WT Week 3

Radio Frequency II

tldr - Problems and (some) solutions for radio wave transmission

Nyquist’s Formula

For a noiseless channel

Where

• B is the bandwidth (Hz)
• n is the number of possible discrete levels in the modulation method used

Example

If the transmitter can generate signals with a bandwidth of 20 MHz, what is the maximum data rate of this channel using the 16-QAM modulation technique?

Signal Power

dBm

Power ratio (dB) of measured power referenced to 1 milliwatt (mW)

• Both dBm and mW are measures of signal power
• Used in radio, microwave and fibre optic networks as a measure of power
• Able to express very large and very small values in a short form (compared to mW)

Converting mW to dBm

Example

Compute the signal strength of 0.0001 milliwatt in dBm

Problems with Radio Wave Transmission

• Attenuation
  • Gradual loss in signal intensity through transmission medium
• Delay Distortion
  • Delay in travelling time in various RF components
• Radio Wave Interference (More below)
• Limited Transmission Bandwidth (More below too)

Problem? 😱

Radio Wave Interference

Radio signals have narrow frequency band ∴ interfering signal near the operating frequency of the radio devices render the device inoperable.

Solution? 🤔

Spread Spectrum

A technique that takes a narrow band of signals and spreads it over a broader portion of the radio frequency band

• Increases immunity to noise
• Increases number of users using the same frequency band
• Increases degree of difficulty to eavesdrop (Side benefit)
• 2 spreading methods: FHSS & DSSS

Frequency-Hopping Spread Spectrum (FHSS)

A method of transmitting radio signals by rapidly switching a carrier among many frequency channels using a random sequence known to both transmitter and the receiver

Advantages 👍🏼

• Lower cost of manufacturing

Disadvantages 👎🏼

• Slower transmission rate
• Limited transmission distance

Usage

• Bluetooth
• Cordless Phone
• WLAN (1 & 2 Mbps)

Example

Decipher the message, given that the random FHSS sequence is 3 5 9 1 6 2 9 2 8.

Answer: F R E Q U E N C Y

Direct-Sequence Spread Spectrum (DSSS)

A spread spectrum modulation technique where data bits are XOR with a chipping code (a.k.a. Pseudo Noise Code or PNcode) to generate DSSS signals to be transmitted

• Receiver will XOR received signals again using the same PNcode to extract original data
• Longer PNcode
  • More bandwidth required
  • High probability to recover original data in case of error

How it works

Narrowband data is transformed into broadband spread spectrum signals with the same total energy ∴ original data is spread over a broader band with lower power. The low-power signals appear as background noise to unintended receivers ∴ resolving narrow frequency band interference.

Advantages 👍🏼

• Greater security as only intended receiver would have the chipping code
• Faster transmission rate

Disadvantages 👎🏼

• Higher cost of manufacturing

Usage

• CDMA & WCDMA
• WLAN (5.5 & 11 Mbps)

Problem? 😱

Limited Transmission Bandwidth

Radio frequencies need to be shared among as many users as possible.

Solution? 🤔

Channel Access Methods

Allow multiple users to access the same frequency band

Frequency Division Multiple Access (FDMA)

A channel access method which sub-divides the frequency band allocated to a network into multiple channels. Each channel is then assigned to a specific user

👎🏼 Users face interference as frequencies are closely grouped

Time Division Multiple Access (TDMA)

A channel access method which allows numerous users to access each channel by allocating a unique time slot to each user within the channel

Code Division Multiple Access (CDMA)

A channel access method which allows multiple users to share a single channel simultaneously by using the spread spectrum technique

• Each transmitter is assigned a code and only the intended receiver has the same code to recover signals

Orthogonal Frequency Division Multiplexing

A method of encoding digital data on multiple carrier frequencies

How it works

Many closely spaced orthogonal sub-carrier signals are used to carry data on several channels. Each sub-carrier is modulated with a conventional modulation scheme (e.g. QAM).

Usage

• Digital TV and audio broadcasting
• Wireless networks
• 4G mobile communications

fin