Cell and Sector Terminology

Cell and Sector Terminology

With cellular radio we use a simple hexagon to represent a complex object: the geographical area covered by cellular radio antennas. These areas are called cells. Using this shape let us picture the cellular idea, because on a map it only approximates the covered area. Why a hexagon and not a circle to represent cells?

When showing a cellular system we want to depict an area totally covered by radio, without any gaps. Any cellular system will have gaps in coverage, but the hexagonal shape lets us more neatly visualize, in theory, how the system is laid out. Notice how the circles below would leave gaps in our layout. Still, why hexagons and not triangles or rhomboids? Read the text below and we'll come to that discussion in just a bit.

Notice the illustration below. The middle circles represent cell sites. This is where the base station radio equipment and their antennas are located. A cell site gives radio coverage to a cell. Do you understand the difference between these two terms? The cell site is a location or a point, the cell is a wide geographical area. Okay?

Most cells have been split into sectors or individual areas to make them more efficient and to let them to carry more calls. Antennas transmit inward to each cell. That's very important to remember. They cover a portion or a sector of each cell, not the whole thing. Antennas from other cell sites cover the other portions. The covered area, if you look closely, resembles a sort of rhomboid, as you'll see in the diagram after this one. The cell site equipment provides each sector with its own set of channels. In this example, just below , the cell site transmits and receives on three different sets of channels, one for each part or sector of the three cells it covers.

Is this discussion clear or still muddy? skip ahead if you understand cells and sectors or come back if you get hung up on the terms at some later point. For most of us, let's go through this again, this time from another point of view. Mark provides the diagram and makes some key points here:

"Most people see the cell as the blue hexagon, being defined by the tower in the center, with the antennae pointing in the directions indicated by the arrows. In reality, the cell is the red hexagon, with the towers at the corners, as you depict it above and I illustrate it below. The confusion comes from not realizing that a cell is a geographic area, not a point. We use the terms 'cell' (the coverage area) and 'cell site' (the base station location) interchangeably, but they are not the same thing.

Mark goes on to talk about cells and sectors and the kind of antennas needed: "These days most cells are divided into sectors. Typically three but you might see just two or rarely six. Six sectored sites have been touted as a Great Thing by manufacturers such as Hughes and Motorola who want to sell you more equipment. In practice six sectors sites have been more trouble than they're worth. So, typically, you have three antenna per sector or 'face'. You'll have one antenna for the voice transmit channel, one antenna for the set up or control channel, and two antennas to receive. Or you may duplex one of the transmits onto a receive. By sectorising you gain better control of interference issues. That is, you're transmitting in one direction instead of broadcasting all around, like with an omnidirectional antenna, so you can tighten up your frequency re-use"

"This is a large point of confusion with, I think, most RF or radio frequency engineers, so you'll see it written about incorrectly. While at AirTouch, I had the good fortune to work for a few months with a consultant who was retired from Bell Labs. He was one of the engineers who worked on cellular in the 60s and 70s. We had a few discussions on this at AirTouch, and many of the engineers still didn't get it. And, of course, I had access to Dr. Lee frequently during my years there. It doesn't get much more authoritative than the guys who developed the stuff!"
Jim Harless, a regular contributor, recently checked in regarding six sector cells. He agrees with Mark about the early days, that six sector cells in AMPS did not work out. He notes that "At Metawave (link now dead) I've been actively involved in converting some busy CDMA cells to 6-sector using our smart antenna platform. Although our technology is vendor specific, you can't use it with all equipment, it actually works quite well, regardless of the added number of pilots and increase in soft handoffs. In short, six sector simply allows carriers to populate the cell with more channel elements. Also, they are looking for improved cell performance, which we have been able to provide. By the way, I think the reason early CDMA papers had inflated capacity numbers were because they had six sector cells in mind."
Mark says "I don't recall any discussion of anything like that. But Qualcomm knew next to nothing about a commercial mobile radio environment. They had been strictly military contractors. So they had a lot to learn, and I think they made some bad assumptions early on. I think they just underestimated the noise levels that would exist in the real world. I do know for sure that the 'other carrier jammer' problem caught them completely by surprise. That's what we encountered when mobiles would drive next to a competitors site and get knocked off the air. They had to re-design the phone.
Now, what about those hexagon shaped cell sites?
Mark van der Hoek says the answer has to do with frequency planning and vehicle traffic. "After much experimenting and calculating, the Bell team came up with the solution that the honeybee has known about all along -- the hex system. Using 3 sectored sites, major roads could be served by one dominant sector, and a frequency re-use pattern of 7 could be applied that would allow the most efficient re-use of the available channels."

