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MICROWAVE LINK (SDH & PDH)

Microwave transmission

Microwave transmission refers to the technique of transmitting information over microwave frequencies, using various integrated technologies. The portion of the microwave spectrum called millimeter wave is highly susceptible to attenuation by the atmosphere (especially during wet weather).

History

Following World War II, which saw the development of high-power microwave emitters known as cavity magnetrons, the idea of using microwaves to transmit power was researched. In 1964, William C. Brown demonstrated a miniature helicopter equipped with a combination antenna and rectifier device called a rectenna. The rectenna converted microwave power into electricity, allowing the helicopter to fly. In principle, the rectenna is capable of very high conversion efficiencies - over 90% in optimal circumstances.

Different Types of Microwave Antenna

INTERFACES IN THE MICROWAVE LINK SYSTEM Standard transfer rates in a radio link are defined

accordingly to PDH and SDH. All the PDH signals and SDH signals are up to 155 Mbps are in principle transferrable in the radio link systems. The interface in a radio link is a definition of the transfer format and physical specification needed for connecting two network elements. The interfaces for different transfer speed are defined in ITU-T recommendation G.703.

The interface of Nokia Metro Hopper is 2 Mbps according G.703 to which at least PCM, GSM and SDH can be connected in Communications laboratory.

Uses of Microwave Communication Used to link BTS-BSC and BSC-MSC. • Communication with satellites • Microwave radio relay links for television and

telephone service providers

In GSM technology MWE links are used to link between 2 sites

Interfaces In Microwave Communication

Microwave communication is established by two main interfaces that are:

• SDH (Synchronous Digital Hierarchy)• PDH (Plesiochronous Digital Hierarchy)

PDH

The Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fiber optic and microwave radio systems. The term plesiochronous is derived from Greek plesio, meaning near, and chronos, time, and refers to the fact that PDH networks run in a state where different parts of the network are nearly, but not quite perfectly, synchronized.

PDH allows transmission of data streams that are nominally running at the same rate, but allowing some variation on the speed around a nominal rate. By analogy, any two watches are nominally running at the same rate, clocking up 60 seconds every minute. However, there is no link between watches to guarantee they run at exactly the same rate, and it is highly likely that one is running slightly faster than the other.

•In case of a PDH microwave link, we aim at receiving and transmitting the signal. For which two antennas are installed ,one for receiving and the other for transmitting.

* In some cases the link is also done by installation of one antenna. Here the BTS receives more than one E1 signals. We have to carry the E1 from the BTS and transfer it to the Indoor Unit (IDU). Through the IDU via microwave we transfer it to the next site.

*The IDU’s are also known as FIU (Flex-bus Interface Unit ). One FIU can trigger a maximum of 16 E1 signals.

FIU 19E OVERVIEW & CONNECTOR DETAIL

Flex-bus Interface Unit(FIU 19)

Procedures for a PDH Link

The microwave is assembled with respect to the features provided for the particular project.

The Out Door Unit (ODU) is mounted on the Feed Horn. A feed horn is a device which is an interface between

the transmitting / receiving signal. The polarization is set as Horizontal or Vertical with

respect to the project planning. Then after mounting the microwave antenna on the

pole-mount. We are ready for the link, then we have to follow the same procedures for the far end site through which we will be bypassing the E1 signal.

Microwave Antenna

Outdoor Unit

At least 60% of link gets successful with a clear Fresnel Zone. The link is performed by series of hit & trial methods. This is achieved by adjusting the height and azimuth of the antenna.

A perfect link can be achieved upto 60% if the microwave installation is done

Carefully ,considering all aspect required for a link like Fresnel zone etc.

Again the link depends upon the transmission card used at the far end.If FIFA is installed then the E1 is processed at the BTS and the second is grabbed from FB2 port of the FIFA. It is bypassed directly through microwave or in some cases another FIU is installed and then the E1 is bypassed to the next site.

If FIPA is used then it can be directly transmitted through it, as it can bypass 8 E1 as it is also known as integrated card.

Towards Backbone Site

Far end Site

In some cases we have to carry the E1 from the back bone site to the fresh site.Here we have to install two microwaves in two directions ,one towards the backbone site where we will receive the signal and one towards the fresh site to transmit it.

