Cellular Architecture with Frequency ReusePattern more than One (TDMA based system)

Course: S-72.333 Post-graduate course in Radio Communications 2001-2002Author: Liang YongStudent No.: 49329FContact info: yliang@cc.hut.fi

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Table of contents

TABLE OF CONTENTS....................................................................................................................... 1

ABBREVIATIONS ................................................................................................................................ 3

1 INTRODUCTION............................................................................................................................... 3

2. MAIN CONCEPTS............................................................................................................................ 4

3. CELL SECTORING.......................................................................................................................... 6

3.1 CELL SECTORING WITH WIDE-BEAM DIRECTIONAL ANTENNAS ................................ 63.2 SECTORING WITH SWITCHED-BEAM ANTENNAS............................................................. 73.3 TRUNKPOOL TECHNIQUES..................................................................................................... 8

4. CELL SPLITTING ............................................................................................................................ 8

4.1 REUSE PARTITIONING ........................................................................................................... 10

5. HIERARCHICAL ARCHITECTURE .......................................................................................... 11

6. MACRODIVERSITY ARCHITECTURES................................................................................... 13

7. SUMMARY ...................................................................................................................................... 13

REFERENCE ....................................................................................................................................... 13

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ABBREVIATIONS

AoA Angle-of-ArrivelBS Base stationCCI Cochannel InterferenceC/I Carrier/InterferenceDCA Dynamic Channel AssignmentQoS Quality of ServiceMS Mobile StationTDMA Time Division Multiple Access

1 Introduction

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In the early phase, mobile radio system normally used a high power transmitter withan antenna mounted on a tall tower. This approach gave very good coverage, but itwas very difficult to reuse these same frequencies, therefore that the network capacityis low. As the demand for mobile service increased, achieving high network capacityby the same radio spectrum was more important than covering large areas [1].

In order to solve the capacity problem, the cellular concept was proposed in 1970. Asthe population grows, cells can be added to accommodate that growth. Frequenciesused in one cell cluster can be reused in other cells. Conversations can be handed offfrom cell to cell to maintain constant phone service as the user moves between cells(Figure 1.1).

Figure 1.1. Mobile telephone systems use a cellular architecture

The cellular concept employs variable low power transmitters, which allow cells to besized according to the subscriber density and demand of a given area. As thepopulation grows, cells can be added to accommodate that growth. . Frequencies usedin one cell cluster can be reused in other cells. Small cells will increase the networkcapacity, but on the other hand will increase the cochannel interference (CCI),therefore affect the quality of service (QoS).

In order to achieve high capacity while satisfying quality of service expectations, acellular architecture must be defined so as to be flexible to accommodate systemgrowth.

2. MAIN CONCEPTS

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The motivation behind implementing a cellular mobile system is to improve theutilization of spectrum efficiency. The frequency reuse is one concept, and cellsplitting is another concept. Cell splitting will be discussed in Chapter 4.

Frequency Reuse

Figure 2.1 Illustration of the cellular frequency reuse concept. Cells with thesame alphabet use the same channel set.

Frequency reuse is the core concept of the cellular mobile radio system. The totalavailable channels are divided into a number of channel sets (Actually, frequencyreuse pattern is equally to the number of channel sets). Each channel set is assigned toa cell. Cells are assigned a group of channels that is completely different fromneighbouring cell. The same set of channels can be reused in another cell providedthat the reuse distance D is fulfilled (Figure 2.1). The reuse distance is the minimumseparation of identical channels that have the same carrier frequency, at which there isacceptable interference:

RND 3= (2-1)

Where N is the number of channel sets (cells in a cluster) (in Figure 2.1, N=7), R isthe radius of a cell [2].

Reduction of Interference

Reusing an identical frequency channel in different cells is limited by cochannelinterference between cells, and the cochannel interference can become a majorproblem. One way to reduce cochannel interference (CCI) is to keep the separationbetween two cochannel cells by a sufficient distance. Another way for controling CCIis to use directional antenneas at the base station (BSs), and we call it cell sectoring.Chapter 3 discusses cell sectoring in detail.

The adjacent channel interference, coming from neighbouring-channels and next-channels, is another consideration in channel assignment. The adjacent channel

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interference depends on the separation of two adjacent channels, the characteristic ofreceiver filters, and the distance of two adjacent channel users. The near-end ratiointerference can occur among the neighbouring channels. Therefore, if one channel isassigned to a cell, its adjacent channels cannot be assigned to the same cell.