A cell cluster. Note how neatly seven hexagon shaped cells fit together. Try that with a triangle. Clusters of four and twelve are also possible but frequency re-use patterns based on seven are most common.

Mark continues, "Cellular pioneers knew most sites would be in cities using a road system based on a grid. Site arrangement must allow efficient frequency planning. If sites with the same channels are located too closely together, there will be interference. So what configuration of antennas will best serve those city streeets?"

"If we use 4 sectors, with a box shape for cells, we either have all of the antennas pointing along most of the streets, or we have them offset from the streets. Having the borders of the sites or sectors pointing along the streets will cause too many handoffs between cells and sectors -- the signal will vary continously and the mobile will 'ping-pong' from one sector to another. This puts too much load on the system and increases the probablity of dropped calls. The streets need to be served by ONE dominant sector."

Do you understand that? Imagine the dots below are a road. If you have two sectors facing the same way, even if they are some distance apart, you'll have the problems Mark just discussed. You need them to be offset.

Fuente: http://www.privateline.com/mt_cellbasics/

Cesar Augusto Suarez
CI 17394384
CAF

Cellular System Components

Cellular System Components

The cellular system offers mobile and portable telephone stations the same service provided fixed stations over conventional wired loops. It has the capacity to serve tens of thousands of subscribers in a major metropolitan area. The cellular communications system consists of the following four major components that work together to provide mobile service to subscribers.

• public switched telephone network (PSTN)
• mobile telephone switching office (MTSO)
• cell site with antenna system
• mobile subscriber unit (MSU)

PSTN

The PSTN is made up of local networks, the exchange area networks, and the long-haul network that interconnect telephones and other communication devices on a worldwide basis.

Mobile Telephone Switching Office (MTSO)

The MTSO is the central office for mobile switching. It houses the mobile switching center (MSC), field monitoring, and relay stations for switching calls from cell sites to wireline central offices (PSTN). In analog cellular networks, the MSC controls the system operation. The MSC controls calls, tracks billing information, and locates cellular subscribers.

The Cell Site

The term cell site is used to refer to the physical location of radio equipment that provides coverage within a cell. A list of hardware located at a cell site includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems.

Mobile Subscriber Units (MSUs)

The mobile subscriber unit consists of a control unit and a transceiver that transmits and receives radio transmissions to and from a cell site. The following three types of MSUs are available:

• the mobile telephone (typical transmit power is 4.0 watts)
• the portable (typical transmit power is 0.6 watts)
• the transportable (typical transmit power is 1.6 watts)
• The mobile telephone is installed in the trunk of a car, and the handset is installed in a convenient location to the driver. Portable and transportable telephones are hand-held and can be used anywhere. The use of portable and transportable telephones is limited to the charge life of the internal battery.

Digital Systems

As demand for mobile telephone service has increased, service providers found that basic engineering assumptions borrowed from wireline (landline) networks did not hold true in mobile systems. While the average landline phone call lasts at least 10 minutes, mobile calls usually run 90 seconds. Engineers who expected to assign 50 or more mobile phones to the same radio channel found that by doing so they increased the probability that a user would not get dial tone—this is known as call-blocking probability. As a consequence, the early systems quickly became saturated, and the quality of service decreased rapidly. The critical problem was capacity. The general characteristics of time division multiple access (TDMA), Global System for Mobile Communications (GSM), personal communications service (PCS) 1900, and code division multiple access (CDMA) promise to significantly increase the efficiency of cellular telephone systems to allow a greater number of simultaneous conversations. shows the components of a typical digital cellular system.

The advantages of digital cellular technologies over analog cellular networks include increased capacity and security. Technology options such as TDMA and CDMA offer more channels in the same analog cellular bandwidth and encrypted voice and data. Because of the enormous amount of money that service providers have invested in AMPS hardware and software, providers look for a migration from AMPS to digital analog mobile phone service (DAMPS) by overlaying their existing networks with TDMA architectures.

