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LTE implementation into an Existing GSM 1800 Network Using Active Antenna Systems

Executive Summary

The introduction of LTE is on the agenda of most Mobile Network Operators (MNOs). Band 3 (1800 MHz) is par- ticularly attractive since it is now perceived as the roaming band for voice and data services in the future and most countries have already allocated this band to MNOs globally. This paper describes the network optimization issues MNOs are facing when introducing LTE into an existing GSM network at the same band. Due to the very different characteristics of GSM and LTE standards in the network, one of the major challenges is optimizing both GSM and LTE with conventional passive antenna system solutions, which simply provide the same tilt for both standards. To analyze these limitations, an LTE rollout is simulated at 1800 MHz. Based on the assumption that an operator owns a spectrum of 20 MHz within band 3 and that 10 MHz will be considered for LTE, the paper describes the optimization options for a mixed GSM/LTE network with basic passive antennas and compares this to a scenario where Active Antenna Systems (AAS) are used. AAS have features such as independent up/down-link tilt or tilt per carrier that help MNOs optimize their networks. As a result, AAS can significantly reduce CAPEX in an initial roll-out phase as they require 24 % less sites to be upgraded to LTE in an existing site-grid in order to achieve the same coverage and capacity requirements, compared with the use of passive antenna systems. Those remaining 24 % of sites can be used later for capacity improvements in the network where the traffic grows fastest.

Contents

Introduction

The Challenge of Deploying LTE at 1800 MHz

Active Antenna Systems and their Significance for Multi-Standard Environments

LTE 1800 Co-Site with GSM 1800 Simulation Strategy

Study Results Baseline network Scenario 1 Scenario 2 Scenario 3

Conclusions

References

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Introduction

With the arrival of the 4th generation mobile communica-tions standard LTE, operators can finally deliver multi-Mbit data rates that today’s consumers demand. LTE theoreti-cally offers peak download rates of up to 299.6 Mbit/s and upload rates of up to 75.4 Mbit/s [5]. While these data rates will be lower in practice, they are still much higher than those offered by 3rd generation standards like WCDMA.

Besides higher data rates, LTE also offers operators and their customers features such as low data transfer latency rates (5ms under optimal conditions), support for all cell sizes (from tens of meters up to 100km radius) and a large number of active clients (greater than 200), along with increased spectrum efficiency (1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz wide cells are supported) and others [5].

Because of these obvious advantages, commercial deploy- ments of LTE are gaining speed all around the world. At the time of writing, Verizon is reported to have over 3 million LTE subscribers in the US followed by NTT DoCoMo with nearly 400,000 subscribers in Japan (Source: [6]).

LTE can be deployed in a large variety of frequency bands ranging from 700/800 MHz to 1800 MHz (band 3), 2100 MHz and 2600 MHz. The emphasis of this paper is on an LTE rollout in the 1800 MHz band. For the operator, this band is attractive for a number of reasons:

· It is available to a large number of operators around the world. This also makes it attractive as a roaming band.

· Most operators own a large enough bandwidth within band 3 to make it possible for them to reduce GSM in that band (and possibly move it to lower bands) and introduce LTE in the freed-up bandwidth.

· Some 2nd wave operators own small or no low-band spectrum (< 1 GHz) and for them an LTE rollout in the 1800 MHz band is the best compromise between coverage, capacity and cost compared to using higher- frequency bands [2].

The Challenge of Deploying LTE at 1800 MHz

When introducing LTE into an existing GSM network at 1800 MHz, two problems arise:

1. To free up bandwidth for LTE, the bandwidth used for GSM needs to be reduced. This means fewer unique frequencies available for the GSM network and therefore most likely more interference between sites.

2. As GSM has been rolled out at 1800 MHz for more than a decade, these networks will be heavily optimized. Adding LTE into this band is likely to require a re-opti- mization of the network. Because of their very different structures, both standards (GSM and LTE) will require different optimizations (tilt settings) at each site to guarantee coverage and maximize capacity. Optimized GSM dense urban networks typically use tilts around

4 degrees. GSM uses different frequencies in different cells and therefore interference is dealt with differently. Considering a capacity optimized LTE network, typical tilt ranges on antenna sites in dense urban areas for LTE are between 8 – 16 degrees in order to best manage interference at the cell edge.

