Good idea，Ｉbelive you that you can sucess if you can do it hard and do it continute.

 Medium access control

Medium access control (MAC) comprises all mechanisms that regulate user access to a medium using SDM, TDM, FDM, or CDM. the physical layer, layer 1, of the ISO/OSI reference model, MAC belongs to layer 2, the data link control layer (DLC). Layer 2 is subdivided into the logical link control (LLC), layer 2b, and the MAC. several MAC mechanisms will be presented for the multiplexing schemes. SDM and FDM are typically used in a rather fixed manner, i.e., a certain space or frequency (or frequency hopping pattern) is assigned for a longer period of time. TDM can be used in a very flexible way, as tuning in to a certain frequency does not present a problem, but time can be allocated on demand and in a distributed fashion. Well-known algorithms are Aloha (in several versions), different reservation schemes, or simple polling. CDM has to assign certain codes to allow the separation of different users in code space.

 SDMA

Space Division Multiple Access (SDMA) is used for allocating a separated space to users in wireless networks. A typical application involves assigning an optimal base station to a mobile phone user. The mobile phone may receive several base stations with different quality. A MAC algorithm could now decide which base station is best, taking into account which frequencies (FDM), time slots (TDM) or code (CDM) are still available (depending on the technology). Typically, SDMA is never used in isolation but always in combination with one or more other schemes. The basis for the SDMA algorithm is formed by cells and sectorized antennas which constitute the infrastructure implementing space division multiplexing (SDM)

 FDMA

Frequency division multiple access (FDMA) comprises all algorithms allocating frequencies to transmission channels according to the frequency division multiplexing (FDM). Allocation can either be fixed (as for radio stations or the general planning and regulation of frequencies) or dynamic (i.e., demand driven). 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 with TDMA. FDM is often used for simultaneous access to the medium by base station and mobile station in cellular networks. Here the two partners typically establish a duplex channel, i.e., a channel that allows for simultaneous transmission in both directions. The two directions, mobile station to base station and 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.

As for example FDM and FDD, Figure 3.3 shows the situation in a mobile phone network based on the GSM standard for 900 MHz. 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.

 TDMA

Compared to FDMA, time division multiple access (TDMA) offers a much more flexible scheme, which comprises all technologies that allocate certain time slots for communication, i.e., controlling TDM. Now tuning in to a certain frequency 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.

Now synchronization between sender and receiver has to be achieved in the time domain. Again this 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 an identification, but are not as flexible considering varying bandwidth requirements.

 CDMA

Finally, codes with certain characteristics can be applied to the transmission to enable the use of code division multiplexing (CDM). Code division multiple access (CDMA) systems use exactly these codes to separate different users in code space and to enable access to a shared medium without interference. The main problem is how to find “good” codes and how to separate the signal from noise generated by other signals and the environment.

A code for a certain user should have a good autocorre-lation2 and should be orthogonal to other codes. Orthogonal in code space has the same meaning as in standard space (i.e., the three dimensional space). Think of a system of coordinates and vectors starting at the origin, i.e., in (0, 0, 0).3 Two vectors are called orthogonal if their inner product is 0. Now let us translate this into code space and explain what we mean by a good autocorrelation. The Barker code (+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1), for example, has a good autocorrelation, i.e., the inner product with itself is large, the result is 11. This code is used for ISDN and IEEE 802.11.

Example explains the basic function of CDMA: -

• Two senders, A and B, want to send data. CDMA assigns the following unique and orthogonal key sequences: key Ak = 010011 for sender A, key BK = 110101 for sender B. Sender A wants to send the bit Ad = 1, sender B sends Bd = 0. To illustrate this example, let us assume that we code a binary 0 as –1, a binary 1 as +1. We can then apply the standard addition and multiplication rules.

• Both senders spread their signal using their key as chipping sequence (the term ‘spreading’ here refers to the simple multiplication of the data bit with the whole chipping sequence). In reality, parts of a much longer chipping sequence are applied to single bits for spreading. Sender A then sends the signal As = Ad*Ak = +1*(–1, +1, –1, –1, +1, +1) = (–1, +1, –1, –1, +1, +1). Sender B does the same with its data to spread the signal with the code: Bs = Bd*Bk = –1*(+1, +1, –1, +1, –1, +1) = (–1, –1, +1, –1, +1, –1).

• Both signals are then transmitted at the same time using the same frequency, so, the signals superimpose in space (analog modulation is neglected in this example). Discounting interference from other senders and environmental noise from this simple example, and assuming that the signals have the same strength at the receiver, the following signal C is received at a receiver: C = As + Bs = (–2, 0, 0, –2, +2, 0).

• The receiver now wants to receive data from sender A and, therefore, tunes in to the code of A, i.e., applies A’s code for despreading: C*Ak = (–2, 0, 0, –2, +2, 0)*(–1, +1, –1, –1, +1, +1) = 2 + 0 + 0 + 2 + 2 + 0 = 6. As the result is much larger than 0, the receiver detects a binary 1. Tuning in to sender B, i.e., applying B’s code gives C*Bk = (–2, 0, 0, –2, +2, 0)* (+1, +1, –1, +1, –1, +1) = –2 + 0 + 0 – 2 – 2 + 0 = –6. The result is negative, so a 0 has been detected.

Work hands on simulation, open issues, as a optimal schedule resources, power control (close loops), roaming with other access methods, efficiency of bits, etc.

Thanks Dr Ravish. The info has REALLY been helpful. I dont know if you can always be of help as I progress with my work sir. Have you ever worked on CDMA?

Welcome , i am always here for all student & member.

Whats the simulation platform for such technology?