Time Division Multiple Access (TDMA)

North American digital cellular (NADC) is called DAMPS and TDMA. Because AMPS preceded digital cellular systems, DAMPS uses the same setup protocols as analog AMPS. TDMA has the following characteristics:

• IS–54 standard specifies traffic on digital voice channels
• initial implementation triples the calling capacity of AMPS systems
• capacity improvements of 6 to 15 times that of AMPS are possible
• many blocks of spectrum in 800 MHz and 1900 MHz are used
• all transmissions are digital
• TDMA/FDMA application 7. 3 callers per radio carrier (6 callers on half rate later), providing 3 times the AMPS capacity

TDMA is one of several technologies used in wireless communications. TDMA provides each call with time slots so that several calls can occupy one bandwidth. Each caller is assigned a specific time slot. In some cellular systems, digital packets of information are sent during each time slot and reassembled by the receiving equipment into the original voice components. TDMA uses the same frequency band and channel allocations as AMPS. Like NAMPS, TDMA provides three to six time channels in the same bandwidth as a single AMPS channel. Unlike NAMPS, digital systems have the means to compress the spectrum used to transmit voice information by compressing idle time and redundancy of normal speech. TDMA is the digital standard and has 30-kHz bandwidth. Using digital voice encoders, TDMA is able to use up to six channels in the same bandwidth where AMPS uses one channel.

Extended Time Division Multiple Access (E–TDMA)

The E–TDMA standard claims a capacity of fifteen times that of analog cellular systems. This capacity is achieved by compressing quiet time during conversations. E–TDMA divides the finite number of cellular frequencies into more time slots than TDMA. This allows the system to support more simultaneous cellular calls.

Fixed Wireless Access (FWA)

FWA is a radio-based local exchange service in which telephone service is provided by common carriers (see Figure 9). It is primarily a rural application—that is, it reduces the cost of conventional wireline. FWA extends telephone service to rural areas by replacing a wireline local loop with radio communications. Other labels for wireless access include fixed loop, fixed radio access, wireless telephony, radio loop, fixed wireless, radio access, and Ionica. FWA systems employ TDMA or CDMA access technologies.

Personal Communications Service (PCS)

The future of telecommunications includes PCS. PCS at 1900 MHz (PCS 1900) is the North American implementation of digital cellular system (DCS) 1800 (GSM). Trial networks were operational in the United States by 1993, and in 1994 the Federal Communications Commission (FCC) began spectrum auctions. As of 1995, the FCC auctioned commercial licenses. In the PCS frequency spectrum, the operator's authorized frequency block contains a definite number of channels. The frequency plan assigns specific channels to specific cells, following a reuse pattern that restarts with each nth cell. The uplink and downlink bands are paired mirror images. As with AMPS, a channel number implies one uplink and one downlink frequency (e.g., Channel 512 = 1850.2-MHz uplink paired with 1930.2-MHz downlink).

Code Division Multiple Access (CDMA)

CDMA is a digital air interface standard, claiming 8 to 15 times the capacity of analog. It employs a commercial adaptation of military, spread-spectrum, single-sideband technology. Based on spread spectrum theory, it is essentially the same as wireline service—the primary difference is that access to the local exchange carrier (LEC) is provided via wireless phone. Because users are isolated by code, they can share the same carrier frequency, eliminating the frequency reuse problem encountered in AMPS and DAMPS. Every CDMA cell site can use the same 1.25-MHz band, so with respect to clusters, n = 1. This greatly simplifies frequency planning in a fully CDMA environment.

CDMA is an interference-limited system. Unlike AMPS/TDMA, CDMA has a soft capacity limit; however, each user is a noise source on the shared channel and the noise contributed by users accumulates. This creates a practical limit to how many users a system will sustain. Mobiles that transmit excessive power increase interference to other mobiles. For CDMA, precise power control of mobiles is critical in maximizing the system's capacity and increasing battery life of the mobiles. The goal is to keep each mobile at the absolute minimum power level that is necessary to ensure acceptable service quality. Ideally, the power received at the base station from each mobile should be the same (minimum signal to interference).

