GTU 3171608 Wireless Communication Summer 2026 Solved Paper & Solutions

A Wireless Local Area Network (WLAN) links computers, phones and other devices inside a small area, such as a home, an office or a campus, using radio waves instead of cables.

• Devices connect through an access point, which is joined to the wired network and the Internet.
• The common standard is IEEE 802.11, known as Wi-Fi. It works in the unlicensed 2.4 GHz and 5 GHz bands.
• The range is tens of metres, and users can move around inside the coverage area.

Bluetooth is a short-range wireless technology. Its primary function in a Personal Area Network (PAN) is to connect personal devices such as a phone, headset, keyboard, speaker or watch without cables, over a distance of about 10 metres.

• It works in the unlicensed 2.4 GHz band and uses frequency hopping to avoid interference.
• Devices form a small network called a piconet. One device is the master and up to seven others are active slaves.
• It needs little power, so it suits small battery-operated devices.

In short, Bluetooth replaces wires between nearby personal devices and lets them exchange data and voice.

Cellular systems have changed from analog voice service to high-speed broadband over four generations.

Generations

Trends from 2G to 4G

The main changes are listed below.

1. Analog to digital: 2G made voice digital. This gave better call quality, more users per channel, encryption and text messages.
2. Circuit switching to packet switching: GPRS and EDGE added packet data to GSM. Users could browse and send email, and pay for data and not for connection time.
3. Higher data rates: 3G used wideband CDMA and gave rates of a few Mbps. 4G uses OFDMA and multiple antennas, and targets about 100 Mbps for moving users and about 1 Gbps for users who stand still.
4. Voice and data on one network: 4G is an all-IP network. Voice is carried as data, for example in VoLTE.
5. More capacity and lower delay: a simpler, flatter core network and wider channels, up to 20 MHz in LTE, give lower delay and support more users.

Frequency reuse means using the same set of radio frequencies in different cells of a cellular system, as long as the cells are far enough apart.

• The available channels are divided among a cluster of $N$ cells, and the same pattern is repeated across the coverage area.
• Cells that use the same frequencies are called co-channel cells. They are kept apart so that their interference is small.
• Reuse lets a limited spectrum serve many more users than one big transmitter could.

The distance-to-frequency reuse ratio, also called the co-channel reuse ratio, is $Q = D/R$. Here $D$ is the distance between the centres of the nearest co-channel cells and $R$ is the cell radius. For hexagonal cells, $Q = \sqrt{3N}$, where $N$ is the cluster size.

Impact on interference:

• A large $D/R$ puts co-channel cells far apart. Co-channel interference is low and call quality is good.
• A small $D/R$ brings co-channel cells close. Interference is high and quality drops.
• The signal to interference ratio grows with $D/R$.

There is a trade-off. A larger $D/R$ needs a larger $N$, so each cell gets fewer channels and the capacity falls.

For hexagonal cells, the co-channel distance is $D = R\sqrt{3N}$.

We count whole cells, so the area holds about 962 cells (ignoring edge effects).

Rayleigh fading is a fast fluctuation of the received signal that happens when there is no direct line-of-sight path between the transmitter and the receiver.

• The receiver gets many scattered copies of the signal. They have random phases and almost equal strength.
• The copies add and cancel each other, so the signal strength changes sharply over a distance of about half a wavelength.
• The envelope of the received signal follows the Rayleigh distribution. Deep fades make the signal drop far below its average.

Multipath fading is the variation of the received signal caused by the signal travelling to the receiver by several paths. The paths come from reflection, diffraction and scattering by buildings, vehicles and the ground.

The copies reach the receiver with different delays, amplitudes and phases. When they add in phase the signal is strong. When they are out of phase they cancel and the signal fades.

Impact on communication systems:

• Signal fading: the signal strength changes quickly, so calls can drop or become noisy.
• Bit errors: deep fades increase the bit error rate.
• Inter-symbol interference (ISI): delayed copies of one symbol spill into the next symbol, which limits the data rate.
• Remedies: equalisers, diversity, error-correcting codes, RAKE receivers and OFDM.

The GSM (Global System for Mobile communication) network has three main subsystems: the Mobile Station, the Base Station Subsystem, and the Network and Switching Subsystem.

Mobile Station (MS)

It is the user's phone. It has two parts: the mobile equipment and the SIM card. The SIM holds the subscriber identity (IMSI) and the secret key.