3. CELL SECTORING

As mentioned in chapter 2, the cell sectoring is employed into the cellular system toreduction the effect of CCI. Two types of directional antennas are discussed here:conventional wide beam directional antennas, and switched beam antennas.

3.1 CELL SECTORING WITH WIDE-BEAM DIRECTIONALANTENNAS

In the initial phase of a system, omnidirectional antennas are used to transmit orreceive signals, and the corresponding cells are called omnidirectional cells. In thelater stages, systems use directional antennas to both increase the capacity of thesystem and reduce the effect of the CCI. The original cells are diveided intosectorized cells, where coverage is confined to individual 120-degree (or 60-degree)sectors rather than the typical full 360-degree coverage (Figure 3.1).

Cell sectoring is a very common method that is employed in cellular systems toimprove the C/I performance. On the forward channel, directional antennas reduce thegeneration of CCI by transmitting the signals to the mobile stations (MSs) with anarrower Angle-of-Arrivel (AoA) spread than omnidirectional antennas. On thereverse channel, directional antennas reduce the effect of the CCI because theyrespond to CCI that is generated with a narrower AoA spread about the MS.

Figure 3.1 Worst case CCI situation on the forward channel with 120° cellsectoring

Let’s analysis the worse case CCI situation on the forward channel with 120° cellsectoring (Figure 3.1). An N-cell reuse cluster with 120° sectors yields an N/3N reuseplan (N cells and 3N sectors). As shown in Fig.3.1, 120° cell sectoring reduces the

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number of first-tier cochannel interferers from six to two. The two first tier interferersare located at approximate distances of D and D+0.7R. The resulting worst case C/I is

αααα

α

−−−−

−

++

=++

=Λ7.0

1

)7.0(

R

D

R

DRDD

R (3-1)

With a path loss exponent = 3.5, the worst case C/I is

( ) dBdB 1.21=Λ (3-2)

Where for the omni-directional antenna case [1], ( ) dBdB 3.14=Λ (3-3)

Therefor, for N=7, 120°sectoring yields a 6.8 dB C/I gain over the case whenomnidirectional antennas are used.

To derive a benefit from sectoring, the carriers that are assigned to each cell must bepartitioned into disjoint sets, such that each sector uses a disjoint set of carriers. Thisfiner partitioning of the carriers results in a loss in trunking efficiency, which we willquantify later. Hence, cell sectoring improves the C/I performance at the cost oftrunking efficiency.

3.2 SECTORING WITH SWITCHED-BEAM ANTENNAS

Switch-beam antennas can be used in place conventional wide-beam directionalantennas, to improve both the coverage and system capacity. Switched-beam antennasare a simple type of smart antenna where multiple antenna beams are used within eachcell sector, and beam steering is achieved through a simple beam selectionmechanism.

Figure 3.2 Trunkpool schemes for switched-beam antennas.

The MS antennas are assumed to be omni-directional. With omni-directional BSantennas, there are six first-tier cochannel interferers for both the forward and reversechannels. The number of first-tier interferers is reduced to two with 120° sectoring.With a switched-beam smart antenna, the number of first-tier cochannel interferers onthe forward channel is a random variable ranging from 0 to 6, due to the narrow-beam

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directional antennas and the dependency of the activated beam on the MS location. Ifthe CCI form the antenna sidelobes is ignored, there is at most one interferer on thereverse channel when the smart antenna beamwidth is less than 40°.

3.3 TRUNKPOOL TECHNIQUES

Although narrow-beam directional antennas in switched-beam smart antenna systemscan reduce unnecessary spillage of radiation and mitigate the effects of channel timedispersion, switched-beam smart antennas will have more frequent handoffs (due tointer-sector handoffs) that result in reduced trunking efficiency (offered traffic perchannel) [1].

To overcome the trunking efficiency degradation caused by narrow beam sectoring,sector-trunkpool and moni-trunkpool (Figure 3.2) load sharing schemes are suggested.

(1) Sector-trunkpool

There are a few beams in each sector. All the channels assigned to one sector areshared by all the beams within that sector. Each sector antenna acts as commonaperture for one of these beams. No handoffs are needed unless the MS crosses sectoror cell boundaries. The trunking efficiency will remain the same as the referencesystem, which each wide-beam sector has a unique channel assignment.

(2) Omni-trunkpool

Any of the channels assigned to a cell can be assigned to any one of the activatedbeams. No handoffs are needed unless an MS crosses a cell boundary.

The omni-trunkpool technique is helpful for increasing the trunking efficiency whensmart antenna systems are employed.

4. CELL SPLITTING

When the traffic load in an area increases, a cell can be split into smaller cells in orderto improve the utilization of spectrum efficiency (reuse frequency more often).Usually the new radius is one half the original radius. For example, the radius of a cellreduces by half and splits an old cell into four new small cells (Figure 4.1). The areaof the split cell is ¼ of the area of the parent cells , then the traffic load is increasedfourfold in the same area, if each new cell carry the same maximum traffic load of theold cell.

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Figure 4.1 Cell splitting

There are two ways of splitting: In Figure 4.2a, the original cell site is not use, whilein Figure 4.2b, it is. In Figure 4.2a, a split cell labelled 1’ is located at the midpointbetween two cells labelled 1. In Figure 4.2b, a split cell is overlaid in its parent cell,and we call it inner cell.

Because the split cells are smaller, the transmit power can be reduced. To estimatedthe transmit power requirements in the split cells, we note that the received power fora MS located at the corner of a parent cell is

( ) β−Ω=Ω 000 RAR (4-1)

While the received power at the boundary of a split cell is ( ) β−Ω=Ω sss RAR (4-2)

Where 0Ω and sΩ and 0R and sR are the transmit power and cell radius associated

with the parent cells and split cells, respectively. The new transmit power can beobtained as follows:

β−

Ω=Ω

00 R

Rss (4-3)

If =4, then ,16/0Ω=Ω s since 2/0RRs = . Hence, the split cells can reduce their

transmit power levels by 12dB.

(a)

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(b)

Figure 4.2 Cell splitting is used to accommodate traffic load by introducingsmaller cells.

To avoid violation of the reuse constraint for introducing of the split cells. A verystraight forward approach is channel segmenting, where the channel sets in thecochannel cells are divided into two groups: the split cells are assigned one group ofchannels, while the parent cochannel cells are assigned the other group of channels.However, this arrangement sacrifices trunking efficiency because the parent cellscannot use the channels assigned to the split cells. Therefore, channel segmenting isnot a good option.

Another solution is shown in Fig. 4.2b, where overlaid inner cells are introduced intothe parent cells. MSs located within the overlaid inner cells and the split cells use onegroup of channels, while MSs located within the outer cells use the other group ofchannels. Whenever an MS moves between the inner and outer areas of a cell ahandoff must be executed, to avoid violations of the cochannels reuse constraint.

4.1 REUSE PARTITIONING

Halpern [5] suggested an overlay/underlay scheme based on the concept of reusepartitioning, where multiple cochannel reuse factors are used in the same deployment.An inner cell is created within each of the existing cells as shown in Fig. 4.3. channelsare assigned to the inner and outer cells according to a 3-cell and 7-cell reuse plan,respectively, although other reuse plans could be used. Handoffs are required when aMS crosses the boundary between an inner and outer cell. The reduced radii of theinner cells leads to an increase in cell capacity.

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Figure 4.3 Overlaid inner cells with reuse partitioning

Cell splitting can be used with reuse partitioning. In Fig.4.4, a split cell is addedbetween the parent B2 cells. And the split cell also uses reuse partitioning. Tomaintain the C/I at an acceptable level, some of the channels in the B2 cells aremoved to the inner cells and denoted by B2′. Furthermore, the closest co-channelinner cells A1 must have their channels partitioned in a similar fashion. Therefore, forusing cell splitting with the reuse partitioning scheme, the drawback is that the cellsmust be divided into many concentric rings that use disjoint cannel sets, and handoffsmust occur when a MS crosses the boundary between two rings.

Figure 4.4 Cell splitting can be used in combination with reuse partitioning

5. HIERARCHICAL ARCHITECTURE

Although the cell splitting technique will increase the network capacity, it also willcause a large number of handoffs among the different small cells for high-mobilityMSs (e.g. MS in moving cars). Since each handoff requires additional work by thenetwork, it is not particularly desirable to increase the number of such events. Toachieve the conflicting goals of maximizing network capacity (which implies use of

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small cells) and minimizing network control (which favours large cells), a hierarchicalarchitecture is proposed.