Fuente: http://www.gsmfavorites.com/documents/introduction/gsm/

Cesar Augusto Suarez
CI 17394384
CAF

Introduction to Cellular Communications

Introduction to Cellular Communications 

1. Mobile Communications Principles

Each mobile uses a separate, temporary radio channel to talk to the cell site. The cell site talks to many mobiles at once, using one channel per mobile. Channels use a pair of frequencies for communication—one frequency (the forward link) for transmitting from the cell site and one frequency (the reverse link) for the cell site to receive calls from the users. Radio energy dissipates over distance, so mobiles must stay near the base station to maintain communications. The basic structure of mobile networks includes telephone systems and radio services. Where mobile radio service operates in a closed network and has no access to the telephone system, mobile telephone service allows interconnection to the telephone network.

Early Mobile Telephone System Architecture

Traditional mobile service was structured in a fashion similar to television broadcasting: One very powerful transmitter located at the highest spot in an area would broadcast in a radius of up to 50 kilometers. The cellular concept structured the mobile telephone network in a different way. Instead of using one powerful transmitter, many low-power transmitters were placed throughout a coverage area. For example, by dividing a metropolitan region into one hundred different areas (cells) with low-power transmitters using 12 conversations (channels) each, the system capacity theoretically could be increased from 12 conversations—or voice channels using one powerful transmitter—to 1,200 conversations (channels) using one hundred low-power transmitters. Figure 2 shows a metropolitan area configured as a traditional mobile telephone network with one high-power transmitter.

2. Mobile Telephone System Using the Cellular Concept

Interference problems caused by mobile units using the same channel in adjacent areas proved that all channels could not be reused in every cell. Areas had to be skipped before the same channel could be reused. Even though this affected the efficiency of the original concept, frequency reuse was still a viable solution to the problems of mobile telephony systems.

Engineers discovered that the interference effects were not due to the distance between areas, but to the ratio of the distance between areas to the transmitter power (radius) of the areas. By reducing the radius of an area by 50 percent, service providers could increase the number of potential customers in an area fourfold. Systems based on areas with a one-kilometer radius would have one hundred times more channels than systems with areas 10 kilometers in radius. Speculation led to the conclusion that by reducing the radius of areas to a few hundred meters, millions of calls could be served.

The cellular concept employs variable low-power levels, which allow cells to be sized according to the subscriber density and demand of a given area. As the population grows, cells can be added to accommodate that growth. Frequencies used in one cell cluster can be reused in other cells. Conversations can be handed off from cell to cell to maintain constant phone service as the user moves between cells.

The cellular radio equipment (base station) can communicate with mobiles as long as they are within range. Radio energy dissipates over distance, so the mobiles must be within the operating range of the base station. Like the early mobile radio system, the base station communicates with mobiles via a channel. The channel is made of two frequencies, one for transmitting to the base station and one to receive information from the base station.

3. Cellular System Architecture

Increases in demand and the poor quality of existing service led mobile service providers to research ways to improve the quality of service and to support more users in their systems. Because the amount of frequency spectrum available for mobile cellular use was limited, efficient use of the required frequencies was needed for mobile cellular coverage. In modern cellular telephony, rural and urban regions are divided into areas according to specific provisioning guidelines. Deployment parameters, such as amount of cell-splitting and cell sizes, are determined by engineers experienced in cellular system architecture.

Provisioning for each region is planned according to an engineering plan that includes cells, clusters, frequency reuse, and handovers.

Cells

A cell is the basic geographic unit of a cellular system. The term cellular comes from the honeycomb shape of the areas into which a coverage region is divided. Cells are base stations transmitting over small geographic areas that are represented as hexagons. Each cell size varies depending on the landscape. Because of constraints imposed by natural terrain and man-made structures, the true shape of cells is not a perfect hexagon.

Clusters

A cluster is a group of cells. No channels are reused within a cluster. Figure 4 illustrates a seven-cell cluster.

Frequency Reuse

Because only a small number of radio channel frequencies were available for mobile systems, engineers had to find a way to reuse radio channels to carry more than one conversation at a time. The solution the industry adopted was called frequency planning or frequency reuse. Frequency reuse was implemented by restructuring the mobile telephone system architecture into the cellular concept.

The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area. Cells are assigned a group of channels that is completely different from neighboring cells. The coverage area of cells is called the footprint. This footprint is limited by a boundary so that the same group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere.

Cells with the same number have the same set of frequencies. Here, because the number of available frequencies is 7, the frequency reuse factor is 1/7. That is, each cell is using 1/7 of available cellular channels.

