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An Efficient Residential LTE Small Cell using a “Designated” Wireless Local Loop Band
2017 ETCM
Second Ecuador Technical Chapters Meeting Salinas – Ecuador October 2017
Authors and their Institutions An
Efficient Residential LTE Small Cell using a “Designated” Wireless Local Loop Band
Technicolor Luis
Montalvo, Ph.D. Eric Gautier
Escuela Politécnica Nacional Darin
Sayed Thomas Borja Cecilia Paredes, Ph.D. Ivan Bernal, Ph.D. (Presenter) ETCM 2017
Presentation of Technicolor
Technicolor Rennes (France)
A pole of excellence in creation, distribution and content access technologies. It is the largest R&D Center which regroups more than 500 talented employees coming from 25 different nations.
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Escuela Politécnica Nacional
Founded in 1869, it is the oldest institution for technical education in Ecuador.
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Motivation
Traffic demand for broadband services, such as high definition video content, is growing exponentially.
Cost effective solutions to address this demand are needed.
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Motivation
The spectrum is a limited, expensive and regulated resource. Mobile Network Operators are focusing on technologies that will use the 5 GHz band.
The spectral efficiency achieved by old technologies is very low.
Digital Enhanced Cordless Telecommunications (DECT) is such a technology. ETCM 2017
DECT
DECT is a leading standard for short-range digital cordless telecommunications.
It is described in a series of standards published by ETSI. In many cases, it is just a single telephone handset and a single base station (BS) connected to the external interface of the access network. DECT uses the 1880/1900 MHz band in Europe.
License-free band. Countries outside Europe have allocated the 1900/1920 and 1910/1930 MHz bands to DECT-similar services.
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Proposed Idea
Re-farm the DECT band, at the residential level, to optimally share it between:
A short range cordless telephony service for a few DECT handsets; and, A residential LTE small cell service for mobile devices, such as smart-phones and tablets.
The simplest implementation of our proposal:
A short-range cordless telephony service for up to 12 DECT handsets; and, An LTE small cell capable to provide user data bitrates from 45 Mbps to 450 Mbps. ETCM 2017
Points backing up our proposal
User habits are shifting away from fixed telephony towards wireless services using smart-phones. The current trend:
Move away from standard voice calls to instant messaging and VoIP using applications such as Skype, WhatsApp, even when the user is at home. In the USA, two out of five homes are equipped only with cellular phones.
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Points backing up our proposal
DECT market slightly grows because of new applications like home automation, home security or climate control. Contrary, LTE users are expected to grow exponentially.
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Basics of DECT
The Customer Premises Equipment (CPE) that provides Internet Services to the end user usually includes a DECT base station (BS) to support short-range cordless telephony service for DECT handsets at home.
The CPE may also contain a WiFi AP for wireless broadband access.
In our paper, we do not include WiFi in our discussion as we do not modify it.
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DECT Time-Frequency Grid DECT divides the 20 MHz - bandwidth into ten 2 MHz channels. Each 2 MHz channel is structured into 10 ms frames capable of carrying 12 digital conversations.
In most of the residential configurations, a user may need one single 2 MHz channel.
18 MHz of precious high quality frequency spectrum is unused in the residential environment.
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DECT Time-Frequency Grid
DECT has two operational modes: FDD and TDD
In this paper we concentrate on TDD.
Each 10 ms frame is divided into 24 slots (ts = 0.42 ms) with two slots allocated to each telephone call.
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DECT Spectral Efficiency 120 simultaneous telephone conversations can be hold with each conversation occupying a duplex channel. DECT Throughput Capacity: 7.68 Mbps
Using 7.68 Mbps as the Capacity and 20 MHz as the bandwidth, the spectral efficiency is 0.384 (bit/s)/Hz per cell. ETCM 2017
Basics of DECT
The initial DECT standard was further developed into NG-DECT (New Generation DECT) standard.
NG-DECT targets VoIP applications while keeping compatibility with all previous developments.
A packet data service was standardized in DECT Packet Radio Service (DPRS). The use of the DECT band is being reengineered for new services.