Base Station Subsystem (BSS)

• Base Transceiver Station (BTS): the radio equipment and antenna of a cell. It talks to the mobile over the air interface.
• Base Station Controller (BSC): controls many BTSs. It manages radio channels, power control and handover.

Network and Switching Subsystem (NSS)

• Mobile Switching Centre (MSC): sets up, routes and ends calls. It also does handover between BSCs.
• Home Location Register (HLR): the permanent database of subscribers and their services.
• Visitor Location Register (VLR): a temporary database of the subscribers now in the area of an MSC.
• Authentication Centre (AuC): stores the keys used for authentication and encryption.
• Equipment Identity Register (EIR): lists valid, stolen or barred handsets by their IMEI.

Interfaces

The Um interface is the radio link between the MS and the BTS. The Abis interface joins the BTS and the BSC. The A interface joins the BSC and the MSC.

Operation subsystem

The Operation and Maintenance Centre (OMC) monitors and manages the network.

In short, FDMA separates users by frequency, and CDMA separates users by code.

GSM protects the network and the user with authentication, encryption and identity protection.

Authentication uses a challenge and response.

1. The network sends a random number RAND (128 bits) to the mobile.
2. The SIM uses the secret key Ki and the algorithm A3 to compute the response SRES.
3. The network computes SRES in the same way in the Authentication Centre and compares both values.
4. If they match, the user is accepted. Ki never travels over the air.

Encryption

• The SIM uses RAND, Ki and the algorithm A8 to make a ciphering key Kc.
• The stream cipher A5 uses Kc to encrypt the data between the mobile and the BTS.

Identity protection: a temporary identity TMSI is used in place of the permanent IMSI, so a listener cannot track the user. The SIM is also protected by a PIN.

The IS-95 (cdmaOne) system is a digital cellular system that uses CDMA. All users share a 1.25 MHz channel and are separated by codes.

Major components

Channels

• Forward link (BTS to MS): pilot, sync, paging and forward traffic channels. Walsh codes separate the channels.
• Reverse link (MS to BTS): access and reverse traffic channels. A user-specific long code separates the users.

How the parts work together

1. The mobile switches on and finds the strongest pilot channel. It reads the sync channel to get system timing.
2. It listens to the paging channel for calls.
3. To make a call it sends a request on the access channel.
4. The BTS assigns a traffic channel. The BSC sets up the call with the MSC, which connects it to the PSTN or to another mobile.
5. During the call, the BSC controls power control and soft handoff between cells.

The reverse channel carries information from the mobile to the base station. It has two functions.

• Access channel: the mobile uses it to start a call, answer a page, register its location and send short messages.
• Reverse traffic channel: it carries the voice or data of the user, and signalling, to the base station during a call.

In IS-95 each mobile uses its own long code, which lets the base station separate the users who send in the same band.

Wi-Fi is based on the IEEE 802.11 standards.

• It is a wireless LAN technology with a range of tens of metres.
• It uses the unlicensed 2.4 GHz and 5 GHz bands.
• Data rates range from 11 Mbps in 802.11b to over 1 Gbps in the latest versions.
• It uses CSMA/CA for channel access and is used in homes, offices and hotspots.

WiMAX is based on the IEEE 802.16 standards.

• It is a wireless metropolitan area network, used for broadband access where cables are not available.
• It uses licensed and unlicensed bands, for example 2.3, 2.5 and 3.5 GHz.
• The range can reach several kilometres, up to about 50 km in the best conditions.
• Its MAC is connection-oriented, and it supports quality of service for voice and video.

Soft handoff is a handoff in which the mobile connects to a new base station before it leaves the old one. The mobile is connected to two or more base stations at once, on the same frequency. It is also called "make before break". It is possible in CDMA because all cells use the same frequency.

How it works

1. The mobile keeps a list of pilot signals. A pilot above the add threshold (T_ADD) is added to the active set.
2. The mobile sends and receives through all base stations in the active set at the same time.
3. On the forward link, the mobile combines the signals with its RAKE receiver.
4. On the reverse link, the base stations pass their copies to the controller. It picks the best frame.
5. When a pilot stays below the drop threshold (T_DROP) for a set time, it is removed from the active set.

Benefits

• Better call quality: combining signals from different base stations gives diversity against fading and shadowing.
• Fewer dropped calls: the link is never broken during the change of cell, so the call continues smoothly.
• Higher reliability: if one link is lost, the other links keep the call alive.
• Lower power: the mobile can use less transmit power, which reduces interference and raises capacity.