Cell Splitting

Unfortunately, economic considerations made the concept of creating full systems with many small areas impractical. To overcome this difficulty, system operators developed the idea of cell splitting. As a service area becomes full of users, this approach is used to split a single area into smaller ones. In this way, urban centers can be split into as many areas as necessary to provide acceptable service levels in heavy-traffic regions, while larger, less expensive cells can be used to cover remote rural regions.

Handoff

The final obstacle in the development of the cellular network involved the problem created when a mobile subscriber traveled from one cell to another during a call. As adjacent areas do not use the same radio channels, a call must either be dropped or transferred from one radio channel to another when a user crosses the line between adjacent cells. Because dropping the call is unacceptable, the process of handoff was created. Handoff occurs when the mobile telephone network automatically transfers a call from radio channel to radio channel as a mobile crosses adjacent cells.

During a call, two parties are on one voice channel. When the mobile unit moves out of the coverage area of a given cell site, the reception becomes weak. At this point, the cell site in use requests a handoff. The system switches the call to a stronger-frequency channel in a new site without interrupting the call or alerting the user. The call continues as long as the user is talking, and the user does not notice the handoff at all.

Fuente: http://www.gsmfavorites.com/documents/introduction/gsm/

Cesar Augusto Suarez
CI 17394384
CAF

As wireless communication systems evolve

As wireless communication systems evolve

As wireless communication systems evolve, service quality and capacity are of primary importance. To ensure reliable communication over a mobile radio channel, a system must overcome multipath fading, polarization mismatch, and interference. The trend towards low power hand held transceivers increases all of these challenges. Even as more spectrum is allocated, demand for higher data rate services and steadily increasing numbers of users will motivate service providers to seek ways of increasing the capacity of their systems.

Antenna arrays can improve reliability and capacity in two ways. First, diversity combining or adaptive beamforming techniques can combine the signals from multiple antennas in a way that mitigates multipath fading. Second, adaptive beamforming using antenna arrays can provide capacity improvement through interference reduction. The use of adaptive arrays is an alternative to the expensive approach of cell splitting, which increases capacity by increasing the number of base station sites. Most adaptive arrays that have been considered for such applications are located at the base station and perform spatial filtering. They cancel or coherently combine multipath components of the desired signal and null interfering signals that have different directions of arrival from the desired signal.

Multi-polarized adaptive arrays, sometimes called polarization-sensitive adaptive arrays, can also match the polarization of a desired signal or null an interferer having the same direction of arrival as the desired signal, if the two signals have different polarization states. If base stations or mobile units in a peer-to-peer system can match the polarization states of hand-held transceivers, link quality and reliability will be enhanced, and power consumption in the hand-held units will be reduced, increasing battery life. It is possible that 100% or greater increase in system capacity can be achieved through a combination of spatial and polarization reuse. Because they offer large untapped potential performance gains, multi-polarized adaptive arrays should be studied extensively to determine what performance improvements are feasible. Currently, however, little is known about the performance of multi-polarized adaptive arrays in mobile communication systems.

Multi-polarized arrays have been considered as a means of rejecting jammers in military applications. The potential of multi-polarized arrays for interference rejection in wireless communication systems has been investigated in recent years. This research indicates that 20 to 35 dB of interference rejection is possible if interfering and desired signals differ in either polarization state or angle of arrival.

However, neither measurements nor simulations have been reported that show the performance of multi-polarized adaptive arrays in typical mobile multipath channels. Some researchers have proposed diversity combining at handheld radios and shown that significant performance gains can be achieved. The use of adaptive antennas on handheld radios is a new area of research. In 1988, Vaughn [1.7] concluded that with then-current technology, adaptive beamforming would work for units moving at pedestrian speeds but would be difficult for high-speed mobile units. In 1999, Braun, et al reported experiments in which data was recorded using a two-element handheld antenna array, and processed using diversity and optimum beamforming techniques.

While this was the first publication of its kind, some assumptions were made in the experiments and data processing that limit the applicability of the results.

This dissertation evaluates the performance improvement that can be achieved using co-polarized and dual-polarized antenna arrays at handheld receivers. This was done by measuring and modeling the performance of small handheld array configurations to determine the degree of diversity gain and interference rejection they can provide in typical mobile radio channels. Multi-polarized geometrically based multipath propagation models were developed for use in this study. A software package, the vector multipath propagation simulator (VMPS) was developed that can model the transmission, reflection, and reception of polarized waves and account for the effects of element and array pattern and orientation. Design guidelines for multi-polarized arrays and recommendations for their integration into new wireless communication systems are presented based on the results of this study. An overview of the dissertation follows.

