Space Division Multiple Access is used to allocating a separated space to users in wireless networks. In this system, the base station has no information on the position of the mobile units within the cell and radiates the signal in all directions within the cell in order to provide radio coverage. A MAC algorithm could now decide which base station is best, taking into account which frequencies, time slots or codes are still available. It is a combination of one or more schemes. The basis for the SDMA algorithm is formed by cells and sectorized antennas which constitute the infrastructure implementing space division multiplexing. Show
TDMA: Time division multiple access offers a much more flexible scheme, which comprises all technologies that allocate certain time slots for communication, i.e., controlling TDM. If we tune into a certain frequency that is not necessary, i.e. the receiver can stay at the same frequency the whole time. Using only one frequency, and thus very simple receivers and transmitters, many different algorithms exist to control medium access. As already, mentioned, listening to different frequencies at the same time is quite difficult, but listening to many channels separated in time at the same frequency is simple. Almost all MAC schemes for wired networks work according to this principle, e.g., Ethernet, Token Ring, ATM etc. The synchronization between sender and receiver has to be achieved in the time domain. It can be done by using a fixed pattern similar to FDMA techniques, i.e., allocating a certain time slot for a channel, or by using a dynamic allocation scheme. Dynamic allocation schemes require an identification for each transmission as this is the case for typical wired MAC schemes (e.g., sender address) or the transmission has to be announced beforehand. MAC addresses are quite often used as identification. This enables a receiver in a broadcast medium to recognize if it really is the intended receiver of a message. Fixed schemes do not need identification, but are not as flexible considering varying bandwidth requirements. FDMA: Frequency division multiple access (FDMA) comprises all algorithms allocatingfrequencies to transmission channels according to the frequency divisionmultiplexing (FDM). Channels can be assigned to the same frequency at all times, i.e., pure FDMA,or change frequencies according to a certain pattern, i.e. FDMA combined withTDMA. FDM is often used for simultaneous access to the medium bybase station and mobile station in cellular networks. Here the two partners typicallyestablish a duplex channel, i.e. a channel that allows for simultaneoustransmission in both directions. The two directions, mobile station to base stationand vice versa are now separated using different frequencies. This scheme is then called frequency division duplex (FDD).
The two frequencies are also known as uplink, i.e., from mobile station to base station or from ground control to satellite, and as downlink, i.e., from base station to mobile station or from satellite to ground control. All uplinks use the band between 890.2 and 915 MHz, all downlinks use 935.2 to 960 MHz. According to FDMA, the base station, shown on the right side, allocates a certain frequency for up- and downlink to establish a duplex channel with a mobile phone. Up- and downlink have a fixed relation. If the uplink frequency is fu = 890 MHz + n·0.2 MHz, the downlink frequency is fd = fu + 45 MHz, i.e fd = 935 MHz + n·0.2 MHz for a certain channel n. The base station selects the channel. Each channel (uplink and downlink) has a bandwidth of 200 kHz. CDMA: Code Division Multiple Access takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it outover the entire available bandwidth. Multiple calls are overlaidon each other on the channel,with each assigned a unique sequence code. CDMA is a form of spread spectrum, which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range.
