WiMAX in 700 MHz Band

8/14/2019 WiMAX in 700 MHz Band

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Overcoming the Challenges of WiMAX Deployment in 700MHz Band

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Overcoming the Challenges of WiMAX

Deployment in 700MHz Band

1. AbstractThe economics of the wireless systems is the major consideration when choosing the

technology, the deployment strategy and the modes of operation. It determines the

investment required to deploy a network, the expense required to operate it and the

operators profit margin. In the context of WiMAX technology, a careful evaluation

of the economics of the network is even more crucial. Since WiMAX technology was

born into a tough world where competing alternatives such as ADSL, fiber and high

speed cellular technologies already exist, WiMAX needs to have a significant

advantage over these incumbent technologies in order to justify the initial deployment

cost, the operation cost and to successfully compete over time in a reality of ARPU

erosion.In order to substantiate the WiMAX business case, efficient ways for maximizing

spectral efficiency are required. Two main techniques are proposed by the WiMAX

forum to enhance spectral efficiency: Aggressive frequency reuse schemes and the use

of multiple antenna techniques MIMO (Multiple Input Multiple Output) and AAS

(Adaptive Antenna Systems).

WiMAX in 700MHz has some unique characteristics that make the deployment of a

WiMAX network at this frequency band even more challenging. The main challenges

are the high capacity required by the large cells characterizing the 700MHz

deployment, the large size antenna arrays that might be required if MIMO and AAS

are chosen as the capacity enhancing techniques and on top of all that the small

spectrum slices assigned to operators in this band.

Experienced wireless operators, not without reason, express their concern about

whether aggressive reuse schemes will prove themselves in Real World

deployments and what impact might the co-channel interference have on overall

system capacity and QoS. In addition, there is a reasonable concern that MIMO will

not be effective in many 700MHz links where LOS (Line of Sight) or NLOS (Near

Line of Sight) exists and both MIMO and AAS antennas will not be a realistic option

in 700MHz deployment due to the size of the antenna structure.

The Cross Sector Interference Cancellation (XSIC) capacity enhancement technology,

developed by Pallasium, addresses these concerns and provides a significant capacity

boost to WiMAX networks, using small size conventional sector antennas. It enables

aggressive reuse schemes and eliminates the dependency on MIMO for capacity

enhancement. However, XSIC can also operate over MIMO, allowing the benefit of

both worlds.

XSIC software improves network economics and user experiences through

interference cancellation and gains in spectral efficiency, client data rates, and overall

system capacity.


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2. The Challenges of 700 MHz WiMAXDeploymentWiMAX in the 700MHz band is mainly used for providing fixed and nomadic

services in suburban and rural areas. In North America the 700MHz band may also be

used for public safety application, including mobile subscribers.

Due to the propagation characteristics of signals in 700MHz, a given area can be

covered with large cells, resulting in potentially fewer base stations. However, since

each cell serves a large number of subscribers, capacity soon becomes the limiting

factor. In many 700MHz systems where relatively small slices of spectrum are

assigned to the operator, capacity becomes a bottleneck even sooner. In order to

exploit the long range propagation of the 700MHz signal and enable large size cells

incorporating spectral efficiency enhancers becomes crucial. Unfortunately, the

MIMO - WiMAX leading spectral efficiency enhancer, exhibits poor performance in

700MHz WiMAX due to the high LOS (Line of Sight) or NLOS (Near Line of Sight),

typical to such deployments. On top of that, the antenna arrays required for MIMO

(Multiple Input Multiple Output) and AAS (Adaptive Antenna Systems) in the

700MHz band are, in many situations, too large to be a realistic option.

Reuse Strategies

Following the WiMAX Forum, the nomenclature for describing the frequency reuse

pattern in this paper is (c, n, s); where c is the number of base station sites in a cluster,

n is the number of unique frequency channels required and s is the number of sectors

per base station site.

Cell and Sector Reuse 1 (1,1,1)

According to this strategy all available sub-channels are assigned to all sectors and

cells with no spatial separation. The concept assumes that some amount of co-channel

interference is tolerable as long as the interference is equally spread between the

subscribers. To equally share the interference between the subscribers and avoid a

situation where a single subscriber is severely affected, WiMAX, in FUSC and PUSC

Fi ure 1 : Fre uenc Reuse o 1 with 3-Sector Base 1,1,3

Source:

WiMAX Forum


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modes, assigns sub-carriers to sub-channels randomly. Although this method works

quite well in an unloaded network, it looses its effectiveness as the network load

increases. As more sub-carriers are simultaneously in use, more co channel collisions

occur; resulting is both throughput degradation and lower probability for adequate

SINR, a fact that hampers significantly the ability of providing adequate QoS.

