Information rate that can be transmitted over a given bandwidth
Spectral efficiency,spectrum efficiency orbandwidth efficiency refers to theinformation rate that can be transmitted over a givenbandwidth in a specific communication system. It is a measure of how efficiently a limited frequency spectrum is utilized by thephysical layer protocol, and sometimes by themedium access control (thechannel access protocol).[1]
Thelink spectral efficiency of a digital communication system is measured inbit/s/Hz,[2] or, less frequently but unambiguously, in(bit/s)/Hz. It is thenet bit rate (useful information rate excludingerror-correcting codes) ormaximum throughput divided by thebandwidth in hertz of acommunication channel or adata link. Alternatively, the spectral efficiency may be measured inbit/symbol, which is equivalent tobits perchannel use (bpcu), implying that the net bit rate is divided by thesymbol rate (modulation rate) or line code pulse rate.
Link spectral efficiency is typically used to analyze the efficiency of adigital modulation method orline code, sometimes in combination with aforward error correction (FEC) code and other physical layer overhead. In the latter case, a "bit" refers to a user data bit; FEC overhead is always excluded.
Themodulation efficiency in bit/s is thegross bit rate (including any error-correcting code) divided by the bandwidth.
Example 1: A transmission technique using onekilohertz of bandwidth to transmit 1,000 bits per second has a modulation efficiency of 1 (bit/s)/Hz.
Example 2: AV.92 modem for the telephone network can transfer 56,000 bit/s downstream and 48,000 bit/s 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 56,000/3,100 = 18.1 (bit/s)/Hz downstream, and 48,000/3,100 = 15.5 (bit/s)/Hz upstream.
An upper bound for the attainable modulation efficiency is given by theNyquist rate orHartley's law as follows: For a signaling alphabet withM alternative symbols, each symbol representsN = log2M bits.N is the modulation efficiency measured inbit/symbol orbpcu. In the case ofbaseband transmission (line coding orpulse-amplitude modulation) with a baseband bandwidth (or upper cut-off frequency)B, thesymbol rate can not exceed 2B symbols/s in view to avoidintersymbol interference. Thus, the spectral efficiency can not exceed 2N (bit/s)/Hz in the baseband transmission case. In thepassband transmission case, a signal with passband bandwidthW can be converted to an equivalent baseband signal (usingundersampling or asuperheterodyne receiver), with upper cut-off frequencyW/2. If double-sideband modulation schemes such asQAM,ASK,PSK orOFDM are used, this results in a maximum symbol rate ofW symbols/s, and in that the modulation efficiency can not exceedN (bit/s)/Hz. If digitalsingle-sideband modulation is used, the passband signal with bandwidthW corresponds to a baseband message signal with baseband bandwidthW, 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 ofM = 16 alternative symbols, withN = 4 bit/symbol or bpcu. Since QAM is a form of double sideband passband transmission, the spectral efficiency cannot exceedN = 4 (bit/s)/Hz.
Example 4: The8VSB (8-level vestigial sideband) modulation scheme used in theATSC digital television standard givesN=3 bit/symbol or bpcu. Since it can be described as nearly single-side band, the modulation efficiency is close to 2N = 6 (bit/s)/Hz. In practice, ATSC transfers a gross bit rate of 32 Mbit/s over a 6 MHz wide channel, resulting in a modulation efficiency of 32/6 = 5.3 (bit/s)/Hz.
Example 5: The downlink of a V.92 modem uses a pulse-amplitude modulation with 128 signal levels, resulting inN = 7 bit/symbol. Since the transmitted signal before passband filtering can be considered as baseband transmission, the spectral efficiency cannot exceed 2N = 14 (bit/s)/Hz 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.
If aforward error correction code is used, the spectral efficiency is reduced from the uncoded modulation efficiency figure.
Example 6: If a forward error correction (FEC) code withcode rate 1/2 is added, meaning that the encoder input bit rate is one half the encoder output rate, the spectral efficiency is 50% of the modulation efficiency. In exchange for this reduction in spectral efficiency, FEC usually reduces thebit-error rate, and typically enables operation at a lowersignal-to-noise ratio (SNR).
An upper bound for the spectral efficiency possible withoutbit errors in a channel with a certain SNR, if ideal error coding and modulation is assumed, is given by theShannon–Hartley theorem.
Example 7: If the SNR is 1, corresponding to 0decibel, the link spectral efficiency can not exceed 1 (bit/s)/Hz for error-free detection (assuming an ideal error-correcting code) according to Shannon–Hartley regardless of the modulation and coding.