Far end Site

Near end Site

Backbone site

Flexi Hopper Indoor Units With Flexi Hopper there are many different indoor unit types available, both

stand-alone and integrated into Nokia 2G/3G BTS one indoor unit supports multiple outdoor units (2 – 4) indoor unit also supplies DC power to the outdoor unit (25W max per OU) every indoor unit supports all radio frequency bands (7 – 38 GHz) every indoor unit supports all radio transmission capacities (2x2M –

16x2M) site add/drop capacity varies according to the indoor unit configuration indoor units include 2M-level cross-connection capability between their

Flexbus (OU) interfaces and their E1 interfaces

No E1-cabling at all is required on a site where Nokia base station integrated indoor units are used this may also eliminate the need for an expensive site support cabinet,

reducing site space and allowing more flexibility in site selection

FIFA is the indoor unit integrated into Nokia Flexi EDGE BTS it can support two outdoor units, and can add/drop up to 16 E1 signals

towards the BTS

Flexbus Cable Flexbus provides a single-cable interconnection

between an indoor unit and an outdoor unit, or an indoor unit and another indoor unit bidirectional coaxial cable, operating in full-duplex

mode carries digital baseband signals and control data

between IU and OU also feeds DC power to the OU

Flexbus can be used also to interconnect one Flexi Hopper IU to any other Flexi Hopper IU

Flexi Hopper outdoor units (1)

Flexi Hopper radios are available in the following frequency bands:

7, 8, 13, 15, 18, 23, 26, 28, 32 and 38 GHz each frequency band is divided into a number

of sub-bands, with various duplex spacings

Every radio can support all transmission capacities: 2 / 4 / 8 / 16 x 2 Mbit/s capacity is programmable with element

manager no need to replace radio when capacity

changes

Radios have low power consumption: 25W max per OU eliminates the need for a separate power supply contributes to higher reliability and longer running times on battery backup.

Outdoor units are small, lightweight, and easy to install radios can be installed on a roof, wall, or tower alignment unit can be mounted on either side of a pole.

Flexi Hopper outdoor units (2) OU reliability is enhanced by the mechanical

construction compact sandwich structure – no cables inside stacked PCB’s with pin/header connections high degree of circuit integration

ModemIFU

MWU

IFU cover

Heat sink

Antenna filter

Power Supply

Flexi Hopper antennas (1)

FlexiHopper is available with either integrated or separate antennas

Integrated 20/30/60 cm antennas

•antennas are integrated onto FlexiHopper’s own alignment unit

Integrated 90/120/180 cm antennas with snap-on adapter

•antennas are supplied with their own alignment unit

•FlexiHopper radios are integrated onto them using a snap-on adapter

Separate 240/300 cm antennas with waveguide antennas are supplied with their own alignment unit.

Flexi Hopper radios are installed separately using Flexi Hopper alignment unit, and connected to the antenna via a flexible waveguide.

Any size antenna may be mounted separately and connected to the radio with a flexible waveguide.

Dual-polarized antennas can only be mounted separately, as they must be connected to two different outdoor units simultaneously.

Flexi Hopper antennas (2)

Flexi Hopper antennas support both vertical and horizontal polarization. They are delivered with vertical polarization.

They can be configured with horizontal polarization by rotating the antenna feeder 90o

Integrated radios must also be rotated 90o (with handle and connectors facing sideways instead of down).

Antennas and radios are fitted with guides to prevent installation in conflicting polarizations

Dual modulation modesFlexi Hopper radios support both 4-state and 16-state modulation.

4-state modulation is π/4-DQPSK (differential quadrature phase shift keying)

16-state modulation is 32 TCM (Trellis coded modulation)

Modulation mode is selectable with the element manager.

16-state modulation is an optional feature – it enables doubling the transmission capacity in the same bandwidth as 4-state modulation

16-state modulation provides a capacity upgrade while maintaining the same bandwidth to save frequency spectrum

Note that 16-state modulation operates only at 8x2M and 16x2M capacities

16 x 2M28

16 x 2M8 x 2M14

8 x 2M4 x 2M7

2 x 2M3.5

16-state4-state

Channel bandwidth

(MHz)

Modulation

FEC and interleaving (1)

Flexi Hopper radios use forward error correction (FEC) and interleaving to improve signal quality

With FEC, redundant information is inserted into the transmitted data stream according to an algorithm which allows the receiver to detect and correct errors in real time – up to a certain threshold – without the need for retransmission FEC uses Reed-Solomon coding – RS(63,59) – which

provides 4 redundancy symbols for every 59 data symbols (6.4% coding redundancy)