Cell Reuse 1 / Sector Reuse 3 (1,3,3)

According to this strategy the available spectrum is divided between the three sectors

in a way that each sector uses one third of the available spectrum as shown in Figure

2. Sector reuse 3 eliminates co-channel interference at the sector boundaries and

significantly decreases co-channel interference between neighboring cells due to the

increased spatial separation for channels operating at the same frequency.

Neglecting for a moment the co-channel interference, sector reuse 1 has the potential

of providing three times higher capacity than sector reuse 3. This is due to the fact that

the same spectrum is reused three times in the same cell.

However, in the real world, a significant overlap exists between sectors and between

cells, resulting in co-channel interference in the overlapping areas. If a way will befound to cancel the co-channel interference in the overlapping areas, a significant

capacity boost may be achieved.

3. XSIC Capacity Enhancement with NoAdditional Antennas and RadiosThe Cross Sector Interference Cancellation (XSIC) technology, developed by

Pallasium, addresses these concerns and provides a significant capacity boost to

WiMAX networks, using small size conventional sector antennas. It enables

Figure 2 : Cell reuse 1 / sector reuse 3 (1,3,3)

Source:

WiMAX Forum


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aggressive reuse schemes and eliminates the dependency on MIMO for capacity

enhancement. XSIC can also operate over MIMO, allowing the benefit of both

worlds.

How Does XSIC Work ?

XSIC provides the operator with an efficient capacity enhancement tool using simple

sector antennas. Capacity gain is achieved by allowing the operator to reuse the same

spectrum by all sectors (sector reuse -1) while the XSIC algorithm eliminates the co-

channel interference in the overlapping areas between the sectors. In contrast with

AAS and MIMO, which become highly questionable in 700MHZ deployment, XSIC

does not require additional antennas and transceivers. Its algorithm utilizes only the

existing conventional sector antennas.Figure 3 illustrate cross sector interference in a three-sector cell. The desired signal is

represented by a solid line and the interfering signal is represented by a dashed line.

The signal originating in sector-1 is colored blue and the signal originating in sector-2

is colored green.

Figure 4 shows the same scenario with the XSIC algorithm activated. The cross sector

interference is canceled by steering a null toward the interferer in the pertinent

pattern, so that each subscriber receives only the signal from the sector it is affiliated

to.

As opposed to the AAS and MIMO methods, keeping large distances between the

sector antennas is not required. On the contrary, it is better for the sector antennas tobe with as little spacing as possible.

Using XSIC in a Single Cell Network

To avoid cross sector interference in conventional single cell deployment, sector reuse

is employed. In a 3-sectors cell, for instance, each sector uses a third of the available

sub-carriers. When employing XSIC algorithm in such a cell, frequency reuse-1 plan

Figure 3: Cross sector interference

in a conventional sector

Figure 4: Cross sector cancellation

when XSIC is employed


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can be implemented where all three sectors use all sub-carriers. By utilizing the same

spectrum three times at the same cell capacity a gain of about 300% can achieved.

XSIC Capacity Improvement in a Single Cell System

To demonstrate the improvement achieved by Pallasium's XSIC technology in a

single cell deployment the following reuse schemes and scenarios were investigated:

Sector reuse-3 (each sector uses one third of the spectrum without XSIC) Full reuse-1 (the three sectors use the same spectrum without XSIC) Full reuse-1 with XSIC (the three sectors use the same spectrum with XSIC

employed).

Full details on the scenarios' parameters and simulations conditions are provided in

Appendix A.

The following table summaries the simulation results for single cell scenarios, in

terms of spectral efficiency. It compares the spectral efficiency of sector reuse 1 and

sector reuse 3 plans without XSIC to a reuse 1 plan while XSIC is activated. Spectral

efficiency is provided per two criteria: calculated for both criteria: Equal Time and

Equal Data. (According to Equal Time criteria, throughput is calculated where

each subscriber is granted the same activity time. According to Equal Data criteria,

throughput is calculated where each subscriber is granted the same data volume).

