Cordless Systems and Wireless Local Loop

Class Contents

Cordless systems.Time Division DuplexDECT Frame FormatDECT Operation

Wireless Local LoopRole of WLL

Class Contents

Propagation Considerations for WLLMultichannel Multipoint Distribution ServiceLocal Multipoint Distribution Service

IEEE 802.16 Fixed Broadband Wireless Access Standards IEEE 802.16 Architecture IEEE 802.16 Services

Cordless Systems

Technology used to bring wireless access into the residence or office

Cordless Telephone Technology Digital Cordless Telephones and

Standards

Cordless Systems Standards

Developed to widen the range of capabilities in two directionsMultiple User Support (single BS)Operation Environments

Residential (voice and data) Office (voice and data or use of cellular

configuration with PBX (private branch exchange switch for multiple users (hundreds or thousands))

Telepoint (BS in public place)

Cordless Systems Standards – Considerations that drive designs Modest Range of the handset to BS (200 m).

Power less by an order of magnitude with respect to cellular systems

Inexpensiveness of the handset and BS. (This dictates the use of simple technical approaches)

Limitation in Frequency Flexibility.

Cordless Standards

Digital Enhanced Cordless Telecommunications (DECT) - Europe

Personal Wireless Telecommunications (PWT) – US

Approach used: Time Division Duplex (TDD)

DECT and PWT characteristicsDECT PWT

Bandwidth 20 MHz 20 MHz

Band 1.88 to 1.9 GHz 1.91 to 1.92 GHz

Access Method TDD/TDMA/FDMA TDD/TDMA/FDMA

Carrier Bandwidth 1.728 MHz 1.25 MHz

Number of Carriers 10 8

Channels per Carrier 12 12

Handoff Yes Yes

Transmitted data rate 1.152 Mbps 1.152 Mbps

Speech rate 32 kbps 32 kbps

Mean output power 10 mW 10 mW

Peak output power 205 mW 90 mW

Maximum Cell radius 30 to 100 m 30 to 100 m

Time Division Duplex

Also know as TCM (Time Compression Multiplexing)

Data are transmitted one direction at a time

Alternation is made on transmissions in both directions

Time to send 1 block:

Block Transmission Rate:

Effective data rate (block of B bits)

Time Division Duplex

gpb TTTt

gpb TTT 2

1

gpb TTT

BR

2

Actual data rate on the medium:

Combining with data rate for a block B

TDD

The actual data rate on the link is more than double the effective data rate seen by the two sides

bT

BA

b

gp

T

TTRA 12

TDD – Choice of Block Size

Block size is a compromise between competing requirements:

If B is increased, the actual data rate, A, decreases (implementation becomes easier)

If B is increased, so is the signal delay due to buffering (undesirable for voice traffic)

DECT – Frame Format Preamble (16 bits): Serves to alert receiver and allow it to “warm

up”. Sync (16 bits): Used to enable the receiver to synchronize on the

beginning of the time slot. A field (64 bits): Used for network control. B field (320 bits): Contains user data X field (4 bits): Consists of four parity check bits, which enables

terminals and base stations to monitor the quality of signal transmission.

Guard (60 bits): This is a 52-s guard time, corresponding to Tg.

DECT - Operation

Protocol Architecture: physical layer: data

are transmitted in the TDMA-TDD frames over one of 10 RF carriers.

MAC layer: selects the physical channel and then establishes or releases connections on those channels

DECT - Operation MAC layer services:

Broadcast (field A) Connection Oriented

(Transfer of user data in field B)

Connectionless (support individual DECT messages in field A)

Data Link Control Layer: Provides for reliable

transmissions using data link control procedures (error detection and ARQ)

DECT - Operation Services Above data link control layer:

Call control: Manages circuit switched calls, including connection set-up and release.

Supplementary Services: Services independent of any call that supports operation.

Connectionless message service: Support connectionless messages.

Connection-oriented message service: Support of connection-oriented messages.

DECT - Operation Mobility Management: Handles functions necessary for

the secure provision of DECT services. Mobility management is organized into seven groups of

services: Identity procedures: Used for the mobile unit to identify itself to the

BS Authentication procedure: Establishes that the mobile unit is a

valid network user Location procedure: Used in systems with multiple base stations

to track location of mobile unit. Access rights procedure: Establishes that the mobile unit has the

right to gain access to a specific type of local or global network. Key allocation procedure: Used to exchange information about

the parameters of the mobile unit and network operation. Ciphering-related procedure: Encryption and decryption

operations.

Wireless Local Loop

Traditional end-user connection (local loop or subscriber loop): provided by wired systems (twisted pair, coax, optical fibre)

Subscriber Demand in capacity (Internet support in particular) has rendered twisted pair technology inadequate

Wireless Technology for subscriber access: WLL (Wireless Local Loop) or fixed wireless access.

Entity Technology Application

Telephony Broadcast Computer

Public telephone Twisted pair, ISDN, xDSL

One and two lines Video on demand High-speed asymmetrical access

Cable Operator Coaxial Cable One and two lines 50+ channels High-speed asymmetrical

Cellular provider Cellular and Cordless

One line No Limited but mobility

3G Cellular provider

Cellular One line No High-speed asymmetrical access

Narrowband WLL operator

Wireless Two lines No 64-kbps access

Broadband WLL operator

Wireless Yes 50+ channels High-speed asymmetrical or symmetrical access.

Terrestrial Broadcast

Analogue and Digital TV

No 5 to 10 channels Some download potential

Satellite Broadcast Analogue and Digital

No 50+ channels No

End User Access Alternatives

WLL Division in cells, each with its own antenna.

Subscriber has fixed antenna in LoS with BS

Link between BS and switching centre

ISP may be connected at the switch or to the switch by a high-speed link.

Two Level Hierarchy

WLL Vs Wired Solutions

Cost: Wireless systems are less expensive than wired systems.

Installation time: WLL systems typically can be installed rapidly. The key stumbling blocks are obtaining permission to use a given frequency band and finding a suitable elevated site for the BS antennas.

Selective installation: Radio units are installed only for those subscribers who want the service at a given time. With a wired system, cable is laid out in anticipation of serving every subscriber in a local area

WLL Advantages:

WLL – Propagation Considerations Frequency Allocated for WLL systems: 2 to 40

GHz (millimetre wave region).

Reasons for use: Wide range of unused frequencies above 25 GHz Wide channel BW (higher data rates) Small size transceiver

Disadvantages Free Space Loss Increases Rainfall attenuation is considerable Multipath losses can be high.

WLL – Line of Sight Considerations

Obstructions must be avoided along or near the LoS.

There should be a space around the LoS path which should be clear of obstacles.

Used Criterion: First Fresnel Zone.

Fresnel ZonesThe definition of Fresnel Zones is based on the theory that any small element of space in the path of an electromagnetic wave may be considered the source of a secondary wavelet, and that the radiated field can be built up by the superposition of all these wavelets.

On the basis of this theory, it can be shown that objects lying within a series of concentric circles around the direct line of sight between two transceivers have a constructive or destructive effect on communications

Fresnel Zones Objects that fall within the first circle, the first

Fresnel zone, have the most serious negative effects

KmKm

KmKm

GHzm DS

DS

fR

1

3.17

It has been found thatif there is no obstructionwithin about 0.6 times the radius of the firstFresnel Zone, at any pointbetween the two transceivers, the attenuation due to obstructions is negligible.

Atmospheric Absorption

there is a favourable window for communication roughly from 28 GHz to 42 GHz, where the attenuation is on the order of 0.13 dB/Km, and another favourable window from 75 GHz to 95 GHz, where the attenuation is on the order of 0.4 dB/Km.