Note that thegoodput (the amount of application layer useful information) is normally lower than themaximum throughput used in the above calculations, because of packet retransmissions, higher protocol layer overhead, flow control, congestion avoidance, etc. On the other hand, a data compression scheme, such as theV.44 orV.42bis compression used in telephone modems, may however give higher goodput if the transferred data is not already efficiently compressed.
The link spectral efficiency of a wireless telephony link may also be expressed as the maximum number of simultaneous calls over 1 MHz frequency spectrum in erlangs per megahertz, orE/MHz. This measure is also affected by the source coding (data compression) scheme. It may be applied to analog as well as digital transmission.
In wireless networks, thelink spectral efficiency can be somewhat misleading, as larger values are not necessarily more efficient in their overall use of radio spectrum. In a wireless network, high link spectral efficiency may result in high sensitivity to co-channel interference (crosstalk), which affects the capacity. For example, in acellular telephone network with frequency reuse,spectrum spreading andforward error correction reduce the spectral efficiency in (bit/s)/Hz but substantially lower the required signal-to-noise ratio in comparison to non-spread spectrum techniques. This can allow for much denser geographical frequency reuse that compensates for the lower link spectral efficiency, resulting in approximately the same capacity (the same number of simultaneous phone calls) over the same bandwidth, using the same number of base station transmitters. As discussed below, a more relevant measure for wireless networks would besystem spectral efficiency in bit/s/Hz per unit area. However, in closed communication links such as telephone lines and cable TV networks, and in noise-limited wireless communication system where co-channel interference is not a factor, the largest link spectral efficiency that can be supported by the available SNR is generally used.
System spectral efficiency or area spectral efficiency
In digitalwireless networks, thesystem spectral efficiency or area spectral efficiency is typically measured in(bit/s)/Hz per unit area, in(bit/s)/Hz percell, or in(bit/s)/Hz per site. It is a measure of the quantity of users or services that can be simultaneously supported by a limited radio frequency bandwidth in a defined geographic area.[1] It may for example be defined as the maximum aggregatedthroughput orgoodput, i.e. summed over all users in the system, divided by the channel bandwidth and by the covered area or number of base station sites. This measure is affected not only by the single-user transmission technique, but also bymultiple access schemes andradio resource management techniques utilized. It can be substantially improved by dynamicradio resource management. If it is defined as a measure of the maximum goodput, retransmissions due to co-channel interference and collisions are excluded. Higher-layer protocol overhead (above themedia access control sublayer) is normally neglected.
Example 8: In a cellular system based onfrequency-division multiple access (FDMA) with afixed channel allocation (FCA) cellplan using afrequency reuse factor of 1/4, each base station has access to 1/4 of the total available frequency spectrum. Thus, the maximum possible system spectral efficiency in(bit/s)/Hz per site is 1/4 of the link spectral efficiency. Each base station may be divided into 3 cells by means of 3 sector antennas, also known as a 4/12 reuse pattern. Then each cell has access to 1/12 of the available spectrum, and the system spectral efficiency in(bit/s)/Hz per cell or(bit/s)/Hz per sector is 1/12 of the link spectral efficiency.
The system spectral efficiency of acellular network may also be expressed as the maximum number of simultaneous phone calls per area unit over 1 MHz frequency spectrum inE/MHz per cell,E/MHz per sector,E/MHz per site, or(E/MHz)/m2. This measure is also affected by the source coding (data compression) scheme. It may be used in analog cellular networks as well.
Low link spectral efficiency in (bit/s)/Hz does not necessarily mean that an encoding scheme is inefficient from a system spectral efficiency point of view. As an example, considerCode Division Multiplexed Access (CDMA)spread spectrum, which is not a particularly spectral-efficient encoding scheme when considering a single channel or single user. However, the fact that one can "layer" multiple channels on the same frequency band means that the system spectrum utilization for a multi-channel CDMA system can be very good.
Example 9: In theW-CDMA 3G cellular system, every phone call is compressed to a maximum of 8,500 bit/s (the useful bitrate), and spread out over a 5 MHz wide frequency channel. This corresponds to a link throughput of only 8,500/5,000,000 = 0.0017 (bit/s)/Hz. 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 = 0.17 (bit/s)/Hz per site, and 0.17/3 = 0.06(bit/s)/Hz per cell or sector.
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.
Spectral efficiency of common communication systems
Service
Standard
Launched, year
Max.net bit rate per carrier and spatial stream, R (Mbit/s)