FEC is continuously on

With interleaving, symbols in one transmit data block are distributed over several adjacent blocks, to minimize vulnerability to burst-type errors in 4-state modulation, interleaving is selectable

between Off, 2-depth and 4-depth modes; in 16-state modulation, interleaving is fixed to 4-depth mode

with 2-depth interleaving, symbols are distributed over two blocks; with 4-depth interleaving, symbols are distributed over four blocks

4-depth mode provides maximum error-correction effectiveness

When FEC is used together with interleaving, burst-type errors can also be corrected more effectively

ATPC and ALCQ (1)

ATPC and ALCQ are both techniques that enable the radio transmitter to increase or decrease the transmit power automatically, based on feedback about the received signal quality at the other end of the hop ATPC – Automatic Transmit Power Control ALCQ – Adaptive Level Control with Quality measurement

When ATPC or ALCQ is in use, the radio always tries to transmit at the minimum power needed for adequate reception at the far end the idea is to monitor the received signal level together with the bit

error ratio (BER) of the receiver, and to adjust the far-end transmitter output power to adapt to the variable fading conditions

the maximum transmit power allowed for the hop, however, is specified (set) with the element manager

The controlled use of transmit power reduces interference between neigh-boring systems, and allows more efficient utilization of radio frequencies than the constant power level approach

FEC and interleaving (2)

FEC encoding

Interleaving

De-interleaving

FEC decoding: X errors detected, N corrected

Corrected frame

Tx data

Interference

Radio frame

ATPC and ALCQ (2) ATPC controls the Tx power level so as to maintain the far-end Rx

signal level above a certain user-defined threshold this threshold is based on the modulation method and capacity

being used

ALCQ is more sophisticated, and also monitors the BER of the receiver in addition, ALCQ applies a novel pseudo-error monitoring

mechanism developed by Nokia the bit errors detected by the FEC decoder, which are invisible

to the user, are interpreted as pseudo-errors and used as an additional input for ALCQ operation

this method can respond to the degradation of Rx signal quality before actual bit errors occur over the radio relay

The selection between ATPC and ALCQ (or the choice not to use transmit power control at all) can be made with the element manager

Installation

Installation with

vertical polarisation

Installation with

horizontal polarisation

SDH Synchronous Digital Hierarchy (SDH) are standardized

multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or light-emitting diodes (LEDs). Lower rates can also be transferred via an electrical interface. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting larger amounts of telephone calls and data traffic over the same fiber wire without synchronization

The basis of Synchronous Digital Hierarchy (SDH) is synchronous multiplexing - data from multiple tributary sources is byte interleaved. In SDH the multiplexed channels are in fixed locations relative to the framing byte. De-multiplexing is achieved by getting out the required bytes from the digital stream by demultiplexing the

stream and then multiplexing it again

SDH Rates SDH is a transport hierarchy based on multiples of

155.52 Mbit/s The basic unit of SDH is STM-1:

STM-1 = 155.52 Mbit/s

STM-4 = 622.08 Mbit/s

STM-16 = 2588.32 Mbit/s

STM-64 = 9953.28 Mbit/s

Each rate is an exact multiple of the lower rate therefore the hierarchy is synchronous.

Procedures for SDH Link

Some of the essential requirements for a SDH link are

MUX FIU FIU can bypass a maximum of 16 E1 signals. A MUX can trigger a maximum of 63 E1 signals. The FIU and the MUX are connected through

Optical fiber cables. A site is rated as a SDH node depending upon the

number of sites in its coverage. In SDH we have an electrical interface for the link.

SDH

MUX

OFC

IF cable

SDH Rack

The procedure implemented for a SDH link are the same as in PDH link, but here we don’t have to install two microwaves for

receiving and transmitting.

In this case already 63 E1 have been dumped in it, and from that node the signal is distributed to different sites ahead. First the signal is received through the microwave ,dumped and processed in the MUX. The signals are dumped in the sections of the KRONE plate .

After the initial installation of both FIU and Micro-wave antenna and alignment, the FIU is connected to the MUX and the KRONE where two tests are done:

• LED test Here a LED is used to trace the TX port and the RX port of

PCM cable at KRONE, the LED glows at the TX and vice-versa.

• Loop test After the Led test the loop test is done to check the

dumped E1. It is done by creating a loop and breaking it. If the NOC receives the break then we conclude the E1 is

dumped.

Loop Break

NOC receives the break then we conclude the E1 is dumped.