In CDMA, each phone's data has a unique code. All of the users transmit in the same wide-band chunk of spectrum. Each user's signal is spread over the entire bandwidth by a unique spreading code. At the receiver, that same unique code is used to recover the signal. Because CDMA systems need to put an accurate time-stamp on each piece of a signal, it references the GPS system for this information. Between eight and 10 separate calls can be carried in the same channel space as one analog AMPS call. In circuit oriented systems, the bandwidth is divided into FDMA or TDMA subchannels that are assigned on demand. The satellite SPADE system, for example, has a pool of FDMA subchannels that are allocated on request. It uses one subchannel operated in a TDMA fashion with one slot per frame permanently assigned to each user to handle the requests and releases of FDMA circuits. Intelsat's MAT-1 system uses the TDMA approach. The TDMA subchannels are periodically reallocated to meet the varying needs of earth stations. The Advanced Mobile Phone Service (AMPS), introduced by Bell Laboratories, is another example of a centrally controlled FDMA system. The uniqueness of this system, however, lies in an efficient management of the spectrum based on space division multiple access (SDMA). That is, each subchannel in the pool of FDMA channels is allocated to different users in separate geographical areas, thus considerably increasing the spectrum utilization. To accomplish space division, the AMPS system has a cellular structure and uses a centralized handoff procedure (executed by a central office) that reroutes the telephone connections to other available subchannels as the mobile users move from one cell to another. Polling SystemsIn packet oriented systems, polling consists of having a central controller send polling messages to the terminals, one by one, asking the polled terminal to transmit. If the polled terminal has something to transmit, it goes ahead; if not, a negative reply (or absence of reply) is received by the controller, which then polls the next terminal in sequence. Polling requires this constant exchange of control messages between the controller and the terminals and is efficient only if (1) the round-trip propagation delay is small, (2) the overhead due to polling messages is low, and (3) the user population is not a large bursty one. Adaptive Polling or ProbingThe primary limitation of polling in lightly loaded systems is the high overhead incurred in determining which of the terminals have messages. A modified polling technique called probing, based on a tree searching algorithm, helps decrease this overhead. This technique assumes that the central controller can broadcast signals to all terminals. First the controller interrogates all terminals, asking if any of them has a message to transmit, and repeats this question until some terminals respond by putting a signal on the line. When a response is received, the central station divides the population into subsets (according to some tree structure) and repeats the question to each of the subsets. The process is continued until the terminals having messages are identified. When a single terminal is interrogated, it transmits its message. This probing technique can be made adaptive by having the controller start a cycle by probing groups of smaller size as the probability of terminals having messages to transmit increases. Split-Channel Reservation Multiple Access (SRMA)An attractive alternative to polling is the use of explicit reservation techniques. In dynamic reservation systems, it is the terminal that makes a request for service on some channel whenever it has a message to transmit. The central scheduler manages a queue of requests and informs the terminal of its allocated time. In SRMA, the available bandwidth is divided into two channels, one used to transmit control information and the second used for the data messages themselves. The request channel is operated in a random access mode (ALOHA or CSMA). Upon correct reception of the request packet, the scheduling station computes the time at which the backlog on the message channel will empty and transmits back to the terminal an answer packet containing the address of the terminal and the time at which it can start transmission. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780750672917500285 Intercell Interference Coordination: Towards a Greener Cellular Network*Duy Trong Ngo, ... Tho Le-Ngoc, in Handbook of Green Information and Communication Systems, 2013 6.4.1 System Model for Downlink CoMPIn order to present a common framework for the downlink CoMP transmission, let us consider a multiuser multicell network with Q coordinated cells concurrently serving K UEs, as illustrated in Figure 6.1. Unlike the multiple access techniques, namely CDMA and OFDMA, considered in Section 6.3, this CoMP system model utilizes the Space Division Multiple Access (SDMA) technique for an additional degree of freedom. Herein, it is assumed that each BS and each UE is equipped with M transmit and 1 receive antennas, respectively. It should be noted that while the discussion in this part is limited to the case of single antenna at the UEs, it can be straightforwardly extended to systems with multiple receive antennas. Denote Qas the set of BSs and Kas the set of UEs. In addition, denote Qi⊆Qas the set of BSs serving UE i and Kq⊆Kas the set of UEs being served by BS q. Note that unlike the system model presented in Section 6.3, the intended data signals at a UE might be sent from multiple BSs, i.e., |Qi|>1under this CoMP framework. Consider the downlink transmission to a particular UE, say UE i; its received signal