إنّ تزايد الطلب على معدلات النقل في الاتصالات دفع الباحثين لإيجاد طرق جديدة لتلبيّة هذه المتطلبات، ومع محدودية المصادر الترددية (عرض الحزمة) والزمنية تمّ التوّجه لإيجاد مصادر جديدة للنظام تساعد على زيادة معدل النقل وتحسين جودة النظام في الوقت نفسه بغ
ية توفير عرض الحزمة والاستطاعة المستهلكة في الأنظمة التقليدية الحالية التي تحوي هوائي إرسال واحد وهوائي استقبال واحد.
ظهرت أنظمة الاتصالات متعددة المداخل والمخارج MIMO لتقدّم الكثير من التحسينات والميّزات الجديدة لشبكات الاتصالات اللاسلكيّة وأدخلت أنظمة الاتصالات بعصر جديد يستخدم النظام فيه عدة هوائيات في الإرسال والاستقبال للاستفادة من خصائص القناة متعددة المسارات Multipath Channel عبر تقديم ربحين: ربح التنويع Diversity Gain وربح التنضيد Multiplexing Gain حيث يعمل ربح التنويع على زيادة تحسين جودة وأداء النظام، بينما يعمل ربح التنضيد على زيادة معدل نقل النظام، كما أدخلت مؤخرا ميزات أخرى على هذا النظام أهمها تقنية تشكيل الحزمة Beamforming التي تعمل على تركيز الحزمة بالاتجاه المطلوب لتقليل التداخل بين الأجهزة وزيادة جودة الإشارة.
وظفت هذه الميزات في الكثير من أنظمة الاتصالات ومنها أنظمة Wi-Fi والأنظمة الخلوية LTE و5G التي أدخلت مفاهيم جديدة على أنظمة MIMO أهمها: تقنية Massive MIMO الذي ساعد على تحقيق معدلات وكفاءات فائقة وتقنية Virtual MIMO التي تعمل على تشكيل مصفوفات إرسال/استقبال من عدة أجهزة متفرقة.
سوف نقوم في هذا البحث بشرح أساسيات القناة متعددة المسارات وأنظمة MIMO، كيفية نمذجتها رياضياً، ميزات وأرباح هذه المنظومة، بنية المنظومة، أهم تطبيقات هذه المنظومة وخصائصها الجديدة وأخيراً تحدياتها وطرق تطويرها.
Antennas have become an indispensable basic need, especially in the area of wireless
networks, and have developed in the past few decades in a very large way in terms of profit
and resistance, bandwidth, bandwidth, radiation pattern, etc. in line w
ith new and
increasing requirements.
In this study, we are interested in studying a broad band antenna where this antenna is very
important in systems that operate on a set of frequencies rather than at a fixed frequency.
The traditional antennas that are designed at a specific frequency change their response
significantly if used on a wide bandwidth around the designed frequency
In this study, the LPDA (Log periodic dipole Array) antenna was designed and studied in a
broadband field and compared to the antenna reference of wire antennas. It is a halfwavelength
antenna λ/2. The study of the effect of frequency change in the antenna
radiation pattern and the radiation beam Half-power level and front-to-back ratio (F/B).
Objective: This research aimed to evaluate the three-dimensional
effects of a Twin Block (TB) appliance on the pharyngeal airway
passage (PAP) in a sample of Class II patients with mandibular
retrusion using cone-beam computed tomography (CBCT).
The RME force is not only concentrated on widening the maxilla,
it is concomitant changes occur in craniofacial sutures, from here
the need for this study was originated.
Aim of the study: To investigate the immediate effect of rapid
maxillary expansion on the spheno-occipital Synchondrosis in
children using cone beam computed tomography.
We present in this paper a study aimed to identify actual situation of
information systems security management in The Ministry of
Communications and Technology and entities associated, and the
ways to improve it.
Recently PGB (photonic band gap) filters show a great
development. Here we will present two different PGB filters with
lattice etched in the ground plane. The rejection band characteristic
is function of the number and shape of elements etched. Th
ese
filters are modelling by the FWCIP method.
The current researches are moving towards more development in order to provide the
growing the needs of users such as support real-time applications, quality of service,
particularly; the high data rate transfer and other. That prompts the network
service
providers to integrate many properties for different networks resource, and support
providing the service "anywhere and anytime".
Hence, the importance of this research, which aims to study the vertical handover as
very important and necessary step to provide the mobility of mobile nodes between the
different networks by using Media Independent Handover (MIH) IEEE802.21standard
which is developed in January 2009. In this paper, the performance of vertical handover
between these two networks is evaluated taking into account many parameters such as
packet loss, handover latency, and throughput, using NS2 simulator (Network Simulator
version2) which includes a support for MIH technology by the National Institute of
Standard and Technology (NIST).
Fractal antenna are considered actually as the most important large band antenna, due
to their design parameters that are based on fractal geometry. The long distance between
two specific points of antenna and hisself similarity allow to obtain a m
ultiple resonance
frequencies (Wide Band), which is used for several application. The using of dipole
antenna in form of Van-Koch curve offer multi-resonance antenna, the Nec simulator is
used to design this antenna. The proposed antenna had been fabricated and measured in the
antenna laboratory at two central frequencies 1GHz and 10 GHz. Both theoretical and
experimental results were very closed in spite of imperfect experiment condition. The Van-
Koch antenna fabrication and measurement in laboratory gives the possibility to designing
and applying this antenna type, additionally we can study the change of antenna parameters
( gain, rayon diagram … ) when the fractal parameters varies.
Novel rejected and passed band devices are obtained using
techniques in two dimensional Photonic Band Gap microstrip
structures. The microstrip is engraved in a PBG dielectric
constituted of a via holes array and enclosed in a metallic box. The
b
oundary element method (BEM) and the fast wave concept
iterative process (FWCIP) are combined to evaluate the
characteristic of this structure.
This research present broadband over power lines (BPL) technology as an effective option for Syrian Arab republic since this technology basically based on existing network which supply the electricity to the public use.
