Cordless Systems and Wireless Local Loop

Class Contents Cordless systems.  Time Division Duplex  DECT Frame Format  DECT Operation Wireless Local Loop  Role of WLL

Class Contents  Propagation Considerations for WLL  Multichannel Multipoint Distribution Service  Local Multipoint Distribution Service IEEE Fixed Broadband Wireless Access Standards  IEEE Architecture  IEEE 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 directions  Multiple 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 characteristics DECTPWT Bandwidth20 MHz Band1.88 to 1.9 GHz1.91 to 1.92 GHz Access MethodTDD/TDMA/FDMA Carrier Bandwidth1.728 MHz1.25 MHz Number of Carriers108 Channels per Carrier12 HandoffYes Transmitted data rate1.152 Mbps Speech rate32 kbps Mean output power10 mW Peak output power205 mW90 mW Maximum Cell radius30 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

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

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.

EntityTechnologyApplication TelephonyBroadcastComputer Public telephoneTwisted pair, ISDN, xDSL One and two linesVideo on demandHigh-speed asymmetrical access Cable OperatorCoaxial CableOne and two lines50+ channelsHigh-speed asymmetrical Cellular providerCellular and Cordless One lineNoLimited but mobility 3G Cellular provider CellularOne lineNoHigh-speed asymmetrical access Narrowband WLL operator WirelessTwo linesNo64-kbps access Broadband WLL operator WirelessYes50+ channelsHigh-speed asymmetrical or symmetrical access. Terrestrial Broadcast Analogue and Digital TV No5 to 10 channelsSome download potential Satellite BroadcastAnalogue and Digital No50+ channelsNo 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 Zones The 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 It has been found that if there is no obstruction within about 0.6 times the radius of the first Fresnel Zone, at any point between 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 15 0 C with a water vapour concentration of 7.5 g/m 3. Graph Shape remains constant, but values change drastically with temperature and relative humidity Temperature ( 0 C) \ Rel.Humidity 0 %50 %100 % Clear Air Absorption At 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

Effect of Rain and Polarization Frequency (GHz) ahah avav bhbh bvbv

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 to Licensed MDS and MMDS; two bands of 60 MHz each to Unlicensed ISM to Licensed MMDS; eight bands of 6 MHz each to Licensed MMDS to Licensed MMDS to Licensed MMDS to Unlicensed ISM-UNII to Unlicensed ISM to Licensed to Licensed LMDS (Block A) to Licensed LMDS (Block A) to Licensed LMDS (Block B) to Licensed LMDS (Block A) to Licensed LMDS (Block B) to 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 MMDS  Less bandwidth. Residential subscriber are principal users Disadvantage of LMDS  Short range from BS (larger number of BS required to service a given area).

IEEE Fixed Broadband Wireless Access Standards A need was recognized within the Industry to develop standards for LMDS WLL. IEEE 802 committee set up the 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 radios  Use 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 standardizes the air interface and related functions associated with LMDS.

Working Groups IEEE : Air Interface for 10 to 66 GHz IEEE : Coexistence of Broadband Wireless Access Systems IEEE : Air Interface for Licensed frequencies, 2 to 11 GHz

IEEE Architecture An 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 Architecture In OCI terms, higher layer protocols are independent of network architecture. IEEE is concerned with the lowest two layers of the OSI model:  Physical Layer  Medium Access Control (MAC) layer

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

IEEE Architecture Transmission Layer:  Choice of transmission medium and frequency band are critical in wireless and must be specified.

IEEE 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 Architecture Convergence Layer: Provides functions specific to the service being provided:  Encapsulate PDU framing of upper layers into the native MAC/PHY frames.  Map an upper layer’s address into addresses  Translate upper layer QoS parameters into native MAC format.  Adapt the time dependencies of the upper layer traffic into the equivalent MAC service

IEEE Services Requirements for the IEEE standards are defined in terms of bearer services that the system must support. A bearer service refers to the type of traffic generated by a subscriber network or core network

IEEE 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 link must support the various QoS services defined for ATM Internet protocols: Supports the transfer of IP datagrams. The link must provide efficient timely service.

IEEE 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 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 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.