yican be modeled as (6.13)yi=∑q=1QhqiHxqi+∑j≠iK∑q=1QhqiHxqj+zi, where xqiis an M×1complex vector representing the transmitted signal at BS q for UE i,hqi∗is an M×1complex channel vector from BS q to UE i, and ziis AWGN with power σ2. Let sqibe the parameter indicating the assignment of UE i to BS q, where sqi=1if BS q transmits data to UE i, and sqi=0otherwise. This assignment is assumed to be known systemwide. Also note that Kqand Qiare now defined asKq=sqii=1Q|sqi=1and Qi=sqiq=1Q|sqi=1. In the coordinated beamforming design under consideration, the transmitted signal xqican be represented as xqi=sqiwqiui, where uiis a complex scalar representing the signal intended for UE i, and wqiis an M×1beamforming vector designed for UE i. Without loss of generality, let E[|ui|]=1. It is easy to verify that the SINR at UE i is indeed (6.14)γi=∑q=1QsqihqiHwqi2∑j≠iK∑q=1QsqjhqiHwqj2+σ2. Note that the term ∑j≠iK∑q=1QsqjhqiHwqj2denotes the interuser interference, including both intracell and intercell interferences, measured at UE i. This combined interference term is simply treated as background noise at the UE. In what follows, the CoMP downlink transmission is investigated under two design criteria: (i) CoMP for power minimization with guaranteed quality-of-service (QoS) requirements expressed in terms of the targeted achievable SINRs at the UEs, and (ii) CoMP for rate maximization under power constraints at the BSs. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780124158443000061 Different views of delay in resource allocation for wireless systems1Ana I Pérez-Neira, Marc Realp Campalans, in Cross-Layer Resource Allocation in Wireless Communications, 2009 7.4.1 Queueing delay parametersThe queueing delay depends on several system specifications affecting the time each packet remains in the system queue and for each one of the users. These factors [16] are: the arrival process, the service time distribution, the available number of links, the queue's maximum allowed length, the total number of users in the system and the service policy. Each of these concepts is now discussed. 7.4.1.1 The arrival processThe information packets are generated at a data source where the source statistics differ among the several applications, users and/or environments; each one defined by some distribution for the packets’ arrival process. This characterization plays a major role in the queueing delay, as it defines the average packets’ load that comes from the source and is stored at the system queue, thus denoting the input to the queue. Obviously, the more packets the source generates, the higher the queueing delay each packet suffers in the system, where the packets arrival process ak(t) for the kth user is defined as (7.11)ak(t)=∑p=1Pk(t)bp with Pk(t) as the number of arriving packets for a given time instant while bp defines the number of bits in each packet. Both Pk(t) and bp can be described through several distributions, thus defining a global distribution for the arrival process, which can follow a Poisson, Markov, deterministic, exponential or general distribution, among others. The value of ak(t) also describes the interarrival time between the packets, which mainly accounts for several scenarios: 1.A memory-less system with independent and identical packet distributions, where the distribution of ak(t) follows a Poisson function. 2.Future input values depending on the current situation with the arrival process being defined through an exponential distribution; this is modeled by a Markov chain. 3.A special Markov case when the future values can be only close (i.e. neighbor Markov states) to the current situation as denoted by a birth–death process. 7.4.1.2 Service time distributionService time distribution represents the amount of packets that can be serviced to the user per each time interval, defining the output of the system queue. Even the service time can be known for certain scheduling schemes, but the characterization of the service rate is very difficult for the wireless channels, due to the multipath, fading and the random channel fluctuations, therefore being a challenging aspect to quantify the queueing system delay. In all other scenarios (computer, wireline, etc.) the queueing delay has closed form expressions, but the already mentioned handicaps for communication through the wireless link make the queueing delay for the wireless channels, and more specifically for the multi-user wireless channels, more difficult to formulate. The distribution for the wireless channels is denoted with a general distribution as its characterization is not known, but, obviously, if the channel can absorb a larger number of packets, then the queueing delay decreases. 7.4.1.3 The available number of linksAnother important factor to determine the queueing delay is the number of links that are available for the transmitter side, where any multiple access scheme is always beneficial, as several packets can travel in the wireless channel at the same time. Therefore, code division multiple access (CDMA), frequency division multiple access (FDMA) and space division multiple access (SDMA) can substantially increase the output from the queue, thus motivating their use in the wireless channel to compensate for the bad channel performance, which is why special attention has been given to the role of SDMA throughout this book. 7.4.1.4 Queue's maximum allowed lengthThis parameter also reflects the number of accepted users (K), as a high number of accepted users in the system translates into a large number of information sources to the system queue, thus increasing the queueing delay for all the users. The number of users actually defines the system stability, as the system cannot account for a very high number of users because the average arrival rate would be larger than the average service rate, thus the packets remain in the system making the system queue increase until infinity, causing system instability [18]. To satisfy the maximum queue restriction, an optimization of the number of accepted users is usually carried out by a CAC unit [1,19], where a short allowed queue size would accept a lower number of information sources as compared to a system that enables a large queue. 