DECT Ultra Low Energy for applications such as home automation and security and climate control. ETCM 2017
Basics of DECT
In this paper, we consider the simplest DECT standard, knowing that the structure of the Physical Layer remains the same in the most evolved versions of the standard for backcompatibility reasons.
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Basics of 3GPP LTE
LTE Release 10 lists the “E-UTRA operating bands” allocated to FDD and TDD operational modes.
In this paper, we are interested in the E-UTRA Band 35, used for TDD, from 1850 MHz to 1910 MHz, that partially overlaps with the DECT band.
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Basics of 3GPP LTE
LTE has two different multiple access schemes:
OFDMA for the downlink (DL) SC-FDMA (Single Carrier – FDMA) for the uplink (UL)
In this paper we focus on the DL (OFDMA).
However, the allocation of the frequency resources is similar.
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LTE Frequency Organization
In OFDMA, a user is allocated a number of subcarriers for a given amount of time.
The total number of available sub-carriers depends on the available bandwidth (BW). The BW is a subset or a combination of integer multiples from {1.4; 3; 5; 10; 15; 20} MHz.
In LTE, a PRB (Physical Resource Block) is the smallest resource allocation element.
12 consecutive 15 KHz sub-carriers (180 KHz).
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LTE Time Organization
Transmission is organized in frames of 10ms.
Each frame is divided into 10 subframes of 1 ms. A subframe is further divided into 2 slots of 0.5ms. The duration of a PRB is 0.5 ms. The slots consist of either 6 or 7 OFDM symbols.
Depending on whether the normal or extended cyclic prefix is employed.
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LTE Time-Frequency Resource Grid
LTE uses a set of special reference signals (pilots), embedded in the PRBs for carrier offset estimation, channel estimation, timing synchronization etc.
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LTE Cell Throughput Capacity The “Cell Throughput Capacity” is equal to the “Cell Peak Capacity” minus the LTE communications overhead due to the control, reference and synchronization signals, etc. Overhead is commonly estimated as 25% of the “Cell Peak Capacity”. Therefore, “Cell Throughput Capacity” is estimated to be 0.75 times the LTE “Cell Peak Capacity”.
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LTE Cell Throughput Capacity
Duration of one LTE slot is 0.5 ms. RE: Resource Element. Nbits_RE: Number of bits per RE (depends on the type of modulation). NRE_slot: Number of RE in one slot.
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Example of computing “Cell Throughput Capacity”
Consider:
Bandwidth: 20 MHz. Number of PRBs: 100. High order modulation: 256QAM so the number of bits per RE is 8.
So, the number of RE in a 0.5ms slot is 8400. “Cell Throughput Capacity”:
0.75 * 134.4 Mbps = 100.8 Mbps.
If a 4x4 MIMO technique were used, the “Cell Throughput Capacity” could be increased to 403.2 Mbps (4 * 100.8 Mbps). ETCM 2017
Example of computing “Cell Spectral Efficiency”
Consider:
A bandwidth of 20 MHz 256QAM Without MIMO
“Cell Throughput Capacity” is 100.8 Mbps “Cell Spectral Efficiency” is 5.04 (bit/s)/Hz.
It is more than 13 times DECT’s “Cell Spectral Efficiency”.
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PROPOSED DECT RE-FARMING Reserve a fraction of the spectrum to serve the DECT legacy users and re-purpose the remaining portion for the new 3GPP compliant small cell service. The re-farmed part of the spectrum should be assigned to the users of the new service.
Allocate resources in such a way that the QoS suitable for each user application is warranted. Achieved this with a classic appropriate bandwidth manager.
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Static DECT Band Re-farming For optimum coexistence with the legacy DECT users, we reserve the lowest 2 MHz frequency band for DECT and assign the remaining 18 MHz to the new LTE service. Assign the 18 MHz to the new LTE users with linear combinations of integer multiples of bandwidths from the set:
{1.4; 3; 5; 10; 15; 20} MHz. Examples:
6 * 3MHz 3MHz + (3 * 5MHz) 3MHz + 5MHz + 10MHz 1.4 MHz may be used but some BW may be wasted
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Static DECT Band Re-farming
Example> The remaining 18 MHz are assigned to six 3 MHz re-farmed LTE channels.