Table 1:Single Cell Simulation results comparison

Equal Time Equal DataSystem

Bit/Cell/Hz Improvement Bit/Cell/Hz Improvement

Baseline Reuse1 1.9 - 0.96 -

Baseline Sector Reuse-3 1.46 -23% 1.29 34%

XSIC Reuse-1 4.34 128% 3.94 310%

1/3

spectrum

1/3spectrum

1/3spectrum

Full

spectrum

Fullspectrum

Fullspectrum

With no XSIC, sector reuse

3 is required to avoid cross

sector interference

With XSIC, the same spectrum

can be reused in all sector.

Resulting in close to 3 times

capacity improvement.

Figure 5: Using XSIC in a single cell 3-sector system.


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As expected, the fact that XSIC enables using three times the same spectrum, provides

a significant spectral efficiency improvement over reuse-1 and reuse-3 scenarios:

128% for Equal Time criteria and 310% for Equal Data criteria.

Using XSIC in a Multi Cell Network

The prevalent reuse strategy in a multi-cell WiMAX network is Cell Reuse 1 / Sector

Reuse 3, shown in Figure 6-a. Using XSIC, a novel topology can be created which

significantly improves spectral efficiency. The new topology sets the frequency reuse

in cells structure rather than in sectors. In this topology cells are arranged in clusters

of three. Each cell uses one third of the available spectrum. On the other hand, each of

the three sectors of the cell uses the full spectrum assigned to this cell. Due to the use

of XSIC technology inter-sector interference are avoided. Figures 6-a and 6-b

illustrate both cell reuse 3 and sector reuse 3 approaches. As can be seen in figure 6-a

and 6-b, the reuse distance in the cell frequency reuse-3 is higher by approximately

75% than sector frequency reuse-3, resulting in a corresponding spectral efficiency

improvement of more than 160 %.

.

Throughput Enhancement Using XSIC

Adaptive modulation assigns each subscriber the highest possible modulation scheme

allowed by the quality of the link. In the presence of co-channel interference, the

quality of the link degrades and a lower modulation scheme is assigned to the

subscriber. The following figures show the size and location of the areas where each

modulation scheme is provided.

Figure 6-a: Conventional

approach: Cell reuse 1 / Sector

reuse 3. (1,3,3)

Figure 6-b: Better reuse scheme:using XSIC: Cell reuse 3 /

Sector reuse 1, (3,3,1)

Figure 6: Using XSIC in a multi cell network


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Figures 7 and 8 show the offered throughput (in terms of modulation scheme) in reuse

(1,3,3) vs. reuse (3,3,1) with XSIC. As can be seen, larger areas can be served with

higher modulation schemes.

Figure 8: Offered modulation schemes with reuse (3,1,3) with XSIC

Figure 7: Offered modulation schemes with conventional reuse

1,3,3


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The results shown in Figure 7 and 8 are summarized in the following table.

Table 2: Offered Modulation Scheme with and without XSIC

Reuse Plan Under

QPSK 1/2

QPSK 1/2 to

QAM16 1/2

QAM16 1/2

QAM 64 2/3

QAM(?)

2/3 andabove

Cell reuse 1 / sector

reuse 3 NO XSIC

23% 35% 20% 22%


reuse 3 with XSIC

9% 19% 36% 36%

Spectral Efficiency Enhancement Using XSIC

The following table summaries the performances of the different reuse schemes and

relative performance improvement compared to the Baseline reuse-1 system.Spectral efficiency is provided per Equal Time and Equal Data criteria.

Table 3: XSIC spectral efficiency improvement in a (3,1,3) reuse scheme

Equal Time Equal DataReuse Scheme


Conventional (1,3,3)

reuse scheme

0.68 -21% 0.31 0%

XSIC employed on(3,1,3) reuse scheme

1.02 18.6% 0.52 67.7%

As can be seen in Table 3 significant improvement is achieved by use of XSIC

technology over conventional reuse 1 and reuse 3 plans.

