Broadband Communication Networks PDF

Broadband Communication Networks Assoc.Prof.Dr.ibrahim khider September 2021 Outline 1 -Block-Based Transmissions Denition Cyclic Prex Types 2 OFDM Systems Introduction System Model Uses of OFDM Advantages 3- OFDMA Subcarrier Allocation  Block-Based Tr...

Broadband Communication Networks Assoc.Prof.Dr.ibrahim khider September 2021 Outline 1 -Block-Based Transmissions Denition Cyclic Prex Types 2 OFDM Systems Introduction System Model Uses of OFDM Advantages 3- OFDMA Subcarrier Allocation  Block-Based Transmissions Denition In a block-based transmission, the entire sequence of modulated symbols is rst divided into multiple blocks, each is preprocessed further using linear transforms, and guard symbols are inserted between two consecutive blocks. To reduce the computational complexity of equalization, block-based transmissions have been proposed Two types of guard symbols are applicable in block-based transmissions. Zero padding inserts zeros between two consecutive blocks Cyclic prex (CP) which is the copy of the last portion of the signal block  Block-Based Transmissions The Cyclic Prefix Instead of transmitting block x directly, a new block is generated and transmitted through the channel. The new block is formed by appending the last P symbols of x to the head of itself. The portion of the first P symbols in the new block is the cyclic prefix (CP). To transmit multiple blocks consecutively, the figure in next slide shows the structure of continuous transmission with CP. Block-Based Transmissions  Block-Based Transmissions The Cyclic Prefix some signals received at the beginning of a block can be affected by symbols transmitted from the previous block. This phenomenon is called interblock interference (IBI).  Block-Based Transmissions Overview of Block-Based Transmissions Various block-based transmission schemes can be developed, including but not limited to the following Orthogonal frequency-division multiplexing (OFDM) Orthogonal frequency-division multiple access (OFDMA) Single-carrier cyclic prefix (SCCP) system Interleaved frequency-division multiple access (IFDMA) Cyclic prefix based code-division multiple access (CP- CDMA) Multicarrier code-division multiple access (MC-CDMA)  Block-Based Transmissions Overview of Block-Based Transmissions OFDM and SCCP systems, however, are designed to support single-user communication only Therefore, to support multiple users in sharing the same radio resource, they have to be used in conjunction with other multiple-access schemes, such as TDMA or FDMA. OFDMA, IFDMA, SC-FDMA, CP-CDMA, and MC- CDMA are designed to support multiple users to share the same radio resource simultaneously  OFDM Systems Introduction Orthogonal frequency-division multiplexing (OFDM) is a discrete Fourier transform (DFT)-based multicarrier modulation (MCM) scheme The OFDM concept is based on spreading the high speed data to be transmitted over a large number of low rate carriers. The carriers are orthogonal to each other and frequency spacing between them are created by using the Fast Fourier transform (FFT) OFDM Systems Introduction OFDM Systems System Model  OFDM Systems System Model A sequence of data is first serial-to-parallel (S/P)- converted to form a single data block of bits Next, each bits group is used to form to form a complex number using PSK or QAM modulation with N subcarriers, in order to be transmitted Next, each modulated symbol is converted using the inverse fast Fourier transform (IFFT) Next, parallel-to- serial, then the CP is added in the guard period between successive blocks and constructed OFDM symbols  OFDM Systems OFDM At the receiver side, after filtering, the receiver first discards the received signal samples associated with the CP portion. The received signal samples associated with the data block are then fed to the FFT operation. The output of the FFT is passed through the equalizer to recover the data symbols Next, each modulated symbol is demodulated, and parallel-to-serial conversion is applied to recover the original data stream OFDM Systems OFDM  OFDM Systems Uses of OFDM OFDM has been widely adopted in various communications systems, including the digital audio broadcast (DAB) digital terrestrial video broadcast (DVB-T) standards in Europe and Japan, the IEEE 802.11a/11n wireless local area network (WLAN) and the asymmetric digital subscriber loop (ADSL)  OFDM Systems Advantages The basic idea of OFDM is to transform a frequency-selective fading channel into several parallel frequency at fading subchannels on which modulated symbols are transmitted. Orthogonal Frequency-Division Multiple Access Orthogonal frequency-division multiple access (OFDMA), is a combination of OFDM and FDMA Orthogonal Frequency-Division Multiple Access Subcarrier Allocation Each user is assigned to a set of subcarriers called a subchannel. suppose that there are N subcarriers and U users in the system. – N subcarriers are divided into S subchannels, in which one subchannel consists of P = N=S subcarriers. – It is obvious that the system can, at maximum, support only U S users simultaneously. In a subcarrier assignment scheme, each users subchannel occupies a group of P adjacent subcarriers. This scheme is called localized subcarrier allocation.