Having WILL Power

In the developing countries, the
teledensity is far below the figure of 20 percent recommended by ITU. These countries
include much of South America and Africa, parts of the Middle East, India, and remote
parts of Russia and China. These countries have limited teledensity because of lack of
financial resources for building up a large wired network. Moreover, in such countries,
people have restricted their interaction and needs within their own groups and
communities, thus limiting their social and economic growth.

alt="https://img-cdn.thepublive.com/filters:format(webp)/vnd/media/post_attachments/52209add08b427e3371501d3378c4351fbafe1647b9fb8dda52e6f07500c1c98.gif (40705 bytes)" align="right" hspace="6" vspace="6"> face="Times New Roman">Under these circumstances, the key requirements of a network are
low capital and easy and faster implementation. Furthermore, difficult terrain and
climatic conditions also impose restrictions on the implementation of wired networks. For
such countries, high bandwidth and multi-port lines per house are not in much demand. The
key issues for all such countries in this category are faster and low-cost implementation
of telecommunication infrastructure expecting economic growth, with the requirements
ranging from simple voice only capability to high-speed Internet and multimedia
requirements.

Indian telecom scenario is a typical case
of this category. In India, average teledensity is less than 2 percent. Rural teledensity
is less than 0.5 percent. This is with the factor that around 50 percent of the Indian
villages are without a telephone number.

The New Challenges COLOR="#000000">

Copper has provided access to the
telephone network for the past 100 years. Though sufficient changes have taken place in
the telecom scenario, the copper local loop has remained largely unaffected. Though
radio-based access is not new and is in use since long, the local loop distribution from
the radio terminal to a house is still based on copper cable.

This is where Wireless In Local Loop
(WILL) is replacing copper, because of low-cost, fast and easier, deployment, and greater
ease of reach to difficult geographic terrain. DoT in India has already taken a decision
to introduce WILL-based systems. It is estimated that over the next five years, the
domestic WILL market will have between two million and 2.5 million subscribers. This is
based on the expectations that 20-25 percent of all new lines being rolled out will use
WILL technologies.

However, initially WILL had to prove
itself as a viable alternative to the existing copper. The quality of service in terms of
traffic handling capacity as well as voice quality needed to be equivalent to that in a
copper loop. The key requirements are:

• Plain old telephony,
  with voice quality as good as wire-line system

• N-ISDN, with data rates
  of 144 kbps providing at least two lines, either of which can be used for voice or data. SIZE="1">

• Fax transmission SIZE="1">

• Supplementary
  services–call divert, follow me, call waiting, barring of incoming and outgoing
  calls, etc.

• Internet service SIZE="1">

• Leased line (supporting analog
  as well as digital standards).

Separate links and circuits have to be
provided for high-speed Internet access and leased lines. For voice telephony in local
loop, toll quality voice is required. For obtaining this either 64 kbps PCM or 32 kbps
ADPCM should be used. Traffic generation per telephone subscriber should be assumed from
0.05 erlang to 0.15 erlang with call blocking rate less than 1 percent.

The system should support high subscriber
density ranging from 1,000 subscribers per sq. km to 100,00 subscriber per sq. km. For
supporting all these factors, bandwidth availability is the key issue. SIZE="4" COLOR="#016077">

Bandwidth Requirements and Micro Cells SIZE="2" COLOR="#000000">

Bandwidth requirements depend mainly on
the number of subscribers to be served and traffic generated. src="infrast2.gif" width="314" height="223" alt="infrast2.gif (10240 bytes)" align="left"
hspace="6" vspace="6">

Assuming the use of 32 kbps ADPCM voice
coding, 30 kHz of frequency spectrum will be required for each way. Thus for two-way
communication two channels will be required raising the bandwidth requirement to 60 kHz.
Assuming that with CDMA or DCA, TDMA (1/1) frequency reuse pattern is adapted. Other
assumption includes 1,000 subscribers per sq. km., with 0.1 erlang traffic. Let us say
that we wish to serve subscribers within a radius of 4 km. The total area served will be
around 50 sq. km. and the total number of subscribers served by that base station will be
50,000 (50 x 1,000 subscribers per sq. km.). With 0.1 erlang traffic per subscriber the
number of channels required per cell is approximately 5,000 (50,000 x 0.1). The total
bandwidth required per cell will be 300 MHz (5,000 x 60 kHz) per cell. Moreover, as the
subscriber density increases, the bandwidth requirements will also increase.

