Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
1999-09-28
2001-05-15
Vo, Don N. (Department: 2631)
Pulse or digital communications
Spread spectrum
Frequency hopping
C375S133000
Reexamination Certificate
active
06233270
ABSTRACT:
FIELD OF INVENTION
The present invention relates the field of telecommunications. More particularly, the present invention relates to synchronized, cellular radio telecommunications systems.
BACKGROUND
Frequency hopping is often employed in cellular, radio telecommunications systems, such as the Global System for Mobile Communication (GSM), to improve system performance. In general, frequency hopping improves system performance by introducing frequency diversity and interference diversity, as will be explained in detail below. Frequency hopping is a well-known technique.
In a radio telecommunications system, frequency diversity is achieved by transmitting each radio telecommunications signal on a sequence of frequencies over time. Each radio signal is transmitted over a sequence of frequencies because radio signals are often subject to amplitude variations called Rayleigh fading. However, Rayleigh fading generally affects radio signals carried on some frequencies more so than other frequencies. Thus, transmitting a radio telecommunications signal over a sequence of different frequencies increases the likelihood that the signal will be received correctly, as it is unlikely that Rayleigh fading will significantly impact each and every frequency over which the radio telecommunications signal is being transmitted. Accordingly, signal quality is improved and overall system performance is enhanced.
On the other hand, Interference diversity works as follows. In addition to fading, a radio signal is often subject to varying degrees of interference caused by traffic on the same frequency (i.e., co-channel interference) and traffic on an adjacent frequency (i.e., adjacent channel interference). If co-channel and/or adjacent channel interference is substantial, the signal quality associated with the radio signal may be severely impacted, so much so, that the connection may be dropped. In theory, frequency hopping, through the introduction of interference diversity, spreads the co-channel and adjacent channel interference amongst numerous end-users, such that the co-channel and adjacent channel interference experienced by a particular end-user is diversified. The overall effect is to raise the signal quality across the network, thereby improving overall system performance.
While frequency hopping improves system performance by improving signal quality, frequency reuse is designed to improve system performance by increasing system capacity. More specifically, frequency reuse permits two or more cells to simultaneously use the same frequency, or group of frequencies, so long as the distance (i.e., the “reuse distance”) between the two cells is sufficient to minimize any co-channel interference that might otherwise have an adverse affect on signal quality. However, as the demand for cellular service increases, reuse distances are likely to decrease. And, as reuse distances decrease, co-channel interference is likely to increase.
To limit co-channel interference, fractional loading may be employed. In a cellular network that employs fractional loading, the number of transceivers installed in each cell is less than the number of frequencies allocated to each cell. With synthesizer frequency hopping, each transceiver hops on all the allocated frequencies, but at any instant in time, the number of frequencies being transmitted by any cell is at most equal to the number of installed transceivers. Since each cell in the conventional, unsynchronized network is typically given a different frequency hopping sequence, at any given instant, potentially interfering cells are unlikely to be emitting exactly the same frequencies. The average interference in the network is thereby reduced.
Another technique that is employed to improve overall system performance is known as base station synchronization. In a Time Division Multiple Access (TDMA) system such as GSM, base stations may be synchronized or unsynchronized with respect to each other. In an unsynchronized system, each base station independently transmits and receives radio communication bursts. Consequently, a radio communication burst associated with one base station will overlap in the time domain with two sequential radio communication bursts associated with each of a number of proximally located base stations. In a synchronous system, base stations, typically in groups of three or more, transmit and receive radio communication bursts in a synchronized manner with respect to each other. Thus, the radio communication bursts transmitted by one base station are aligned in the time domain with the radio communication bursts transmitted by the other base stations affiliated with the group of synchronized base stations. Likewise, radio communication bursts received by one base station are aligned in the time domain with the radio communication bursts received by the other base stations affiliated with the group. In general, synchronization provides an element of control, whereby a system or network operator is better able to manage the level of co-channel and adjacent channel interference through careful allocation of frequencies and frequency offsets. However, in order to achieve this additional control, in a system that employs frequency hopping techniques, it is necessary for all synchronized cells to follow the same frequency hopping sequence (i.e., a reference frequency hopping sequence). By providing a mechanism to better control interference through proper and prudent frequency and frequency offset allocation, system performance may be significantly improved. Frequency offset management is particularly useful if fractional loading is employed, as will be discussed further below.
FIG. 1
illustrates a subset of cells A, B, C, A′, B′ and C′ associated with a synchronized, cellular radio telecommunications system
100
. System
100
, as shown, employs a frequency reuse plan, and more particularly, a one-reuse plan, as all frequencies are potentially used by each cell. Thus, in accordance with the example illustrated in
FIG. 1
, a mobile station operating in cell A may simultaneously operate over the same frequency as a mobile station operating in cell A′.
In addition, system
100
employs fractional loading and offset management. In accordance with fractional loading and offset management, the group of cells comprising cells A, B and C is allocated a common set of frequencies. If, for example, twelve frequencies 1-12 are allocated, there are inherently twelve frequency offsets 0-11. Then, in any one cell, a fraction of the twelve frequency offsets is assigned. For instance, cell A may be assigned frequency offsets 1, 4, 7, 10, where it will be understood that it is preferred to have at least a few frequency units between each frequency offset assigned to each cell so as to mitigate adjacent channel interference.
In accordance with conventional frequency hopping techniques, a reference frequency hopping sequence is established for the entire system. A mobile station, at handover or call set-up, is then assigned an available frequency offset associated with the cell in which the mobile station is operating. The mobile station hops through a sequence of frequencies that are, over time, offset from the reference frequency hopping sequence by a fixed amount that is equal to its assigned frequency offset. In accordance with the GSM standard, each frequency offset is referred to as a Mobile Allocation Index Offset (i.e., MAIO).
To better illustrate conventional frequency hopping,
FIG. 2
depicts an exemplary, reference frequency hopping sequence, over a time period t
1
-t
10
, for the telecommunications system
100
. As shown, the reference frequency hopping sequence over the time period t
1
-t
10
is [9,5,11,1,3,9,12,10,7,8]. If a first mobile station operating in cell B is, for example, assigned frequency offset zero, the first mobile station will hop through the sequence [9,5,11,1,3,9,12,10,7,8] over the time period t
1
-t
10
. If a second mobile station operating in cell A is assigned frequency offset sev
Craig Stephen G.
Edgren Erik
Magnusson Sverker
Thurfjell Magnus
Burns Doane Swecker & Mathis L.L.P.
Telefonaktiebolaget LM Ericsson (publ)
Vo Don N.
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