Spectral efficiency

Spectral efficiency (alternatively, spectrum efficiency or bandwidth efficiency) refers to the information rate that can be transmitted over a given bandwidth in a specific communication system. It is a measure of how efficiently limited frequency spectrum is utilized by the physical layer protocol, and sometimes by the medium access control (the channel access protocol).

Spectral efficiency (alternatively, spectrum efficiency or bandwidth efficiency) refers to the information rate that can be transmitted over a given bandwidth in a specific communication system. It is a measure of how efficiently limited frequency spectrum is utilized by the physical layer protocol, and sometimes by the medium access control (the channel access protocol).

Link spectral efficiency

Example 2: A V.92 modem for the telephone network can transfer downstream and upstream over an analog telephone network. Due to filtering in the telephone exchange, the frequency range is limited to between 300 hertz and 3,400 hertz, corresponding to a bandwidth of 3,400 − 300 = 3,100 hertz. The spectral efficiency or modulation efficiency is downstream, and upstream. An upper bound for the attainable modulation efficiency is given by the Nyquist rate or Hartley's law as follows: For a signaling alphabet with M alternative symbols, each symbol represents N = log 2 M bits. N is the modulation efficiency measured in bit/symbol or bpcu. In the case of baseband transmission (line coding or pulse-amplitude modulation) with a baseband bandwidth (or upper cut-off frequency) B, the symbol rate can not exceed 2B symbols/s in view to avoid intersymbol interference. Thus, the spectral efficiency can not exceed 2N (bit/s)/Hz in the baseband transmission case. In the passband transmission case, a signal with passband bandwidth W can be converted to an equivalent baseband signal (using undersampling or a superheterodyne receiver), with upper cut-off frequency W/2. If double-sideband modulation schemes such as QAM, ASK, PSK or OFDM are used, this results in a maximum symbol rate of W symbols/s, and in that the modulation efficiency can not exceed N (bit/s)/Hz. If digital single-sideband modulation is used, the passband signal with bandwidth W corresponds to a baseband message signal with baseband bandwidth W, resulting in a maximum symbol rate of 2W and an attainable modulation efficiency of 2N (bit/s)/Hz. Example 3: A 16QAM modem has an alphabet size of M = 16 alternative symbols, with N = 4 bit/symbol or bpcu. Since QAM is a form of double sideband passband transmission, the spectral efficiency cannot exceed N = . Example 4: The 8VSB (8-level vestigial sideband) modulation scheme used in the ATSC digital television standard gives N=3 bit/symbol or bpcu. Since it can be described as nearly single-sideband, the modulation efficiency is close to 2N = . In practice, ATSC transfers a gross bit rate of over a 6 MHz wide channel, resulting in a modulation efficiency of . Example 5: The downlink of a V.92 modem uses a pulse-amplitude modulation with 128 signal levels, resulting in N = 7 bit/symbol. Since the transmitted signal before passband filtering can be considered as baseband transmission, the spectral efficiency cannot exceed 2N = over the full baseband channel (0 to 4 kHz). As seen above, a higher spectral efficiency is achieved if we consider the smaller passband bandwidth. Example 9: In the W-CDMA 3G cellular system, every phone call is compressed to a maximum of (the useful bitrate), and spread out over a 5 MHz wide frequency channel. This corresponds to a link throughput of only . Let us assume that 100 simultaneous (non-silent) calls are possible in the same cell. Spread spectrum makes it possible to have as low a frequency reuse factor as 1, if each base station is divided into 3 cells by means of 3 directional sector antennas. This corresponds to a system spectrum efficiency of over 1 × 100 × 0.0017 = per site, and per cell or sector. The spectral efficiency can be improved by radio resource management techniques such as efficient fixed or dynamic channel allocation, power control, link adaptation and diversity schemes. A combined fairness measure and system spectral efficiency measure is the fairly shared spectral efficiency.

Comparison table

Examples of predicted numerical spectral efficiency values of some common communication systems can be found in the table below. These results will not be achieved in all systems. Those further from the transmitter will not get this performance. |} N/A means not applicable.

See also

Baud CDMA spectral efficiency Channel capacity Comparison of mobile phone standards Cooper's Law Goodput Network throughput Orders of magnitude (bit rate) Radio resource management (RRM) Spatial capacity

References

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