This second point can be a major headache for operators rolling out LTE, as it generally means giving priority to either GSM or LTE or making compromises for both. This is because standard passive antennas – which are used on most sites today – only offer a static tilt that has to be used for all standards running on the site. In contrast, Active Antenna Systems (AAS) provide a tilt-per-standard feature, which allows each standard to be optimized separately. In this paper, this issue is investigated further.

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Active Antenna Systems and their Significance for Multi-Standard Environments

Active Antenna Systems (AAS) are a new category of mobile communications antenna. In contrast to a passive antenna and remote radio head (RRH) combination, active antennas have a transceiver for each of the antenna elements in a macro antenna. This makes possible a number of innovative features:

· Beam Forming and Electronic Tilt. The multiple beams from each antenna element superimpose to form a specific beam pattern at a sufficient distance from the antenna (the far field). As the phase and amplitude of a transmitted signal at each transceiver element can be controlled com- pletely independently, the beam that is formed can be shaped, made wider or narrower and can effectively be tilted electronically without the use of mechanical elements like RETs. In a similar fashion, the receive signal can be made directional by a phase adjusted re-combination of the indi- vidual receive signals. This means the operator has complete control over the shape of the beam transmitted and received.

· Separate Rx/Tx Tilt. Receive and transmit beams can be shaped and tilted completely independently from one another allowing, for example, to tilt the transmit beam away from the cell edge to reduce interference with the neighboring cell.

· Tilt-per-Carrier/Standard. The main beam can be split into many sub-beams that can all have a different shape and tilt. This enables a completely separate tilt for each carrier or standard transmit- ted and received on the antenna. Also, this enables standard specific network optimization and indi- vidual optimization of overlay/underlay network structures or hot spots. Tilt per carrier/standard will be further analyzed in this paper.

· Dynamic coverage adoption where traffic goes· Fewer dropped calls, improved throughput· Macro Cell optimization – fewer small cells

· Dramatic cell edge improvement · Up to 40 % better throughput · Up to 60 % terminal battery life· Reduction of number of sites by up to 25 %

· Independent optimization per service (e.g. GSM vs. UMTS) leads to improved interference and reduced number of sites

Beam Forming and Electronic Tilt

Separate Rx/Tx Tilt

Tilt-per-Carrier/Standard

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· Vertical Sectorization. The main beam is split into two or more beams so that each beam has a different vertical tilt. This effectively creates sepa- rate inner and outer cells. Vertical sectorization is useful when users are not distributed homo- genously throughout the cell but are grouped towards the cell center and cell edge. This can be used permanently or temporarily depending on the capacity demand of the network.

· Self-Healing. Full AAS1 have, by their very nature, built-in redundancy. The many transceiver ele- ments are all exactly equal and one or more failures can easily be compensated by adjusting the antenna pattern. This effectively results in a manageable degradation of the AAS, which extends its MTBF and therefore reduces OPEX for the operator.

· Digital Architecture. By its very nature, an AAS reduces the amount of analog signals in the access network. Only the electronics very close to each antenna element are dealing with analog signals, the rest of the AAS is fully digital. The AAS will be connected to a base-band unit (BBU) via a digital fiber-optic CPRI or OBSAI connection. This “digi- tization” has a number of advantages:

· Reduction of losses and interference due to analog effects (like signal losses in cables). · Every parameter of the AAS can easily be changed digitally and in real-time from any network operations center (no site visit necessary). MNOs therefore gain full digital access and control up to each radiator for antennas in their network. Alarms can also be sent out to the operators notifying them of problems. · AAS is an enabler for SON as it delivers the “hooks” for a SON system to actually optimize the network.

1 Full AAS refers to an AAS which has one transceiver element per antenna element. There are also Semi AAS, where many antenna elements are served by one (bigger) transceiver. These types of AAS generally have worse self- healing performance than full AAS.

