Multiple access has to be performed in systems where a medium is shared for transmission and reception by multiple users or entities. Those entities or users, sometimes also called nodes, are accessing the very same medium to transmit their information, other than, for example, in CS communication schemes. For example, within classical CS communications like Plain Old Telephone Systems (POTSs), each communication node gets a dedicated resource (a telephone landline) to dedicatedly access for the complete communication session. On the other hand, it is necessary to apply a multiple access method when multiple nodes share the same medium for their information transfer, in order to prevent or detect collisions on the shared medium.
This implies that the users share the same resources for communication in a certain way. Multiple access methods are mainly used with PS data transmission, as multiple nodes usually share the same resources for efficiency reasons. Packet Switched (PS) data transmission is mostly characterized by bursty traffic patterns. There are different schemes which can be applied to share those resources. These access schemes are known as multiple access methods.
Multiple access methods introduce rules for accessing the shared medium, generally resources. Care has to be taken not to use the same resources by two or more nodes at once, because this would result in distortion of the transferred information.
Transmission/reception resources are one or multiples of the following: time, code, frequency, space, etc.
One of the first radio-based multiple access scheme is Aloha. Early research in these schemes was carried out at the University of Hawaii in the early 1970s. In the Hawaiian language Aloha means "Hello" which indicates a fundamental mechanism: the university ran several campuses on different islands where an early radio-based packet data network was established. Stations immediately transmitted packets to be sent and waited for a fixed time (double the round-trip time of the most distant stations in the network plus the transmission and processing time of packets) for an acknowledgment (ACK) from the receiving station. If an ACK was received, the packet was retransmitted. Aloha shows that many collisions of packets occur when applying this scheme.
Most modern access methods use mechanisms of avoiding, detecting, or preventing collisions within the shared medium, in order to reach a certain system efficiency. A basic method to avoid collisions is sensing the medium before starting a transmission, to avoid interrupting or interfering with an ongoing transmission of other communication peers.
This scheme is known as Carrier Sense Multiple Access (CSMA). Sensing the medium before transmission adds Collision Avoidance functionality (i.e., CSMA/CA). In addition to sensing the medium before transmission, one has to take care with detecting collisions when two terminals have started a transmission at the same time. This scheme, for example, is used with Ethernet local area networks, CSMA/CD. If a collision is detected from both transmit entities, a collision resolution mechanism must be applied. Both peers select a random time in a defined range in order to restart their transmission after this randomly selected period of time. The process starts by sensing the medium again, as the other collision peer (or a new transmission of a third node) could already have (re)started its transmission as it has selected a shorter back-off period. The efficiency can be increased by introducing discrete back-off slots.
A special effect of wireless networks without infrastructure has to be taken care of. As mentioned above, a station listens to the channel before sending data to avoid a collision with an ongoing transmission from two other stations at that time. But this behavior is not fully sufficient to avoid collisions at all stations within the transmission range. If a node within the transmission range receives a data frame from another station which is not in the range of a node also trying to allocate the channel, this node will interfere with reception of the other node without warning. This unrecognized collision scenario is called the hidden terminal effect, because the sending node is "hidden" or out of range.
Figure 1 shows a typical scenario for the hidden terminal effect. The circles around the stations demonstrate the transmission ranges of the nodes. Node A has a link to node B but does not know about the existence of node C, which is the hidden terminal from the point of view of node A. Node B has a link to both node A and node C. In this scenario node C attempts to transmit data to node B and indicates this with a Request to Send (RTS) packet with the destination address of node B. The designated destination node B confirms this request with a Clear to Send (CTS) packet to node C; this CTS packet is also received by node A. Thereby, node A detects that there is another station while transmitting, until the reception node B sends an ACK packet to complete this transmission. Within that time, node A will not initiate any transmission, not to node B nor to any other possible node, in order not to corrupt reception at node B.
Note that this effect only occurs in mobile networks without fixed infrastructure, for example, mobile ad-hoc networks. Thus, the hidden terminal effect does not affect LTE.
One way of sharing the same resources between communication entities is to introduce a master entity, which takes care of the usage of the shared medium. This scheme is widely used within mobile cell phone networks, as the base station controls and grants the access of resources within its cell. Within a Time Division Multiple Access (TDMA) mobile network, one frequency resource is divided into time slots which are used by different users. Mobile networks of the second generation (GSM) share eight time slots within a certain frequency band. Figure 2 illustrates the DL frame of a TDMA system which uses FDD. Thus, UL and DL utilize different frequency bands and both use TDMA. A Guard Period (GP) used between time slots serves to reduce the risk of multi-user interference. The training sequence in the middle of the time slot is used to estimate the wireless channel conditions. This information is extrapolated time-wise to the adjacent data sections.
Non-overlapping frequency bands are assigned to different UEs within Frequency Division Multiple Access (FDMA). A single user allocates one frequency resource which is used for the complete active time. Guard bands need to be designed for the system in order to prevent multi-user interference. Systems (as GSM) often use a mixture of TDMA and FDMA, as multiple frequency channels with, for example, eight time slots serve as cell resources. Figure 3 depicts a FDMA system with five users assigned to five different frequency bands.
UMTS users share orthogonal codes which are used to spread transmission data in order to be transmitted on the same frequency resources within one cell. Picture this as a metaphor: each user uses the same time, space, and frequency resources, but communicates in a different language. This multiple access method is known as Code Division Multiple Access (CDMA) and illustrated in Figure 4.
The shared resources in LTE are very small frequency bands and small transmission time slots. Thus, the method combines FDMA and TDMA behavior, but in a very agile way. Frequency and time resources are often reassigned for diversity or efficiency reasons during ongoing user transmissions. This method is known as Orthogonal Frequency Division Multiple Access (OFDMA).
Within mobile cell phone network systems, time slot resources, frequency resources, and code resources are controlled by the base station. This controlled multiple access method is known as scheduling. This scheduling case allows active transmission collision protection, as well as other parameters, to be taken into account when granting transmissions between users. These parameters can be fairness, QoS requests of nodes, medium/channel quality or (charging) policies.
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