<<<All Things Wireless>>>
The real benefits of OFDM become apparent in a frequency selective channel. The introduction of the cyclic prefix (guard period) allows us to remove the Intersymbol Interference (ISI) in the time domain and frequency domain equalization allows us to overcome the channel variations in the frequency domain.
Read more64-QAM is an important component of 4G/5G Air Interface that promises higher data rates and spectral efficiencies. Combined with OFDM and MIMO it successfully combats the detrimental effects of the wireless channels and provides data rates in excess of 100Mbps (peak data rate). Here, we discuss a simple example of 64-QAM modulation with OFDM in an AWGN channel. We assume a bandwidth of 1.25MHz which corresponds to an FFT size of 128.
Read more64-QAM is an important modulation scheme being used in WiMAX and LTE. It allows for transmission of 6 bits symbol which results in higher bit rate and spectral efficiency. The calculation of bit error rate of 64-QAM is a bit tricky as there are many different formulas available with varying degrees of accuracy. Here, we first calculate the bit error rate (BER) of 64-QAM using a simulation and then compare it to the theoretical curve for 64-QAM. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % FUNCTION TO CALCULATE 64-QAM BER USING SIMULATION % n_bits: Input, number of bits % EbNodB: Input, energy per bit to noise […]
Read moreWe have previously looked at a simple OFDM modulation and demodulation scheme. We saw that the BER performance of OFDM in AWGN was the same as the BER performance of the underlying modulation scheme (QPSK in this case). We will now continuously improve upon our basic simulation to get a more realistic picture. In this regard we introduce the cyclic prefix which is used in OFDM to overcome Intersymbol Interference. The duration of the cyclic prefix is 0.8usec (16 samples at 20MHz) resulting in a symbol duration of 4usec (IEEE 802.11a). Given below is the code for OFDM modulation and demodulation […]
Read moreOFDM modulation works on the principle of converting a serial symbol stream to a parallel symbol stream with each symbol from the parallel set modulating a seperate carrier. The spacing between the carriers is 1/T where T is the duration of the OFDM symbols (without cyclic prefix). This guarantees orthogonality of the carriers i.e. there is no interference between carriers. The addition of orthogonal carriers modulated by parallel symbol streams is equivalent to taking the IFFT of the parallel symbol set. At the receiver the inverse operation of FFT is performed and the parallel symbol streams are converted to serial symbol streams. The main advantage of this scheme is that one […]
Read moreT-Mobile claims to be serving 4G to 152 markets with 170 million POPs across the US. The technology that T-Mobile is using is HSPA+ which achieves data rates comparable to those achieved by 4G LTE technology being used by Verizon. With this technology (HSPA+21 and HSPA+42) end users can experience average download speeds of 5Mbps and peak download speeds of 12Mbps. However the future of T-Mobile hangs in the balance now as the AT&T and T-Mobile merger is being debated at the FCC. T-Mobile LTE Coverage Although the above figure seems to suggest that T-Mobile 4G service is available throughout […]
Read moreWe have previously looked at the birds eye view of 4G LTE coverage within the US. We know that Verizon 4G services are now available to more than 50% of the US population. However, geographically, the service is only available in very small islands of population. Now, we take a closer look at 4G LTE coverage within California. LTE Coverage in CA We see that the coverage is available in most of the population centers such as Sacramento, San Francisco, Oakland, San Jose, Fresno and Bakersfield. Further south the coverage is also available in areas around Los Angeles and San […]
Read moreVerizon Wireless 4G LTE is now available to 160 million people with coverage in 117 cities within the US. This has been achieved within eight months of the initial deployment. Verizon hopes to increase the coverage to 185 million people by the end of 2011. The company claims that with its current deployment strategy users can experience data rates of 5-12Mbps on the downlink and 2-5Mbps on the uplink. When users do not have access to the 4G LTE network the phones will automatically switch to 3G which is available around most of the US. LTE Coverage This push to […]
Read moreLTE devices operate in either of two modes: TDD (time domain duplex) or FDD (frequency domain duplex). In FDD transmission and reception takes place at different frequencies whereas in TDD transmission and reception takes place at the same frequency but different time slots. The are separate frequency allocations for these two modes but some bands are common between the two modes. Given below are the frequency allocations for FDD. LTE FDD There is suitable separation between the uplink and downlink frequency bands (at least 10MHz) so that the transmitted signal does not feedback into the receiver. It is extremely difficult […]
Read more