Employing XSIC on a full reuse 1 (1,1,3) scheme

An aggressive full reuse 1 (1,1,3) scheme is promoted by several vendors. According

to this strategy, all available sub-channels are assigned to all sectors and cells with no

spatial separation. This concept assumes that some co-channel interference is tolerable

as long as the interference is equally spread between the subscribers and not

concentrated in one or in a few of them. This method has the highest potential for

sub-carries collision resulting in co-channel interference. When XSIX is employed

on a full reuse 1 scheme, a significant improvement in terms of SINR, spectral

efficiency and capacity is gained.

Table 2 summarizes the simulation results in terms of percentage of subscribers for

each range of modulation scheme.


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Figures 9 and 10 show the offered throughput (in terms of modulation scheme) in

reuse (1,1,3) vs. reuse (3,1,3) with XSIC. Comparing Figure 9 to Figure 10 it is easy

to see that with XSIC employed on reuse (3,3,1), larger areas can be served with


The results shown in Figure 9 and 10 are summarized in the following table:

Table 4: Offered Modulation Scheme with and without XSIC

Reuse Plan Under

QPSK 1/2

QPSK 1/2 to

QAM16 1/2

QAM16 1/2

QAM 64 2/3

QAM(?)

2/3 and

above


reuse 1 No XSIC

63% 24% 11.5% 1.5%


reuse 1 with XSIC

52% 23% 14% 11%

XSIC In Full Reuse Scheme In a Fixed System.

Many 700MHZ systems serve fixed subscribers with directional antennas. In such

systems, the inter-cell interference level is significantly reduced and the benefits of

XSIC technology are even more prominent as shown in the following data.

Adaptive modulation assigns each subscriber the highest possible modulation scheme

allowed by the quality of the link. In a presence of co-channel interference, the quality

of the link degrades and a lower modulation scheme is assigned to the subscriber. The

following diagrams show the size and location of the areas where each modulation

scheme is provided.

Figure 9: Offered modulation schemes in

reuse 1,1,3) without XSIC CPE Omni)

Figure10: Offered modulation schemes

in reuse (1,1,3) with XSIC (CPE Omni)


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The results shown in Figure 11 and 12 are summarized in the following table:

Table 5: Offered Modulation in Reuse1 with fix directional SUs antennas

System UnderQPSK 1/2

QPSK 1/2 toQAM16 1/2

QAM16 1/2QAM 64 2/3

QAM 2/3and above

Baseline Reuse1 34% 36% 25.5% 4.5%

XSIC Reuse1 5% 15% 36% 44%

Figures 11 and 12 show the offered throughput (in terms of modulation scheme) in

reuse (1,1,3) vs. reuse (3,1,3) with XSIC. Comparing Figure 11 to Figure 12 it is easy

to see that with XSIC employed on reuse (1,1,3), larger areas can be served with


The following table provides the spectral efficiency improvement when XSIC is

employed on a system serving fixed subscribers. Spectral efficiency is provided per

Equal Time and Equal Time criteria:

Figure 11: Offered modulation schemes

with reuse (1,1,3) without XSIC (CPE

directional).

Figure 12: Offered modulation schemes

with reuse (1,1,3) with XSIC (CPE

directional).


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Table 6: XSIC spectral efficiency improvement in a (1,1,3) reuse scheme when

CPEs use directional antennas

Equal Time Equal DataSystem


Baseline Reuse1 1.71 - 0.87 -

XSIC Reuse1 3.52 106% 2.31 165%

4. SummaryThe XSIC technology presented in this document, addresses the challenges involved

with WiMAX deployment in the 700MHZ band. It provides the operator an efficient

capacity enhancement tool using simple sector antennas. Capacity gain is achieved by

allowing the operator to reuse the same spectrum by all sectors (sector reuse -1) while

XSIC algorithm eliminates the co-channel interference in the overlapping areas. As

opposed to AAS and MIMO, which become highly questionable in 700MHZ

deployment, XSIC does not require additional antennas and transceivers. Its algorithm

utilizes only the existing conventional sector antennas.

XSIC principle of operation was explained and data was provided showing a

significant capacity enhancement, in various reuse plans, translated to lower cost of

Mbps per square area, fewer base stations per network and lower operation

expenditure.


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Appendix A Simulation AssumptionsThe performance improvement achieved by XSIC technology in 700MHZ

deployment scenarios is demonstrated by means of simulation. The simulations were

conducted on two network types: single cell and multi cell topologies.

Single Cell Scenario

In this scenario a single 3 sector 10KM radius macrocells has been assumed. With this

scenario no inter-cell interference exist.