7.4.1.5 The total number of users in the system (Nt)The CAC can choose the selected users among the total number of users in the system, but the total number of available users can be lower than the optimum number of users in the system, thus giving more importance to the parameter Nt. Notice that a smaller number of users in the system always induce a lower queueing delay for all the system, but a large number of serviced customers (users) are always beneficial to the operators, as income is increased. Therefore, a trade-off between the two effects is generated, which motivates the requirement for the CAC unit in the system. 7.4.1.6 The service policyIn the first part of this chapter, the access delay was presented where the scheduling policy that is employed by the system plays a major role in defining the users’ access delay. Now, in the section of queueing delay, another form of scheduling, but now among the packets present in the queue, also has an important impact over the queueing delay. As there exist several packets in the queue, some policy is required to select the packet that will be the first for transmission (i.e. the one that enters the access delay measurements). This policy is actually defined through the service policy implemented over the queue. Even it seems logical to think that the first packet in the queue is the one that has to enter the service, but several policies are proposed (and implemented) to account for different restrictions and system requirements [16,17], for example: First Come First Serve (FCFS), Last Come First Serve (LCFS), Shortest Processing Time First (SPT) and Biggest In First Served (BIFS), among others. All these serving philosophies are not channel aware policies, so that the packet is selected from the queue without any regard of the user channel characteristics. In the last section of this chapter, when presenting the stability operating conditions, the optimal service policy is shown to be channel aware in selecting the packet from the queue at the same time as the user with best channel conditions [18]. Notice that in multi-user systems, each packet from the source to its destination goes through two selection procedures: the service policy at the system queue and the scheduling step to select the user who accesses the channel, where each procedure has its own targets and restrictions. The packets from the different sources can be put all together in a joint queue for all the system, as previously discussed in this section, but it can be also put in several queues, for example a single queue for each application and/or for each accepted user in the system. An important aspect regarding the two already discussed types of delay (access and queue delays) is their relation to the throughput concept that was discussed in a previous chapter. A system that shows a high throughput for its wireless channel, then a larger number of packets, can be transmitted and the system queueing delay decreases for that system. Therefore, the throughput is beneficial for the queueing delay. On the other hand, for some scheduling philosophies, the increase in the system throughput comes at the expense of the access delay. For example, in the spatial multiplexing and scheduling with CSI policy, increasing the number of available users improves the system throughput [9], but the access delay is then increased as each user has to wait for a longer time until it is again serviced when K is large [19]. Therefore, a trade-off appears between throughput and access delay, making the throughput increase harmful for the access delay. View chapterPurchase book Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9780123741417000075 Medium access control protocols for safety applications in Vehicular Ad-Hoc Network: A classification and comprehensive survey Which one is used for allocating a separated space to user in wireless network?Space division multiple access or spatial division multiple access is a technique which is MIMO (multiple-input multiple-output) architecture and used mostly in wireless and satellite communication. It has the following features. All users can communicate at the same time using the same channel.
What is TDMA CDMA and FDMA?FDMA stands for Frequency Division Multiple Access. TDMA stands for Time Division Multiple Access. CDMA stands for Code Division Multiple Access. In this, sharing of bandwidth among different stations takes place. In this, only the sharing of time of satellite transponder takes place.
What is CDMA and TDMA?TDMA stands for Time Division Multiple Access. CDMA stands for Code Division Multiple Access. 2. In this, only the sharing of time of satellite transponder takes place. In this, there is sharing of both i.e. bandwidth and time among different stations takes place.
Which multiple access technique divides the channel capacity into separate areas?1.1 Frequency division multiple access. FDMA works on the principle of dividing the total bandwidth of the communication channel into a number of discrete segments, and allocating each segment exclusively to a user.
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