· Up to 65 % capacity improvement in high band, 40 % in low band, fewer small cells· Special coverage solution for e.g. outdoor high rise building coverage

· OPEX saving: MTBF of > 500 k hours, still no outage· In line with market leading requirements, site visit is still a recommendation, not mandatory

For the purpose of this paper we want to emphasize the tilt-per- standard feature of AAS as it allows a different tilt for each stand- ard that is transmitted and received by the antenna. When running GSM and LTE together in the 1800 MHz band, the tilt-per-standard feature allows a different tilt for each of the standards and there-fore completely independent optimization.

Beam Forming and Electronic Tilt

Self-Healing

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In order to fulfill this functionality, an AAS needs to allow electronic tilting over at least a 16 degree range between maximum and minimum tilt angle. Within this tilt range, the upper side lobes should be suppressed sufficiently in order to avoid interference with neighboring cells. Side lobe suppression (SLS) of 17 dB is generally seen as sufficient. Finally, the power of the main lobe should not change significantly over the tilt range to maintain maximum efficiency. Ubidyne’s AAS solutions fulfill these requirements and should therefore help operators with multi-standard requirements.

LTE 1800 Co-Site with GSM 1800 Simulation Strategy

For this paper, an LTE rollout into an existing GSM network in the 1800 MHz band is simulated using Mentum Planet [7] and Symena’s Capesso [8] net-work planning and optimization tools. The rollout scenario is simulated for the Paris metropolitan area. A map of the simulated area together with positions of the antennas is shown in Figure 2. The full simu-lated network consists of 700 sites. However, further simulations only take into account the inner 315 sites marked by the red line in Figure 22 and consider the outer network as interferer.

In the simulated scenario, it is assumed the operator owns 20 MHz of spectrum in the 1800 MHz band which is initially used for GSM-only. The intention is to introduce LTE in a 10 MHz bandwidth. Therefore, 10 MHz of the 20 MHz bandwidth is “cleared” for LTE and the other 10 MHz used for GSM.

2 The area within the red border was derived from GSM coverage border as shown in Figure 3.

Figure 2: Overview of the inner network covering 315 sites

Figure 1: The base station evolution

Passive antenna with RRH

CPRI/OBSAI/ORI

Active Antenna System

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Initially, the baseline has been defined. For this purpose, the 20 MHz spectrum of the GSM 1800 network has been reduced to 10 MHz and then optimized to reach the optimal performance as close as possible to the larger spectrum.

3 In reality, AAS will have higher EIRP compared to passive antennas as there will be no feeder loss.

2 The area within the red border was derived from GSM coverage border as shown in Figure 3.

Network planning tools:

Propagation Model: Technologies: Spectrum:

City:

Analysis Area:

GSM Frequency Plan:

EIRP (for active and passive antenna)3

Simulation Environment:

Mentum Planet 5.3/5.4, Capesso 4.17.2 (ACP)

Okumura Hata based model

GSM 1800/ LTE 1800 (FDD)

20 MHz is used for the original network. The spectrum was divided to 10 MHz for GSM and 10 MHz LTE carrier with 200 kHz guarding channel.

Paris (artificial network)

793.91 km²

Frequency reuse, no frequency hopping is used.

61.75 dBm

Outdoor

Table 1: Parameters and constraints of the network simulation

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GSM 315 sites, LTE 315 sites

GSM 315 sites, LTE 240 sites

Passive antennas

Scenario 1

Scenario 3

AAS

N/A

Scenario 2

Table 2: Scenario overview

After the initial baseline has been established, LTE is introduced into the network. Here, three different scenarios are simulated:

1. LTE is introduced into the GSM network using passive antennas. Both GSM and LTE use the same tilt.

2. LTE is introduced into GSM network using active antennas (different tilt for GSM and LTE). The aim is to give the best network coverage for LTE with the fewest sites possible. The tilt for LTE is fixed at 2°.

3. For comparison purposes, it is also investigated what happens to the network of scenario 2 using only passive antennas, without the benefit of tilt-per-standard (i. e. same tilt for GSM and LTE).

Study Results

Baseline networkThe GSM baseline network receive quality indicator is RxQual < 4 for 96 % of the area used for the inves-tigation. The coverage is simulated to be 99.7 % of the area with a receive power threshold of -102 dBm. The coverage map is shown in Figure 3.