Fuente: http://scholar.lib.vt.edu/theses/available/etd-04262000-15330030/unrestricted/ch1.pdf

Cesar Augusto Suarez 

CI 17394384

CAF

The Capacity in Wireless Communication Systems

The Capacity in Wireless Communication Systems

Multiple input multiple output (MIMO) take numerous benefits over conventional wireless systems in either data rate or reliable link. A seminal work demonstrated, the wireless channel capacity namely Shannon capacity is limitation and the bandwidth of wireless systems is very scarce. Thus, the applicable approach of this phenomenon technology is implementation of various techniques and algorithm exploit to wireless systems. The performance of MIMO systems depend on some term i.e. array gain, spatial multiplexing and diversity and so on. Channel characteristic play a significant role and consider as deterministic as well as random in wireless systems.

In this paper we have to explore wireless systems capacity is limitation and capacity can be obtain by using number of transceiver. The capacity is explored when the channel is known and unknown for transmitter and receiver. The MIMO channel is also random channel for different capacity i.e. 10% outage, Ergodic and theier number of transmitter and receiver. However, the signal attitude of real wireless systems is abnormal so it's distributed as Rayleigh in Line of Sight (LOS) case are well result. Moreover, we have to define the channel model as SISO, SIMO, MISO and MIMO systems and their input output relations and also mention as frequency selective channel. Thus MIMO is the best candidate for next generation wireless standard and guarantee achieve to best capacity in wireless communication systems.

MIMO is an abstract mathematical model of general matrix systems more specifically it produce array of antenna at both sides respectively transmitter and receiver.

Before starting MIMO technology, to take flavor about some others systems like SISO, SIMO, MISO and MIMO. Conventionally SISO (single input single output) provide single antenna at transmitter and receiver respectively. On the other hand SIMO referred single transmitter and multiple receiver is called SIMO (Single Input Multiple Output) systems. To do this trend the use of multiple antennas at transmitter and single receiver in wireless link MISO (Multiple Input Single Output) systems. MIMO (Multiple Input Multiple Output) provide same fashion in this scenario. Lastly, in this technology included MU (multi user)-MIMO whether provide a system, user can also communicate with base station by using multiple antennas.

Array gain

Array gain is employed at the both side receiver and transmitter for increased average signal to noise ratio (SNR) at the receiver those signal comes from coherent combining effect in the multiple antennas. Channel knowledge is required for transmitter/receiver to obtain array gain and depends on number of transmitter/ receiver antenna. If the transmitters know the channel then transmitter will weight the transmission with weights, depending on the channel coefficients, so that there is coherent combining at the single antenna receiver. The array gain in this system is called transmitter array gain. Alternatively, if we have only one antenna at the transmitter and no knowledge of the channel and a multiple antenna receiver, which has perfectly knowledge of the channel, the receiver can suitably weight the incoming signals so that they coherently add up at the output (combining), thereby enhancing the signal and is known receiver array gain. So in MIMO systems provide both side array gains is available.

Diversity

In wireless channel, signal is always fluctuate and create fading if the signal fluctuate very fast then it's create fast fading, however diversity is one kind of technique that is capable to combat fading in wireless links. Multipath fading is common scenario in wireless channel causing by Receian or Rayleigh fading. If the signal strength is very low normally it given fade and increased high bit error rate (BER). Diversity techniques involve with time, frequency and space.

Temporal diversity:

It provides the replica of the transmitted signal across the time by using channel coding and time interleaving. In this situation for diversity needs channel sufficient variations in time. We can achieve diversity when the channel coherence time smaller than desired interleaving symbol so it is assumed that interleaved symbol is independent of the previous symbol, thus makes a completely the new replica of the signal.

Frequency Diversity:

Signal is always fluctuate into the channel. It transmitted by using different types of frequency and reached at the receiver by using multipath, if the coherence bandwidth of the channel is less than compared with signal bandwidth then we can apply this technique to get the replicas of the accurate signal and thus established a reliable link in wireless channel.