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Static DECT Band Re-farming The legacy DECT and re-farmed DECT frames have both 10ms duration. However, their respective sub-frames have different duration.
Legacy DECT sub-frame duration is 0.417 ms Re-farmed DECT sub-frame duration (LTE) is 1 ms.
In the previous figure, both frames are shown time aligned, however, this is not required.
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Proposed Idea – Advanced version
A more elaborate implementation would be a dynamic re-farming of the DECT band, with a 2 MHz - bandwidth granularity, so that different residential configurations could be covered:
From devoting the whole DECT band to DECT services. To sharing the DECT band between a short-range cordless telephony service for a maximum of 12 DECT handsets and an LTE small cell with a minimum user data bit-rate capacity of 45 Mbps (same as the simplest implementation) and a maximum of 450 Mbps. ETCM 2017
Dynamic DECT Band Re-farming Dynamically adapt the spectrum assignment between the legacy and re-farmed spectrum as a function of the number of DECT legacy handsets present. The “Dynamic Re-farming” idea could be applied at the frequency dimension only, or at the frequency and time frame dimensions.
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A SMALL CELL BASED ON DECT RE-FARMING
We present two possibilities for applying our proposal to the implementation of a residential 3GPPcompliant LTE small cell.
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A SMALL CELL:
Basic Architecture
We have a macro cell covering a residential small cell. Our proposal concerns only the small cell.
We keep the macro cell as specified by 3GPP.
The base station of the proposed small cell is collocated with the CPE (GW).
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A SMALL CELL:
The UE will have to be modified to include two LTE interfaces:
UE changes
One tuned for the currently specified LTE bands. One tuned for the DECT band (1880/1900 MHz).
The mechanisms used in DECT to associate a handset to a base station could be adapted to our proposal:
A smart-phone is associated to the small cell base station only the first time it is used with the DECT re-farmed small cell.
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A SMALL CELL:
CPE changes
CPE (gateway) has to provide the additional LTE interface for the small cell base station. Gateways already include a DECT base station, but it must be modified to comply with the DECT spectrum refarming and with the legacy DECT standard.
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A SMALL CELL: small cell BS-eNB link changes
The physical link between the residential small cell base station (GW) and the eNB of the macro cell could be fiber or wireless.
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A SMALL CELL: SECOND ALTERNATIVE
DECT Re-farmed and WiFi Aggregation
Our proposal is open to the possibility of using other frequency band aggregation schemes such as: LAA
(Licensed Assisted Access) LWA (LTE WLAN Aggregation) LWIP (LTE Wi-Fi radio level integration with IPsec tunnel).
This solution lets aggregate WiFi, LTE, and the DECT re-farmed bands.
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ANALYSIS OF OUR DECT RE-FARMING PROPOSAL
The DECT re-farmed “Cell Throughput Capacity” is a function of the throughput provided by the DECT re-farmed part plus the throughput provided by the DECT legacy part:
For LTE, for a channel bandwidth of 18 MHz, the number of useful PRBs is 90.
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ANALYSIS OF OUR DECT RE-FARMING PROPOSAL
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Conclusions
Our proposal can help both Fixed and Mobile Network Operators handle the capacity challenge in the residential environment for the next years minimizing harm to the legacy DECT service.
The deployment of our proposal might require approval from the regulator entities.
Our proposal guarantees:
A high quality “Cordless Digital Voice” service to up to 12 DECT handsets A new LTE service capable to provide a maximum user throughput of up to 454 Mbps. ETCM 2017
Conclusions
Better spectral efficiency than the legacy DECT service
A factor of 60 could be obtained (22.7 (bit/s)/Hz compared to 0.384 (bit/s)/Hz in legacy DECT.
Better throughput
A factor of 60 and we could provide additional 450Mbps to the current LTE UE.
Small impact on the mobile operator infrastructure, because no modifications are required to the core network. Possibility of incorporating our proposal into LTE aggregation solutions, such as LAA.
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