Basic Repeater Presentation

Repeaters in wireless networks: from coverage to capacity Dr Mike Lewis SDR repeater architect

www.axellwireless.com

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Topics • Repeaters in wireless systems • Design challenges in repeaters • Capacity-centric versus coverage-centric design


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Repeaters in wireless systems


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“Gap-filling” coverage extension Donor Antenna Service Antenna seitilicaF laireA detimiL

Donor BTS Repeater


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Tunnel coverage Donor BTS

Repeater located externally or in equipment room Donor Antenna

Aerial Facilities Limited

Radiating cable in tunnel


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In-building coverage Donor BTS Direct signal from the BTS cannot penetrate the building; signal is distributed within the building via panel antennas and radiating cable

Donor antenna and remote repeater

80% of mobile phone calls originate in buildings! www.axellwireless.com

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Fibre-fed systems (active DAS)

Repeater

Fibre input

7/16 type Connect or

Indoor 1

Repeater

Fibre input


Indoor 2

Lightning protection EMP

Hydrid

Controller HI B

2 x 30dB Coupler

POWER SUPPLY

POWER SUPPLY +5V +10 V +15 V

+5V +10 V +15 V

c

Master Unit

Controller

c

HI B



Repeater

Fibre input


Indoor 3

Repeater

Fibre input



Indoor 4

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In building Coverage with active DAS


On-board repeaters


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Why use repeaters? • Why not just install another base station? – Cheaper! Cost of a base station may not be motivated if not enough users being served. • Rural areas / coverage black spots • Roll-out of new technology with few users

– No need for data connection to core network • Remote sites (or moving ones!)

– Operator-neutral • Example: coverage for a shopping mall or stadium, installed by building owner

• Multi-hop relaying introduced in LTE – Not clear if this will be cost-effective www.axellwireless.com

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REPEATER DESIGN CHALLENGES


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Basic repeater Downlink path Control logic

ADC

Donor (BTS side) www.axellwireless.com

LNA

MCPA

ADC

Control logic

RMS power

MCPA

LNA

Uplink path

Server (mobile side)

RMS power

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Overlap-zone interference • In overlap area: – Direct signal coming from base station – Delayed signal from the repeater

• Signal from repeater appears as strong ISI – Often find some pathological region where signals almost equal

Donor Antenna

Service Antenna

Donor BTS

Repeater

 Must minimize repeater delay – “Repeater” multipath adds to “real” multipath for receiver to handle •

Goals: < 6µs for GSM, 14µs TETRA

– Delay also has an effect on max cell size (round-trip time)


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TX – RX isolation Path loss -ISOLATION (dB)

GAIN (dB)

• High gain typically desirable (80-90dB) • Finite isolation between transmit and receive antennae 𝐼𝑠𝑜𝑙𝑎𝑡𝑖𝑜𝑛 𝑚𝑎𝑟𝑔𝑖𝑛(𝑑𝐵) = 𝐼𝑠𝑜𝑙𝑎𝑡𝑖𝑜𝑛 − 𝐺𝑎𝑖𝑛

• Isolation margin ≤ 0dB: oscillation • Low isolation margin: self-interference www.axellwireless.com

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Frequency-shifting repeater

F1

900 MHz c

F9 F5 Link Antenna

RF Link Path F1

1800 MHz

F5

Link Antenna

900 MHz

Remote Repeater

Donor Repeater

BTS

• •

Frequency shift gives very large isolation (due to filter rejection) Allows the use of omni antennas for coverage


Repeaters versus base station design • Both repeaters and base stations need to meet similar regulatory requirements • Base station is master of system timing – Processing delay allowed for as part of mobile standards – Repeater has to minimize delay

• Power control algorithms seek to normalize power into base station – Repeater located “out in the cell” and can see a wide range of signal levels


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Repeaters versus base station design • Base station works with baseband data – Can perform various data-aided algorithms to compensate for non-ideal RX path, or precompensate for TX path – Repeater has (almost) no knowledge of the underlying signals

• Repeater output power levels typically lower than for (macro) base stations – But wide bands, many carriers, high dynamic range and less scope for crest factor reduction


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Digital repeaters • Traditional repeaters have been purely analogue (other than supervision) – Filtering fixed, defined by e.g. SAW filters – Each passband has a separate down / upmixer and IF path

• Proliferation of standards and multioperator scenarios require more flexibility – And need very sharp filtering

GSM filter


G B

WCDMA Filter

G B

GSM Filter

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Traditional repeater Downlink path Control logic

ADC

Donor (BTS side) www.axellwireless.com

LNA

MCPA

IF stages replicated once per passband ADC

Control logic

RMS power

MCPA

LNA

Uplink path


RMS power

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Digital repeater Downlink path

Donor (BTS side)



Uplink path

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Advantages of digital repeater • Easy to adapt to new requirements – FPGA-based processing is a “blank sheet” – Filter bandwidths, frequencies all flexible

• Better support for multiple passbands – Cheaper, smaller, lower power

• Filtering numerically defined – Can implement stable, sharp filters


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Advantages of digital repeater • Access to signal in digital form – Easy to perform measurements on signal, implement gain control algorithms – Can use digital techniques to correct for analogue imperfections • Digital pre-distortion for PA linearization • Feedback cancellation • …


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Disadvantages of digital repeater • High performance A/D and D/A needed – Wide signal band, and high-IF sampling used due to image rejection issues • Zero-IF not practical: I/Q compensation not enough due to high gain from input to output

– High dynamic range

• Aliasing needs to be managed • Processing delay of FPGA needs to be carefully managed … but high performance mixed signal components and FPGAs now affordable. www.axellwireless.com

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CAPACITY-CENTRIC VERSUS COVERAGE-CENTRIC DESIGN


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The capacity problem… • Coverage used to be the main focus of mobile operators – Explosion of data usage is changing that

• Need enough carrier frequencies to meet bandwidth requirements – Or users get kicked off onto older technologies

• Only solution: have denser base station sector coverage – Higher frequency re-use, fewer users per sector www.axellwireless.com

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Base station hotels – the solution to site acquisition issues

• Shortage of suitable sites • Requirements on air conditioning • Transmission needs • Many base stations will be needed in the mature network



Phase 1: few users www.axellwireless.com


Phase 2: more users



Bill Boards

Phase 3: many users www.axellwireless.com

London 2012 • Axell providing a coverage system across the park for both Cellular and public safety • There is one BTS hotel for the 4 operators with 600 base stations in it • Solution involves over 300 fibre-fed repeaters across the campus


Venues for Axell Wireless

Possible new

Current orders for the

requirements

Olympic park


Repeaters versus RRHs • Remote radio heads: base station transceiver at end of digital fibre link – How can a repeater-based distributed antenna system compete? – Also: micro / nano / pico base stations?

• Multi-operator support! – For in-building applications, stadia, tunnel systems: typically the site owner installs the equipment. – Don’t want to install separate equipment per operator: costly and space issues www.axellwireless.com

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Conclusions • Traditional repeaters will always have their place in wireless networks, but… • Distributed antenna systems blur the borders between the base station and repeater network • Requirement to support multiple operators will drive DAS business • Managing capacity the next big challenge


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THANK YOU!


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Basic Repeater Presentation

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