Spatial (Antenna) Diversity:

It can mitigate fading in wireless channel and associated with time/frequency diversity. This diversity can be applied when the antenna spacing is larger than the coherence space. If the MIMO channel fade is independently and transmitted signal suitably constructed, the receiver can also received signal coherently and reduce the signal amplitude then we can get MTxMRx(The number of transmitter and Receiver) order diversity. This diversity depend design of the transmitted signal and Space-Time Coding (STC) can be done. Spatial diversity can be categorized receive and transmit diversity

Receive Diversity:

At the receiver end using maximum ratio combining (MRC) to improve signal quality but it's very costly in wireless communication systems that's why transmit diversity is becoming a popular and it's less complexity to implement at the transmitter side and also exciting topics in MIMO systems. Receive Diversity improve capacity and range capability at the base station, except cost it's very efficient technique to mitigate fading within a signal.

Transmit Diversity:

Earlier we have to mention why it is very popular for researchers and wireless companies. Transmit diversity is applicable when multiple antennas are used at the transmitter. It's a suitable signal construction. A significant effort has been devoted in 3GPP to develop efficient transmit diversity solutions to enhance downlink capacity. Transmit diversity methods also provide space diversity for terminals with only one receive antenna, and in that sense retain the complexity at the base station. Typically, in 3G base stations, the transmitting antenna elements are relatively close to each other. [13] In later section we will discuss more about diversity with space-time coding.

Spatial Multiplexing:

Spatial multiplexing offers a linear (in minimum number of transmit and receive antenna) increase capacity without additional power expenditure and bandwidth. It is only provide MIMO channels [5, 6]. This is commonly known spatial multiplexing gain and is considered for two transmit and receive antennas. It can be extended in MIMO channel. Let us consider 2×2 MIMO systems, in this case, we want to send bit stream, at first bit stream will split and modulated then transmitted simultaneously from both antennas. Channel knowledge is available at the receiver so it can completely decoded data thus provide receiver diversity whether transmitter has no knowledge about channel. In such event transmitter cannot provide diversity and data stream is completely different from each other so they carry totally different data. Thus, spatial multiplexing increases data capacity in MIMO systems.

Multi Antenna System Model

We consider the number of transmitte antenna (i=1,2…………….MT) and the number of reciver antenna (j =1,2……….MR) respectively. Hence the create MIMO channel denoted Hij.

It gives us MT×MR complex matrix is called MIMO channel . However, if consider signal s is transmitted from ith transmit antenna. At the receive end, will get a complex weighted version of the transmitted signal. As we know jth receiver antenna by hji, where hij is the path gain or channel response between receive antenna jth and transmit antenna ith. The vector [h1i, h2i……..hMRi] Tis the signature induced by the ithtransmit antenna across the receive antenna array. Using this assumption, MIMO channel H for MT transmitter antenna and MR receive antenna can be represent as

The channel defines the input-output relation of the MIMO system and is also known as the channel transfer function. We assume that channel is Gaussian distributed (i.i.d.) means Gaussian variables. Hence the systems consider channel is unknown at the transmitter and assumed that the signals transmitted from each antenna have same power . So the covariance matrix of this transmitted signal is given by Where is the power across the transmitter irrespective of the number of antennas and is an identity matrix.

Hence we can ignore the signal attenuation, scatterings, and so on. In this scenario the channel matrix as deterministic as

If the channel is random, so this result can be apply for normalization.

The channel realization in real wireless communication systems is very difficult.

In the receiver, the channel estimation can be found at the receiver to send training sequence from the transmitter. On the other hand, the transmitter can get the channel information via feedback information. Hence the channel matrix is known for receiver but unknown for transmitter.

If there is no correlation of components n. the covariance matrix is can be obtain as

Where is the identical noise power for each receiver.

For Simplicity, if we send signal vector from ithtransmitter antenna array (xi) then the signal received at the receiver antenna array is . At the receiver end is applied maximum likelihood (ML) algorithm over receiver antennas. We assumed that each received power level denoted by and the total power of receive antenna is equal to the total transmitted power.

Where n is the additive white noise random variable with MR×1 column matrix distributed elements with zero mean complex Gaussian random variables with variance 0.5 per real dimension.

