Lecture Notes: EEEC6440315 Communication Systems - Spectral Efficiency

Lecture Notes: EEEC6440315 Communication Systems - Spectral Efficiency

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  EEEC6440315 COMMUNICATION SYSTEMS SpectralEfficiency FACULTY OF ENGINEERING AND COMPUTER TECHNOLOGY BENG (HONS) IN ELECTRICALAND ELECTRONIC ENGINEERING Ravandran Muttiah BEng (Hons) MSc MIET
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  Methods Advantages DisadvantagesApplications DSB-SC Lower power consumption The modulation system is simple Complex detection Analog TV systems: to transmit color information SSB-SC Better management of the frequency spectrum. Low power consumption The generation of exact SSB is rather difficult. Complex detection Two way radio Frequency division multiplexing VSB-SC It is a compromise between DSB and SSB. Therefore it is easier to generate than SSB-SC Demodulation system is still complex. Its bandwidth is about 25 % greater than SSB-SC Analog TV broadcast systems Items 1~3 with carrier Envelope detection is possible Low power efficiency Wireless broadcasting systems 8,16 level VSB-SC ATSC digital television standard FM Increased immunity to noise Require a larger bandwidth Radio broadcasting; direct- satellite broadcasting PM Increased immunity to noise More complex receiving hardware Data communications systems ASK Bandwidth efficiency Low power efficiency PSK Power efficiency. System is the simplest Low bandwidth efficiency Low speed communication 𝑀 − ary PSK Require more bandwidth Transmitted power is lower V − 32 Telephone data/fax modems. Satellite transmission of DTVB programs Table 1 1
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  Methods Advantages DisadvantagesApplications QAM An attractive alternative to SSB-SC. Sharp cutoff band-pass filters are not required Compared with SSB, QAM is more exacting in terms of the carrier frequency and phase or the requirement for a distortion- less transmission medium Used in cable communication for the transmission of DTV and internet traffic between cable modem and modem termination systems SHD AM/FM RECEIVERS Increased selectivity against adjacent channels Interference from signal frequencies close to the intermediate frequency Radio broadcasting LINE CODES Power efficiency. Error detection and correction capability. Zero PSD at 𝑓 = 0 Transmit only one bit of information over the interval of 𝑇b second Optical communications 𝑀 − ary PAM Transmits more bits in a symbol time Require more power 𝑀2 less system complexity High rate data links 2
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  Abbreviation ATSC Advanced TelevisionSystems Committee DSB Double sideband DTV Digital television SC Suppressed carrier SHD Superheterodyne SSB Single sideband VSB Vestigial sideband Table 2 3
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  Determining Spectral Efficiency Spectralefficiency is a method of how quickly data can be transmitted in an assigned bandwidth, and the unit of measurement is bits/s/Hz (b/s/Hz). Each type of modulation has a maximum theoretical spectral efficiency measure. Signal to Noise Ratio (SNR) is another important factor that influence spectral efficiency. It also can be expressed as the Carrier to Noise Ratio (CNR). The measure is the BER for a given CNR value. BER is the percentage of errors that occur in a given number of bits transmitted. As the noise become larger compared to the signal level, more errors occur. Some modulation methods are more immune to noise than others. Amplitude modulation methods like ASK/OOK and QAM are far more susceptible to noise so they have a higher BER for a given modulation. Phase and frequency modulation (BPSK, FSK, etc.) far better in a noisy environment so they require less signal power for a given noise level. 4
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  5 Bandwidth efficiency orsometimes called information density is often used to compare the performance of one digital modulation technique to another. In essence, it is the ratio of the transmission bit rate to the minimum bandwidth required for a particular modulation scheme. Bandwidth efficiency generally is normalized to a 1 Hz bandwidth and, thus indicates the number of bits that can be propagated through a medium for each hertz of bandwidth. Mathematically, bandwidth efficiency is, BW Efficiency = transmission rate (bps) minimum bandwidth (Hz) = bits sec hertz = bits second cycles second = bits cycle
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  Bandwidth Efficiency ForPopular Digital Modulation Methods Type of Modulation Bandwidth Efficiency (bits/s/Hz) FSK < 1 (depends on modulation index) GMSK 1.35 BPSK 1 QPSK 2 8PSK 3 16QAM 4 64QAM 6 OFDM > 10 (depends on the type of modulation and the number of subcarriers) Table 3 6
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  Other Factors EffectingSpectral Efficiency While modulation plays a key role in the spectral efficiency we can expect, other aspects in wireless design influence it as well. For example, the use of Forward Error Correction (FEC) techniques can greatly improve the Bit Error Rate (BER). Such coding methods add extra bits may be used, so that errors can be detected and corrected. These extra coding bits add overhead to the signal, reducing the net bit rate of the data, but that’s usually an acceptable tradeoff for the single- digit dB improvement in CNR. Such coding gain is common to almost all wireless systems today. Digital compression is another useful technique. The digital data to be sent is subjected to a compression algorithm that greatly reduces the amount of information. This allows digital signals to be reduced in content so they can be transmitted as shorter, slower data streams. 7