Abobe 10 GHz, Radio Waves are subject to molecular absorption

Absorption as function of frequency is very uneven

Atmospheric Absorption Graph only shows absorption at an atmospheric pressure of 1013

mb at 150C with a water vapour concentration of 7.5 g/m3.

Graph Shape remains constant, but values change drastically with temperature and relative humidity

Temperature (0C) \ Rel.Humidity 0 % 50 %

100 %

00 0.02

0.05 0.08

100 0.02

0.08 0.14

200 0.02

0.12 0.25

300 0.02

0.20 0.44

400 0.01

0.33 0.79

Clear Air AbsorptionAt 28 GHz in dB/Km

Effect of Rain

Rain is one of the most serious concerns for millimetre wave propagation

The presence of raindrops can severely degrade the reliability and performance of communications links and, during periods of heavy rain, may outweigh all other factors

Effect of Rain

Formula for estimation of attenuation due to rain (dB/Km):

Rain Rate (R) measured in mm/hr a and b depend on the distribution of drop sized

on frequency and polarization of electromagnetic wave

bRaA

Effect of Rain and PolarizationFrequency

(GHz)ah av bh bv

1 0.0000387 0.0000352 0.912 0.880

2 0.000154 0.000138 0.963 0.923

6 0.00175 0.00155 1.308 1.265

10 0.0101 0.00887 1.276 1.264

20 0.0751 0.0691 1.099 1.065

30 0.187 0.167 1.021 1.000

40 0.350 0.310 0.939 0.929

50 0.536 0.479 0.873 0.868

Effect of Rain – Zone Climate

Rainfall Intensity Exceeded (mm/hr) for various regions

Approaches for WLL Most Interesting approaches are: MMDS and

LMDS

Multichannel Multipoint Distribution Service (MMDS): Can be used to support two-way services. It is an alternative for broadband data services such as Internet access. MMDS has been used to compete with cable TV providers and to provide service in rural areas not reached by broadcast TV or cable. For this reason MMDS is also known as wireless cable.

Approaches for WLL

Local Multipoint Distribution Service (LMDS): Relatively new WLL service used to deliver TV signals and two-way broadband communications, operating at millimeter frequecies.

Frequency (GHZ) Usage

2.1500 to 2.1620 Licensed MDS and MMDS; two bands of 60 MHz each

2.4000 to 2.4823 Unlicensed ISM

2.5960 to 2.6440 Licensed MMDS; eight bands of 6 MHz each

2.6500 to 2.6560 Licensed MMDS

2.6620 to 2.6680 Licensed MMDS

2.6740 to 2.6800 Licensed MMDS

5.7250 to 5.8750 Unlicensed ISM-UNII

24.000 to 24.250 Unlicensed ISM

24.250 to 25.250 Licensed

27.500 to 28.350 Licensed LMDS (Block A)

29.100 to 29.250 Licensed LMDS (Block A)

31.000 to 31.075 Licensed LMDS (Block B)

31.075 to 31.225 Licensed LMDS (Block A)

31.225 to 31.300 Licensed LMDS (Block B)

38.600 to 40.000 Licensed

ISM = Industrial, Scientific and Medical

LMDS = Local Multipoint Distribution Service

MDS = Multichannel Distribution Service

MMDS = Multichannel Multipoint Distribution Service

UNII = Unlicensed National Information Infrastructure

Fixed wireless communications bands (FCC allocation)

Comparing MMDS and LMDS Advantages of MMDS over LMDS

MMDS signals have larger wavelength (greater than 10 cm) and can travel farther without losing significant power.

MMDS can operate in considerably larger cells, thereby lowering base station equipment costs.

Equipment at lower frequencies is less expensive, yielding cost savings at both subscriber and base station.

MMDS signals don’t get blocked as easily by objects and are less susceptible to rain absorption.

Comparing MMDS and LMDS Advantages of LMDS

Relatively high data rates, in the Mbps range.Capable of providing video, telephony, and

data.Relatively low cost in comparison with cable

alternatives .