MIMO Capacity

MIMO channel H affected by large number of scatters like the superposition of delayed, reflected, scattered (buildings, vehicle and other terrain objects) in the wireless spectrum. So any receive antenna received transmitted signal with several multi-path component. In such an event the replica of transmitted signal at each antenna will be complex random variable. The element of channel matrix H can be assumed to be independent, zero mean, complex Gaussian random variables that are distributed by Rayleigh (Raleigh fading). When signal introduce rich multipath with large delay spread then H varies as a function of time, the channel delay spread, which is a measure of the difference in the time of arrival of various multipath components at the receiver antenna, is less than the symbol rate. This assumption guarantees flat fading.

The capacity of MIMO channel is explain.

To control radio frequency spectrum in time varying channel with multipath propagation environment is really difficult for both case forward (base station to mobile) and reverse (mobile to base station).Actually, receiver signal is generally weaker than transmitted signal due to the propagation phenomena like slow fading, propagation loss and fast fading. The mean propagation comes from angles of spreading by water and foliage and effect of ground reflections, slow fading arise by building and natural features and fast fading caused by multipath scattering. All fades expressed by Rayleigh fading [15]. So needless to say that channel is always unpredictable normally its behavior is random. On the other hand bandwidth is limited. In this event, a very essential systems designed was required in wireless communication that will done fill up all of requirement within a systems. MIMO is phenomenon's that fill up all necessity in Wireless industry. According to MIMO definition we can get highest capacity in wireless channel. How we can get highest capacity in multi antenna system and several types of channel behaviors detailed can be found [5] within an Additive Gaussian channel with fading and without fading. This seminal paper also provides computational procedure for these dump antenna systems. Now we have to discuss MIMO capacity within an information theory. Before then, how we can achieve a sufficient data transmission within a MIMO systems possibly 1 Gb/s [2]. Let us consider a system to achieve this rate. When spectral efficiency 4 b/s/Hz over 250 MHz. Bandwidth then we can achieve 1 Gb/s. In real systems to get 250 MHz bandwidth available in 40-Ghz frequency, normally frequency bands below should be 6 GHz. A potential paper proposed [2], where MIMO wireless constitutes technological breakthroughs that will allow1 Gb/s within NLOS environment. To do this, need 10×10 array of antenna at the both sides. In SISO systems to get 1 Gb/s need 220 MHz bandwidth whether in MIMO systems require only 20 MHz bandwidth and also does not need additional transmit power or receive SNR to deliver such performance gains. Thus MIMO provide a very strong and high data capacity rate in wireless systems.

However, consider provide rich capacity in several system that is exploit a MIMO channel and apply with signal scheme STC in practical wireless systems.

If channel is Rayleigh fading, in SISO systems provide capacity

Where h is channel with additive white Gaussian and complex value, is the SNR for any MR antenna, in such case if we add more antenna at the receiver side to get more capacity is given (SIMO case)

Where hiis the channel gain with number of MR receive antenna. It is also provide receiver diversity. In contrast of this system we can say MISO case whether add more antennas at the transmitter, whether transmitter has no knowledge about channel. In such event MISO is given capacity

Where hi is AWGN channel with number of MT antenna. It can worked as a transmit diversity.

Lastly at the both side multi antenna (MIMO) systems is given tremendous capacity

Where (*) means transpose-conjugate and H is the MT×MR channel matrix. H* is the conjugate transpose of H. Till now this capacity is best capacity for MIMO systems.

Generally receiver has perfect knowledge for the channel but it can be implementation in different channel situation when channel is unknown and known to the transmitter.

Conclusion

The increasing demand for the development of wireless communication systems for high data rate transmission and high quality information exchange leads to the new challenging subject in communication research area. MIMO principles are able to provide future wireless communication systems with significant increased capacity or higher link reliability using the same bandwidth and transmit power as today. From the literature review, significant performance improvement possible over traditional wireless communication systems by using several kind of STC technique, that will drive in MIMO systems. This technique guaranteed maximum code rate, excellent diversity, rich coding gain and lastly not least reliable wireless communications. A good tutorial can be found for MIMO STC. However, Space-time coding is poised to play an important role in MIMO systems. Furthermore, MIMO technology is a strong candidate for 4G and beyond. Numerous vendors, such as Airgo, Lucent, are promoting MIMO as the IEEE802.11 standard, 802.11n, which the activities will complete by 2006.

Fuente: http://cnx.org/content/m14048/latest/

Cesar Augusto Suarez 

CI 17394384

CAF