Three baseline tests have been defined:

Sector Frequency reuses-1 without XSIC. Sector frequency reuse-3 without XSIC. Single omni sector.

One scenario with XSIC was tested:

Frequency reuse-1 with XSIC.Multi Cell Scenario

The simulation was performed over 19 hexagonal 3 sector 10KM radius macro-cells,

where interference is evaluated in the center cell. 2-Baseline scenarios without XSIC

have been defined:

Frequency reuse-1. Sector frequency reuse-3.

Three topologies with XSIC were tested against base line:

Cell/Sector frequency reuse-1 with XSIC. Sector frequency reuse 1-Cell frequency reuse-3 with XSIC. Fractional cell reuse-3 with XSIC a frequency reuse saving scheme

recommended by the WiMAX forum.

Simulation Conditions

Network Topology

Basic 3 sectors 19 hexagonal cells canonic network topology was employed.

Table 1:Network Topology Parameters

Number of cells 19

Sectors per Cell 3

Sectors boresight angles (300,150

0,-90

0) or (0

0,120

0,-120

0)

Cell Radius 10000m


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Equipment Model

The equipment model assumes all BS/MSs have identical characteristics.

BS is modeled with a single 900

antenna per sector.

Table 2:BS Model

Tx Power 36dBm

Antenna Boresight Gain 16dBi

Antenna 3dB Beamwidth1

900/120

0

Front to Back power ratio 25dB

Noise Figure 4 dB

Number of Antennas per

Sector1

Note:

1) 900antennas are used in network simulation to reduce cross cell interferenceswhen using sector reuse-3. 120

0antennas are used in a single cell deployment to

increase coverage at sectors fringes.

MSs are modeled with a single omni-directional antenna and 24 dBm TX power.

Table 3:MS Model

Tx Power 24dBm

Noise Figure 6 dB

Number of Antennas 1


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Channel Model

Table 4:Channel Model

Pathloss Single slop model (COST 231 HATA)

PL = 34.5 + 34LOG10(d)

Where dis distance in meters.

Lognormal distributed random variable with zero

mean and 8 dB standard deviation.

Shadowing

Shadowing of a given user at sectors of thesame cell is 100% correlated.

Shadowing of a given user at sectors ofdifferent cells is 50% correlated.

Implementation Loss 1 dB

WiMAX PHY Parameters

The simulation is performed with the following WiMAX Air Interface parameters.

Table 5:WiMAX Channel Parameters

Channel Bandwidth 10 MHz

Frame Duration 5 ms

Sub-Channelization mode PUSC

DL/UL Data Symbols1 DL:28 UL:12

Sounding symbols per Frame 2

DL/UL Data Symbols with Sounding2

DL:26 UL:12

Notes:

1) Only data carrying symbols without preamble FCH and mapping were used.2) When Sounding is used the UL/DL symbols allocation is changed so only the

DL is influenced.

Adaptive Modulation

The assumption is that adaptive modulation is applied according to the MS SINR,

with modulation and coding available from QPSK 1/12 to QAM64 3/4. In addition

HARQ is employed with up to 3 repetitions.


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Table 6:Modulation SNR and capacity

Data rate Per Sub-

channel1

Modulation

Required SNR [dB]

Bits/QAM SymbolWithout

Sounding

With

Sounding

Implementation

Loss

QPSK 1/18 HARQ3 -9.11 .055 7466.6 6933.3

QPSK 1/12 HARQ2 -7.34 .083 11200 10400

QPSK 1/12 -4.34 .166 22400 20800

QPSK 1/8 -2.57 .25 33600 31200

QPSK 1/4 0.43 .5 67200 62400

QPSK 1/2 3.43 1 134400 124800

QPSK 3/4 6.09 1.5 201600 187200

QAM16 1/2 9 2 268800 249600

QAM16 3/4 11.58 3 403200 374400

QAM64 2/3 16.15 4 537600 499200

QAM 64 3/4 17.2 4.5 604800 561600

1 dB

Note:

1) Data rate is calculated per sub-channel per second according the number ofdata carrying OFDM symbols in the DL sub-frame according to Table 5.

XSIC Performance Assumption

The XSIC algorithm reduces interferences between different sectors of the same cell

by 15dB and up 20 maximum of -25dB.

Documents

WiMAX in 700 MHz Band