Comparing MMDS and LMDS

Disadvantage of MMDSLess bandwidth. Residential subscriber are

principal users Disadvantage of LMDS

Short range from BS (larger number of BS required to service a given area).

IEEE 802.16 Fixed Broadband Wireless Access Standards

A need was recognized within the Industry to develop standards for LMDS WLL.

IEEE 802 committee set up the 802.16 working group in 1999 to develop broadband wireless standards.

About the standards:

The charter for the group is to develop standards that:Use wireless links with microwave and

millimetre wave radiosUse licensed spectrum (typically)Are metropolitan in scale

About the standards:

Provide public network service to fee-paying customers (typically)

Use point-to-multipoint architecture with stationary rooftop or tower-mounted antennas

Provide efficient transport of heterogeneous traffic supporting quality of service (QoS)

Are capable of broadband transmission ( 2 Mbps)

In essence, IEEE 802.16 standardizes the air interface and related functions associated with LMDS.

Working Groups IEEE 802.16.1: Air Interface for 10 to 66 GHz IEEE 802.16.2: Coexistence of Broadband Wireless

Access Systems IEEE 802.16.3: Air Interface for Licensed frequencies, 2

to 11 GHz

IEEE 802.16 Architecture An 802.16 wireless service provides a

communications path between a subscriber site, which may be either a single subscriber device or a network on the subscriber’s premises and a core network.

Examples of core networks are the public telephone network and the Internet.

IEEE 802.16 Architecture

In OCI terms, higher layer protocols are independent of network architecture.

IEEE 802.16 is concerned with the lowest two layers of the OSI model:Physical LayerMedium Access Control (MAC) layer

IEEE 802.16 Architecture

Physical Layer Functions: Encoding/decoding signals Preamble generation/removal (synchronization) Bit transmission/reception

IEEE 802.16 Architecture

Transmission Layer: Choice of transmission medium and frequency band

are critical in wireless and must be specified.

IEEE 802.16 Architecture

Medium Access Control Layer (MAC) – service to subscribers: On transmission, assemble data into a frame with address and

error detection fields. On reception, disassemble frame, and perform address

recognition and error detection. Govern access to wireless transmission medium

IEEE 802.16 Architecture

Convergence Layer: Provides functions specific to the service being provided: Encapsulate PDU framing of upper layers into the native 802.16

MAC/PHY frames. Map an upper layer’s address into 802.16 addresses Translate upper layer QoS parameters into native 802.16 MAC format. Adapt the time dependencies of the upper layer traffic into the

equivalent MAC service

IEEE 802.16 Services

Requirements for the IEEE 802.16 standards are defined in terms of bearer services that the 802.16 system must support.

A bearer service refers to the type of traffic generated by a subscriber network or core network

IEEE 802.16.1 Bearer Services

Digital audio/video multicast: Transports one way digital audio/video streams to subscribers

Digital telephony: Supports multiplexed digital telephony streams

ATM: Provides a communications link that supports the transfer of ATM cells as part of an overall ATM network. The 802.16 link must support the various QoS services defined for ATM

Internet protocols: Supports the transfer of IP datagrams. The 802.16 link must provide efficient timely service.

IEEE 802.16.1 Bearer Services

Bridged LAN: A bridge LAN service enables transfer of data between two LANs with switching at the MAC layer.

Back-haul: For cellular or digital wireless telephone networks. An 802.16 system may be a convenient means to provide wireless trunks for wireless telephony base stations.

Frame relay: Similar to ATM. Frame relay uses variable-length frames in contrast to the fixed-length cells of ATM.

IEEE 802.16.3 Bearers Services

Voice Transport: A packet-based (as opposed to circuit switched) service that provides equivalent service to that of the PSTN.

Data Transport: Provides support for IP-based traffic, including IP-based QoS requirements.

Bridged LAN: Similar to IP-based support. A bridged LAN service enables transfer of data between two LANs with switching at the MAC layer.