A hierarchical architecture may consist of two or more layers of cell, with the layerhaving the smallest cells the lowest in the hierarchy. They may be formed of apicocellular layer to give service to indoor environment and a microcellular layer toprovide service to indoor/outdoor environments. Both could be overlaid by amacrocellular layer that deals with requests from users in outdoor areas in cities orrural areas. Finally, at the highest hierarchical level, there could be communicationsatellite beams overlaying all the previous terrestrial layers or some clusters of them.Small cells provide greater spectral reuse and larger capacity and allow the use oflow-power lightweight handsets. They also provide coverage to points that larger cellscannot reach due to natural or man-made obstacles. Large cells are used to coverlarger areas with low-cost implementation, to cover spots that are difficult for radiopropagation in small cells, and to provide overflow groups of channels for clusters ofsmall cells when heavily loaded. Figure 5.1 shows an example of a four-levelhierarchical architecture.

Figure 5.1 Four-level hierarchical architecture

Hierarchical architectures are used to improve coverage, to increase capacity, tobalance the load between layers, and give service to uses with different mobilitycharacteristics (i.e., slow or fast). The speed of the user determines which cellularlayer is the most adequate to provide service. Once a layer is selected, efforts aremade to attend the user in that layer. Where there is a lack of resources in theappropriate layer, the next higher layer in the hierarchy with larger cells would attendthe call.

It is common to associate people working in indoor environments with picocells,pedestrians and users in cars traveling at low speeds with microcell, users in carsmoving at high speeds with macrocells, and users traveling in ships and airplanes withthe highest levels of the hierarchy or satellite. However, there is no clear classificationof when a user should be considered as a slow-moving user or a fast-moving user. Thethreshold to differentiate one from the other can even depend on the offered load, andcan change during the day. Thresholds are normally proposed by system operators todifferentiate users and depend mainly on the probability density function of their

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speed, the time required to establish a call or a handoff, and the cells’ radii. Theclassification of users most commonly adopted in a hierarchical architecture formedof two layers, microcells and macrocells, is the one that considers only slow and fastmoving users with the division between the two decided by operational realities.

For a Time Division Multiple Access (TDMA) hierarchical architecturs, a key issuefor introducing microcells (picocell) is the partitioning of the frequency resourcesamong the hierarchical layers. In order no to violate frequency reuse constraint,careful frequency planning is needed when microcells are introduced.

6. MACRODIVERSITY ARCHITECTURES

Macrodiversity architectures are another method for achieving high capacity, wherethe same signal is received by, and perhaps transmitted by, multiple BSs. At any time,the BS with the best quality measure is chosen to serve the MS. The selection ofbranch (BS) is based on the local mean power (local-mean-based selection scheme)rather than the instantaneous power, because the branch selection algorithm cannotreact to the rapidly varying instantaneous signal power.

Macrodiversity is an effective method for combating shadow and envelope fading. Infact, cellular handoff algorithms implement macrodiversity. The soft handofftechniques used in Code Division Multiple Access (CDMA) systems are a wellknown method for realizing macrodiversity. In TDMA systems that use hard handoffalgorithms will not yield as much macrodiversity gain due to latencies in the order ofa few seconds in the hard handoff algorithms. The requirement for hard handoff inTDMA systems arises a result of the non-universal frequency reuse. However, ifDynamic Channel Assignment (DCA) techniques are used, then TDMA systems canrealize benefits from macrodiversity similar to those obtained in CDMA systems [2].

7. SUMMARY

In this paper, cellular architectures aspects of TDMA digital cellular system areconsidered, more specially, it concentrate on the methods for achieving high networkcapacity, for example, cell splitting, hierarchical architecture have been discussed indetail. To control the CCI, cell sectoring is one of the simplest methods, mainly focuson sectoring with switched-beam antennas. Trunkpool techniques are discussed aswell. When design a cellular system, frequency planning must be consider carefully toavoid violations of the cochannel reuse constraint. This paper proves a guideline forcell planning.

Reference

1. Gordon L.Stuber, “Principles of Mobile Communication (Second Edition)”,Kluwer Academic Publishers, pp.16, 2001.

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2. Lee, William C. Y., “Mobile Cellular Telecommunications Systems,” McGraw-Hill Book Company, 2ed., 1995.

3. Ni, Shaoji, "Dynamic Channel Allocation Algorithms in Cellular Systems(Licentiate Thesis)", Communications Lab of HUT, Oct. 1997.

4. S.C.Swales, M.A.Beach, D.J.Edwards, and J.P.McGeehan, “The performanceenhancement of multibeam adaptive base station antennas for cellular land mobileradio systems,” IEEE Trans. Veh. Technol., Vol. 39, pp. 56-67, February 1990.

5. S.W.Halpern, "Reuse Partitioning in Cellular Systems," in IEEE Veh. Technol.Conf.,Toronto, Ontario, Canada, pp. 322-327. 1983.