Multiplex communications – Duplex – Communication over free space
Reexamination Certificate
2000-04-25
2002-05-14
Chin, Wellington (Department: 2664)
Multiplex communications
Duplex
Communication over free space
C370S347000, C370S350000, C370S507000, C370S509000, C455S422100, C455S502000
Reexamination Certificate
active
06388997
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the present invention pertains to communications and, more particularly, to an air interface structure and protocol suitable for use in a cellular communication environment.
2. Description of Related Art
A growing demand for flexible, mobile communication has led to development of a variety of techniques for allocating available communication bandwidth among a steadily increasing number of users of cellular services. Two conventional techniques for allocating communication bandwidth between a cellular base station and a set of cellular user stations (also called “mobile stations”) are frequency division duplex (FDD) and time division duplex (TDD).
As used herein, FDD refers to a technique for establishing full duplex communications having both forward and reverse links separated in frequency, and TDD refers to a technique for establishing full duplex communications having both forward and reverse links occurring on the same frequency but separated in time to avoid collisions. Other techniques for communication are time division multiple access (TDMA), wherein transmissions by a plurality of users are separated in time to avoid conflicts, frequency division multiple access (FDMA), wherein transmissions by a plurality of users are separated in frequency to avoid conflicts, and time division multiplex (TDM), wherein multiple data streams are time multiplexed together over a single carrier. Various combinations of FDD, TDD, FDMA, and TDMA may also be utilized.
In a particular FDD technique, a base station is allocated a set of frequencies over which it may transmit, using a different frequency slot for each user station, and each user station is allocated a different frequency over which it may transmit to the base station. For each new user in contact with a base station, a new pair of frequencies is required to support the communication link between the base station and the new user station. The number of users that can be supported by a single base station is therefore limited by the number of available frequency slots.
In a particular TDD technique, the same frequency is used for all user stations in communication with a particular base station. Interference between userstations is avoided by requiring that user stations transmit at different times from one another and from the base station. This is accomplished by dividing a time period into a plurality of time frames, and each time frame into a plurality of time slots. Typically, the base station communicates with only one user station during a time slot, and communicates with all the user stations sequentially during different time slots over a single time frame. Thus, the base station communicates with a particular user station once during each time frame.
In one version of the described system, the base station is allocated a first portion of each time slot during which the base station transmits to a particular user station, and the user station is allocated a second portion of the time slot during which the user station responds to the base station. Thus, the base station may transmit to a first user station, await a response, and, after receiving a response from the first user station, transmit to a second user station, and so on, until the base station has communicated with all user stations sequentially over a particular time frame.
Time division duplex has an advantage over FDD and FDMA of requiring use of only a single frequency bandwidth. However, a drawback of many conventional TDD or TDMA systems is that their efficiency suffers as cell size increases. The reduction in efficiency stems from the relatively unpredictable nature of propagation delay times of transmissions from the base station over air channels to the user stations, and from the user stations over air channels back to the base station. Because user stations are often mobile and can move anywhere within the radius of the cell covered by a base station, the base station generally does not know in advance how long the propagation delay will be for communicating with a particular user station. In order to plan for the worst case, conventional TDD systems typically provide a round-trip guard time to ensure that communication will be completed with the first user station before initiating communication with the second user station. Because the round-trip guard time is present in each time slot regardless of how near or far a user station is, the required round-trip guard time can add substantial overhead, particularly in large cells. The extra overhead limits the number of users, and hence the efficiency, of TDD systems.
FIG. 1
is an illustration of the basic round trip timing for a TDD system from a base station perspective. A polling loop
101
, or time frame, for a base station is divided into a plurality of time slots
103
. Each time slot
103
is used for communication from the base station to a particular user station. Thus, each time slot comprises a base transmission
105
, a user transmission
107
, and a delay period
106
during which the base transmission
105
propagates to the user station, the user station processes and generates a responsive user transmission
107
, and the user transmission
107
propagates to the base station.
If the user station is located right next to the base station, then the base station can expect to hear from the user station immediately after finishing its transmission and switching to a receive mode. As the distance between the user station and the base station grows, the time spent by the base station waiting for a response grows as well. The base station will not hear from the user station immediately but will have to wait for signals to propagate to the user station and back.
As shown in
FIG. 1
, in a first time slot
110
the user transmission
107
arrives at the base station at a time approximately equidistant between the end of the base transmission
105
and the start of the user transmission
107
, indicating that the user station is about half a cell radius from the base station. In a second time slot
111
, the user transmission
107
appears very close to the end of the base transmission
105
, indicating that the user station is very close to the base station. In a third time slot
112
, the user transmission
107
appears at the very end of the time slot
112
, indicating that the user station is near or at the cell boundary. Because the third time slot
112
corresponds to a user station at the maximum communication distance for a particular base station, the delay
106
shown in the third time slot
112
represents the maximum round-trip propagation time and, hence, the maximum round-trip guard time.
In addition to propagation delay times, there also may be delays in switching between receive and transmit mode in the user station, base station, or both, which are not depicted in
FIG. 1
for simplicity. Typical transmit/receive switching times are about two microseconds, but additional allocations may be made to account for channel ringing effects associated with multipath.
As cell size increases, TDD guard time must increase to account for longer propagation times. In such a case, guard time consumes an increasingly large portion of the available time slot, particularly for shorter round trip frame durations. The percentage increase in time spent for overhead is due to the fact that TDD guard time is a fixed length, determined by cell radius, while the actual round trip frame duration varies according to the distance of the user station. Consequently, as cells get larger, an increasing amount of time is spent on overhead in the form of guard times rather than actual information transfer between user stations and the base station.
One conventional TDD system is the Digital European Cordless Telecommunications (DECT) system developed by the European Telecommunications Standards Institute (ETSI). In the DECT system, a base station transmits a long burst of data segmented into time slots, with each time slot having data associa
Chin Wellington
Lyon & Lyon LLP
Phan M.
Xircom Wireless, Inc.
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