What is Spectral Efficiency?

There is an extension of this post under the title Spectral Efficiency Revisited

 

Spectral efficiency, one of the crucial term for every type of network whether it is wi fi network, or wimax or cellular network. The main aim of the designer is to increase the spectral efficiency as much as possible and that is why there are terms like cell splitting, cell sectoring etc. in cellular system. In this post, spectral efficiency is defined in brief and its importance is elaborated.

There are many ways to define spectrum efficiency. These definitions are given below in accordance.

1. As per ITU – R Recommendations
   

SM.1046-1 on “DEFINITION OF SPECTRUM USE AND EFFICIENCY OF A RADIO SYSTEM” mentions

” Efficient use of spectrum is achieved by (among other things) the isolation obtained from antenna directivity, geographical spacing, frequency sharing, or orthogonal frequency use and time-sharing or time division and these considerations reflected in definition of spectrum utilization. Therefore, the measure of spectrum utilization- Spectrum Utilization factor, U, is defined to be the product of the frequency bandwidth, the geometric (geographic) space, and the time denied to other potential users:

U = B . S . T

where, B: Frequency bandwidth, S: geometric space (usually area) and T: time”.

1. According to ITU-R Recommendation SM.1046-1
   

   “According to the definition of SUE (or Spectrum efficiency as a shortened term) of a radio communication system, it can be expressed by:
   

   SU E= M / U = M / (B . S . T)
   

   where, M: amount of information transferred over a distance”.
   

2. For cellular mobile system, Spectral Efficency is defined as
   

   SUE= (Traffic in Erlang) / (Amount of spectrum in MHz X Area in Sq. Kms.)
   

   For a specified Grade of Services
   

   The definition takes into account the traffic carrying capacity of a technology to replicate the available spectrum in the unit area for a specified QoS (Quality of Services).
   

3. There is one more definition of Spectral Efficiency as given in wiki,
   

   “In digital wireless networks, the system spectral efficiency or area spectral efficiency is typically measured in bit/s/Hz/area unit [(bit/s)/Hz per unit area], bit/s/Hz/cell [(bit/s)/Hz per cell] or bit/s/Hz/site [(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. It may for example be defined as the maximum throughput or goodput, summed over all users in the system, divided by the channel bandwidth. “
   

   Also,
   

   “The system spectral efficiency of a cellular network may also be expressed as the maximum number of simultaneous phone calls per area unit over 1 MHz frequency spectrum in (E/MHz)/cell (erlangs per megahertz per cell), (E/MHz)/sector, (E/MHz)/site, or (E/MHz)/km².”
   

   Spectral Efficiency is calculated to figure out how efficiently the spectrum allotted is being utilized and the main goal of service provider is to accommodate maximum number of users in the limited bandwidth. That is why spectral efficiency calculation plays very important role in wireless network planning.