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  For example, voicesignals are compressed for digital cell phones and Voice over Internet Protocol (VoIP) phones. Music is compressed in MP3 or AAC files for faster transmission and less storage. Video is compressed so high-resolution images can be transmitted faster or in bandwidth- limited systems. Another factor affecting spectral efficiency is the use of Multiple-Input Multiple-Output (MIMO), which is the use of multiple antennas and transceivers to transmit two or more bit streams. A single high-rate stream is divided into two parallel streams and transmitted in the same bandwidth simultaneously. By coding the streams and their unique path characteristics, the receiver can identify and demodulate each stream and reassemble it into the original stream. MIMO, therefore, improves data rate, noise performance, and spectral efficiency. Newer Wireless LAN (WLAN) standards like 802.11n and 802.11ac/ad and cellular standards like Long Term Evolution (LTE) and WiMAX use MIMO. 8
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  Implementing Modulation AndDemodulation In the past, unique circuits implemented modulation and demodulation. Today, most modern radios are Software-Defined Radios (SDR) where functions like modulation and demodulation are handled in software. Digital Signal Processing (DSP) algorithms do the job previously assigned to modulator and demodulator circuits. The modulation process begins with the data to be transmitted being fed to a DSP device that generates two digital outputs, which are needed to define the amplitude and phase information required at the receiver to recover the data. The DSP produces two baseband streams that are sent to Digital-to-Analog Converters (DACs) that produce the analog equivalents. These modulation signals feed the mixers along with the carrier. There is a 90° shift between the carrier signals to the mixers. The resulting quadrature output signals from the mixers are summed to produce the signal to be transmitted. If the carrier signal is at the final transmission frequency, the composite signal is ready to be amplified and sent to the antenna. This is called direct conversion. Alternately, the carrier signal may be at a lower Intermediate Frequency (IF). The IF signal is upconverted to the final carrier frequency by another mixer before being applied to the transmitter PA. 9
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  At the receiver,the signal from the antenna is amplified and downconverted to IF or directly to the original baseband signals. The amplified signal from the antenna is applied to mixers along with the carrier signal. Again, there is a 90° shift between the carrier signals applied to the mixers. The mixers produce the original baseband analog signals, which are then digitized in a pair of Analog-to-Digital Converters (ADCs) and sent to the DSP circuitry where demodulation algorithms recover the original digital data. There are three important points to consider. First, the modulation and demodulation processes use two signals in quadrature with one another. The DSP calculations call for two quadrature signals if the phase and amplitude are to be preserved and captured during modulation or demodulation. 10
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  Second, the DSPcircuitry may be a conventional programmable DSP chip or may be implemented by fixed digital logic implementing the algorithm. Fixed logic circuits are smaller and faster and are preferred for their low latency in the modulation or demodulation process. Third, the PA in the transmitter needs to be a linear amplifier if the modulation is QPSK or QAM to faithfully reproduce the amplitude and phase information. For ASK, FSK, and BPSK, a more efficient nonlinear amplifier may be used. 11
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  The Pursuit OfGreater Spectral Efficiency With spectrum being a finite entity, it is always in short supply. The Communications Commission and other government bodies have assigned most of the electromagnetic frequency spectrum over the years, and most of that is actively used. Shortages now exist in the cellular and land mobile radio sectors, inhibiting the expansion of services such as high data speeds as well as the addition of new subscribers. One approach to the problem is to improve the efficiency of usage by squeezing more users into the same or less spectrum and achieving higher data rates. Improved modulation and access methods can help. One of the most crowded area of spectrum is the Land Mobile Radio (LMR) and Private Mobile Radio (PMR) spectrum used by the federal government, state governments and local public safety agencies like fire and police departments. Currently, they’re assigned spectrum by FCC license in the 150 to 174 MHz VHF spectrum and 421 to 512 MHz UHF spectrum. 12
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  (1) Erhan A.Ince, Communication Systems, Eastern Mediterranean University, 2003. References 13