Since in the further analysis we will look into the benefits of the feature tilt-per-carrier, the distribution of tilts used is shown in Figure 4 (blue graph). As it can be seen, the average tilt for GSM is about 4°.

Figure 3: Baseline network GSM coverage (315 sites)

Figure 4: Tilt distribution for GSM baseline network

Prob

abili

ty (%

)

Tilt angle (°)

100 %

90 %

80 %

70 %

60 %

50 %

40 %

30 %

20 %

10 %

0 %

GSM baseline tiltScenario 2 LTE tilt

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-2 -1 0 1 2 3 4 5 6 7 8 9 10 11

Scenario 1

One 10 MHz LTE carrier is introduced into the net-work using standard passive antennas. All 315 sites are upgraded. As a passive antenna does not allow separation of tilts for GSM and LTE, the GSM tilt is also applied to the LTE network. A coverage map is shown in Figure 5. The network simulation results are summarized in Table 3 (scenario 1).

As expected, the coverage for GSM stays the same as for the baseline network at 99.7 %. For the newly introduced LTE, the coverage is simulated to be 89 %. The tilt range as given in Figure 4 varies from -2° to +10°.

Figure 6: Scenario 2 network LTE coverage (240 sites)

Scenario 2

In this scenario, the coverage improvement impact of AAS is considered. It is therefore expected for the amount of LTE upgrades to reduce, meaning not all of the 315 sites need to be upgraded to LTE.

Again, the GSM tilt is kept the same as in the baseline simulation. However, because of the additional degree of freedom an active antenna adds, it is now possible to have different tilts for the LTE carrier. In this scenario, the LTE tilt is fixed at two degrees in order to simplify this roll-out scenario. This corres-ponds to the orange graph in Figure 4. It needs to be remarked that there is further room for performance optimization of this scenario 2 due to the assumption of a single tilt of 2° for all sites.

The optimization target for this scenario is to achieve LTE coverage similarly to that achieved in scenario 1, but with fewer sites. Indeed, the simulation shows coverage of 90 % (coverage graph in Figure 6 and summary of results in Table 3) with only 240 sites rather than 315, a 24 % reduction in sites.

Figure 5: Scenario 1 network LTE coverage (315 sites)

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Figure 7: DL data rate for scenario 2

Figure 7 shows the down-link data rate for this sce-nario. The blue curve shows the statistical cumulative distribution of the LTE data rate of scenario 2, the red curve corresponds to the cumulative distribution of the down-link data rate for LTE of scenario 1. It can be seen that the scenario 2 data rate is slightly lower than the data rate of scenario 1. For an new LTE roll-out, MNOs seek initially to gather traffic by providing coverage and later to improve the capacity. With this background, the throughput still is good enough for an initial network roll-out and MNOs can gain a benefit of 24 % reduction in sites.

Scenario 3

For comparison, it is interesting to see what happens if only 240 sites are rolled out in the passive antenna case. This is summarized in Table 3 (scenario 3). In this case, the coverage simulation for LTE shows a reduction to 84 %. This means using active antennas for a roll-out as shown in scenario 2, gains 6 % in coverage over a roll-out using passive antennas. As a conclusion, passive antenna systems with the limita-tion of fixed tilts per carrier or standard drive the cost of initial network roll-outs as demonstrated in the simulated scenarios. This leads also to the conclusion that there is headroom in scenario 2, since only 240 out of the 315 available sites have been upgraded. With the implementation of active antennas, MNOs therefore can save sites to be upgraded in an early roll-out phase, can improve the network capacity by adding LTE towards non-equipped sites easily in a later phase and by that get better performance at same coverage requirements.

Figure 8: Scenario 3 network LTE coverage (240 sites)

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52,

54,

56,

58,

510

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,548

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,552

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100 %

90 %

80 %

70 %

60 %

50 %

40 %

30 %

20 %

10 %

0 %

DL data rate (Mbps)

CDF

area

(%)

240 LTE sites with roll out tilt (2°)315 sites GSM tilt

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4 All coverages calculated with RxLevel (GSM)/Reference Signal Strength (LTE) threshold at -102 dBm

Passive antenna

315 sitesRxQual ≤ 4 for 96 % of areaCoverage4 = 99.7 %Tilt range: -2° to +10°

SCENARIO 1:

GSM:315 sitesCoverage = 99.7 %Tilt: Same as baseline

SCENARIO 3:

GSM:315 sitesCoverage = 99.7 %Tilt: Same as baseline

Active antenna

N/A

SCENARIO 2:

GSM:315 sitesCoverage = 99.7 %Tilt: Same as baseline

BASELINE: GSM (10 MHz bandwidth and tilt optimized)

GSM + LTE 10 MHz bandwidth each (full 315 sites)

GSM+LTE 10 MHz bandwidth each (reduced sites for LTE)

Table 3: Summary of findings

LTE:240 sitesCoverage = 90 %Tilt: 2° (fixed)

LTE:315 sitesCoverage = 89 %Tilt: Same as baseline

LTE:240 sitesCoverage = 84 %Tilt: Same as baseline

Conclusions

It has been shown that the independent-tilt-per-standard feature of Ubidyne’s AAS has significant benefits for ope-rators running different standards on the same spectrum. This manifests itself either in a lower number of sites that need to be upgraded to LTE or a higher network capacity when upgrading all sites as compared to a network using only passive antennas.

Therefore, CAPEX and OPEX savings are possible with AAS in an initial phase of the network roll-out, which is critical for MNOs financials (huge investment in a new techno- logy, without revenue from those services yet). If, in the initial roll-out phase, the operator chooses to upgrade only a subset of the GSM sites to LTE (24 % savings) with AAS, it is still possible to achieve the same coverage as compared to a network with passive antennas. Also, less LTE sites mean lower operating expense of the overall network because of reduced maintanance and power requirements.

At a later phase, when the capacity demand in the LTE network increases, the operator can still choose to upgrade the remaining GSM-only sites, but will have deferred that investment and provide higher capacity to customers. Also, there is a possibility to switch off the additional LTE sites during times of lower capacity demand (e. g. at night) which will deliver further savings on operating expenses.

There is more to gain from other AAS features such as separate Rx/Tx tilt and self-healing capabilities that have not been taken into account in this paper, but which will have further CAPEX and OPEX benefits for MNOs. Just by looking at the independent-tilt-per-standard feature, we have shown a tremendous benefit for an operator using AAS when upgrading a GSM network towards GSM and LTE in the same spectrum. These additional features will further improve the network, provide more freedom to MNOs in regards to optimization and coverage solutions and offer the opportunity for full digital remote control and automization of networks (SON).

References

[1] CommScope: “Active Antennas: The Next Step in Radio and Antenna Evolution”[2] GSA Report: “Embracing the 1800MHz opportunity: Driving mobile forward with LTE in the 1800 MHz band”[3] http://www.telecoms.com/30086/lte1800-lte-deployments-in-1800mhz-band/[4] http://www.cellular-news.com/story/53499.php[5] http://en.wikipedia.org/wiki/LTE_(telecommunication)#Features[6] “LTE Strategies for MNOs” presentation at the LTE World Summit 2012 by Frederic Pujol of idate.org[7] Mentum Planet for LTE http://www.mentum.com/index.php?page=lte-technology-module&hl=en_US[8] http://www.symena.com

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About Ubidyne

Ubidyne is the leader in active antenna technology for wireless communications. The company’s break-through active antenna architecture provides the highest integration possible enabling OEMs to improve time to market and reduce development effort while enabling mobile network operators to reduce network complexity, reduce total cost of ownership and provide innovative network optimization features.

Ubidyne was founded by mobile phone and other industry insiders in 2006. It is registered in Delaware (USA) and has its R&D center and main office in Ulm (Germany) as well as a sales office in Shanghai (China).

If you are interested in Ubidyne’s technology or products please contact us at marketing@ubidyne.com or visit our website www.ubidyne.com

Magirusstrasse 43 I D-89077 Ulm I Germany

Information provided in this document is proprietary to Ubidyne GmbH. Ubidyne reserves the right to alter this description in the course of improving its products and business. Detailed technical information can be found in the specification documents. Ubidyne assumes no responsibility for any errors that may appear in this document. Contracts, price details, delivery schedule and options selected are covered by separate agreements. Copyright © 2012 Ubidyne. All rights reserved.