| The signal may be described in several | | | | Signal insulation can also be accomplished by |
| domains, and an appropriate manipulation of | | | | assigning each signal a different code (a |
| the signals within these domains may lead to | | | | key, a password)—the physical channel. |
| effective use of the transmission medium. The | | | | A code is built as a sequence of symbols |
| signal domains most commonly used for access | | | | belonging to an alphabet. In an ideal |
| purposes are frequency, time, code, and | | | | situation, these codes must present zero |
| space. | | | | cross-correlation so that they can be |
| | | | univocally discriminated (e.g., for different |
| Frequency Domain | | | | sources, different passwords are assigned). |
| | | | For a finite alphabet, the number of codes is |
| The radio spectrum is a precious resource, | | | | obviously finite. Therefore, the larger the |
| the quintessence of wireless communication | | | | alphabet, the larger the number of orthogonal |
| systems. Because radio propagation does not | | | | codes in the alphabet and the longer each |
| recognize geopolitical boundaries and because | | | | code. For a given transmission rate, the |
| political, economic, and social aspirations | | | | longer the code, the longer the time to |
| may vary from country to country, | | | | transmit the code and the longer the time to |
| international cooperation leading to an | | | | detect the code. Should it be transmitted, |
| intelligent and efficient use of the | | | | and detected, in a shorter period of time, |
| frequency spectrum is mandatory. Because of | | | | the transmission rate must be increased as |
| the explosion of the demand for and the | | | | well as the required bandwidth. Therefore, |
| diversification of wireless services, special | | | | for a limited bandwidth the number of |
| attention have been given to | | | | orthogonal codes (code slots) is also |
| spectrumallocation issues.Wireless | | | | limited. |
| communications systems have been driven to | | | | |
| use high frequencies due to the congestion at | | | | Space Domain |
| the lower portion of the frequency spectrum, | | | | |
| where the available bandwidth is insufficient | | | | Signal insulation in the space domain can be |
| to satisfy the great demand for mobile | | | | achieved in two possible dimensions: distance |
| services. | | | | and angle—the physical channels. The |
| | | | distance dimension exploits the fact that the |
| However, dealing with high frequencies | | | | propagation loss increases with the increase |
| usually leads to intricate problems that are | | | | of the distance between transmitter and |
| severely aggravated by the mobility of the | | | | receiver. Thus, signals using the same |
| users. Services are assigned fixed bandwidth, | | | | frequency but transmitted by sources |
| not necessarily in a contiguous fashion. For | | | | sufficiently apart from each other may not |
| competition purposes, for a given service the | | | | strongly interfere with each other. In the |
| frequency band is split into subbands, each | | | | same way, a given signal may reach the |
| of which is allotted to different service | | | | receiver through different paths (due to |
| operators. Each one of these bands is then | | | | multiple reflections, for example). Each |
| further split into two halves, one for the | | | | multipath signal suffers different |
| forward link and the other for the reverse | | | | attenuations and different delays, according |
| link. Subsequent divisions are carried out to | | | | to the length of the path traveled. |
| form the nonoverlapping frequency slots | | | | Therefore, both attenuation and delay, |
| (channels). The channel bandwidth (channel | | | | jointly or independently, can be used to |
| spacing) is determined according to criteria | | | | detect each multipath signal. The angle |
| such as the services to be provided and the | | | | dimension exploits the fact that, by |
| available technology. Each channel—the | | | | illuminating wedges of a circular area, |
| physical channel—is identified by a | | | | signals simultaneously using the same |
| carrier placed in the middle of the channel | | | | frequency may be discriminated by these very |
| band. | | | | wedges within which they are located. Smart |
| | | | antennas may be used to keep track of these |
| Time Domain | | | | signals. |
| | | | |
| Signal insulation in the time domain is | | | | Brief Remarks on Signal Domains |
| accomplished by allowing the information to | | | | |
| use the frequency band during a specific | | | | The most commonly used and most |
| period of time (time slot)—the | | | | straightforward way of accomplishing radio |
| physical channel. Nonoverlapping time slots | | | | signal insulation is by assigning different |
| constitute the orthogonal channels. For any | | | | frequency carriers to different signals. This |
| given piece of information, the aim is to | | | | technique is widely employed by both analog |
| transmit the information in as short a period | | | | and digital wireless systems. Insulation in |
| of time as possible, so that more information | | | | the time domain has been boosted by the |
| can be conveyed in the same frequency band; | | | | digital technology and is widely used in |
| this is achieved by including more time slots | | | | wireless communications. Insulation in the |
| per carrier. As before, this certainly | | | | code domain is a well-known technique that |
| depends on the services to be provided and on | | | | has long been used for military as well as |
| the available technology. Access in the | | | | satellite communications applications. The |
| timedomaincharacterizes the transmission | | | | move toward high-capacity wireless systems |
| occurring in bursts because for the same | | | | has found great support in this technique. |
| source the information will occupy the | | | | Insulation in the space domain is widely used |
| carrier only in specific periods of time. | | | | in wireless communications. More |
| | | | specifically, the cellular concept with its |
| Code Domain | | | | frequency reuse philosophy constitutes an |
| | | | example of such an application. |