OFDMA is essentially a hybrid of FDMA and TDMA: Users are dynamically assigned subcarriers (FDMA) in different time slots (TDMA) . The advantages of OFDMA start with the advantages of single-user OFDM in terms of robust multipath suppression and frequency diversity. In addition, OFDMA is a flexible multiple-access technique that can accommodate many users with … Read more
Diversity Reception: Multiple Antennas at Base Station Each Antenna is Affected by Multipath Differently Due to Their Different Location Allows Selection of the Signal Least Affected by Multipath Fading If Diversity Antennas are Good, Why Not Use Base Stations as a Diversity Network? – Soft Handoff The concept of diversity reception has been well known … Read more
Time diversity is a technique common to most digital transmission systems. The Rake Receiver is used to find and demodulate multipath signals that are time delayed from the main signal. Rake Receiver to Find and Demodulate Multipath Signals. Data is Interleaved : Spreads Adjacent Data in time to Improve Error Correction Efficiency Convolutional Encoding : … Read more
Frequency diversity is inherent in a spread spectrum system. A fade of the entire signal is less likely than with narrow band systems. Combats Fading, Caused by Multipath Fading Acts like Notch Filter to a Wide Spectrum Signal May Notch only Part of Signal Fading is caused by reflected images of an RF signal arriving … Read more
Make use of a number of receive antennas that are well separated (coherence distance) to generate independent receptions of the transmitted signal. Selection diversity: choose received signal with largest received power, S/N, etc. Switched diversity: choose alternate antenna if signal falls below a certain threshold. Linear combining: linearly combine a weighted replica of all received … Read more
We know that we normally have several independent paths from TX to Rx of different lengths, these are affected and delayed differently, Signals that start out together at the transmitter will be separated in time when they reach the receiver Diversity doesn’t work with Line of Sight, This is exploiting Multipath rather than fighting it … Read more
Hopping can lower the influence of Rayleigh fading to signal strength. This influence equals to frequency diversity. One voice code block of GSM is transmitted in 8 bursts separately after channel coding. Since the Rayleigh fading is based on frequency, in other words, the valley point of signal fading appears in different locations at different … Read more
Subscriber Types Subscriber Equipment may take different form factors. It may resemble alert 3G phones, or maybe a CPE device, or a device connected to a USB dongle or USB port. RF different features, such as different transmission power levels, different Receive diversity schemes, different antenna patterns and gains, it would need Appropriate to perform … Read more