Communications: directive radio wave systems and devices (e.g. – Directive – Including a steerable array
Reexamination Certificate
2002-08-23
2004-10-05
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a steerable array
C342S372000
Reexamination Certificate
active
06801160
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to antennas, and more particularly to phased array antennas.
BACKGROUND OF THE INVENTION
The broad popularity and ever increasing demand for mobile communication services has given rise to many techniques and innovations geared toward increasing the capacity and communication quality of wireless networks. A wireless mobile network includes one or more base stations spread over a coverage region and a group of mobile subscribers such as cellular phone users. A base station provides a local link between the mobile subscribers and a traditional telephone line. In this context, capacity is the amount of information (i.e., number of bits) that can be exchanged in a given unit of time (or per unit bandwidth) in a given area. In more simplistic terms, the capacity of a wireless network dictates the maximum number of simultaneous wireless telephone conversations that can take place in a given geographical region.
Traditionally, subscribers communicating with the same base station have been differentiated from one another by frequency, as in FDMA (frequency division multiple access); by time, as in TDMA (time division multiple access); or by code, as in CDMA (code division multiple access). A traditional base station antenna broadcasts signals in a fixed direction covering a fixed, broad sector. In such a case, most of the energy is wasted, never reaching a subscriber. Furthermore, given a fixed, broad beam antenna, it is not possible to take advantage of the fact that subscribers in a given region are spatially diverse (i.e., their angular direction with respect to the nearest base station is different).
More recently, “smart antennas” have been introduced as a means of increasing capacity. For purposes of discussion herein, a “smart antenna” is any antenna that is capable of controlling the direction of its transmitted energy or the direction from which it receives energy. Throughout this description, it should be kept in mind that discussions relating to transmitting or transmissions apply with equal veracity to reception of electromagnetic energy or signals. In order to avoid prolixity, the present description will focus primarily on transmission characteristics of antennas, with the proviso that it is understood that reception of energy or signals is also inherently described. Smart antennas have been used in many applications over the years including phased array radar systems and in other communication systems. However, until recently, the use of “smart antennas” as a building block in a mobile communication network has been prohibitively expensive.
One proposed form of “smart antenna” is used in conjunction with digital beam forming techniques. Such an antenna includes multiple radiating columns with energy being received (or transmitted) through each column. By applying amplitude weights to the energy passing through each column the resulting radiation pattern can be very specifically tailored so that significantly more signal is transmitted or received in, or from, certain angular directions relative to other angular directions. Another way to state this is that by applying appropriate amplitude weights to the energy passing through each column, pattern nulls can be created at specific angular directions while other angular directions benefit from the full antenna gain. This ability to steer pattern nulls is useful, for example, when a given communications system is operating at what would otherwise be its full capacity.
A simplified example focusing on two subscribers competing for the same conventional channel in a TDMA system will be presented here. A conventional channel is defined as the unique combination of a carrier frequency (one of 126) and a time slot. If all conventional channels in a given sector are used up, any additional request for service must either be denied or placed on a channel that is already in use by another subscriber. Without a smart antenna, two subscribers on the same channel would immediately become interference for one another. In such a case neither would be able to communicate effectively with the base station. However, by using a null steering smart antenna it is possible to take advantage of spatial diversity and create additional channels. These additional channels are known as space division multiple access (SDMA) channels. To create such a channel the base station radio communicating with the first subscriber places a null on the second subscriber to attenuate the interfering signal. Likewise, the base station antenna communicating with the second subscriber must place a null on the first subscriber. In doing so, two independent, SDMA channels are created permitting reuse of the existing FDMA and TDMA channels. This null steering takes place very quickly (in tens of microseconds) and is synchronized with ongoing communications protocols. In this example two “smart antennas” located at the same base station essentially carve out two spatially discriminated channels permitting two users in the same area (or cell) to share a single conventional channel.
The principle illustrated in the above example is simplified in order to facilitate understanding the invention, and fails to address some of the realities that may significantly limit the utility of adding null steering capability to a mobile communication network. One such limiting reality is signal spreading. Signal spreading refers to the fact that the received communication signal in a mobile network may arrive at a base station (for example) from many different angles. Signal spreading results because radio frequency energy naturally follows all available reflection paths between a mobile subscriber and a base station (this is commonly referred to as “multi-path” propagation, or simply “multi-path”). To make matters worse, the angle from which the strongest signal arrives is not necessarily constant and, in fact, may change very rapidly as a function of time. Thus, because of signal spreading, a single spatially narrow null will not always provide enough isolation to create an independent channel. Furthermore, even if a spatial channel can be created at times, it is not possible to guarantee that the channel can be maintained continuously. This is because the signal-spreading signature changes rapidly as subscribers move and as other objects in the physical channel move. Given enough angular separation between signals, it may be possible to detect and compensate for all of these changes in real time and realize spatially independent channels. However, in general, multi-path effects will tend to significantly limit this otherwise theoretically clean method of significantly improving capacity using SDMA. And in any event, the null steering and angle of arrival algorithms required to actually realize capacity improvements are very computationally intense. Such algorithms are very costly to develop, require significant hardware upgrades for implementation in today's networks, and also require significant computer resources.
High costs and other uncertainties associated with digital beam forming have provided impetus for developing lower cost measures for exploiting spatial subscriber diversity. One such measure includes dividing a broad sector into multiple fixed sectors. An independent fixed antenna with a narrower beam is then used to service each of the smaller sectors. Each sectored beam originates from an independent aperture at the base station. An antenna with a narrower beam has more gain and is not as susceptible to interference as a broad beam antenna. Thus, the use of a narrow beam improves both the signal to noise ratio and the signal to interference ratio (sometimes referred to as “carrier to interference ratio” in telecommunication systems) within the network. In most cases, mobile communication networks are interference limited. Thus, an improved signal to interference ratio can be exploited to increase network capacity. A typical mobile communications network is divided into many cells with frequencies being re-used among
du Toit Cornelius Frederik
Henderson Herbert Jefferson
Karasack Vincent G.
Law Offices of Donald D. Mondul
Phan Dao
LandOfFree
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