It can be seen that micro cells help to
serve larger subscriber density. Assume that cell radius has been reduced to 500 m. For a
subscriber density of 1,000 subscriber per sq. km., the number of subscribers served per
base station will be approximately 800 subscribers.

With 0.1 erlang traffic per subscriber,
the bandwidth required will be approximately 4.8 MHz per cell (800 x 0.1 x 60) only.

Thus, we can see that micro cells are the
solution for serving large number of subscribers. But micro cell architecture has its own
problems like inter-cell interference and restrictions on frequency re-use. However, these
problems can be overcome by proper cell planning and location of base station. SIZE="4" COLOR="#016077">

TDMA or CDMA

Since the beginning of the Nineties, there
is a debate going on about whether CDMA or TDMA is the best access technique. There are
various factors which are to be considered for these two techniques, viz. capacity
comparison, range, sectorization, frequency planning, and micro cell structure. SIZE="2">

Capacity Comparison

The capacity of a TDMA system can be
calculated in terms of numbers of channels per MHz as given by N = (1/B)/K. Where
"B" is the bandwidth per channel in MHz and "K" is the cluster.

And in the case of CDMA system, the
capacity can be calculated as N = (G/SIR)(1/a)(f.h.p). Where "a" is the voice
activity factor, "f" is the inter-cell interference, "h" is the
handover loss, and "p" is the factor relating to power control inefficiency. In
case of WILL systems, handover loss will be 1 as there is no mobility.

Typically, for a CDMA system, the capacity
is around two times the capacity of an equivalent TDMA system. However, by using Dynamic
Channel Allocation (DCA) TDMA, the capacity benefits of CDMA reduce to 1.4.

In DCA system, each cell has the access to
all the frequencies. Before transmitting, each cell measures the interference on each
frequency and dynamically allocates the channel with the lowest interference. This enables
TDMA system to use the same frequencies in all the cells.

It should, however, be noted that although
CDMA-based systems are having more capacity than TDMA systems, it is difficult to have a
concrete comparison, until there is widespread deployment of CDMA systems. Moreover, in
rural areas or low capacity areas, high-system capacity may not be of great concern. Thus,
considering all the factors including cost and the business potential, the choice can be
made between the technologies.

Network Roll-out Strategy

The network roll-out, after selection of
technology, starts with radio coverage planning, selection of the number of cells,
selection of cell sites, connecting cell sites with main network, deployment of microwave
links or leased links for interconnection, and interface with switching and backbone
systems.

alt="https://img-cdn.thepublive.com/filters:format(webp)/vnd/media/post_attachments/be868bcbbbe961de02029a33fd6b9ad403ebdfa8417d9c921296acdf86a2db7b.gif (19855 bytes)" align="right" hspace="6" vspace="6"> face="Times New Roman">Number of Cells: Selecting the number of cells is fundamentally
based on the business plan. The key inputs from the business model are the number of
houses in the area, density of houses, expected penetration, traffic per subscriber, and
the projected growth.

Depending upon the subscriber traffic and
subscriber density, the number of traffic channels can be calculated. The number of calls
required for capacity is given by Number of cells = (Traffic channels required/Traffic
channels per cell), where Number of channels = Number of subscribers x Busy hour traffic
per subscriber.

Number of channels is also linked with the
call blocking rate. For a particular blocking rate and erlang traffic, the number of
channels required can be determined from erlang B table. The erlang formula is :

Where P face="Times New Roman">B is the blocking
probability, A is the offered traffic in erlangs and N is the Number of traffic channels
available.

Selection of Cell Site: Site selection
can be based on two factors. Coverage area and capacity calculations. In case, if more
coverage is required in low subscriber density area, high buildings should be selected for
micro-cells, whereas in case if high capacity is the criterion, low height buildings
should be selected. Radio coverage can be planned by using proper planning tools. WILL
systems are highly based on Line of Sight (LoS) model. The general inputs for these
planning tools include: