)² .


The system throughput ( Sum-Rate) is given by


The ASE of a cell is defined as the throughput/Hz/unit area that is supported by a cell’s resources

Case Study: AWGN TDMA System Uplink

K usersCell Radius RAll user assigned equal time slots Tk = 1/ KAll user transmit the same power P6 interferers (blue dots) to B.S. of the center cell


Using Simplified Path Loss Model

Setting do = 1 m for our calculations yields


The received signal power of the kth user (red dot) by his base station is

Interfernce caused by 6 interferers is

where 2 ≤ γ ≤ 4.


The maximum rate for the kth user in the cell Rkis

and the ASC1 is

where Ck = Rk.

Plots [1] of Ae versus D for γ=4 and γ=2 with the cell radius normalized to R=1


If all interferers are at a distance D − R/2 from the base station of the center cell, then the ASE2 is

Plots of ASE2 along with ASE1 for γ = 4

CONCLUSION

Well designed cellular systems are interference-limited.The dynamic nature of cellular system in load and channel conditions requires a dynamic resource allocation to efficiently utilize these resources.Shannon’s Capacity of channels with interference (Cellular Systems) is a long-standing open problem.The optimization of reuse distance D relative capacity limits is based on ASE.

References

[1] Andrea Goldsmith, "Wireless Communications", Cambridge University Press, 2005.

[2] David Tse; Pramod Viswanath, "Fundamentals of Wireless Communication", Cambridge University Press, 2004.

[3] Simon Haykin, "Communication Systems", John Willy & sons, 2001.

[4] William Stallings, "Data and Computer Communications", Pearson Education, 2004.

[5] Lucent Technologies; Bell Labs Innovations, "GSM Introduction WL9001", 1998.

Documents

Cellular Systems and Infrastructure-Based Wireless Networks · · 2005-12-28Introduction WL9001", ... with unity channel gain within a cell and a ... CONCLUSION Well designed